Permalloy Maximum Permeability Evaluation: Principles, Methods, and Standards

Permalloy maximum permeability evaluation is a core DC magnetic characterization service for nickel-iron soft magnetic alloys widely used in shielding, precision transformers, and magnetic sensors. Using ring-specimen and permeameter methods in accordance with IEC 60404-4, ASTM A596/A341, and GB/T 3657, accredited third-party laboratories determine the maximum permeability, coercivity, and hysteresis loop of grades such as 1J50, 1J79, and 1J85. This article explains the testing principles, specimen requirements, procedures, typical acceptance values, and practical considerations for reliable measurement.

Technical Overview

Permalloy is the generic name for a family of nickel-iron soft magnetic alloys, typically containing 35% to 85% nickel, often with additions of molybdenum, chromium, or copper to refine their magnetic properties. These alloys are characterized by very high permeability, low coercivity, and low losses under weak-field magnetization, making them the material of choice for high-sensitivity magnetic components. Common commercial grades include 1J50 (approximately 50% Ni, balance Fe), 1J79 (79% Ni-4% Mo), and 1J85/1J86 (80% Ni-5% Mo, the so-called molybdenum permalloy or supermalloy class), which are covered by specifications such as GB/T 14986 and ASTM A753/A753M.

Maximum permeability (µmax) is defined as the highest value of the ratio B/H reached along the normal (initial) magnetization curve, which is traced from a completely demagnetized state. As the applied magnetic field strength H increases from zero, the permeability rises from the initial permeability µi, passes through a peak value µmax at a specific field strength, and then declines as the material approaches magnetic saturation. Because µmax directly governs shielding efficiency, transformer accuracy, and achievable inductance per turn, it is the single most important acceptance parameter for permalloy products.

• 1J50 / 50% Ni-Fe (similar to ASTM A753 Type 4750): µmax typically 35,000–60,000, saturation flux density about 1.5 T
• 1J79 (79% Ni-4% Mo): µmax specified at 100,000 or higher, typically 120,000–200,000 after optimal annealing, with saturation flux density around 0.75 T
• 1J85 / 1J86 (80% Ni-5% Mo supermalloy class): µmax of 150,000 and above, exceeding 300,000 with optimized heat treatment, coercivity below about 1 A/m

Accurate evaluation of µmax requires a closed magnetic circuit, a well-defined demagnetized starting state, and quasi-static magnetization conditions. The relevant international and national standards include IEC 60404-1 (classification), IEC 60404-4 (DC measurement of soft magnetic materials), ASTM A596/A596M (DC properties of toroidal cores), ASTM A341/A341M (permeameter method), GB/T 3657 (DC magnetic measurement of soft magnetic alloys), and GB/T 14986 (specification of soft magnetic alloy products).

Testing Samples

Ring (Toroidal) Specimens from Cold-Rolled Strip

The ring specimen is the preferred and most accurate sample form for permalloy. Strip with a thickness of 0.02 mm to 0.35 mm is stamped, wound, or stacked into a toroid with electrical insulation between layers to suppress eddy currents. To keep the internal field uniform, the radial width should not exceed one eighth of the mean diameter, and typical laboratory rings have outer diameters of 40 mm to 60 mm with a suitable stacking height. After final annealing, a primary (magnetizing) winding and a secondary (search) winding are uniformly applied around the toroid.

Solid Ring and Machined Specimens

For bars, rods, forgings, and powder-metallurgy parts, solid ring specimens are machined directly from the product. Because machining introduces residual stress that severely degrades permeability, machined specimens must be re-annealed before testing so that the measured properties represent the fully softened magnetic condition.

Strip and Bar Specimens for Permeameter Testing

When a ring cannot be prepared, for example for long bars or wide strips, straight specimens can be tested in a permeameter that closes the magnetic path through a high-permeability yoke. This approach is more convenient but introduces additional uncertainty, so it is normally used for comparative or screening purposes rather than final certification.

Heat-Treatment Condition

All permalloy specimens must be tested in their final magnetic state, which is established by high-temperature annealing at 1050 °C to 1200 °C in dry hydrogen or vacuum followed by a precisely controlled cooling rate. For 79% and 80% Ni-Mo grades, the cooling rate controls atomic ordering and therefore the balance between initial and maximum permeability. Some square-loop grades additionally require magnetic field annealing. Specimens must be handled without bending, clamping, or winding tension after annealing, since even elastic stress can reduce µmax by a significant margin.

Testing Items

A complete DC magnetic evaluation of permalloy normally reports the following quantities, with maximum permeability as the core item:

• Maximum permeability µmax, together with the magnetic field strength at which the maximum occurs
• Initial permeability µi at a specified weak field, for example 0.08 A/m or 0.4 A/m
• Flux density at specified field strengths, such as B at 80 A/m or 800 A/m, and saturation flux density Bs
• Coercive field strength Hc and remanence Br determined from the DC hysteresis loop
• Squareness ratio Br/Bm for square-loop applications
• Optional AC characteristics, including AC permeability and specific total loss versus frequency and flux density, measured per IEC 60404-6 or GB/T 3658 ring methods
• Test conditions, including specimen geometry, number of turns, demagnetization procedure, and ambient temperature

Testing Methods

Reference Methods

The primary reference method is the DC ring method described in GB/T 3657, IEC 60404-4, and ASTM A596/A596M. For non-ring geometries, the permeameter method of ASTM A341/A341M or the permeameter clauses of IEC 60404-4 are applied. The measurement can be performed point by point (ballistic method) or continuously with an electronic hysteresigraph.

Measurement Principle

The magnetizing field strength is calculated from the primary current as H = N1·I / lm, where N1 is the number of primary turns and lm is the mean magnetic path length, lm = π·(OD + ID)/2 for a ring. The flux density is obtained by integrating the induced voltage of the search coil, B = 1/(N2·Ae)·∫V·dt, where Ae is the cross-sectional area of the specimen. The relative permeability at each point is then µ = B/(µ0·H), with µ0 = 4π × 10⁻⁷ H/m, and the peak of the µ–H curve is reported as µmax.

Demagnetization and Neutralization

Because µmax is defined on the normal magnetization curve, the specimen must first be brought into a truly demagnetized (neutral) state. In practice the specimen is saturated and then demagnetized with an alternating field whose amplitude decays slowly to zero, or thermally demagnetized. After demagnetization, a stabilization waiting time of several minutes is observed to allow magnetic after-effects to settle before data acquisition begins.

Point-by-Point (Ballistic) Procedure

• Apply a saturating field, then demagnetize the specimen and allow the stabilization period to elapse.
• Apply an increasing sequence of field steps, cyclically commuting each current level to establish a symmetric magnetization condition for every point on the normal curve.
• At each step, record the flux change with a calibrated fluxmeter or ballistic galvanometer and compute B and µ.
• Continue stepping until the permeability has clearly passed its peak and is decreasing toward saturation.
• Plot µ versus H, identify the maximum value, and if necessary add intermediate field steps around the peak to resolve µmax accurately.

Continuous (Electronic Hysteresigraph) Procedure

A modern DC hysteresigraph ramps the magnetizing current slowly while a drift-compensated fluxmeter samples B continuously, allowing automatic tracing of the magnetization curve, the µ–H curve, and the full hysteresis loop. The sweep rate must be slow enough that eddy-current effects remain negligible, which is especially important for strip thicknesses above 0.1 mm. Periodic zero checks and demagnetization between runs are essential for high-permeability alloys, whose signals are large but whose coercivities are very small.

Permeameter Method

In the permeameter method, the strip or bar specimen is clamped between pole pieces or yokes of high permeability to form a closed circuit, and the field in the specimen is derived from calibrated sensors rather than from winding geometry. Air-gap and yoke corrections are applied, and the achievable uncertainty is higher than for the ring method, so this route is mainly used when toroidal preparation is impractical.

Influencing Factors and Best Practices

• Temperature is maintained at 23 °C ± 5 °C, and rapid temperature cycling of the specimen is avoided.
• Residual stress from winding, clamping, or transport must be eliminated, as it can depress µmax substantially.
• Instrument chains are verified with certified reference materials traceable to national metrology institutes.
• Uniform winding distribution and correct mean path length calculation are critical for accurate H values.
• The report should always state the field strength at which µmax occurs, since the peak position is sensitive to heat treatment and thickness.

Applications

Permalloy maximum permeability evaluation supports material acceptance, process development, and component design across many industries:

• Magnetic shielding enclosures, chambers, and cable screens made of mu-metal, where the shielding factor scales directly with permeability
• Precision current and voltage transformers, including residual-current device (RCD/GFCI) cores that must respond to very weak leakage currents
• Audio, instrumentation, and pulse transformers requiring high inductance and low distortion at low signal levels
• Chokes, magnetic amplifiers, and transductor cores operating in weak-field regimes
• Fluxgate magnetometers, magnetically shielded sensors, and other high-sensitivity detection instruments
• Relay armatures, reed-switch shields, and magnetic head laminations
• Incoming material inspection, supplier qualification, research on heat-treatment optimization, and failure analysis, with certification available from accredited third-party laboratories

FAQ

What is the difference between initial permeability and maximum permeability? Initial permeability µi is the slope of the magnetization curve at very small field strengths near zero, while maximum permeability µmax is the largest B/H ratio reached anywhere on the normal magnetization curve, typically at moderate fields well below saturation. High-molybdenum grades such as 1J85 excel in both, whereas simpler 50% Ni-Fe grades trade lower µmax for higher saturation flux density.

Why must the specimen be demagnetized before testing? The normal magnetization curve, and therefore µmax, is defined only from a magnetically neutral state. Any remanence shifts the apparent permeability, usually lowering the measured µmax and distorting the position of the peak. Standards therefore mandate demagnetization followed by a stabilization waiting time before measurement.

Can maximum permeability be measured on an open magnetic path? No. Demagnetizing fields in an open circuit invalidate the simple B/H calculation. A closed magnetic path is required, which in practice means either a ring specimen or a specimen inserted into a calibrated permeameter yoke.

How does heat treatment affect the results? Annealing temperature, atmosphere, and especially cooling rate control the defect density and atomic ordering of the alloy. A poorly executed or contaminated anneal can reduce µmax by a factor of several, so evaluation laboratories must test specimens in their final, correctly heat-treated condition.

Is DC permeability the same as AC permeability? No. AC permeability depends on frequency and flux density and is reduced by eddy currents and relaxation effects. DC maximum permeability characterizes the intrinsic quasi-static material quality, while AC testing per IEC 60404-6 or GB/T 3658 is added when the component operates at power or audio frequencies.

What specimen size is needed for a ring test? Typical laboratory rings have outer diameters of 40 mm to 60 mm with a radial width not exceeding one eighth of the mean diameter and a stack height of a few millimetres to tens of millimetres. Strip, wire, and machined solid rings can all be accommodated, and accredited third-party laboratories can advise on minimum sample dimensions for a given grade and thickness.