NdFeB vs. SmCo vs. Ferrite vs. Alnico

Every commercially significant permanent magnet belongs to one of four material families. The master comparison:

Ranges span common commercial grades. Max operating temperatures assume favorable permeance coefficients — actual limits depend on the magnetic circuit.

Developed in the early 1980s, sintered NdFeB is the strongest permanent magnet material available and now accounts for the large majority of high-performance magnet applications. For a given flux requirement, an NdFeB magnet is a fraction of the size and weight of any alternative — which is why it enabled compact BLDC motors, small speakers, hard drives, and modern EV traction motors.

Strengths

• Highest energy product (up to ~55 MGOe) — smallest magnet for the job
• Wide grade range: strength (N35–N55) and temperature class (up to 230 °C) selectable independently
• Mature, high-volume supply chain; broadest range of shapes, sizes, and magnetization patterns — including true radial orientation for rings
• Best cost per unit of flux in compact designs, despite high cost per kilogram

Limitations

• Corrodes rapidly uncoated — coating is mandatory in nearly all applications
• Significant reversible flux loss with temperature (−0.12%/°C); high-temperature classes require costly heavy rare earths
• Brittle; cannot be conventionally machined after sintering
• Rare earth price volatility and geographically concentrated supply

For a deeper treatment — grades, BH curves, sintered vs. bonded, manufacturing — see Magnets 101 and the sintered vs. bonded comparison.

SmCo predates NdFeB by roughly a decade and remains the material of choice when the environment defeats neodymium. Two families exist: SmCo5 (1:5) and Sm2Co17 (2:17), with 2:17 offering higher energy products and dominating current use.

Strengths

• Operates continuously to 250–350 °C depending on grade — far beyond any NdFeB class
• Roughly 3–4× better thermal stability of output than NdFeB (−0.035%/°C vs. −0.12%/°C) — valuable in precision sensors and instruments
• Excellent bare corrosion resistance — usable uncoated in most environments, including many vacuum and aerospace applications
• High intrinsic coercivity; very resistant to demagnetization

Limitations

• Most expensive family per kilogram — samarium and especially cobalt are costly inputs
• Lower energy product than NdFeB (up to ~33 MGOe) — larger magnet for the same flux at room temperature
• The most brittle of the four families; prone to chipping in handling and assembly

Hard ferrite (strontium or barium hexaferrite) is by far the most-produced magnet material on Earth by tonnage. It is a true ceramic made from iron oxide — no rare earths, no strategic metals, no coating required.

Strengths

• Lowest cost per magnet and per kilogram by a wide margin
• Completely corrosion-proof — it is already an oxide; no coating needed ever
• Stable supply chain with no rare earth exposure
• Operates to 250 °C; high electrical resistivity means negligible eddy-current losses in AC fields

Limitations

• Weak per unit volume — roughly one-tenth the energy product of NdFeB; magnets must be large
• Worst reversible temperature coefficient of Br (−0.19%/°C), and coercivity falls as temperature drops — ferrite can demagnetize in extreme cold under load
• Brittle and abrasive to machine

Alnico (aluminum-nickel-cobalt-iron) was the dominant strong magnet from the 1930s through the 1970s. It survives today in applications the newer materials still can't touch: extreme temperature and extreme output stability.

Strengths

• Highest usable temperature of any commercial magnet — continuous operation to ~450–550 °C
• Best thermal stability of the four (−0.02%/°C) — the reference material for precision instruments, aerospace sensors, and guitar pickups where tone consistency matters
• High remanence (up to ~13.5 kG in grade 5-7) — strong surface field from the right geometry
• Mechanically the most robust family; can be cast into complex shapes

Limitations

• Very low intrinsic coercivity (<2 kOe) — easily demagnetized by opposing fields, shock, or even improper storage; requires long magnet geometries and careful circuit design (high permeance coefficient)
• Low energy product (≤7.5 MGOe) — big magnets for modest flux
• Cobalt content keeps cost meaningful despite the mature process

Temperature is the fastest filter in material selection. Find your maximum continuous operating temperature:

Remember that all "max operating temperature" ratings assume a reasonable permeance coefficient. A thin magnet in open circuit has a lower real limit than the datasheet number; a magnet in a closed circuit can often exceed it. When in doubt, give your supplier the geometry and the circuit and ask for the load-line check.

Cost comparisons between magnet materials mislead when done per kilogram. The meaningful metric is cost per unit of delivered flux in your envelope:

• If space is constrained (compact motors, sensors, wearables): NdFeB usually wins on total cost despite the highest-looking unit price, because the magnet is 5–10× smaller.
• If space is free (large DC motors, separators, holding fixtures, loudspeakers): ferrite usually wins — a bigger, cheaper magnet delivers the flux for less money.
• SmCo and alnico are never the cheap option. They are bought for temperature and stability. If your application doesn't need those, you're overpaying.

Total cost of ownership also includes what surrounds the magnet: NdFeB's coating, ferrite's larger housing and structure, alnico's remagnetizing risk, SmCo's assembly scrap from chipping. For the full breakdown of what's inside an NdFeB quote specifically, see Magnet Pricing Explained.

NdFeB's corrosion sensitivity is manageable — that's what the coating system is for — but in continuously wet, chemically aggressive, or hydrogen-bearing environments, the coating becomes a reliability item to be engineered and tested, not a checkbox. In those environments SmCo or ferrite may be the lower-risk system choice even at a magnetic performance penalty.

Signals that your current material is the wrong one:

• Ferrite design keeps growing. If the magnet has become the largest component in the assembly, price an NdFeB version — the size reduction often pays for the material.
• NdFeB in a hot application keeps climbing temperature classes. Once you're pricing EH grades, compare against SmCo 2:17 at operating temperature — SmCo may deliver more flux for similar money with no coating risk.
• Field returns show flux loss. Irreversible loss points to an under-specified temperature class or an unfavorable load line — a grade fix or a geometry fix, sometimes a material change.
• Coating failures in aggressive environments. If NdFeB coating qualification keeps failing salt spray or chemical exposure, a bare SmCo or ferrite solution may cost less than the next coating iteration.
• Alnico device demagnetizes in service. Modern high-coercivity materials (NdFeB/SmCo) eliminate the problem — with a redesign of the magnetic circuit, since the materials are not drop-in equivalents.