Permalloy Hysteresis Loop Evaluation: Standards, Methods, and Key Magnetic Parameters

Permalloy hysteresis loop evaluation is the core testing process used to characterize the d.c. and a.c. magnetic properties of nickel-iron soft magnetic alloys, including saturation flux density, coercivity, remanence, and permeability. Accurate measurement of the hysteresis loop is essential for material qualification, quality control, and the design of high-performance magnetic components such as transformer cores, current sensors, and magnetic shielding. This article provides a comprehensive overview of permalloy hysteresis loop evaluation, covering sample requirements, testing items, measurement methods, relevant standards, and typical applications.

Technical Overview

Permalloy refers to a family of nickel-iron soft magnetic alloys, typically containing 72 to 83 percent nickel with the balance iron, often supplemented with small additions of molybdenum, copper, or chromium. Classic grades such as 78Ni-Fe (mumetal-type) and 81Ni-2Mo-Fe (supermalloy-type) are distinguished by extremely high permeability, very low coercivity, low core loss, and near-zero magnetostriction. These characteristics make permalloy the material of choice for precision magnetic components operating at low field strengths.

The hysteresis loop, or B-H curve, represents the relationship between the magnetic flux density (B) inside the material and the applied magnetic field strength (H) over a complete magnetization cycle. The loop encloses an area that corresponds to the energy dissipated as heat per cycle, known as the hysteresis loss. For permalloy, the loop is characteristically narrow and steep, reflecting the ease with which the material can be magnetized and demagnetized.

Hysteresis loop evaluation involves sweeping the applied field through a full cycle—typically from positive saturation to negative saturation and back—while recording the resulting flux density. From this loop, engineers extract the parameters that define the material's performance in service. Deviations in loop shape can reveal improper annealing, mechanical stress, impurities, or processing defects, making loop measurement a powerful diagnostic and quality assurance tool.

Evaluation is performed in accordance with internationally recognized standards, including IEC 60404-4 and IEC 60404-6, ASTM A596/A596M, and the corresponding GB/T standards for magnetically soft materials. Measurements may be conducted under quasi-static (d.c.) conditions or at power and audio frequencies (a.c.), depending on the intended application of the material.

Testing Samples

Toroidal Ring Cores

Toroidal, or ring, specimens are the preferred geometry for permalloy hysteresis evaluation because the closed magnetic path eliminates demagnetizing effects and air gaps. Strip-wound ring cores are fabricated by winding annealed permalloy tape around a mandrel. The primary and secondary windings are applied directly to the core, and the mean magnetic path length is calculated from the inner and outer diameters. Standards such as IEC 60404-6 recommend ring specimens with an outer-to-inner diameter ratio close to unity, ideally not exceeding 1.25, to keep field uniformity errors within acceptable limits.

Sheet and Strip Samples

Flat sheet or strip samples may be evaluated when toroidal geometry is impractical, particularly for thin permalloy foils used in shielding and recording head applications. These samples are usually cut into strips and assembled into an Epstein frame or measured with a permeameter that closes the magnetic circuit externally. Edge burrs and residual stress from cutting must be removed by stress-relief annealing before measurement, since mechanical deformation can severely degrade the permeability of permalloy.

Wire and Bar Samples

Permalloy wires, used in magnetic sensors and pulse components, and solid bars can be tested using permeameter methods in which the specimen is clamped in a closed yoke. The yoke completes the magnetic circuit while the field is applied along the axis of the sample. This configuration is covered under ring and permeameter procedures in ASTM A596/A596M.

Thin Film Samples

Sputtered or electrodeposited permalloy thin films, common in microelectromechanical systems and integrated inductors, are characterized using a vibrating sample magnetometer or a magneto-optical Kerr effect hysteresis loop tracer. These techniques require only small coupon samples and provide in-plane loop data at the nanometer film thickness scale.

Testing Items

Hysteresis loop evaluation yields a complete set of d.c. and a.c. magnetic parameters that define the performance envelope of permalloy materials and components.

• Saturation magnetic flux density (Bs): the maximum flux density achieved at the specified maximum field, typically 0.7 to 0.9 T for standard permalloy grades.
• Remanence, or residual flux density (Br): the flux density remaining when the applied field returns to zero after saturation.
• Coercive force, or coercivity (Hc): the reverse field required to reduce the flux density to zero; for well-annealed permalloy, Hc values below 1 A/m are achievable.
• Initial permeability (μi): the permeability measured as the field approaches zero, a signature property of permalloy that can exceed 20,000 to 50,000 for high-nickel grades.
• Maximum permeability (μm): the peak slope of the virgin magnetization curve.
• Squareness ratio (Br/Bs): the rectangularity of the loop, critical for pulse transformer and magnetic amplifier applications.
• Hysteresis loss (Wh): the energy dissipated per unit volume per cycle, equal to the loop area, reported in J/m³ or as specific loss in W/kg at a given frequency.
• AC permeability and AC loss: frequency-dependent properties measured at specified frequencies and flux densities.
• Magnetization curve parameters: including the field at which technical saturation is reached and the differential permeability over defined segments.

Testing Methods

DC Hysteresisgraph Ring Method

The d.c. hysteresisgraph, described in IEC 60404-4 and ASTM A596/A596M, is the primary method for quasi-static loop measurement. A ring specimen is wound with a primary magnetizing winding and a secondary sensing winding. A controlled, slowly ramped current generates the field H, computed from the ampere-turns divided by the mean magnetic path length, while an integrating fluxmeter connected to the secondary winding measures the flux density B. The field sweep rate is kept sufficiently slow to exclude eddy current effects, ensuring a true quasi-static loop. The specimen is demagnetized prior to testing by a gradually decaying alternating field.

AC Hysteresis Loop Measurement

For applications at power and audio frequencies, the loop is measured according to IEC 60404-6, covering the range from 20 Hz to 200 kHz. A sinusoidal magnetizing current is driven through the primary winding, and the induced secondary voltage is integrated to obtain the flux waveform. Digital acquisition and phase-sensitive detection allow simultaneous determination of AC hysteresis loss, complex permeability, and loop shape. Testing is performed at specified frequencies and peak flux densities, and results are reported with the waveform factor and total harmonic distortion of the flux.

Permeameter Method

When ring specimens cannot be produced, permeameter measurements using rod or strip specimens closed through a laminated yoke provide an alternative. Field strength is measured with a calibrated Hall probe or an H-coil positioned adjacent to the specimen surface. This method is suitable for material screening and process control, although corrections for yoke reluctance may be required for high accuracy.

Vibrating Sample Magnetometer Method

For thin films, ribbons, and small coupons, a vibrating sample magnetometer measures the magnetic moment as the sample oscillates within a pick-up coil assembly while a superconducting or electromagnet source sweeps the field. This method provides the full hysteresis loop over fields up to several tesla and is widely used in research and development for permalloy film characterization.

Sample Conditioning and Demagnetization

Because permalloy is highly stress-sensitive, all specimens must be annealed in a protective hydrogen or high-vacuum atmosphere to develop their full permeability, and handled without bending or shock thereafter. Thermally demagnetized or field-demagnetized initial states are mandatory before each measurement. Test temperature is controlled and reported, since both coercivity and permeability vary measurably with temperature.

Applications

Permalloy hysteresis loop evaluation directly supports the design, qualification, and acceptance of components across multiple industries.

• Precision current transformers and current sensors, where high permeability at low fields ensures accurate ratio and phase performance.
• Audio and broadband transformers that exploit the low hysteresis distortion of permalloy cores.
• Magnetic shielding enclosures and cables for electronic equipment, relying on high initial permeability to divert weak interference fields.
• Fluxgate magnetometers and navigation sensors, where low coercivity and a well-controlled loop enable high sensitivity.
• Magnetic amplifiers, transductors, and pulse transformers that require square-loop permalloy grades with high squareness ratios.
• Transformer and inductor cores for aerospace, telecommunication, and medical electronics, where low core loss at operating frequencies is essential.
• Magnetic recording heads and MEMS devices using sputtered permalloy thin films evaluated by VSM loop tracing.
• Material development and failure analysis, where loop shape changes reveal stress, impurity, or annealing-related degradation.

FAQ

What is the typical hysteresis loop shape of permalloy?

Permalloy exhibits a very narrow, steep hysteresis loop with extremely low coercivity, often below 1 A/m for supermalloy grades, and a high saturation flux density of roughly 0.7 to 0.9 T. Special square-loop permalloy compositions produce a rectangular loop for switching applications.

Which standards govern permalloy hysteresis loop measurement?

The principal standards are IEC 60404-4 for d.c. magnetic properties using ring specimens and permeameters, IEC 60404-6 for a.c. properties from 20 Hz to 200 kHz using ring specimens, ASTM A596/A596M for electronic hysteresigraph methods, and the corresponding GB/T national standards. Testing should be performed by accredited third-party laboratories or in-house facilities with traceable calibration.

Why must permalloy samples be annealed before measurement?

Permalloy derives its exceptional permeability from a carefully controlled annealing treatment that develops the ordered atomic structure and relieves internal stress. Cold working, cutting, or winding introduces stress that raises coercivity and depresses permeability, so final annealing in hydrogen or vacuum followed by careful handling is essential for representative results.

What is the difference between DC and AC hysteresis loop evaluation?

DC, or quasi-static, evaluation uses a slowly swept field to capture the intrinsic hysteresis loop free of eddy current effects. AC evaluation applies a sinusoidal excitation at the service frequency and includes dynamic contributions from eddy currents and domain wall damping, yielding higher measured losses and frequency-dependent permeability.

How much sample material is required for testing?

A standard ring core evaluation typically requires a tape-wound toroid with an outer-to-inner diameter ratio near 1.1 to 1.25 and sufficient cross-sectional area to produce measurable induced voltage, usually a few square millimeters or more. Thin film or small coupon testing by VSM requires only millimeter-scale specimens.

What accuracy can be expected from hysteresis loop measurements?

With calibrated fluxmeters, certified reference materials, and proper demagnetization, reproducibility of key parameters such as coercivity and saturation flux density within a few percent is achievable. Measurement uncertainty budgets must account for path length tolerances, winding uniformity, temperature, and waveform distortion.