Definition: What Makes a Magnet "Permanent"?

A permanent magnet is a material that generates its own persistent magnetic field without requiring an external power source. Unlike an electromagnet — which needs continuous electrical current — a permanent magnet retains its magnetization indefinitely under normal operating conditions.

The defining characteristic of permanent magnets is their ability to resist demagnetization. This resistance is quantified by a property called coercivity (H_c) — the higher the coercivity, the harder it is to demagnetize the magnet. Materials with high coercivity are called "hard" magnetic materials, while easily magnetized (and demagnetized) materials are called "soft."

The Atomic Origin of Magnetism

Magnetism at the atomic level arises from two sources: the spin of electrons around their own axis, and the orbital motion of electrons around the nucleus. Each electron behaves like a tiny bar magnet. In most materials, electrons pair up with opposite spins and cancel each other out, producing no net magnetic moment.

In ferromagnetic materials — iron, nickel, cobalt, and certain rare-earth alloys — unpaired electrons create a non-zero magnetic moment. Furthermore, quantum mechanical exchange interactions cause neighboring atoms to align their magnetic moments in the same direction, forming regions of uniform magnetization called magnetic domains.

Magnetic Domains

In an unmagnetized piece of iron, domains exist with their magnetization pointing in random directions. The net magnetization of the entire piece is therefore zero. When exposed to an external magnetic field, domains aligned with the field grow at the expense of misaligned domains — a process called domain wall motion — until the material becomes magnetized.

Hard vs. Soft Magnetic Materials

How Are Permanent Magnets Manufactured?

The manufacturing process for sintered neodymium magnets — the most common high-performance permanent magnets — involves several carefully controlled steps:

1. Alloy preparation: Neodymium, iron, and boron are melted together in precise ratios (approximately Nd₂Fe₁₄B) using vacuum induction melting.
2. Milling: The alloy is broken into fine powder (3–5 microns) using hydrogen decrepitation followed by jet milling in an inert atmosphere.
3. Pressing: Powder is aligned in a strong magnetic field and pressed — either die-pressed (axial) or isostatically pressed — to create a "green" compact.
4. Sintering: The compact is sintered at ~1050–1100°C in a vacuum furnace, achieving >99% theoretical density.
5. Machining: The brittle sintered block is cut and ground to final shape using diamond-tipped tools.
6. Coating: A protective coating (typically Ni-Cu-Ni) is applied to prevent corrosion.
7. Magnetizing: The finished part is exposed to a strong pulsed field (>3T) to magnetize it fully along its easy axis.

Frequently Asked Questions

The Six Core Properties on Every Magnet Datasheet

Remanence (B_r) — The "Strength" Number

Remanence is the magnetic flux density remaining in a fully saturated magnet after the external magnetizing field is removed. It represents the maximum flux the magnet can produce in a closed circuit (zero air gap). A higher B_r means more flux is available for a given magnet volume.

For N52 grade NdFeB, B_r ≈ 1.42–1.48 T. For a typical ceramic ferrite, B_r ≈ 0.38–0.40 T — nearly 4× lower, which is why neodymium magnets produce much stronger fields for the same size.

Coercivity (H_cj) — Resistance to Demagnetization

Coercivity measures how strongly the magnet resists being demagnetized by an opposing magnetic field. There are two coercivity values on datasheets:

• H_cb (Inductive coercivity): The field required to reduce B to zero in the demagnetization curve. Less commonly used for magnet selection.
• H_cj (Intrinsic coercivity): The field required to reduce the intrinsic magnetization M to zero. This is the value that matters for demagnetization resistance — always check H_cj when specifying.

Maximum Energy Product (BH_max) — The Figure of Merit

BH_max is the most commonly cited single-number measure of a magnet's "strength." It represents the maximum product of B and H on the demagnetization curve and describes how much magnetic energy can be stored per unit volume. The grade number in NdFeB naming (e.g., N42, N52) directly encodes this value in MGOe.

Curie Temperature (T_c) and Maximum Operating Temperature

The Curie temperature is the point at which thermal energy overcomes magnetic order and the material becomes paramagnetic (loses all permanent magnetism). For NdFeB, T_c ≈ 310–320°C.

However, the maximum operating temperature (T_max) is substantially lower because irreversible flux loss begins well below T_c. For standard N-grade NdFeB, T_max is just 80°C. Higher-coercivity grades extend this:

Temperature Coefficients (α and β)

Magnetic properties change reversibly with temperature according to temperature coefficients. For NdFeB:

• α (alpha) — Temperature coefficient of B_r: Typically –0.11 to –0.13 %/°C. B_r drops ~11% for every 100°C rise.
• β (beta) — Temperature coefficient of H_cj: Typically –0.55 to –0.65 %/°C. Coercivity drops ~55% for every 100°C rise for standard grades.

These reversible losses recover when the magnet cools back down. Irreversible losses — caused by operating above T_max — do not recover without re-magnetization.

What Is a Neodymium Magnet?

Neodymium magnets — officially designated Nd₂Fe₁₄B — are a family of rare-earth permanent magnets first developed independently by General Motors and Sumitomo Special Metals in 1984. They are the strongest type of permanent magnet commercially available today, with energy products up to 52 MGOe — more than 10× that of ceramic ferrite magnets.

The alloy consists primarily of neodymium (Nd), iron (Fe), and boron (B) in a tetragonal crystal structure that creates extremely high magnetocrystalline anisotropy. Small additions of dysprosium (Dy) or terbium (Tb) are used in high-temperature grades to boost coercivity.

Sintered vs. Bonded NdFeB

Grade System Overview

NdFeB grades follow the pattern [N][energy product][temperature suffix]. The energy product number runs from 28 to 55 MGOe; the temperature suffix indicates the operating temperature range. Common grades stocked by Radial Magnets:

Strengths and Limitations

China's Role in the NdFeB Supply Chain

China produces approximately 85–90% of the world's NdFeB magnets and controls a dominant share of global rare earth mining and processing capacity. This concentration creates procurement risk for manufacturers dependent on a single source region.

Radial Magnets works directly with qualified magnet manufacturers and maintains buffer inventory to mitigate supply disruption risk. For critical applications, we recommend reviewing our Rare Earth Supply Chain guide for strategic stocking considerations.

At-a-Glance Performance Table

When to Choose Each Material

Choose NdFeB when…

• Maximum energy density is required (smallest possible magnet for the job)
• Operating temperature stays below 80°C (or up to 230°C with high-coercivity grades)
• Cost efficiency matters at moderate-to-high volumes
• A protective coating can be applied and maintained

Choose SmCo when…

• Operating temperatures exceed 200°C
• High corrosion resistance is needed without a coating
• The application is in aerospace, defense, or downhole oil & gas
• Cobalt supply risk and cost premium are acceptable

Choose Alnico when…

• Extremely high temperature stability is required (>450°C)
• The magnet must be cast into a complex shape
• Low coercivity is acceptable (Alnico demagnetizes easily — it must be shunted when disassembled)
• Applications in musical instrument pickups, instruments, and meters

Choose Ceramic Ferrite when…

• Cost is the primary driver and performance requirements are modest
• Large volume and bulk weight are acceptable
• Inherent corrosion resistance is needed without coating
• Applications in refrigerator magnets, loudspeakers, DC motors, and magnetic separators

Why Coatings Are Critical for NdFeB Magnets

Sintered neodymium magnets are composed of Nd₂Fe₁₄B grains bound by a neodymium-rich intergranular phase. This Nd-rich phase is highly reactive — in the presence of moisture, oxygen, or salt, it oxidizes preferentially, causing the magnet to swell, crack, and eventually disintegrate. A magnet left uncoated in a humid environment may degrade within days.

Unlike SmCo or ferrite, NdFeB always requires a protective coating for any real-world application. The coating choice should be matched to the environmental exposure, temperature range, dimensional tolerance requirements, and any mating surface considerations (electrical contact, bonding, etc.).

Coating Comparison Table

The Standard: Nickel-Copper-Nickel (Ni-Cu-Ni)

The industry default for NdFeB magnets is a triple-layer electroplated coating: an initial nickel layer, a middle copper layer for adhesion, and an outer nickel layer for hardness. This system provides:

• Bright metallic appearance and smooth surface finish
• Good wear resistance for assembly handling
• Electrical conductivity (useful for some applications, problematic for others — eddy currents)
• 24–96 hours salt spray per ASTM B117

Ni-Cu-Ni is the right choice for the vast majority of indoor industrial applications where moisture exposure is incidental rather than sustained.

High-Humidity and Outdoor Applications: Epoxy + Nickel

For outdoor installation, marine environments, or applications involving regular water exposure, a combination of Ni-Cu-Ni undercoat plus an epoxy topcoat provides substantially better protection. The epoxy creates a polymer barrier that dramatically slows moisture ingress, extending useful life in humid environments from weeks to years.

Why Temperature Grade Matters

All NdFeB magnets experience a loss of coercivity (H_cj) as temperature rises. At sufficiently high temperatures, the working point on the demagnetization curve crosses into the irreversible loss region — and the magnet permanently loses flux. This is why the grade suffix is not a minor detail: choosing the wrong grade for a hot application causes field decay over time that no amount of re-magnetizing will prevent without first fixing the root cause.

Grade Reference Table

The Role of Dysprosium and Terbium

Higher-temperature grades achieve their enhanced coercivity by substituting dysprosium (Dy) or terbium (Tb) for some of the neodymium in the Nd₂Fe₁₄B lattice. Both are heavy rare earth elements (HREEs) that dramatically increase magnetocrystalline anisotropy — and therefore H_cj — at the cost of slightly reducing B_r.

This is why high-temperature grades have a trade-off: a given energy product (e.g., 42 MGOe) requires a larger magnet at SH grade than at N grade, because B_r is slightly lower. Supply of Dy and Tb is concentrated in China and subject to significant price volatility.

How to Select the Correct Grade

1. Determine maximum operating temperature — measure or simulate the magnet temperature at worst-case load and ambient conditions.
2. Add a 20°C safety margin — always spec to a grade rated at least 20°C above your measured maximum.
3. Perform load-line analysis — check that the working point at maximum temperature remains above the knee of the demagnetization curve.
4. Verify with supplier — request full B-H curve data at operating temperature, not just room-temperature datasheet values.

The Complete Magnet Specification Checklist

A complete magnet specification eliminates ambiguity and ensures the part you receive performs as designed. Missing even one of these parameters can result in incorrect quotes, long revision cycles, or worse — parts that fail in service.

1. Geometry and Dimensions

• Shape: Disc, ring, block, arc, cylinder, sphere, or custom
• Dimensions with tolerances: All critical dimensions in mm with ± tolerances (e.g., Ø12.5 ±0.05 mm × 3.0 ±0.05 mm)
• Drawing: Always provide a dimensioned 2D drawing or 3D model (STEP preferred)
• Surface finish: Specify if Ra or flatness is critical for assembly

2. Material and Grade

• Material: NdFeB, SmCo, Alnico, or Ferrite
• Grade: Specific grade (e.g., N42SH) or minimum property requirements (B_r ≥ X, H_cj ≥ Y, BH_max ≥ Z)
• Maximum operating temperature
• Sintered or bonded (for NdFeB)

3. Magnetization Direction

• Axial (through thickness), diametric, radial, multi-pole, or custom orientation
• Mark the north pole face on the drawing
• Specify if the magnet should be delivered magnetized or unmagnetized

4. Coating

• Coating type (Ni-Cu-Ni, Zn, epoxy, gold, Parylene, etc.)
• Thickness range or maximum (important for press fits and assemblies)
• Salt spray requirement (hours per ASTM B117)

5. Performance Requirements

• Minimum surface gauss or tesla at a specified test point
• Flux per pole (for multipole rings)
• Magnetic moment (for torque calculations)

6. Quality and Qualification Requirements

• Inspection standards (IEC 60404, customer-specific)
• PPAP level (for automotive)
• Certificate of conformance / material traceability
• RoHS / REACH compliance requirements

7. Commercial Information

• Annual volume estimate and initial sample quantity
• Target lead time
• Packaging requirements (tape-and-reel, tray, bulk)
• Delivery destination and Incoterms

Common RFQ Mistakes That Delay Your Quote

Why Electric Vehicles Depend on Rare Earth Magnets

The dominant motor topology in modern battery electric vehicles is the Interior Permanent Magnet Synchronous Motor (IPMSM), where NdFeB magnets are embedded in the rotor lamination stack. This design achieves the highest power density, efficiency, and torque-to-weight ratio of any motor type — critical for maximizing vehicle range.

A typical passenger EV traction motor contains 1–4 kg of sintered NdFeB, depending on power output. A high-performance vehicle motor (300+ kW) may use 3–5 kg. At global EV production volumes, this creates enormous demand — and significant supply chain pressure — on the rare earth sector.

Key Magnet Requirements for EV Traction Motors

Magnet Configurations in EV Rotors

Surface-Mounted Permanent Magnet (SPM)

Magnets are bonded to the outer surface of the rotor core. Simpler design but limited to moderate speeds due to centrifugal force on the magnet retainer. More common in lower-speed applications like hub motors and some commercial vehicles.

Interior Permanent Magnet (IPM)

Magnets are embedded in slots cut into the rotor lamination stack. The steel surrounding the magnets contains them against centrifugal force — enabling very high rotor speeds (>15,000 RPM). The dominant topology in passenger EV traction motors (used by Tesla, BMW, GM Ultium, and others).

Spoke / V-Type / Delta Arrangements

Advanced IPM rotors arrange magnets in spoke, V, or delta patterns to concentrate flux and further boost torque density. These require precision-ground flat bar or custom-profile magnets and very tight positional tolerances.

Other Magnet Uses in an EV

Beyond the traction motor, a modern electric vehicle contains magnets in dozens of subsystems:

• Transmission sensors: Position and speed sensors using disc or ring magnets for commutation feedback
• Steering: Electric power steering (EPS) motor — typically a PMSM with arc segment NdFeB
• Braking: Regenerative braking generator magnets; ABS speed sensors
• HVAC: Compressor motor (scroll compressor driven by 3-phase PMSM)
• Charging: OBC and DC-DC converter transformers using soft magnetic cores
• Latches and closures: Door, frunk, and charge port latches using disc or block holding magnets
• Speakers: High-output cabin audio systems using NdFeB speaker drivers

Personal Safety Rules

• Always wear leather gloves when handling magnets larger than 20mm in any dimension
• Never allow two large magnets to come together freely — bring them together slowly and under control, with a non-magnetic spacer
• Keep magnets away from pacemakers and implantable devices — maintain at least 12" (30cm) from any person with a cardiac device
• Wear safety glasses — magnets can chip and shatter when they collide at high speed
• Never machine or drill magnetized parts — magnetic swarf is a fire and inhalation hazard; machine only unmagnetized blanks
• Neodymium powder and swarf are flammable — use proper containment and do not use standard fire extinguishers on burning NdFeB

Storage Requirements

• Store in cool, dry conditions — humidity accelerates corrosion even on coated magnets
• Keep magnets separated from ferrous tools, fixtures, and components; attraction forces across storage containers can exceed the container's structural strength
• Use interleaving spacers (wood, cardboard, plastic) to prevent magnets from attracting across shelves
• Maintain minimum 1 meter (3 feet) clearance from magnetic storage media, CRT displays, and precision instruments
• Label storage areas with magnetic field warning signs per ANSI Z535 standards

Shipping Regulations

Magnetized products that produce a magnetic field of 0.00525 Gauss or more at 7 feet (2.1m) from the package surface are classified as magnetized material under IATA Dangerous Goods Regulations (Section 2.3.5.9) and ICAO technical instructions. Shipments that exceed this threshold require:

• Magnetic shielding (mu-metal or steel enclosure around the package)
• IATA Class 9 dangerous goods declaration
• Compass deviation testing per IATA standards
• Carrier pre-approval for air shipment

Radial Magnets handles all IATA/ICAO documentation and proper magnetic shielding for international and domestic air shipments. Ground shipping by UPS/FedEx has no magnetic field threshold but still requires proper packaging to prevent magnets from moving and attracting to vehicle structures.

The Geographic Concentration Problem

China dominates every stage of the NdFeB supply chain: rare earth mining (~60–70% of global output), separation and refining (~85–90%), magnet alloy production, and finished magnet manufacturing (~85–90%). This concentration means that tariff changes, export controls, environmental regulations, or geopolitical events in China can disrupt global magnet supply with minimal warning.

The 2010 rare earth crisis — in which China briefly restricted rare earth exports, causing neodymium prices to spike by more than 600% in 12 months — demonstrated how quickly supply disruptions can cascade into production shutdowns for downstream manufacturers. The lesson: treating magnets as a commodity purchased on spot pricing without safety stock is a significant operational risk.

Key Volatility Drivers to Monitor

• Chinese export quotas and tariffs: Periodically adjusted; sudden restriction events are not uncommon
• Heavy rare earth (Dy, Tb) supply: Dysprosium and Terbium are geographically concentrated even within China's producing regions; supply is tighter than Nd
• Energy costs: Magnet production is energy-intensive; electricity price increases in China translate to finished magnet prices
• Environmental compliance: Periodic crackdowns on rare earth mining and processing have caused multi-week production shutdowns
• EV demand growth: Rapidly rising demand from the EV sector tightens available supply for all other magnet users
• Ocean freight and port congestion: 4–6 week transit times from China mean lead times extend quickly during shipping disruptions

Strategic Stocking Recommendations

For manufacturers dependent on NdFeB magnets in production, Radial Magnets recommends:

1. Minimum 90-day safety stock on all critical magnet SKUs — enough to absorb a factory-level supply disruption without production impact
2. Blanket purchase orders with scheduled releases — secure inventory at current pricing with flexible delivery windows
3. Dual qualification — qualify at least two magnet suppliers (and two mills if possible) for any single critical part number
4. Annual spend review — track market pricing quarterly; buy opportunistically during price softness
5. Grade flexibility where possible — where your design can accept N40SH or N42SH, qualifying both gives sourcing flexibility when one grade tightens

What Is PPAP?

The Production Part Approval Process (PPAP), defined by the Automotive Industry Action Group (AIAG) in its PPAP 4th Edition reference manual, is a standardized supplier qualification and approval process used primarily in the automotive industry. PPAP establishes documented evidence that a supplier's manufacturing process can consistently produce parts that meet all engineering design requirements.

For magnet suppliers, PPAP submission is increasingly required not only for direct automotive OEM tiers but also for Tier 2 and Tier 3 suppliers building sub-assemblies (sensors, actuators, motors) that feed the automotive supply chain.

PPAP Submission Levels

Key PPAP Elements for Magnet Parts

Process Flow Diagram

Maps the complete manufacturing sequence from raw material receipt through shipping. For sintered NdFeB magnets, this includes: incoming material inspection → alloy preparation → milling → pressing → sintering → machining → coating → magnetizing → inspection → packaging.

PFMEA (Process Failure Mode and Effects Analysis)

Identifies potential failure modes at each manufacturing step, their effects on the part, and the controls in place to prevent or detect them. For magnets, high-severity failure modes include incorrect grade material, undercured coating, improper magnetization direction, and dimensional non-conformance on critical features.

Control Plan

Documents the inspection and process controls at each stage. For magnets, critical control points typically include: incoming material traceability, dimensional inspection (CMM or optical comparator), magnetic property testing (Gaussmeter or Helmholtz coil), coating adhesion and thickness testing, and final part marking/traceability.

MSA (Measurement System Analysis)

Validates that the inspection gages and measurement systems used are adequate to detect conforming vs. non-conforming parts. For magnets, this includes Gage R&R studies on dimensional inspection and magnetic flux measurement systems.

SPC (Statistical Process Control)

Demonstrates that key processes are in statistical control. Magnets typically require SPC on critical dimensions and magnetic properties (surface gauss or total flux). A minimum Cpk ≥ 1.67 is required for new submissions under AIAG PPAP 4th Edition.

Why Magnetization Is a Manufacturing Step, Not a Material Property

A freshly sintered NdFeB blank has all the microstructure needed for strong permanent magnetism, but it starts life magnetically neutral. The individual crystalline grains are aligned during pressing, but without an applied external field they produce no net magnetic moment. Magnetization — exposing the part to a controlled impulse field exceeding its coercive force — converts a machined blank into a functional magnet.

This matters for procurement: magnets can be shipped unmagnetized to avoid handling hazards and attract-force shipping restrictions, then magnetized in the end-use assembly fixture. Most custom magnets from Radial Magnets ship fully magnetized unless otherwise specified.

Magnetic Domain Alignment

Inside a ferromagnetic material, atoms with unpaired electrons align their magnetic moments within small regions called magnetic domains. In an unmagnetized piece, domains point in random directions and cancel out. During sintering of NdFeB, an aligning field is applied while the material is still in powder form — crystallographically aligning the easy-axis of each grain so they can all be magnetized in the same direction. After sintering and machining, a magnetizing pulse snaps the moments into alignment along that easy axis.

Magnetizing Equipment and Field Requirements

Saturation magnetization of NdFeB requires an applied field of at least 3× the intrinsic coercivity (Hcj). For N-grade at room temperature, Hcj ≈ 10–12 kOe, so a 30–36 kOe impulse field is required. Higher-coercivity grades (UH, EH) need proportionally stronger fixtures.

Magnetization Orientations

• Axial: Field parallel to the cylinder or disc axis — north and south poles on flat faces.
• Diametric: Field perpendicular to the cylinder axis — poles on curved surface.
• Radial: Field radiating outward from center — requires custom radial magnetizing fixtures; common for motor arc segments.
• Multipole: Multiple alternating poles encoded around a ring or disc using a multi-tooth fixture — used in encoders and sensing applications.

Handling After Magnetization

• Store magnetized magnets separated by keeper plates or foam spacers to prevent snap-together damage and cracking.
• Keep magnetized magnets >12" from hard drives, pacemakers, credit card strips, and other field-sensitive devices.
• Machining after magnetization is not recommended — sparks from NdFeB grinding can ignite in the presence of a strong flux field.

The Four Demagnetization Mechanisms

Temperature Demagnetization

NdFeB has large temperature coefficients: Br decreases at approximately −0.12%/°C and Hcj at approximately −0.6%/°C. These are reversible losses — cool the magnet and full flux returns. The irreversible problem occurs when temperature is sustained long enough for grain boundary phases to restructure or when coercivity can no longer prevent domain reversal.

Operating Point and the B-H Curve Knee

The single most important demagnetization analysis is identifying the operating point on the B-H demagnetization curve at maximum temperature, and ensuring it stays above the knee. Below the knee, any additional reverse field causes catastrophic irreversible loss.

• High Pc (thick magnet, short air gap) → operating point high on curve, far from knee → safe
• Low Pc (thin magnet, large air gap, strong opposing field) → risk of demagnetization near or below knee

NdFeB grades with higher suffix letters (H, SH, UH) have higher Hcj, shifting the knee further left — allowing operation in stronger opposing fields without irreversible loss. This is the primary reason to specify a higher grade even when temperature itself isn't the only concern.

Measuring Flux Loss

Post-demagnetization flux measurement is performed with a fluxmeter and Helmholtz coil (for total flux) or a Gaussmeter at a defined measurement point. A control plan should specify measurement frequency and acceptance limits (typically ±5% of nominal flux).

Two Alloy Systems

Why SmCo Instead of NdFeB?

• Operating temperature >150°C: NdFeB EH-grade tops at ~200°C; Sm₂Co₁₇ reaches 350°C with far smaller flux loss per degree.
• Temperature stability in precision instruments: SmCo's Br temp coefficient (−0.03%/°C) is 4× more stable than NdFeB's — critical for sensors, meters, and traveling-wave tubes.
• Corrosive environments without coating: SmCo does not require a protective coating in most industrial and aerospace environments.
• Radiation hardness: SmCo is preferred for space and nuclear applications where radiation degrades NdFeB more rapidly.
• Long-term stability: SmCo shows minimal aging flux loss over decades — important for reference magnets and instruments expected to operate without recalibration for 10–20 years.

SmCo Grade Table

Brittleness and Machining

SmCo is extremely brittle — more so than NdFeB. It chips and cracks easily. All precision grinding should be performed wet with diamond wheels before magnetization. Avoid mechanical clamping that applies point loads — use conformal fixtures.

Composition and Structure

Alnico is an alloy of Aluminum, Nickel, and Cobalt, with iron as the base and small amounts of copper and titanium. Produced either by casting or sintering — cast Alnico achieves higher magnetic properties.

Alnico Grade Comparison

Strengths of Alnico

• Highest operating temperature: Alnico operates up to 450–550°C — far exceeding SmCo (350°C) or NdFeB (230°C).
• No rare earth content: Insulated from rare earth supply chain price volatility.
• Stability over decades: Properly stabilized Alnico shows virtually no aging flux loss over 20–50 years.

Limitations: Low Coercivity

Alnico's critical limitation is very low coercivity — Hcb of 640–1650 Oe vs. 10,000–30,000+ Oe for NdFeB or SmCo. Alnico magnets are easily demagnetized by opposing fields, mechanical shock, or improper storage without keeper plates.

What Is a Ceramic Magnet?

Ceramic (ferrite) magnets are made from iron oxide combined with strontium carbonate or barium carbonate. Strontium ferrite is the dominant commercial type. Raw materials are abundant, inexpensive, and entirely free of rare earth elements — making ferrite immune to rare earth supply chain volatility. Ferrite does not corrode and requires no protective coating.

Ferrite Grade Properties

Unique Temperature Behavior

Ferrite coercivity (Hcj) increases as temperature decreases — opposite to NdFeB, which loses coercivity rapidly on cooling below room temperature.

When to Choose Ferrite

• Cost is primary constraint: Ferrite is 10–30× less expensive per unit energy than NdFeB.
• No coating required: Outdoor, wet, and chemical environments where NdFeB would corrode are natural fits.
• High volume production: Ferrite tooling and production costs are lower; injection-molded bonded ferrite achieves complex shapes economically.

D/H Ratio and Permeance Coefficient

Pull Force vs. Air Gap

Pull force falls off rapidly with air gap. Practical rules for disc magnets:

• Rated pull force is measured steel-to-steel contact with no air gap.
• At 1 mm air gap, pull force typically drops 30–50% of contact value.
• At 3 mm air gap, pull force is often less than 25% of contact value.
• At 10 mm, most small disc magnets produce negligible usable force.

Diametrically Magnetized Discs

Disc magnets can be supplied diametrically magnetized (poles on the curved surface). This is common for angular position sensing — a diametrically magnetized disc rotated above a Hall sensor produces a sinusoidal output proportional to rotation angle — and for BLDC motor feedback.

Magnetization Options for Ring Magnets

Ring Magnets in Encoder Applications

Multipole ring magnets are the sensing element in most incremental and absolute magnetic encoders. Key parameters to specify:

• Pole count: Matched to encoder resolution requirement.
• Pole pitch uniformity: Angular spacing errors translate to position errors. Specify ±2% or better for precision encoders.
• Air gap to sensor: Typically 0.5–1.5 mm; confirm with sensor IC datasheet (peak-to-peak field typically 20–100 mT).

Standard Dimensional Tolerances (Sintered NdFeB)

IPM Rotor Applications

Block magnets are the primary magnet geometry in Interior Permanent Magnet (IPM) motor rotors. Rectangular NdFeB blocks are inserted into slots in the rotor lamination stack, enabling flux concentration configurations (V-shaped, U-shaped, spoke-type) for high power density.

Halbach Arrays

Alternating-angle block magnet assemblies can form a Halbach array — concentrating flux on one side while nearly canceling it on the other. Used in linear motors, magnetic levitation bearings, and high-efficiency voice coil actuators. Proper angular tolerance (≤±0.2°) is critical for Halbach performance.

Pole Arc Ratio

The pole arc ratio is the magnet's subtended angle divided by the pole pitch (360°/number of poles). For an 8-pole motor, pole pitch = 45°. A magnet subtending 40° has a pole arc ratio of 0.89.

Parallel vs. Radial Magnetization

• Parallel: All flux lines are parallel — equivalent to magnetizing a block and bending it. Simple to produce; less costly. Back-EMF waveform is slightly trapezoidal.
• Radial: Flux lines radiate from the rotor center. Produces more sinusoidal back-EMF and more uniform air gap flux density. Requires custom radial fixture; higher cost but improved motor performance.

Quick-Reference: Magnetization Directions by Shape

Diametric Magnetization

For a cylinder or disc, the magnetizing field is applied perpendicular to the axis — across the diameter. When a diametrically magnetized cylinder or disc rotates, a fixed Hall sensor measures a sinusoidal field — making this the standard configuration for rotary angle sensing and BLDC motor Hall feedback.

Multipole Magnetization

A multipole fixture encodes multiple alternating N/S pole pairs into a single magnet face or surface. The number of poles must be specified (always an even number). Pole count options range from 2 to 128+ for encoder rings. The pole pitch must match the sensor IC's optimal field amplitude — consult the sensor datasheet for field strength requirements at your target air gap.

Why NdFeB Corrodes So Readily

Sintered NdFeB is a multiphase material. The main magnetic phase is Nd₂Fe₁₄B, but grain boundaries contain a distinct Nd-rich phase that is:

• Highly chemically reactive — neodymium is a very electropositive rare earth
• Continuously interconnected throughout the magnet cross-section
• An anode relative to the Nd₂Fe₁₄B main phase — galvanic corrosion accelerates the attack

Once moisture reaches the grain boundaries, it attacks the Nd-rich phase, dissolving it and causing individual Nd₂Fe₁₄B grains to separate. The magnet literally crumbles from the inside out.

The Pinhole Problem

A properly applied coating is very effective — but only as good as its most compromised point. Even a single pinhole, scratch, or edge chip can initiate a corrosion cell that propagates laterally under the coating, causing blistering and delamination. This is why edge coverage is critical, and coatings must be matched to the severity of the end-use environment.

The Corrosion's Effect on Magnetic Properties

The corrosion attacks the Nd-rich grain boundary phase, which is critical for maintaining high coercivity. As corrosion progresses, Hcj drops, making the magnet more susceptible to demagnetization. In advanced corrosion, the magnet loses structural integrity and may have only 50–70% of its original flux.

Step 1 — Define the Operating Environment

Step 2 — Check Temperature Limits

Step 3 — Account for Dimensional Budget

Step 4 — Regulatory and Biocompatibility Requirements

• Medical implants: Gold or Parylene C — both pass ISO 10993 cytotoxicity. Nickel is a known allergen — not suitable for implantable use.
• Food contact: PTFE (FDA-compliant grades) or gold.
• RoHS / REACH: All standard coatings are RoHS compliant. Confirm with supplier documentation.

Key Application Categories

Biocompatibility and Coating Requirements

MRI Safety Considerations

• MRI force/torque: Strong static fields (1.5T or 3T) exert translational force and torque on permanent magnets. Implantable magnets must meet ASTM F2052 and F2213 force/torque limits.
• MRI heating: RF pulses induce eddy currents; small magnets typically have minimal heating but must be evaluated per ASTM F2182.
• Demagnetization: Rotating gradient fields can partially demagnetize implanted magnets over time, shifting coupling force in cochlear devices.

Sensing Technologies and Magnet Requirements

Multipole Ring Encoders: Resolution Reference

Key Application Parameters to Specify

• Pole count (always even)
• Ring OD, ID, height and grade
• Required peak field at air gap distance (from sensor IC datasheet)
• Pole pitch uniformity tolerance (typically ±2% for encoder applications)
• Magnetization direction (axial face vs. OD surface)

Why Aerospace Chooses SmCo

• Temperature stability: SmCo's Br temp coefficient (−0.03%/°C) is 4× more stable than NdFeB's, critical for −55°C to +200°C+ operational swings.
• No coating required: Eliminates a potential failure mode from coating degradation in conformal or vacuum environments.
• Radiation hardness: Substantially more radiation-hard than NdFeB — preferred for space particle radiation environments.
• Long flight heritage: SmCo has 40+ years of flight heritage in satellites, gyroscopes, and aircraft actuators.

Common Applications and Typical Grades

Quality Standards

Key Application Specifications

Electropermanent Magnet (EPM) Grippers

EPM grippers combine NdFeB and Alnico magnets that can be switched between "on" (holding) and "off" (released) with a brief current pulse. Unlike electromagnets, they consume power only during switching — not continuously while holding. This makes them ideal for collaborative robot grippers and pick-and-place systems handling ferromagnetic parts.

Linear Motor Tracks

Linear motors eliminate mechanical transmission, enabling higher throughput and positioning accuracy. NdFeB block magnets form the stationary magnet track, assembled from individual blocks in alternating polarity bonded to a steel back-plate. Block magnets must have consistent thickness (±0.05 mm) to maintain uniform air gap and minimize thrust ripple.

Two Fundamentally Different Manufacturing Routes

Neodymium magnets are produced by two distinct processes that result in very different products — even though both start from the same Nd₂Fe₁₄B alloy. Sintered NdFeB is pressed and heat-treated to near full density, producing the strongest permanent magnets available. Bonded NdFeB suspends magnetic powder in a polymer binder and forms it at room temperature or low heat, trading magnetic performance for shape versatility and design freedom.

The choice between them is rarely obvious from a spec sheet alone — it depends on the geometry of your part, your volume, the air gap in your circuit, and how much field strength you actually need.

Side-by-Side Comparison

How Sintered NdFeB Is Made

Sintered magnets are produced by a powder metallurgy process. The alloy is melted, cast, and reduced to a fine powder (3–5 microns) by hydrogen decrepitation and jet milling. The powder is pressed in a strong magnetic field to align the crystalline easy axes — this anisotropy is what gives sintered magnets their high energy product. The pressed compact is sintered at ~1,060–1,100°C in a vacuum furnace, then cut and ground to final shape using diamond tools, coated, and magnetized.

How Bonded NdFeB Is Made

Bonded magnets start from rapidly quenched (melt-spun) or HDDR-processed powder mixed with a polymer binder at 10–30% by volume. The compound is then formed by injection molding, compression bonding, or extrusion. Because the powder is suspended in a binder rather than sintered together, bonded magnets have lower flux density — but the isotropic nature of most bonded grades means the part can be magnetized in any direction after molding, including multipole patterns impossible to achieve with sintered magnets in a single piece.

When to Choose Sintered

• Maximum field strength per unit volume is required
• Part geometry is a standard disc, ring, block, or arc segment machined from sintered blank
• Operating temperature may exceed 120°C
• Large production volumes where per-unit cost must be minimized
• Applications in traction motors, servo motors, or any design where energy density drives system size

When to Choose Bonded

• Part geometry is complex, thin-walled, or requires overmolding with a housing
• Multipole magnetization in a single piece is required (encoder rings, commutation rings)
• Tight tolerances are needed without a grinding step (molded dimensions can be held very accurately)
• Corrosion resistance is needed without a separate coating process
• Operating temperature stays below 120–150°C

When Do You Need a Custom Shape?

The vast majority of magnet applications are served by standard geometries — discs, rings, blocks, and arc segments. Custom shapes should only be considered when a standard shape genuinely cannot meet the application: integrating the magnet with a machined assembly with specific counter-bores; achieving a field pattern requiring a non-standard cross-section; or overmolding a magnet with a housing to reduce assembly steps.

Sintered Custom Shapes: What's Achievable

Sintered NdFeB is machined using diamond grinding and wire EDM after sintering. The brittleness of the material — similar to hardened ceramic — sets the practical limits:

Achievable Features

• Countersunk holes: Flat-bottomed counterbore or through-hole for fastener clearance. Minimum wall: 0.8–1.0 mm.
• Chamfers and bevels: 45° chamfers on edges reduce chip risk. Standard on production parts.
• Stepped profiles: Different OD at each end (stepped cylinder) — useful for pilot diameter fits.
• Trapezoidal cross-sections: Achievable with wire EDM or precision grinding.
• Slots and grooves: Wire EDM minimum slot width ~0.5 mm; aspect ratio (depth/width) max 3:1.
• D-cut cylinders: Flat on one side — common for orientation keying in assemblies.

DFM Rules (Sintered)

Bonded Magnet Custom Shapes: Greater Freedom

Bonded NdFeB (injection molded or compression bonded) offers far greater shape freedom. The polymer binder makes the part tougher and easier to form — complex 3D shapes are achievable in a single molding step.

• Thin walls: Down to 0.3 mm versus 0.8 mm minimum for sintered
• Overmolding: Magnetic compound molded directly around a metal insert, shaft, or housing — integrating magnet and carrier in one step
• Complex 3D profiles: Undercuts, side-cored features, non-planar surfaces achievable with standard injection mold tooling
• Multipole in one piece: Isotropic bonded NdFeB can be magnetized with any multipole pattern post-molding
• Integrated keying features: Flats, notches, and boss features molded in without secondary machining

Why Consumer Electronics Drives Magnet Miniaturization

Consumer electronics OEMs relentlessly pursue thinner profiles, lighter weight, and higher performance — all of which push toward smaller, stronger magnets. NdFeB's unmatched energy density makes it the enabling material for speaker drivers in earbuds under 5g, position sensors in foldable phones, and wireless charging alignment rings embedded invisibly in a 0.3mm glass substrate. Consumer electronics also demands extremely tight tolerances and cosmetic standards that typically exceed automotive dimensional requirements — even though the formal documentation burden is lower.

Key Applications by Device Category

Smartphones

True Wireless Earbuds (TWS) and Headphones

TWS earbuds require the smallest magnets in any consumer product — a typical 6mm driver magnet may weigh under 0.05g. The magnet must deliver a strong, uniform field across the voice coil gap (typically 0.1–0.15mm) to convert electrical signal to acoustic vibration with minimal distortion. N50–N52 pot magnet assemblies are standard for premium acoustic performance (>100 dB/mW sensitivity). Case latch magnets also trigger the Hall-effect sensor that stops playback when the earbud is removed.

Laptops and Tablets

• Lid closure magnets: Block or disc NdFeB (N38–N42) embedded near the screen edge detect lid state via Hall sensor and provide tactile latch force
• Smart connector / magnetic power: Ring magnets ensure correct connector orientation and retention (MagSafe for Mac, Surface Connect, etc.)
• Stylus attachment: Multipole bonded rings or arc segment arrays hold stylus magnetically to device edge

MagSafe and Wireless Charging Alignment

Apple's MagSafe ecosystem popularized embedded multipole magnet arrays for wireless charging alignment — a ring of precisely pitched alternating poles that creates a self-aligning torque when charger and phone approach each other. The Qi2 wireless charging standard (developed by the Wireless Power Consortium based on MagSafe concepts) extends this approach across Android and other platforms. Key requirements: bonded NdFeB ring with precisely controlled pole count and pitch; pole-to-pole field consistency within ±3%; total ring height of 1.5–2.5mm to fit within phone chassis.

What Is a Magnetic Assembly?

A magnetic assembly is a permanent magnet combined with ferrous steel (and sometimes additional magnets) in a housing or circuit designed to direct and concentrate flux for a specific purpose. The assembly achieves substantially higher effective pull force or working field than the raw magnet alone — often 3–5× improvement — by channeling stray field lines into a useful path.

Pot Magnets (Cup Magnets)

A pot magnet consists of a permanent magnet embedded in a circular steel cup, with poles directed to a flat working face. The steel cup concentrates all flux at the face — eliminating leakage through the back and sides — dramatically increasing pull force on a flat steel surface compared to the bare magnet.

Pull force is extremely sensitive to surface conditions. A 0.1mm air gap (paint or surface roughness) reduces pull force by 30–50%. A 1mm gap reduces it by 70–80%. Always specify pull force at the working gap for your application.

Halbach Arrays

A Halbach array arranges permanent magnets with sequentially rotating magnetization directions. The result is that flux concentrates on one side (the "strong side") while nearly canceling on the other — useful where field is needed only on one surface.

• Magnetic levitation (maglev): Halbach arrays generate a pattern that induces currents in a conducting track, creating repulsive lift force — the principle behind hyperloop-style systems and linear motors
• Linear motors: Halbach arrays are the rotor in high-efficiency linear motors for semiconductor manufacturing stages and robotics
• Portable MRI: Halbach cylinders create a strong, uniform bore field without power supply

Magnetic Couplings

Magnetic couplings transmit torque across a gap or through a containment wall without mechanical contact — used to drive pumps, mixers, and compressors in sealed vessels (chemical processing, pharmaceuticals) where shaft seals would leak or contaminate the product. Inner and outer rotors with arc segment NdFeB follow each other magnetically. If torque exceeds the slip rating, the poles slip out of alignment — protecting equipment without mechanical shear pins. Grade N42H or N42SH handles temperatures up to 150°C in typical process equipment.

Electropermanent Magnets (EPM)

An electropermanent magnet contains two permanent magnets of different coercivity. A brief current pulse changes the polarity of the soft magnet — switching between a holding state (full flux at working surface) and released state (flux contained internally). No continuous power is needed to maintain either state. EPM grippers are increasingly used in robotic end-of-arm tooling for metal handling, providing permanent-magnet holding force with electromagnet controllability at far lower power consumption.

Why Tolerances Matter for Magnets

Sintered NdFeB is a brittle, anisotropic ceramic-like material machined by diamond grinding — not a ductile metal shaped by conventional turning. Tolerances are real manufacturing constraints, not arbitrary numbers. Specifying tighter tolerances than necessary increases cost, extends lead time, and can degrade quality by requiring additional machining passes that introduce micro-cracking at the surface. The goal: tolerances exactly as tight as the assembly requires — no tighter.

Standard Tolerances by Shape

Disc and Cylinder Magnets

Ring Magnets

Arc / Segment Magnets

Surface Finish (Ra)

• As-ground (standard): Ra 0.8–1.6 µm — suitable for most industrial applications, bonded with adhesive
• Fine ground: Ra 0.4–0.8 µm — for assemblies requiring good contact fit
• Lapped: Ra 0.1–0.2 µm — for optical or precision mating surfaces; significant add cost

For most magnet applications bonded with epoxy, standard as-ground finish (Ra 0.8µm) is preferred — surface roughness provides mechanical key for the adhesive. Specifying a smooth finish (Ra < 0.4µm) without reason may actually reduce bond strength.

Press Fits and Interference Fits

NdFeB is brittle — excessive press force will crack it. Maximum recommended diametral interference for a sintered NdFeB press fit is 0.01–0.015mm per 10mm of diameter. Beyond this, use an adhesive-assist press or design for a light slip fit with retaining adhesive instead.

Common Tolerance Specification Mistakes

The Two Routes to an NdFeB Magnet

Every neodymium magnet starts with Nd₂Fe₁₄B alloy — but the path from raw material to finished part splits into two fundamentally different processes: sintering and bonding. The choice between them is one of the most consequential decisions in magnet design, affecting energy density, shape capability, dimensional precision, corrosion resistance, and cost.

Sintered NdFeB: Maximum Performance

Sintered NdFeB is produced by milling the alloy into a fine powder (3–5 µm), aligning the particles in a strong magnetic field, pressing them into a compact, and then sintering at ~1,050–1,100°C in vacuum to achieve full density. The result is a fully dense ceramic-like material with energy products from 26 to 52 MGOe.

The key advantage of sintering is that full grain alignment during pressing creates a strongly anisotropic material — every grain's easy axis points in the same direction. This alignment is what produces the extreme energy density that makes sintered NdFeB the strongest permanent magnet commercially available.

Bonded NdFeB: Maximum Shape Flexibility

Bonded NdFeB is produced by mixing NdFeB magnetic powder with a polymer binder (nylon, epoxy, or rubber) and forming the mixture by injection molding, compression molding, or calendering. The process is closer to plastic part manufacturing than to ceramics — tooling produces near-net-shape parts with very little post-process machining required.

The trade-off is energy density: the polymer binder occupies 10–40% of the part volume and contributes no magnetic energy. Bonded NdFeB typically achieves 5–12 MGOe — about one-quarter to one-half the performance of sintered grades. Compression-bonded parts approach 12 MGOe; injection-molded parts are typically 5–8 MGOe.

Additionally, most bonded NdFeB is isotropic — the powder particles are not aligned during forming, so the magnet can be magnetized in any direction after molding. Some compression-bonded variants use pre-aligned powder for anisotropic performance, but these are less common and more expensive.

Head-to-Head Comparison

When to Choose Sintered NdFeB

• Maximum flux density is required — motor rotors, lifting magnets, high-field sensors
• The shape is simple (disc, ring, block, arc) and achievable by grinding
• Operating temperature may approach or exceed 80°C (higher-grade sintered variants available to 230°C)
• The application demands the highest coercivity and demagnetization resistance
• Volume production at lowest per-unit cost

When to Choose Bonded NdFeB

• Complex geometry is required — thin walls, integrated features, non-standard cross-sections
• Multipole magnetization in a single part (encoder rings, stepper motor magnets, brushless DC commutators)
• Very tight dimensional tolerances on molded surfaces without secondary grinding
• Moderate magnetic performance is sufficient (5–12 MGOe)
• Operating temperature stays below the binder's Tmax (~120–150°C for nylon; ~150°C for epoxy)
• The part replaces a machined sintered magnet plus separate plastic assembly features — integration reduces part count

Frequently Asked Questions

Why Shape Matters

The geometry of a magnet determines more than just where it physically fits. Shape directly controls the magnetic circuit's permeance coefficient, the direction and distribution of the external field, the surface-to-volume ratio that governs corrosion risk, and the mechanical stresses the magnet will experience in service. Getting the shape right — and understanding what's manufacturable — is as important as selecting the right grade.

Common Non-Standard Geometries

Trapezoidal and Tapered Magnets

Trapezoidal cross-sections are used in Halbach arrays, spoke-type motor rotors, and linear motors where the magnet must fit a wedge-shaped slot. These are produced by wire EDM or surface grinding from sintered blocks. Key constraint: minimum face width of the narrow end is typically 1.0–1.5 mm for NdFeB due to brittleness.

Stepped and Multi-Thickness Magnets

Magnets with a shoulder or stepped profile allow for integrated mechanical retention without an additional fastener or adhesive. Common in actuator assemblies. Achievable by grinding — but each surface add cost and increases chip risk during machining.

Countersunk and Counterbored Magnets

Through-holes and countersinks for fastener mounting are possible in sintered NdFeB using diamond core drills or wire EDM, but they are expensive and introduce significant cracking risk. Wall thickness around a hole should be at least 2× the hole diameter. For high-volume parts, injection-molded bonded NdFeB with molded-in holes is often a better alternative.

Curved and Contoured Faces

Concave or convex pole faces alter the field distribution and can concentrate flux over a specific target area. Used in magnetic field therapy devices, pick-and-place tooling, and precision positioning systems. Produced by profile grinding on a CNC surface grinder.

Segmented Assemblies (Pseudo-Complex Shapes)

Many "complex" shapes are better approached as assemblies of simple sintered segments bonded into a housing or potted with epoxy. A ring magnet with an internal pocket, for example, is often built as a pressed ring plus a disc bonded in situ — cheaper and more reliable than attempting to machine the feature into a single piece.

Manufacturing Constraints for Sintered NdFeB

True Radial Magnetization — Radial Magnets' Specialty

One of the most technically demanding custom magnet configurations is true radial magnetization in ring and cylinder magnets — where the magnetization direction points radially outward (or inward) from the center axis at every point around the circumference. This is fundamentally different from diametric magnetization (north on one side, south on the other) or multipole magnetization.

True radially magnetized rings are used in brushless DC motors, voice coil actuators, linear motors, and rotary transducers because they produce a uniform radial field around the entire circumference — enabling smooth, cogging-free rotation. Producing them requires special magnetizing fixtures and, often, purpose-built sintered ring tooling. This is a core competency of Radial Magnets, Inc.

See our dedicated article on True Radial Magnetization for full technical detail.

The RFQ Process for Custom Shapes

1. Provide a dimensioned drawing — 2D with all tolerances, or a STEP file. Avoid PDF-only submissions for complex geometry.
2. Flag critical vs. reference dimensions — not every surface is equally important. Tell the supplier which dimensions affect assembly fit and which are reference-only.
3. State the magnetization direction — mark north and south on the drawing.
4. Discuss in-process breakage risk early — for very thin or fragile features, ask for a manufacturability review before committing to tooling.
5. Prototype first — for complex geometry, a small first article run (10–25 pieces) before full production tooling is standard practice.

Why Consumer Electronics Drives Magnet Innovation

Consumer electronics — smartphones, laptops, earphones, wearables, smart speakers — have been one of the primary forces shrinking magnets and pushing performance boundaries over the past two decades. The relentless drive for thinner, lighter, and more powerful devices demands magnets that deliver maximum flux in minimum volume, which means sintered NdFeB at the highest practical grade, often N48 to N52.

At the same time, the enormous production volumes of consumer electronics (hundreds of millions of units per year) make cost reduction equally important. The result is a fascinating engineering environment where N52 magnets measured in millimeters coexist with high-volume ferrite magnets measured in cents per piece.

Loudspeakers and Audio Drivers

Every dynamic loudspeaker — from a phone earpiece to a floor-standing hi-fi speaker — relies on a permanent magnet to create the static field that the voice coil moves against. The magnet's strength directly determines the motor's force factor (BL product), sensitivity, and bass extension.

• Consumer earbuds and headphones: Sintered NdFeB disc or ring magnets, typically 3–10 mm diameter, N42–N52 grade. The trend to smaller, higher-sensitivity drivers has pushed grades higher year after year.
• Phone/laptop speakers: Thin rectangular NdFeB blocks or discs, often with a pole piece to shape the field. Space constraints are extreme — magnets as thin as 0.5 mm are not uncommon.
• Smart speakers (HomePod, Echo): Larger neodymium woofers with ferrite or NdFeB tweeters. Bass-reflex designs use larger magnets; sealed designs prioritize stronger motors for tight bass control.
• Legacy/budget audio: Ceramic ferrite magnets — large, heavy, but extremely cheap. Still dominant in low-cost PC speakers, car door speakers, and budget consumer audio.

Haptic Actuators (Taptic Engines)

Modern smartphone haptics are produced by linear resonant actuators (LRAs) — essentially a small voice coil motor driven at its resonant frequency. Apple's Taptic Engine uses a sintered NdFeB magnet assembly attached to a spring-suspended mass; an electromagnetic coil drives the assembly back and forth at 150–200 Hz to produce precise, programmable tactile sensations.

Haptic magnets must achieve very high flux density in a tiny space (10–25 mm typical longest dimension), require excellent dimensional tolerance for voice coil clearance, and must survive tens of millions of actuation cycles without fatigue or demagnetization. N48 or N50 grade with nickel coating is typical.

Hard Drive Voice Coil Motors (VCMs)

Hard disk drives use a voice coil motor to position the read/write head assembly across the disk surface. The VCM consists of a flat coil in a strong magnetic field created by two opposed sintered NdFeB magnets separated by a steel return path. Despite the growth of solid-state storage, HDD VCMs remain a high-volume market segment for precision sintered NdFeB.

VCM magnets require extremely uniform flux density across the coil travel range, tight dimensional control, and N40–N48 grade. They are typically nickel-plated and delivered magnetized with specific field orientation relative to a reference datum on the part.

Optical Image Stabilization (OIS)

Smartphone cameras use electromagnetically driven lens or sensor shift systems to compensate for hand shake. OIS systems contain an array of small NdFeB magnets (typically 4–8 per camera module) arranged around the moving element, paired with flat coils that drive the controlled movement. The magnets must be very flat, dimensionally precise, and uniform in field strength — even a small variation across the array causes calibration error in the stabilization algorithm.

Wireless Charging and MagSafe-Style Alignment

Apple's MagSafe system uses a ring of small NdFeB disc magnets embedded in the iPhone back cover, arranged to self-align with a corresponding ring in the charger. These magnets serve no energy transfer function — they are purely alignment and retention devices. The ring configuration creates a rotational snap that aligns the coils for maximum wireless charging efficiency.

Qi wireless charging standards do not require magnets, but the trend toward magnet-assisted alignment (MagSafe, Qi2) has created significant demand for thin, precisely oriented N45–N50 disc magnets in the 3–5 mm diameter range.

Other Consumer Electronics Applications

From Raw Magnet to Working System

A bare permanent magnet produces field lines that spread out in all directions — only a fraction of the available flux reaches the intended work surface. A magnetic assembly adds steel components (return path, pole pieces, and housings) that channel flux to maximize useful output at the working face. The result can be 5–10× the holding force per unit of magnet material compared to a bare magnet of the same grade.

Pot Magnets (Mounting Magnets)

A pot magnet — also called a mounting magnet or cup magnet — consists of a neodymium or ferrite disc magnet encased in a mild steel cup. The steel cup forms a magnetic return path around the back and sides of the magnet, concentrating nearly all available flux through the flat face. This dramatically increases the surface flux density and therefore the pull force against a steel target.

Key characteristics:

• Pull force per volume: 5–10× a bare magnet of equal size due to the steel return path
• Shear vs. pull performance: Pot magnets are optimized for direct pull (axial force). Shear force (sliding) is typically 20–30% of pull force — design mechanical stops into any system relying on a pot magnet for shear retention
• Surface sensitivity: Holding force drops sharply with air gap. A 0.5 mm gap can reduce pull force by 50% or more. Rough, painted, or non-ferrous surfaces dramatically reduce performance
• Ferrite vs. NdFeB pots: Ferrite pot magnets are lower cost and more corrosion-resistant but 3–5× weaker per unit volume. NdFeB pot magnets achieve holding forces of 10–1,000+ kg in compact housings

Channel Assemblies (C-Channel and U-Channel Magnets)

A channel assembly consists of one or more magnets mounted in a steel U-channel profile. The channel forms a return path between the magnets and concentrates the field in the open face of the channel. Channel assemblies are used as magnetic rails in linear automation systems, tool holders, clamping fixtures, and material handling systems.

Unlike pot magnets, channel assemblies can cover large continuous lengths — standard channel magnet bars are available in lengths from 50 mm to over 1 meter. Custom lengths with specific magnet pitch patterns are available from Radial Magnets for automation applications.

Halbach Arrays

A Halbach array is a special magnet arrangement where the magnetization direction rotates in a specific sequence around the array. The effect is that magnetic flux is greatly amplified on one side of the array while nearly canceling on the other. Applications include:

• Magnetic levitation (maglev): Repulsion-based levitation using opposing Halbach arrays
• Linear motors: Halbach arrays on the forcer produce stronger flux with less magnet material than conventional layouts
• MRI machines: Permanent magnet MRI systems use Halbach arrays to produce a strong, uniform field over a large bore without superconducting coils
• Magnetic couplings: High-torque contactless couplings and sealed drive-through-wall systems

Magnetic Couplings and Drive-Through-Wall Systems

Magnetic couplings transmit torque or linear force through a physical barrier (a non-magnetic wall) without mechanical contact or seals. This is critical in applications where the driven side must be hermetically isolated — chemical pumps, pharmaceutical processing, underwater actuators, and vacuum systems. The coupling consists of a driving magnet assembly outside the barrier and a driven magnet assembly inside, attracting across the gap. NdFeB is preferred for maximum torque density.

Custom Holding Fixture Design

For automated assembly, inspection, and machining fixtures, magnetic holding is often preferred over mechanical clamping because it:

• Applies distributed, even force across the part surface without distortion
• Allows instant release and repositioning (with switchable electromagnets or with manually activated permanent magnet systems)
• Eliminates the mechanical complexity and maintenance of pneumatic or hydraulic clamps in clean-room environments

Permanent magnet chucks for surface grinders and mill tables are a mature application. Newer switchable permanent magnet (SPM) systems — which use a lever or rotation mechanism to combine two magnet arrangements that cancel each other out when "off" — offer full holding force without any power consumption in the "on" state.

Why Tolerances Matter More for Magnets Than Most Parts

Dimensional tolerance on a magnet affects more than assembly fit. Because magnetic field strength decreases rapidly with distance (roughly as the inverse square of gap distance for simple geometries), even a small variation in magnet thickness or diameter can produce a measurable change in the field at a sensor or across an air gap. Engineers designing tight-tolerance magnetic circuits — position sensors, encoder rings, medical devices — must account for both dimensional variation and its magnetic effect simultaneously.

Furthermore, sintered NdFeB's brittleness means tighter tolerances require more aggressive grinding, which increases both cost and chip/crack risk. Understanding what's standard, what's achievable, and what the cost implications are is essential before you lock a tolerance callout on a drawing.

Standard Machining Tolerances for Sintered NdFeB

Surface Finish (Ra) for NdFeB Magnets

Surface roughness is measured as Ra (arithmetic mean roughness) in µm or µ-inch. For sintered NdFeB ground surfaces:

• Standard ground surface: Ra 0.8–1.6 µm (32–63 µ-inch). This is the default finish from a surface or centerless grinder and is suitable for most applications.
• Fine ground surface: Ra 0.4–0.8 µm (16–32 µ-inch). Achievable with a fine-grit wheel and careful grinding parameters. Suitable for precision sensor applications and bonding surfaces.
• Lapped surface: Ra 0.1–0.4 µm (4–16 µ-inch). Requires a secondary lapping operation after grinding. Used in optical, medical, and precision measurement applications. Adds significant cost and lead time.
• As-sintered surface: Ra 3.2–12.5 µm. Rough and dimensionally imprecise; not suitable for applications with contact or close-tolerance fits. Only acceptable for interior surfaces or non-contact faces.

Edge Conditions and Chamfers

Sintered NdFeB chips easily at sharp edges during handling and assembly. Standard practice is to specify a small chamfer or radius on all edges that will experience contact during assembly:

• Standard edge break: 0.1–0.2 mm chamfer (or R0.1–0.2 mm radius) — adds minimal cost, prevents most handling chips
• Precision chamfer: 0.3–0.5 mm × 45° — specifiable on drawings; adds a grinding pass
• No chamfer: Only acceptable on surfaces that will never contact another component during assembly; document this explicitly to prevent the supplier from applying their default chamfer

Magnetic Property Variation — The Tolerance You Can't See

Dimensional tolerances are only part of the story. Magnetic properties also vary part-to-part within a production lot, and this variation must be included in your design margins:

For tight-tolerance sensor applications, specify a surface gauss range (minimum and maximum) on the drawing as an acceptance criterion. Parts outside this range are rejected at incoming inspection, regardless of whether the grade datasheet values are met.

Neodymium Magnet Standard Grades & Tolerances — Quick Reference

Radial Magnets publishes a detailed grade and tolerance chart at radialmagnet.com/neodymium-magnet-grades-tolerances/ covering standard property ranges for all grades from N35 through N52AH. Bookmark this as a working reference when writing magnet specifications.

What Is True Radial Magnetization?

In a truly radially magnetized ring or cylinder magnet, every point on the magnet is magnetized in the direction pointing radially outward from the center axis — like the spokes of a wheel. The north pole is the entire outer cylindrical surface; the south pole is the entire inner bore surface (or vice versa for inward radial magnetization).

This is fundamentally different from two other configurations that are sometimes confused with true radial magnetization:

• Diametric magnetization: North on one side of the diameter, south on the other — like a bar magnet bent into a ring. Produces a dipole field, not a radially symmetric one.
• Multipole magnetization: Alternating north and south poles around the circumference at a fixed pitch. Still produces pole faces, not a continuous radial field.

True radial magnetization produces a cylindrically symmetric radial field — the field at any point on the outer surface points directly away from the axis, and has the same magnitude everywhere around the circumference. This is exactly what voice coil motors and certain brushless DC configurations require.

Why True Radial Magnetization Is Difficult to Produce

The difficulty of true radial magnetization arises during both the pressing stage and the magnetizing stage of production:

The Pressing Problem

Sintered NdFeB achieves its extreme energy density through crystal alignment — during pressing, the magnetic powder particles are aligned with an external field so all easy axes point in one direction. For a truly radial magnet, the easy axes must point radially outward at every point in the ring — meaning the alignment field must change direction continuously around the circumference of the compact during pressing. This requires a specially designed radial pressing die with a radially oriented alignment coil system — a significant and expensive piece of tooling that most magnet manufacturers do not possess.

The Magnetizing Problem

Even with correctly oriented powder, the magnetizing fixture must apply a field that is radially directed at every point on the ring simultaneously. A simple electromagnet coil does not do this. Special radial magnetizing fixtures — typically involving a central pole piece inside the ring bore and an outer return path — are required. The geometry of this fixture must match the ring's dimensions precisely.

This combination of specialized tooling and fixture requirements is why most magnet manufacturers do not offer true radial magnetization — or offer it only in a limited range of sizes. It is a core competency at Radial Magnets, Inc.

Applications Requiring True Radial Magnetization

Voice Coil Motors (VCMs)

A voice coil motor consists of a cylindrical coil of wire suspended in a radial magnetic field. When current flows through the coil, the Lorentz force pushes it axially — the direction of force depends on current direction, enabling linear back-and-forth motion. The radial field must be uniform around the entire bore for the coil to experience the same force regardless of its rotational position. True radial magnetization is the ideal — and often only practical — way to achieve this uniformity in a single-piece ring magnet.

VCMs appear in hard drive head actuators, autofocus mechanisms in camera lenses, loudspeaker drivers, linear actuators, and vibration shakers.

Brushless DC Motor Rotors

Some BLDC motor designs use a radially magnetized ring magnet rather than individual arc segment magnets on the rotor. This simplifies assembly (one part instead of many), eliminates gaps between segments that cause torque ripple, and produces a more uniform radial field for the stator coils. True radially magnetized ring rotors are used in high-speed spindle motors (hard drives, turbomolecular pumps), precision servo motors, and miniature BLDC motors for robotics and medical devices.

Magnetic Bearings and Couplings

Passive magnetic bearing systems use opposing radially magnetized rings to provide a stable radial restoring force without contact. When two coaxial radially magnetized rings with opposing magnetization directions are positioned adjacent to each other, they repel radially — providing a centering force. This is used in frictionless bearing concepts for flywheels, turbines, and blood pumps where zero contact is required.

Specifications for True Radial Ring Magnets

The Halbach Effect

A Halbach array is a special arrangement of permanent magnets in which the magnetization direction rotates in a specific sequence around the array. Named after physicist Klaus Halbach who described the configuration in the 1980s while working on particle accelerator focusing elements, the arrangement produces a remarkable result: magnetic flux is strongly concentrated on one side of the array and largely cancels on the other.

In a standard one-sided linear Halbach array, four magnets per period are arranged with magnetizations rotating 90° between adjacent elements: up → right → down → left (or its mirror). The flux reinforces strongly on the "strong" side and nearly vanishes on the "weak" side. The surface field on the strong side is approximately 1.4× that of a single magnet, while the weak side sees nearly zero field — a useful property for both performance and safety.

Linear Halbach Arrays

The classic linear Halbach array consists of rectangular NdFeB blocks arranged in a row with progressively rotating magnetization directions. Key parameters:

• Period (λ): The spatial repeat length of the magnetization pattern. Typical: 20–100 mm for linear motor applications.
• Number of magnets per period: 4 (90° rotation) is most common; finer segmentation (8 magnets, 45° rotation per step) produces higher field uniformity and less harmonic content at the cost of more pieces.
• Array length: The longer the array, the more uniform the field in the center; end effects cause field non-uniformity over approximately one period at each end.
• Back iron (or not): Halbach arrays do not require a steel return path — the field is largely self-contained. This eliminates the back iron that would be needed in a conventional magnet rail, reducing weight and the attractive force between the array and any steel structure during assembly.

Cylindrical Halbach Arrays

A cylindrical Halbach array (also called a Halbach cylinder or "magic cylinder") produces a strong, uniform field inside the cylinder bore with near-zero field outside. This is the configuration used in:

• Permanent magnet MRI machines: Open MRI systems based on Halbach cylinders use NdFeB arrays to produce 0.3–1.0 Tesla uniform fields in a patient bore without superconducting coils or large cryogenics infrastructure
• NMR spectrometers: Benchtop NMR instruments use small Halbach cylinder arrays (50–200 mm bore) to achieve the field uniformity required for spectroscopy
• Undulators and wigglers: Particle physics beam line components using Halbach arrays to oscillate electron beams and produce synchrotron radiation

Applications Across Industries

Designing a Halbach Array — Practical Considerations

Magnet Grade Selection

Halbach arrays deliver peak benefit when the individual magnets are as strong as possible — N42 to N52 sintered NdFeB is standard for most applications. The no-back-iron advantage is largest for high-grade magnets; the benefit diminishes for lower-grade magnets where a back iron would still significantly boost the useful field.

Segmentation and Assembly

Each segment in a Halbach array must be magnetized in the correct direction before assembly. For a 4-segment-per-period array, you need four magnet orientations — typically 0°, 90°, 180°, and 270° relative to the array plane. These are standard magnetization directions for block magnets. The assembly process requires careful fixturing because adjacent segments with different magnetization directions attract and repel each other in complex ways. Segments are typically potted or bonded into a non-magnetic housing (aluminum or stainless) before the array is assembled.

End Effects

The field at the ends of a linear Halbach array drops off and becomes non-uniform over approximately one period. Design the usable travel length of a linear motor or the active length of a beam line element to exclude one period at each end. For cylindrical arrays, end effects are less severe but still present at the top and bottom of the cylinder bore.