Permalloy magnetic permeability testing measures the initial, maximum, and frequency-dependent permeability of nickel-iron soft magnetic alloys using wound ring specimens, fluxmeters, and LCR/impedance analyzers in accordance with IEC 60404-6, ASTM A596, ASTM A772, and GB/T 3658. This technical article explains the measurement principles, specimen preparation, key test items, typical permeability values for common permalloy grades, and how the results are applied in magnetic shielding, precision transformers, and sensor design.
Technical Overview
Permalloy is the generic name for a family of nickel-iron (Ni-Fe) soft magnetic alloys containing roughly 35% to 85% nickel, often alloyed with small additions of molybdenum, chromium, or copper. These alloys are distinguished by exceptionally high permeability, low coercivity, low core loss, and near-zero magnetostriction, which makes them the material of choice for magnetic shielding, precision current transformers, and low-level signal applications. Common engineering grades include Ni-50 type alloys (such as grade 1J50, μi typically 3,000 to 8,000), Ni-79Mo4 type alloys (such as grade 1J79, μi typically 20,000 to 40,000), and Ni-80Mo5 type high-permeability alloys (such as grade 1J85, μi often exceeding 40,000 after optimal heat treatment).
The purpose of a permalloy magnetic permeability experiment is to quantify how strongly the material concentrates magnetic flux in response to an applied magnetic field. Permeability is defined as the ratio of magnetic flux density B to magnetic field strength H, that is μ = B/H. In practice, engineers work with the relative permeability μr = μ/μ0, where μ0 = 4π×10⁻⁷ H/m is the permeability of free space. Because permeability in permalloy is strongly dependent on field strength, frequency, temperature, and prior magnetic history, a complete characterization involves several distinct quantities rather than a single number.
- Initial permeability μi: the permeability in the limit of vanishing magnetizing field, typically measured at H ≈ 0.08 A/m or at a very small induction level (e.g., B < 0.1 mT).
- Maximum permeability μmax: the peak of the B/H ratio along the DC magnetization curve.
- Amplitude permeability μa: permeability measured at a specified peak induction under AC excitation.
- Complex permeability μ = μ′ − jμ″: the frequency-dependent decomposition into an energy-storage component (μ′) and a loss component (μ″).
- Effective permeability μe: the apparent permeability of a gapped or non-uniform magnetic circuit.
Measurements are performed by accredited third-party laboratories, material producers, and R&D centers following recognized international and national standards, principally the IEC 60404 series, relevant ASTM standards, and the Chinese national standard GB/T 3658.
Testing Samples
Toroidal Ring Cores
The toroidal (ring) specimen is the preferred geometry for permeability testing because the closed magnetic circuit eliminates demagnetizing effects and provides a well-defined field. Strip-wound ring cores are made from permalloy ribbon of 0.02 mm to 0.35 mm thickness, wound and then annealed. Typical laboratory specimen dimensions are outer diameter 20 mm to 40 mm, inner diameter 12 mm to 30 mm, and stack height 5 mm to 10 mm. For testing, the core is fitted with a primary (magnetizing) winding of N₁ turns and a secondary (measuring) winding of N₂ turns, with turn counts chosen to balance field uniformity, winding resistance, and sensitivity. A uniform magnetizing field is assumed when the ratio of outer to inner diameter does not exceed approximately 1.4; beyond this, a correction factor must be applied.
Strip, Foil, and Ribbon Specimens
Flat permalloy strip and foil in the as-delivered condition can be evaluated using Epstein frame specimens (strips of 300 mm × 30 mm per IEC 60404-2) or single-strip testers. Sheared strip carries edge stress that depresses permeability, so specimens for design-relevant data are normally stress-relief annealed before measurement. Thin foils and amorphous-like ribbons below 50 μm are usually tested as small wound coils or in specially designed fixtures.
Bars, Rods, and machined components
Solid bars, rods, and finished parts such as shield cups or laminated stacks cannot easily be converted into closed ring circuits. These are tested in permeameters or coercion-coercimeters per IEC 60404-4, in which the specimen completes a yoke circuit and the tangential field at the surface is sensed with a Hall probe or a search coil. Because an open magnetic circuit introduces a demagnetization factor, the measured apparent permeability is always lower than the true material permeability and must be interpreted with the specimen geometry in mind.
Wire and Thin-Film Samples
Permalloy wire is commonly assessed through the effective permeability of a small wound toroid made from the wire itself, or by impedance spectroscopy over a coaxial line fixture. Sputtered or electroplated permalloy thin films are characterized with vibrating sample magnetometry (VSM) or ferromagnetic resonance methods, which yield the effective permeability of the film as-deposited or after annealing.
Testing Items
A comprehensive permalloy permeability experiment typically includes the following test items, selected according to the alloy grade and the end application:
- Initial permeability μi at DC or at a reference frequency (commonly 50 Hz, 1 kHz, or 10 kHz) and at a stated low field strength.
- Maximum permeability μmax and the field strength H at which it occurs.
- Amplitude permeability μa as a function of peak flux density (e.g., at 0.1 T, 0.3 T, 0.5 T).
- Permeability versus frequency sweep, from 20 Hz up to 200 kHz per IEC 60404-6 and into the MHz range with impedance analyzers, including complex permeability μ′ and μ″.
- Permeability versus temperature (typically −55 °C to +125 °C) to verify stability in service environments.
- DC hysteresis loop parameters that accompany permeability data: saturation flux density Bs, remanence Br, and coercivity Hc.
- Specific total loss Ps or core loss Pv as a function of frequency and induction.
- Disaccommodation, the slow decay of permeability with time after demagnetization, which is important for precision sensing applications.
Testing Methods
DC Magnetization Curve Method (Ring Specimen with Fluxmeter)
The classical DC permeability experiment follows the approach of ASTM A596/A596M and the DC provisions of IEC 60404-4 and GB/T 3658. The specimen is first fully demagnetized by a slowly decaying AC field or by thermal demagnetization. A stepwise increasing magnetizing current is applied to the primary winding; the resulting change in flux linkage induces a voltage impulse in the secondary winding that is integrated by an electronic fluxmeter to obtain the flux density B. The field strength is calculated directly from H = N₁·I / (2πr), where r is the mean radius of the ring. Plotting B/H point by point yields the magnetization curve, from which μi and μmax are extracted. This method delivers the highest accuracy, typically within ±3%.
AC Inductance Method (LCR Meter or Impedance Analyzer)
For AC permeability, the wound ring core is treated as an inductor. An LCR meter or impedance analyzer measures the inductance L (and loss factor tan δ) at a controlled small AC voltage across a chosen frequency grid, per IEC 60404-6 and ASTM A772/A772M. The effective permeability is then computed from μe = L·le / (μ0·N²·Ae), where le is the mean magnetic path length and Ae is the cross-sectional area of the core. Driving the measurement over a frequency range reveals the roll-off of the real permeability μ′ and the rise of the imaginary component μ″ caused by eddy currents, domain-wall relaxation, and magnetic after-effects. Because winding capacitance influences high-frequency results, low-loss test fixtures and short, shielded leads are essential.
Permeameter Method for Open Specimens
When only bars, strips, or finished components are available, a permeameter per IEC 60404-4 is used. The specimen is clamped in a magnetizing yoke, the tangential field at the specimen surface is measured with a Hall sensor or compensated coil, and a search coil or wrapped winding senses the flux. This method is convenient for production control of bars and wires, but the demagnetization factor of the specimen geometry limits the measurable permeability range and must be reported together with the result.
Hysteresisgraph and VSM Methods
A hysteresisgraph records the full B–H loop of a wound core at controlled field amplitude and waveform, from which amplitude permeability, Br, Hc, and loop loss are derived. For research-grade characterization of small samples and thin films, a vibrating sample magnetometer measures the magnetic moment versus applied field with high sensitivity, and permeability is inferred from the initial slope of the M–H curve, corrected for the sample shape factor.
Specimen Preparation and Conditioning
- Final high-temperature anneal (typically 1,050 °C to 1,200 °C in dry hydrogen or high vacuum) is mandatory to develop the high permeability of Ni-79 and Ni-80 grades; permeability measured before annealing is not representative.
- After annealing, specimens must be handled without bending, clamping, or shock, because mechanical stress of only a few megapascals can reduce μi by an order of magnitude.
- The specimen is demagnetized immediately before every measurement sequence, and a defined waiting time (commonly several minutes) is observed to let disaccommodation effects stabilize.
- Winding wire, turn count, and layer insulation are selected so that the winding does not short the core surface and does not add significant capacitance at the test frequency.
- Test temperature is stabilized and recorded, typically at 23 °C ± 5 °C, since permeability of high-nickel permalloy falls measurably below ambient temperature.
Applications
Permeability data from permalloy experiments directly determine whether a given alloy and heat-treatment condition will meet the requirements of the target application:
- Magnetic shielding for photomultiplier housings, MRI suites, laboratory enclosures, and sensitive electronics, where high initial permeability at very low fields (earth's field level, ~40 A/m and below) provides the attenuation needed.
- Precision current transformers and current comparators, in which the ratio error and phase displacement depend almost entirely on the effective permeability of the Ni-79 or Ni-80 core.
- Fluxgate and magnetoresistive sensor cores, where low coercivity and a well-defined permeability-versus-frequency response set the noise floor and bandwidth.
- Audio, telecom, and pulse transformers, magnetic amplifiers, and transductors that exploit the steep, controllable permeability of Ni-50 permalloy under DC bias.
- Common-mode chokes and EMC filters, where the frequency dependence of complex permeability defines the impedance spectrum of the component.
- Magnetic recording heads and MEMS devices using electroplated Ni-Fe films, where effective film permeability governs frequency response and efficiency.
- Material qualification and incoming inspection in the power electronics, aerospace, and instrumentation industries, where measured μi, μmax, Bs, Br, and Hc values are compared against grade-specific acceptance limits in specifications aligned with IEC 60404-1, ASTM A753, and GB/T 14986.
FAQ
What permeability values can be expected for common permalloy grades?
Representative values after full annealing are: Ni-50 type (1J50) initial permeability 3,000 to 8,000 and maximum permeability 30,000 to 60,000; Ni-79Mo4 type (1J79) initial permeability 20,000 to 40,000 and maximum permeability above 150,000; Ni-80Mo5 type (1J85) initial permeability 30,000 to 70,000 with maximum permeability reaching several hundred thousand. Exact values depend strongly on impurity content, thickness, annealing atmosphere, and cooling rate, so design data should always be taken from measurements on the actual production lot.
Why must the specimen be demagnetized before measuring permeability?
Permeability is not a single-valued property; it depends on the point of the hysteresis loop at which the material sits. A specimen that retains remanence from prior magnetization will show an apparent permeability that can differ from the true value by a large factor. Demagnetization with a slowly decaying alternating field restores the material to a reproducible virgin-like state, which is the reference state required by IEC 60404 and GB/T 3658.
Which standards govern permalloy permeability testing?
The principal standards are IEC 60404-6 (ring specimen method, 20 Hz to 200 kHz), IEC 60404-4 (permeameter methods for DC properties), IEC 60404-2 (Epstein frame for strip), ASTM A596/A596M (DC properties using toroidal specimens), ASTM A772/A772M (AC permeability using sinusoidal current), and GB/T 3658 (Chinese national standard for AC magnetic properties of soft magnetic materials by the ring specimen method). Material classification and nominal properties are covered by IEC 60404-1 and related national material standards.
What measurement uncertainty can be achieved?
For carefully prepared ring specimens measured on calibrated fluxmeters, expanded uncertainty of 3% to 5% (k = 2) is typical for permeability up to moderate levels. For very high-permeability Ni-80 grades, uncertainties grow because the windings no longer create a perfectly uniform field and the air-flux correction becomes significant; laboratories report uncertainty together with the specimen geometry and the applied correction factors.
Why did my measured permeability come out far below the datasheet value?
The most common causes are mechanical stress on the specimen after annealing (tight winding, clamping, or handling), incomplete demagnetization, testing at a frequency or field amplitude different from the datasheet reference condition, or testing a specimen with an unfavorable geometry that introduces a demagnetizing factor. Verifying annealing condition, demagnetization procedure, and test parameters usually resolves the discrepancy.
Can permeability be measured at cryogenic or elevated temperatures?
Yes. Specialized test fixtures with temperature-controlled chambers allow the ring specimen method or impedance method to be run from liquid-nitrogen temperature up to the Curie region. For high-nickel permalloys, permeability decreases noticeably below room temperature and collapses as the material approaches its Curie temperature (approximately 400 °C for Ni-80Mo5 grades), so temperature-dependent data are essential for extreme-environment applications.