Magnetization Patterns: Direction Is Half the Spec
Two magnets with identical dimensions, grade, and coating can behave completely differently depending on how they're magnetized. Magnetization direction and pattern are as much a part of your specification as the diameter — and the most common source of "wrong part" returns we see. Here's the full vocabulary.
the four basic directions
the four workhorse directions. green arrows show magnetization vector m.
axial — through the thickness of a disc, cylinder, or ring. the default for holding, sensing, and coupling applications; poles on the flat faces
diametric — across the diameter of a disc or cylinder. poles on the curved sides — used for rotary sensors, couplings, and stirrers where the part spins about its axis
radial — flux travels from ID to OD (or reverse) uniformly around a ring. true radial magnetization requires special material orientation and fixtures — often approximated with segmented arcs
thickness / length — for blocks: specify which dimension the flux passes through. "magnetized through 1/8 inch thickness" removes all ambiguity
multipole patterns
Instead of one north and one south, a multipole magnet carries alternating poles on a single part — replacing an assembly of discrete magnets with one component. The two standard families:
face poles drive axial-gap encoders and couplings; od poles drive radial-gap sensors and motors.
multipole on face — alternating sectors on the flat face of a ring or disc. used in axial encoders, magnetic gears, torque couplings. spec: pole count, pole accuracy (° mechanical), and which face
multipole on od / id — alternating poles around the circumference. this is the classic sensor ring and BLDC rotor magnet. spec: pole count, pole-to-pole uniformity (% field), transition width
skewed poles — pole boundaries cut at a helix angle to reduce cogging torque in motors. requires custom fixture — call it out explicitly
one-side multipole — refrigerator-magnet style pattern concentrating flux on one face of flexible or bonded material; short pole pitch = strong grip at zero distance, fast falloff
bonded vs. sintered for multipole: isotropic bonded NdFeB and injection-molded ferrite can be magnetized in nearly any pattern the fixture can produce, which is why sensor rings are usually bonded. sintered anisotropic material is largely limited to patterns aligned with its press orientation.
the anisotropy rule — why you can't re-magnetize sideways
Sintered NdFeB, SmCo, and oriented ferrite are anisotropic: during pressing, an applied field aligns the powder's easy axes in one direction, and sintering locks that orientation in permanently. The finished magnet can only be magnetized along that axis. Requesting diametric magnetization on a part pressed for axial orientation doesn't produce a weak diametric magnet — it produces scrap.
Practical consequences:
direction is a manufacturing decision — it's set at pressing, months before magnetization. changing direction means new pressed blanks, not a fixture change
isotropic materials trade strength for freedom — bonded NdFeB (MQ powders) and isotropic ferrite accept any pattern but deliver roughly a third to half the energy product
true radial rings are special — radially-oriented sintered rings exist but require dedicated tooling per size; segmented arc assemblies are the common alternative
how to call it out on a drawing
An unambiguous magnetization spec has five elements:
element
example callout
direction / pattern
"magnetized axially through 0.250 thickness" / "8 poles on od, evenly spaced"
polarity reference
"north pole on surface a per view b" — define north as the flux-exiting face, and mark it
north identification method
"north face identified with dot of white ink" (or notch, chamfer, laser mark)
field acceptance criterion
"surface field 4,450 G ±5% at center of face a, measured with hall probe in contact"
orientation tolerance
"magnetization axis within 3° of geometric axis" — critical for sensor and coupling apps
the classic mistake: specifying "n pole up" with no marked datum on a symmetric part. a disc has two identical faces — without an ink dot, notch, or chamfer tied to a drawing view, "up" doesn't survive the trip through a parts bin. if your assembly is polarity-sensitive, pay for the mark.
pattern selection quick reference
application
typical pattern
material notes
holding / clamping
axial
sintered NdFeB on steel backing
rotary position sensing (hall/tmr)
diametric disc or 2-pole ring
orientation tolerance matters most
speed / encoder rings
multipole on od, 16–120 poles
bonded NdFeB or ferrite for pattern freedom
bldc / pmsm rotors
arc segments or multipole ring, often skewed
sintered for power density; bonded ring for cost
magnetic couplings
multipole on face or od, matched pairs
pole count sets torque vs. gap tradeoff
linear position sensing
axial or through-thickness, tightly toleranced field
100% field testing typical for safety-critical use
not sure how to spec it?
Send us the application and we'll recommend the pattern, orientation tolerance, and acceptance test that fits — and supply parts 100% flux-tested with the north pole marked your way.
