Maximum Magnetic Energy Product (BH)max Testing for Permanent Magnet Materials

Maximum magnetic energy product (BH)max testing determines the energy density and overall magnetic performance of permanent magnet materials such as sintered and bonded NdFeB, SmCo, AlNiCo, and hard ferrites. Accredited third-party laboratories perform this testing in accordance with IEC 60404-5, IEC 60404-8-1, ASTM A977/A977M, GB/T 3217, and JIS C 2501, delivering complete demagnetization curves together with Br, HcB, HcJ, and (BH)max values. The results support material certification, grade verification, incoming inspection, and electromagnetic design across the electric vehicle, wind power, industrial motor, and consumer electronics industries.

Technical Overview

The maximum magnetic energy product, denoted (BH)max, is the most widely used figure of merit for permanent magnet materials. It is defined as the maximum value of the product of magnetic flux density (B) and magnetic field strength (H) at operating points along the second-quadrant demagnetization curve of a fully magnetized specimen. Because (BH)max quantifies the maximum static magnetic energy per unit volume that a magnet can deliver to an external circuit, it directly determines the minimum magnet volume required to produce a given flux in devices such as motors, generators, actuators, and sensors.

The energy product is expressed in kJ/m³ in the SI system or in MGOe (megagauss-oersted) in the CGS system, where 1 MGOe corresponds to approximately 7.96 kJ/m³. For an ideal magnet with a perfectly linear demagnetization curve, the theoretical upper limit of (BH)max equals Br²/(4μ0), where Br is the remanence and μ0 is the permeability of free space. Real materials fall below this limit due to the knee of the demagnetization curve and finite coercivity, so measured values are essential for accurate device design. Accredited third-party laboratories determine (BH)max from complete demagnetization curves measured in accordance with IEC 60404-5, IEC 60404-8-1, ASTM A977/A977M, GB/T 3217, and JIS C 2501.

Testing Samples

Energy product testing can be performed on virtually all commercially available permanent magnet families. Specimens are typically individual, uncoated, and machined to standardized geometry so that closed-circuit measurement conditions are reproducible.

Sintered NdFeB Magnets

Neodymium-iron-boron is the highest performance commercial magnet family, with (BH)max values typically ranging from about 200 to 430 kJ/m³ (25 to 54 MGOe) depending on grade. Testing verifies grade conformance (Br, HcJ, and (BH)max) for grades from N35 through N52 and high-temperature series such as N38SH, N42UH, and N38EH.

Bonded NdFeB and SmFeN Magnets

Compression-bonded and injection-molded magnets combine magnetic powders with polymer binders, offering (BH)max values of roughly 30 to 90 kJ/m³ (4 to 11 MGOe). Testing confirms both magnetic output and lot-to-lot consistency for high-volume precision components.

Samarium-Cobalt Magnets

SmCo5 and Sm2Co17 grades provide (BH)max values of approximately 110 to 265 kJ/m³ (14 to 33 MGOe) combined with excellent thermal stability, making them suitable for aerospace and high-temperature applications up to 250–350 °C.

Hard Ferrite Magnets

Barium and strontium hexaferrite magnets (BaFe12O19 and SrFe12O19) exhibit (BH)max values of about 8 to 40 kJ/m³ (1 to 5 MGOe). They are electrically insulating and cost effective, and are tested in disc, ring, arc, and block geometries.

AlNiCo and FeCrCo Magnets

These cast or wrought alloys offer high remanence but low coercivity, with (BH)max values of approximately 10 to 75 kJ/m³ (1.3 to 9.5 MGOe). Closed-circuit measurement is particularly important for these materials because they self-demagnetize in open-circuit conditions.

Specimen Preparation Requirements

Preferred specimen geometries include cylinders of approximately 10 mm diameter and 5 to 10 mm height, or rectangular blocks of approximately 10 × 10 mm cross-section with parallel, ground end faces. Dimensional accuracy within ±0.02 mm is required because geometric errors propagate directly into B and H values. Specimens may be submitted in the magnetized or demagnetized state; the laboratory saturates the sample immediately before measurement. Samples should be conditioned to room temperature (23 ± 5 °C) prior to testing, and protective coatings are normally removed or masked at the pole faces to ensure direct magnetic contact.

Testing Items

A standard energy product test report includes the following measured and derived parameters:

• Maximum energy product (BH)max, reported in both kJ/m³ and MGOe
• Remanence Br (residual induction)
• Normal coercivity HcB (coercive field strength of the B–H curve)
• Intrinsic coercivity HcJ (coercive field strength of the J–H curve)
• Complete second-quadrant demagnetization curves B(H) and J(H)
• Knee field Hk and squareness ratio Hk/HcJ
• Recoil permeability (relative recoil permeability) of the working region
• Full hysteresis loop parameters where required (saturation behavior, loop squareness)
• Reversible temperature coefficients α(Br) and β(HcJ), and irreversible loss at specified temperatures
• Measurement uncertainty statements traceable to national metrology institutes

Testing Methods

Closed-Circuit Hysteresisgraph Method

The reference method follows IEC 60404-5 and ASTM A977/A977M. The specimen is clamped between the pole faces of a high-permeability electromagnet yoke, forming a closed magnetic circuit that eliminates the internal demagnetizing field. Magnetic flux density B is measured with a tightly wound search (B) coil around the specimen, while field strength H is measured with a calibrated Hall probe or coaxial H-coil at the specimen surface. The specimen is first pulse-magnetized to saturation at a field of at least three times its intrinsic coercivity, after which H is swept through the second quadrant to record the demagnetization curve. The B·H product is computed point by point, and (BH)max is identified as its maximum. Electromagnet pole fields of up to approximately 2 T are sufficient for ferrites, AlNiCo, SmCo, and standard NdFeB grades.

Pulsed-Field Magnetometer Method

For ultra-high-coercivity NdFeB grades (HcJ above roughly 2000 kA/m), which cannot be fully saturated or driven through the full demagnetization curve by electromagnets, a pulsed-field magnetometer (PFM) is used. A capacitor bank discharges into a solenoid, generating pulsed fields of 6 to 10 T with durations of 10 to 80 ms. Induction coils and electronic integration reconstruct the intrinsic J(H) curve, with eddy-current corrections applied for electrically conductive specimens. Results are validated against closed-circuit measurements on standard grades.

Vibrating Sample Magnetometer Method

VSM measurement is an open-circuit technique in which a small specimen vibrates within pickup coils while a controlled field (up to 2–3 T from an electromagnet, or higher with superconducting magnets) is swept. The instrument measures magnetic moment as a function of applied field, and the intrinsic curve is converted to engineering units after geometry and demagnetization-factor corrections. VSM is preferred for research, small or irregular samples, thin films, and material development programs rather than routine certification.

Temperature-Dependent Testing

Cryostats and furnace enclosures mounted between the magnet poles allow demagnetization curves to be recorded from approximately −196 °C (liquid nitrogen) up to +250 °C or higher. From these measurements, reversible temperature coefficients α(Br) and β(HcJ) are calculated, commonly about −0.11 %/°C and −0.6 %/°C respectively for sintered NdFeB, along with irreversible flux losses after temperature cycling. These data are critical for motor and generator design per the temperature stability designations in IEC 60404-8-1.

Calibration, Traceability, and Uncertainty

Hysteresigraphs, Hall probes, search coils, and fluxmeters are calibrated using certified reference magnets and reference instruments traceable to national metrology institutes. Testing is conducted within ISO/IEC 17025-compliant quality systems at accredited third-party laboratories. Typical expanded uncertainties (k = 2) are approximately ±1 % for Br, ±1.5 to 2 % for (BH)max, and ±2 to 3 % for HcJ.

Applications

Energy product testing data are used throughout the magnet supply chain and across many end-use industries:

• Electric vehicle traction motors, auxiliary motors, and e-axle development programs
• Direct-drive and doubly-fed wind turbine generators
• Industrial servo, spindle, stepper, and brushless DC motors
• Loudspeakers, headphones, receivers, and acoustic transducers
• Magnetic sensors, encoders, and Hall-effect assemblies
• MRI systems, magnetic separation equipment, and scientific instrumentation
• Magnetic holding, lifting, and couplings assemblies
• Supplier qualification, incoming inspection, and lot certification for magnet purchasers and manufacturers
• Input data for finite-element electromagnetic simulation, aging studies, and failure analysis

FAQ

What is the difference between HcB and HcJ?

HcB (normal coercivity) is the field at which the flux density B in the magnet falls to zero, defining the practical operating limit of the external circuit. HcJ (intrinsic coercivity) is the field at which the intrinsic magnetization J is fully cancelled, defining the point of irreversible demagnetization. A high HcJ relative to HcB indicates strong resistance to demagnetization under adverse fields and elevated temperatures.

What are typical (BH)max values for common permanent magnet materials?

Sintered NdFeB: 200–430 kJ/m³ (25–54 MGOe); sintered SmCo: 110–265 kJ/m³ (14–33 MGOe); bonded NdFeB: 30–90 kJ/m³ (4–11 MGOe); AlNiCo: 10–75 kJ/m³ (1.3–9.5 MGOe); hard ferrite: 8–40 kJ/m³ (1–5 MGOe).

What sample size and geometry are required?

Standard specimens are cylinders of about 10 mm diameter and 5 to 10 mm height, or blocks of about 10 × 10 mm cross-section with ground, parallel end faces. Non-standard sizes can usually be accommodated, and laboratories can often machine coupons from larger magnets or assemblies on request.

Can assembled or magnetized products be tested?

Full demagnetization curve measurement requires individual specimens in a closed magnetic circuit. Complete magnet assemblies and production units are instead characterized using open-circuit techniques such as Helmholtz coil flux measurements, which verify total flux and remanence-equivalent output for quality control purposes.

How does temperature affect the measured energy product?

Both Br and HcJ decrease reversibly with increasing temperature, which lowers (BH)max; near the knee region, elevated temperature can also cause irreversible losses if the operating point drops below the knee. Temperature-dependent testing quantifies these effects and is strongly recommended for any magnet intended to operate above 80 °C.

What is the typical turnaround time for energy product testing?

At accredited third-party laboratories, standard room-temperature measurements with a full data report are typically completed within 3 to 7 working days after sample receipt. Temperature-dependence studies and programs involving special sample preparation may require additional time.

In summary, (BH)max testing provides the definitive measure of permanent magnet energy capability. Combined with Br, HcB, and HcJ data from the same demagnetization curve, it gives designers and quality engineers a complete, standards-based picture of material performance for certification, simulation, and reliability assurance.