Magnets for Sensors: Selection & Design Guide

A magnetic sensor system is a transducer pair: a magnet that encodes mechanical position into a field pattern, and a sensing element that reads it. Datasheets, app notes, and design attention overwhelmingly go to the silicon half — while accuracy, temperature drift, and unit-to-unit variation in the finished product are usually dominated by the magnet half: its field level at the gap, its pole geometry and placement tolerance, and its thermal behavior.

Treating the magnet as an engineered component — specified by field at the working point, verified by measurement, toleranced where it matters — is what separates a sensor design that calibrates once from one that fights yield forever.

Working-field figures are order-of-magnitude orientations; the sensor datasheet governs. The magnet's job is to place the field at the IC inside that window across all tolerances and temperatures.

Full geometry definitions for each magnetization option — axial, diametric, true radial, multipole — with diagrams, live in the magnetization directions guide; this page focuses on making them work with a sensor.

The design variable is B at the sensor location — not the grade, not the surface field, not the pull force. Work it in this order:

• Establish the real working distance — magnet face to sensing element, including package standoff (the die sits ~0.3–1 mm inside the IC package), housing walls, and the tolerance stack on both sides. The stack usually matters more than the nominal.
• Set the field window: from the sensor datasheet — minimum field at maximum gap and temperature; maximum field at minimum gap (linear devices clip, angle sensors have upper windows too).
• Size geometry before grade: field at distance is driven by magnet dimensions far more than grade steps (a grade step buys ~3–4% field — the grades chart math). Diameter-to-thickness ratio shapes how fast the field falls with gap: larger faces throw field further.
• Then pick the grade and temperature class for the thermal environment (next sections and the temperature guide).
• Prefer designs that tolerate strength variation: angle-based and zero-crossing architectures ride out the ±3–5% Br spread between lots; absolute-threshold designs inherit it as error. If your architecture measures field magnitude directly, budget for lot spread explicitly.

A diametrically magnetized disc on the shaft end, with a 2D/3D Hall or magnetoresistive angle sensor on the rotation axis, delivers absolute 0–360° sensing with a handful of parts — throttle and pedal position, valve actuators, steering, knobs, BLDC commutation.

• Magnet size: typically 3–8 mm diameter discs; the field magnitude at the IC needs to sit in the device's window across the gap tolerance — usually easy to satisfy, which is why this architecture is so forgiving.
• What actually drives accuracy: mechanical placement — eccentricity of the magnet on the rotation axis, tilt, and axial gap variation bend the field direction and appear directly as angle error. Specify magnet OD-to-fixture concentricity and the mounting method (press into a brass/aluminum holder, adhesive per the bonding guide) as carefully as the magnet.
• Clocking: if the electrical zero must align to a mechanical feature, the pole-axis-to-feature angle and tolerance go on the drawing (e.g., "pole axis to flat: 0° ±2°") — and the supplier verifies it in a rotary fixture, per the testing guide.
• Stray fields: for automotive stray-field immunity requirements (ISO 11452-8 class environments), differential/gradiometric sensor variants pair with the same magnets — the magnet spec doesn't change, the IC does.

When the shaft end is occupied — through-shafts, hollow shafts, large-diameter joints — the field moves to a ring around the shaft, read by a sensor at the OD:

• Incremental encoding — OD multipole rings: alternating poles around the circumference; each pole pair is one signal period, so pole pitch sets resolution (a 32-pole ring gives 16 periods/rev; interpolation multiplies from there). Accuracy lives in the pole-transition placement: specify pole count, peak field at the sensor gap, pole-to-pole balance, and maximum transition angle error — the parameters an automated pole scan verifies.
• Speed/direction sensing: the same rings at coarser pitch replace toothed-wheel VR targets in ABS and motor feedback — quadrature from two offset sensors gives direction.
• Uniform-field applications — true radial rings: torque sensors, absolute off-axis schemes, and couplings need a continuous, joint-free radial field. A one-piece true radially oriented ring delivers it; a glued arc-segment "radial ring" puts a flux dip at every joint that appears directly as periodic signal error. Specify one-piece construction explicitly — the comparison table and drawing language are in the directions guide, and manufacturing capability is exactly what the True Radial line exists for.
• Material choice for rings: sintered NdFeB for maximum signal; bonded NdFeB when fine pole pitch, complex patterns, or over-molding onto a hub matter more than raw field (sintered vs. bonded).

• Reed switches actuate on flux along the reed axis, with sensitivity specified in ampere-turns — so the magnet's orientation relative to the reeds matters as much as its strength. A magnet approaching parallel to the reed axis gives one clean operate zone; perpendicular approaches create multiple operate/release lobes that surprise designers in testing.
• Design to operate AND release: the release point (hysteresis) defines the off state — verify the magnet's far position drops below release across tolerances, or the switch never opens.
• Typical pairings: small axial NdFeB discs/blocks for compact security and appliance switches; ferrite or alnico where cost or legacy sensitivity curves apply. Ferrite in cold outdoor duty carries the low-temperature caveat from the material comparison.
• Prototyping tip: reed + magnet interaction maps poorly to hand calculation — a quick bench map of operate/release positions with the real switch beats simulation for these devices.

• NdFeB drifts ~−0.12%/°C in Br — a threshold-based sensor calibrated at 25 °C sees its switch point move with temperature; a 100 °C swing is a ~12% field change. Angle- and ratio-based architectures cancel most of it; magnitude-based ones must budget or compensate for it (many sensor ICs offer programmable TC compensation matched to NdFeB — use it).
• Precision instruments reach for SmCo (−0.035%/°C) or accept the size penalty for 3–4× less drift — the classic aerospace and metering choice from the material comparison.
• Irreversible loss is a calibration killer: one over-temperature event permanently shifts every threshold. Spec the temperature class against the real profile with a load-line check — small sensor magnets are often thin discs, exactly the geometry the temperature guide flags as vulnerable below their rated class.
• Thermal stabilization before calibration: for calibrated devices, pre-bake magnetized parts above worst-case service temperature so the one-time knock-down happens before, not after, calibration — specify it on the drawing.

The sensor-magnet additions to the standard RFQ checklist:

• Field at the working point: B (mT/G) at a defined distance from a defined face, with tolerance and temperature of measurement.
• Configuration & pattern: axial / diametric / true radial one-piece / multipole with pole count and location — plus clocking angle and tolerance where the application needs it.
• For multipole rings: peak field at the sensor gap, pole-to-pole uniformity, and maximum transition angle error.
• Thermal: operating range, temperature class per load-line check, and thermal stabilization requirement for calibrated devices.
• Acceptance test: the measurement method that defines a good part — fixtured field-at-point, helmholtz moment, or pole scan — with limits anchored to first-article hardware.
• Assembly interface: holder material (non-magnetic), mounting method, and concentricity/perpendicularity to the functional axis.

Sensor magnets are a Radial Magnets specialty — from stock diametric discs to custom true radial and multipole encoder rings with pole-scan certification. Send the sensor part number, the gap, and the motion, and our engineers will spec the magnet with you.