Magnetization Directions & Pole Configurations

Sintered NdFeB and SmCo are anisotropic materials. During pressing, the alloy powder is compacted inside a strong magnetic field that physically rotates each particle so its crystalline "easy axis" points the same way. Sintering locks that alignment into the solid part permanently.

The consequences drive everything in this guide:

• The orientation axis is manufactured in, not magnetized in. Final magnetization (a millisecond pulse in a magnetizing fixture) can only charge the magnet along the axis established at pressing. Pulsing across the easy axis yields a weak, unstable magnet.
• A wrong direction cannot be reworked. An axially oriented disc can never become a diametric one. Parts made to the wrong orientation are scrap.
• Direction affects tooling and price. Some orientations press easily in standard dies; others — true radial above all — require specialized pressing and magnetizing equipment that many suppliers do not have.

Axial is the default and by far the most common direction: flux passes through the thickness of the part, giving one full N face and one full S face. Discs, rings, and blocks magnetized "through thickness" are all axial.

• Best for: holding and clamping, sensors triggered face-on, magnetic couplings (face type), speaker motors, attach/detach features.
• Manufacturing: standard pressing and magnetizing; lowest cost and shortest lead time of any orientation.
• Watch for: thin discs (large diameter-to-thickness ratio) have low permeance coefficients — check the load line at your operating temperature.

Diametric magnetization runs across the diameter of a disc, cylinder, or ring: one half of the OD is N, the opposite half is S. Rotating the magnet rotates the external field with it — the basis of nearly all rotary position sensing.

• Best for: rotary encoders reading a 2-pole field (sin/cos Hall or AMR/TMR sensors), stir bars, rotors for small stepper and BLDC designs, rotary couplings.
• Manufacturing: standard capability; slightly less common than axial but not a cost premium in most sizes.
• Watch for: the angular position of the poles relative to a keyway, flat, or fixture feature. If your assembly needs the pole axis clocked to a mechanical feature, the drawing must define that angle and its tolerance — see section 06.

In a true radially oriented ring, the easy axis points along the radius at every position around the circumference: flux exits uniformly through the entire OD and returns through the entire ID (or vice versa). The result is a rotationally uniform field with no preferred angular position.

True radial vs. segmented assemblies

Because true radial pressing requires specialized dies that generate a radial orienting field — capability many magnet factories lack — the industry workaround is to grind arc segments and glue them into a ring. The two are not equivalent:

A multipole magnet carries alternating N and S poles on a single part — around the OD, around a face, or along the ID. The magnetizing fixture defines the pattern; on isotropic (bonded) material almost any pattern is possible, while sintered anisotropic material supports patterns compatible with its orientation.

What a multipole spec must define

• Pole count — total poles (always even).
• Pole location — OD, ID, one face, or both faces.
• Pole geometry — equal sectors is the default; state any skew or unequal spacing.
• Angular reference — pole position relative to a keyway, hole, or mark, with tolerance, if the application needs clocking.
• Field spec at the working distance — peak field and pole-to-pole uniformity measured at the sensor air gap, plus allowable pole-transition angle error for encoder applications.

A magnetization callout is complete when a stranger could magnetize and inspect the part from the drawing alone. Include:

• An arrow on the part view showing the orientation axis, labeled "magnetization direction." For axial parts, state which face is N (if it matters — say so if it doesn't).
• A text note naming the configuration: "Magnetized axially, thru thickness"; "Magnetized diametrically"; "True radially oriented, N poles on OD, one-piece construction"; "8-pole, OD, poles equally spaced."
• Pole identification — whether N-pole marking is required (paint dot, laser mark) and on which face/side.
• Angular reference and tolerance for diametric and multipole parts that must clock to a feature (e.g., "pole axis aligned to keyway ±2°").
• Magnetized or unmagnetized at delivery — and if unmagnetized, whether saturation magnetization is done by you or the supplier post-assembly.
• Acceptance test reference — the measurement method and limits that define "correctly magnetized" (next section).

Each configuration has a natural verification method — put the matching one on your drawing:

A single-point gaussmeter reading is a polarity and gross-error check, not a saturation or uniformity test — surface field readings are extremely sensitive to probe position. For the full treatment of measurement methods and writing acceptance limits, see How Magnets Are Tested & Measured.