Understanding the Anisotropic Direction in Sintered NdFeB
Sintered neodymium magnets are anisotropic – they have a preferred direction of magnetization that is set during pressing in a magnetic field. The green compact (unsintered powder) is aligned in a magnetic field of 1.5-2.0 Tesla, then sintered. After sintering, the magnet can only be magnetized along that original alignment direction. Magnetization in any other direction yields a weak, unstable field (typically 10-20% of the aligned direction's Br). Consequently, you cannot "re-magnetize" an axially pressed block into a diametrically magnetized one.
Thus, specifying magnetization direction is a design decision that determines the pressing tooling and manufacturing process. For a given magnet shape, there are usually two possible alignment directions (e.g., through thickness vs. along length). The customer must choose based on the application's required flux path.
Axial, Diametrical, and Radial Magnetization Explained
For a cylindrical magnet (height H, diameter D):
Axial magnetization: magnetic poles on the two circular faces (north on top face, south on bottom face). Field is parallel to the cylinder axis. Used in permanent magnet couplings, axial flux motors, and magnetic springs.
Diametrical magnetization: magnetic poles on opposite sides of the cylinder's outer surface (north on one side, south 180° opposite). Field is perpendicular to the axis, passing across the diameter. Used in single-pair brushless DC fan motors, hall effect sensors, and small rotary encoders.
Radial magnetization: specifically for ring magnets (OD, ID). Poles are on inner and outer cylindrical surfaces (north on OD, south on ID, or vice versa). Flux flows radially. Used in high-speed rotors with multipole rings, and in magnetic bearings.
Multi-pole diametrical/radial: magnets are magnetized with multiple alternating poles around the circumference (e.g., 4-pole, 8-pole, 12-pole). Requires a multi-pole magnetization fixture after pressing.
When to Choose Radial Magnetization for High-Speed Rotors
For high-speed permanent magnet rotors (above 20,000 rpm), surface-mounted arc magnets are subject to centrifugal forces. A multipole radially magnetized ring magnet (one-piece ring with alternating poles on its OD) has no glued segments – it is a monolithic sintered ring. This eliminates the risk of segment detachment. Radial rings also produce a smoother magnetic field profile compared to segmented arcs, reducing torque ripple by 30-50%.
However, radial rings require specialized magnetization fixtures (coils around the ring's circumference). The fixture cost is higher ($2000-5000 per pole count) than for axial/diametrical magnets, but the fixture is reusable for all rings of same dimensions and pole count.
For rotor diameters from 20mm to 150mm, we offer radially magnetized rings with 2 to 16 poles. Minimum wall thickness: 3mm for reliable sintering without cracks.
How Magnetization Direction Impacts Your Tooling Costs
Tooling and manufacturing costs vary significantly:
| Magnetization Direction | Pressing Tooling Complexity | Magnetization Fixture Cost (for custom) | Typical Yield (%) |
|---|---|---|---|
| Axial (through thickness) | Simple (parallel plate mold) | Low ($300-500) | 95-98% |
| Axial (through length) for blocks | Standard | Low | 92-96% |
| Diametrical on cylinder | Simple but requires special orientation | Medium ($800-1500) | 85-90% (higher rejection due to alignment error) |
| Radial ring, single pole (ID/OD) | Complex (requires oriented powder flow) | Medium ($1000-2000) | 80-85% |
| Radial ring, multi-pole (4+ poles) | Very complex | High ($3000-6000) | 70-80% |
| Halbach-type custom angle | Special tooling only | Custom fixture ($5000+) | 60-75% |
For prototyping, we recommend using isotropic bonded magnets or standard magnetization on simple shapes, then moving to anisotropic sintered for production after design validation.
For custom motor magnets requiring specific magnetization directions – axial, diametrical, radial, or multi-pole – please visit our Rotor Magnet product page. We supply magnetization fixtures with each order, enabling you to check polarity before assembly.
To discuss your motor's required flux pattern and field strength, contact our technical team. We provide magnetization direction verification using Hall probes and Helmholtz coils.
Frequently Asked Questions
Q: Can I magnetize the same neodymium ring first axially and then diametrically?
A: No. Once magnetized in one direction, the magnet retains that orientation. Attempting a second magnetization direction demagnetizes the material or creates a complex multi-domain state with 50% lower total flux.
Q: How do I specify magnetization direction on a drawing for a custom block magnet?
A: Use an arrow labeled "Magnetization Direction" on the drawing. Show the arrow relative to the part's dimensions. Provide a 3D CAD file (STEP or STL) with the arrow modeled as a reference. We confirm by return drawing.
Q: What is the maximum number of poles you can magnetize on a 30mm diameter radial ring?
A: Up to 16 poles (22.5° per pole). Minimum pole arc: 5mm at OD. For higher pole counts (e.g., 24 poles on 30mm OD), the magnetizable material between poles becomes too narrow, causing high leakage. Use FE analysis to check.
