Permalloy magnetization curve testing determines the DC and AC magnetic characteristics of nickel-iron soft magnetic alloys, including initial and maximum permeability, saturation flux density, remanence, and coercivity. This guide explains the test principles, specimen requirements, measurement methods, and applicable standards such as IEC 60404 and ASTM A341. Performed by accredited third-party laboratories, the test supports material qualification and component design for transformers, magnetic shields, and precision sensors.
Technical Overview
Permalloy is the generic name for a family of nickel-iron soft magnetic alloys containing approximately 35 to 85 percent nickel, frequently alloyed with molybdenum, chromium, or copper to further enhance permeability and electrical resistivity. Because of their exceptionally high permeability, very low coercivity, and narrow hysteresis loop, permalloy grades such as Ni79Mo4 and Ni80Mo5 are indispensable in applications requiring efficient magnetization at weak field strengths.
The magnetization curve, commonly referred to as the B-H curve, describes how the magnetic flux density B develops inside the material as the applied magnetic field strength H is increased from a demagnetized state to saturation. Together with the corresponding hysteresis loop, the curve yields the key figures of merit for a soft magnetic material: initial permeability, maximum permeability, saturation flux density, remanence, and coercivity. Magnetization curve testing is therefore the primary qualification tool for incoming permalloy strip, wound cores, and finished magnetic components, and it is standardized in the IEC 60404 series as well as in ASTM and GB/T method standards.
Testing Samples
Toroidal Ring Cores
The ring specimen is the preferred geometry for permalloy testing because it forms a closed magnetic circuit with no demagnetizing air gap. Strip-wound toroids or rings machined from solid stock are used, with the ratio of outer to inner diameter kept as close to unity as practicable so that the field is essentially uniform along the magnetic path length. The specimen is fitted with an insulated primary (magnetizing) winding and a secondary (measuring) winding.
Strip Specimens for the Epstein Frame
Flat strips, typically 30 mm wide and 280 mm to 305 mm long, are cut with half of the strips parallel and half perpendicular to the rolling direction, then assembled into a square Epstein frame with double-lapped corners. This configuration is mainly used for evaluating permalloy strip material at power and audio frequencies.
Bar and Rod Specimens
When a ring cannot be prepared, straight bars, rods, or strips may be tested in a permeameter that closes the magnetic circuit through a high-permeability yoke, minimizing demagnetization corrections.
Thin Films and Miniature Specimens
Sputtered or electroplated permalloy films, small ribbons, and powder specimens are characterized by vibrating sample magnetometry, which records the full M-H loop and requires a geometric demagnetization correction.
Before testing, specimens are normally stress-relief annealed at approximately 1050 °C to 1200 °C in a hydrogen or vacuum atmosphere with controlled cooling, and they must be fully demagnetized, typically by a slowly decaying alternating field, immediately before measurement.
Testing Items
A complete magnetization curve test program typically includes the following measured and derived quantities:
- DC normal magnetization curve (B as a function of H from the demagnetized state to saturation)
- Initial permeability at defined weak field strength
- Maximum permeability
- Saturation magnetic flux density Bs
- Remanence Br from the saturated hysteresis loop
- Coercive field strength Hc from the saturated hysteresis loop
- Magnetic induction at specified field strengths, for example B at H = 80 A/m
- Squareness ratio Br/Bs
- AC magnetization curves and amplitude permeability at specified frequencies, commonly 50 Hz to 20 kHz
- Specific total loss derived from the AC hysteresis loop area where required
Testing Methods
DC Ring Specimen Method
The magnetizing current applied to the primary winding is increased in small, controlled steps. At each step, the change of flux linkage in the secondary winding is integrated by a fluxmeter, from which the flux density is calculated as the flux divided by the number of secondary turns and the specimen cross-sectional area. The field strength is calculated from the ampere-turns divided by the mean magnetic path length. Recording B point by point produces the normal magnetization curve, while cycling the specimen between symmetric saturation states produces the full hysteresis loop. This method is covered by IEC 60404-4, ASTM A596/A596M, ASTM A773/A773M, and GB/T 3657.
Permeameter Method
Straight bar or strip specimens are clamped in a yoked permeameter that provides a closed magnetic circuit. The magnetizing field and the induced flux are measured with a coaxial field sensing coil or Hall sensor and a search coil respectively. This approach, standardized in ASTM A341/A341M, is suitable for larger specimens and routine inspection of semi-finished products.
AC Magnetization Curve Measurement
An alternating magnetizing current is applied while the secondary waveform is regulated to maintain a sinusoidal flux density. The flux density is obtained from the mean-area value of the secondary voltage and the field strength from the voltage across a calibrated shunt in the primary circuit. Measurements may be performed with the classical voltmeter-ammeter method or with modern digital sampling B-H analyzers that reconstruct the entire dynamic loop. Applicable standards include IEC 60404-2 for the Epstein frame method, IEC 60404-6 for ring specimens in the range 20 Hz to 200 kHz, and GB/T 3658.
Vibrating Sample Magnetometry
For thin films and miniature specimens, the sample vibrates within pickup coils and the induced signal is compared with a calibrated reference to yield the magnetic moment as a function of applied field. The loop is subsequently corrected for the demagnetizing factor of the specimen geometry.
All measurements should be performed by accredited third-party laboratories operating under ISO/IEC 17025 quality systems, with instruments and reference materials traceable to national metrology institutes. Reported results must state the specimen geometry, winding data, test temperature, demagnetization procedure, and the exact field points at which permeability values were determined.
Applications
Magnetization curve data for permalloy directly govern the design and acceptance of a wide range of high-sensitivity magnetic devices:
- Precision current transformers and protective relay cores for metering and grid protection
- Magnetic shielding enclosures, chambers, and cable screens for sensitive electronic and scientific instruments
- Audio, telecom, and wideband signal transformers requiring high permeability at low drive levels
- Fluxgate magnetometer sensors used in navigation, geophysics, and space instrumentation
- Magnetic amplifiers, saturable reactors, and high-stability inductors
- Magnetic recording heads, magnetoresistive sensor stacks, and microelectromechanical devices
- Incoming material inspection, heat-treatment process validation, and new alloy development in research and production environments
FAQ
Why must the specimen be demagnetized before measuring the magnetization curve?
The magnetization curve is only defined starting from a magnetically neutral state. Any residual magnetization from prior handling, winding, or transport shifts the starting point and falsifies the initial permeability and the shape of the low-field portion of the curve. A slowly decaying alternating field or thermal demagnetization restores the neutral state and ensures reproducible results.
What is the difference between the DC and AC magnetization curves?
The DC (quasi-static) curve reflects only the intrinsic domain processes and is geometry-independent for a closed magnetic circuit. The AC curve additionally contains eddy-current and domain-damping effects, so the apparent permeability decreases and the loop widens with increasing frequency and strip thickness. AC results must therefore always be reported together with the test frequency, waveform control condition, and flux density level.
What are typical magnetic properties of common permalloy grades?
Nickel-rich molybdenum permalloy of approximately Ni79Mo4 composition typically exhibits a saturation flux density of about 0.75 T to 0.8 T, an initial permeability in the range of tens of thousands, and a coercivity below a few amperes per meter when properly annealed. Exact values depend strongly on composition, impurity content, thickness, and final heat treatment, so certified test data should be obtained for each material lot.
Which specimen geometry gives the most reliable results?
The toroidal ring core is the reference geometry because it eliminates the air gap and demagnetizing field. The radial build should be small relative to the diameter, the cross section uniform, and the windings evenly distributed. Epstein strips and permeameter specimens remain useful alternatives for material strip qualification when ring preparation is impractical.
Up to what frequency can permalloy be characterized?
With ring specimens and digital B-H instrumentation, standardized AC measurements cover 20 Hz to 200 kHz in accordance with IEC 60404-6, and specialized setups extend to several hundred kilohertz for very thin foils, provided specimen self-heating and winding capacitance effects are properly controlled.
Can finished components be tested instead of raw material coupons?
Yes. Wound cores, transformer assemblies, and shield parts can be measured directly by adapting the magnetizing and sensing windings to the component terminals, provided the complete magnetic path is accessible and the test conditions, especially the maximum field strength, do not alter the heat-treated state of the part.