Magnetic Property Testing of Permanent Magnet Powders

Magnetic property testing of permanent magnet powders is a critical quality control and research procedure that evaluates key parameters such as remanence (Br), coercivity (Hcb, Hcj), and maximum energy product ((BH)max) for materials including NdFeB, SmCo, ferrite, and SmFeN powders. This article explains the testing standards, sample preparation methods, measured parameters, and typical applications of permanent magnet powder magnetic characterization.

Technical Overview

Permanent magnet powders are the fundamental feedstock for bonded magnets, sintered magnets, magnetic composites, and additive manufacturing processes. Because the magnetic performance of the finished magnet is largely determined by the intrinsic properties of the powder, accurate characterization of the raw material is essential for material development, supplier qualification, and production quality control.

Unlike solid sintered magnets, magnetic powders cannot be tested directly in a closed magnetic circuit. They must first be prepared into measurable specimens, typically by blending the powder with a non-magnetic binder and compacting it into a cylindrical test piece, or by encapsulating a small quantity for open-circuit measurement. The resulting hysteresis loop and derived parameters characterize the magnetic quality of the powder batch.

Testing is generally performed in accordance with internationally recognized standards, including IEC 60404-5 and IEC 60404-8-1, ASTM A977/A977M for high-coercivity permanent magnet materials, GB/T 3217 for permanent magnet (hard magnetic) materials, and related ISO and JIS methods. Accredited third-party laboratories routinely conduct these measurements with traceable calibration and documented measurement uncertainty.

Testing Samples

Common sample types submitted for permanent magnet powder magnetic property testing include the following material families and specimen forms:

Neodymium-Iron-Boron (NdFeB) Powders

Rapid-quenched (melt-spun) NdFeB powder for bonded magnets, gas-atomized spherical powder for additive manufacturing, and jet-milled powder for sintered magnet production. Test specimens are prepared by mixing the powder with epoxy resin, pressing into cylinders of typically 10 mm diameter and 10 mm height, and magnetizing in a pulsed field prior to measurement.

Samarium-Cobalt (SmCo) Powders

Both SmCo 1:5 and 2:17 type powders, usually coarse-crushed sintered-grade material or fine powder for bonded applications. These materials exhibit high intrinsic coercivity and elevated-temperature stability, requiring high magnetizing fields during specimen preparation.

Samarium-Iron-Nitrogen (SmFeN) Powders

A newer class of high-performance rare-earth powder produced by nitriding SmFe2N or related intermetallics, used primarily in bonded and polymer-bonded magnets. Testing verifies the nitrogenation quality through coercivity and remanence values.

Hard Ferrite Powders

Barium and strontium hexaferrite powders for sintered ferrite magnets, injection-molded ferrite compounds, and rubber magnetic compounds. Specimens may be compacted or measured as bonded polymer-bonded rings or discs.

Aluminum-Nickel-Cobalt (AlNiCo) Powders

Powders for casting or bonding into temperature-stable magnet grades, characterized by high remanence but relatively low coercivity, allowing measurement at moderate field strengths.

Testing Items

The core magnetic parameters determined from the hysteresis loop and demagnetization curves of permanent magnet powder specimens include:

• Residual magnetic flux density, or remanence (Br), expressed in mT or kGs
• Magnetic coercive force (Hcb, or bHc), expressed in kA/m or kOe
• Intrinsic coercive force (Hcj, or jHc), a key indicator of powder quality and thermal stability
• Maximum magnetic energy product ((BH)max), in kJ/m³ or MGOe
• Complete B–H and J–H demagnetization curves from saturation to the recoil region
• Squareness factor (Hk/Hcj), reflecting the knee shape of the demagnetization curve
• Recoil permeability (μrec)
• Saturation magnetization and specific magnetic moment (measured by open-circuit methods)
• Curie temperature (Tc) by thermo-magnetic analysis
• Temperature coefficients of Br (α) and Hcj (β) over specified temperature ranges
• Supporting physical characterizations such as apparent density, tap density, compression density, and particle size distribution

Testing Methods

Permanent magnet powder magnetic properties are measured using one of several established techniques, selected according to the coercivity of the material, the required accuracy, and the specimen format:

Hysteresisgraph Method (Closed Circuit)

Following IEC 60404-5 and ASTM A977/A977M, a compacted bonded specimen is placed in an electromagnet yoke to form a closed magnetic circuit. Search coils around the specimen record the B–H and J–H loops, from which Br, Hcb, Hcj, and (BH)max are calculated. This is the standard method for reporting bonded magnet powder properties and is directly comparable to solid magnet data.

Vibrating Sample Magnetometer (VSM)

A small quantity of powder, typically tens of milligrams, is secured in a non-magnetic holder and vibrated within a detection coil system. The VSM measures magnetic moment as a function of applied field, providing high-sensitivity open-circuit results suitable for research, small-volume samples, and high-coercivity materials when equipped with superconducting magnets.

Pulsed Magnetometer Method

High-coercivity powders, particularly NdFeB grades with Hcj exceeding 2000 kA/m, may require pulsed fields for full saturation. Pulsed magnetometers with induction or extraction detection capture the complete loop in milliseconds and are accepted for difficult-to-saturate materials.

Specimen Preparation and Alignment

For anisotropic powders, specimens are oriented in a pulsed or DC aligning field during compaction to reproduce the aligned state of the final magnet. Isotropic powders are compacted without alignment. The binder fraction, typically 2 to 3 percent by mass of epoxy, and the achieved green density are reported alongside the magnetic results because Br scales with packing density and is commonly normalized to full density.

Applications

Magnetic property testing of permanent magnet powders serves a wide range of industrial and scientific purposes:

• Incoming inspection of purchased powder by bonded magnet and sintered magnet manufacturers
• Lot-to-lot quality control and release testing before compaction, molding, or printing
• Research and development of new alloy compositions, grain-boundary engineering, and nitriding processes
• Qualification of feedstock for additive manufacturing of magnets
• Supplier audits and verification of certificates of analysis by accredited third-party laboratories
• Failure analysis of magnets traced back to raw material defects
• Support for export compliance, customer specifications, and IEC 60404-8-1 grade classification

FAQ

Can magnetic powder be tested directly without compaction?

Closed-circuit hysteresisgraph measurement requires a solid specimen of defined geometry, so powders are compacted with a small amount of binder. For direct powder measurement, open-circuit methods such as VSM can be used, but the reported values are specific magnetic moments rather than standard grade parameters.

How much powder is required for a test?

Typically 10 to 30 grams of powder are needed to prepare a standard cylindrical bonded specimen for hysteresisgraph testing. VSM measurements require only tens of milligrams.

What magnetizing field is needed for saturation?

As a general rule, an applied field of at least three times the intrinsic coercivity is required. High-coercivity NdFeB powders may therefore require magnetizing fields above 4000 kA/m, achieved with pulsed or superconducting systems.

Which standards govern the testing?

The principal references are IEC 60404-5 and IEC 60404-8-1, ASTM A977/A977M, GB/T 3217, and the corresponding ISO and JIS permanent magnet measurement standards.

Why are density normalization and alignment important in reporting?

Because Br of a bonded specimen depends on the volume fraction of magnetic powder, results are commonly normalized to full theoretical density to enable comparison with sintered magnet grades, and alignment data indicate whether anisotropic powder orientation was adequate.