Initial Magnetization Curve Measurement: Principles, Standards, and Testing Methods

Initial magnetization curve measurement determines the B–H relationship of a magnetic material starting from a fully demagnetized state, yielding key design parameters such as initial permeability, maximum permeability, and saturation flux density. This technical guide explains the underlying principles, specimen requirements, standard test methods (IEC 60404, ASTM A341, GB/T 3656), measurable parameters, and typical industrial applications of initial magnetization curve determination for soft and hard magnetic materials.

Technical Overview

The initial magnetization curve, also known as the virgin curve, describes the locus of points relating magnetic flux density (B) to magnetic field strength (H) as the applied field is gradually increased from zero on a completely demagnetized specimen until technical saturation is approached. Unlike a hysteresis loop, which is traced between field limits after saturation, the initial curve is measured in a single monotonic excursion from the demagnetized origin and therefore reflects the intrinsic domain processes of the material in its "as-demagnetized" condition.

The curve can be divided into three characteristic regions, each dominated by a distinct magnetization mechanism:

• Initial (Rayleigh) region: at very low fields, magnetization proceeds by reversible domain wall bending and translation, defining the initial permeability μi = lim(H→0) B/H.
• Steep region: irreversible domain wall displacement produces rapid flux growth with pronounced Barkhausen discontinuities; the steepest slope of this region defines the maximum permeability μmax.
• Approach to saturation: further increases in H occur mainly through rotation of magnetization vectors against anisotropy, and the curve flattens toward the saturation flux density Bs.

Quantitatively, the measurement relies on the fundamental relations B = μ0(H + M), the amplitude permeability μ = B/H, and the relative permeability μr = μ/μ0. Determination of the initial magnetization curve is standardized internationally in the IEC 60404 series (notably IEC 60404-2, IEC 60404-4 and IEC 60404-6), in ASTM standards such as ASTM A341/A341M and ASTM A596/A596M, in the Chinese national standard GB/T 3656, and in related JIS standards for sheet and ring specimens. Accredited third-party laboratories routinely perform these measurements for material qualification, design data generation, and research.

Testing Samples

Ring (Toroidal) Specimens

Ring-shaped specimens are the preferred geometry for soft magnetic materials because they form a closed magnetic circuit with negligible demagnetizing fields and stray flux. The field strength is calculated directly from the magnetizing current using Ampere's law, H = N·I / (2πr), where r is the mean radius. To ensure acceptable field uniformity, standards typically require the ratio of outer to inner diameter to be close to unity (commonly OD/ID ≤ 1.1 for the highest accuracy, with corrections applied when this ratio is exceeded).

Strip Specimens for the Epstein Frame

Electrical steel sheet and strip are tested as sheared strips, typically 305 mm × 30 mm, assembled into a standard 25 cm Epstein double-lapped square with a specified test mass (conventional 10 kg scale or reduced 0.5 kg scale per IEC 60404-2). This format is used for AC magnetization characteristics of grain-oriented and non-oriented laminations.

Bar and Rod Specimens

Cylindrical bars, rods, or strips that cannot be wound directly are measured in a permeameter or hysteresisgraph, in which the specimen completes a closed magnetic circuit through high-permeability yokes. Field strength is measured on the specimen surface with a Hall probe or a coaxial search coil in accordance with IEC 60404-4.

Bulk and Powder Specimens

Permanent magnet materials, sintered parts, and irregular bulk samples are characterized with magnetometer-based methods such as vibrating sample magnetometry (VSM) or closed-circuit hysteresisgraphs; open-circuit results must be corrected for the demagnetizing factor. Soft magnetic composites (SMC) and bonded powders are usually pressed into rings so they can be tested with the standard ring method.

Testing Items

A complete initial magnetization curve determination typically reports the following parameters:

• The full tabulated B–H data set and the plotted initial magnetization curve from H = 0 to the specified maximum field.
• Initial permeability μi (and relative initial permeability μri), extrapolated to zero field from the Rayleigh region.
• Maximum permeability μmax and the field H(μmax) at which it occurs.
• Magnetic flux density B at specified field strengths, for example B at H = 100 A/m, B at H = 800 A/m, or B at H = 4000 A/m, depending on the material class.
• Saturation flux density Bs, or the practical saturation value B at a high specified field (such as B8000 or B16000).
• Optional derivatives of the curve, including the differential permeability μd = dB/dH and the location of the knee point used in core design.
• If the test is extended beyond the initial curve, the complete static hysteresis loop with remanence Br and coercivity HcB/HcJ can be added in the same run.

Testing Methods

Ring Specimen Method (DC, Fluxmetric)

The specimen is fitted with a primary (magnetizing) winding and a secondary (search) winding. After thorough demagnetization, the magnetizing current is increased in a staircase or slow ramp; the field strength is computed from the current and the mean magnetic path length, while the flux density is obtained by integrating the induced voltage in the search coil with a calibrated fluxmeter, B = (1/(Ns·A))·∫V·dt. Air-flux and zero-drift corrections are applied as specified in IEC 60404-6 and ASTM A596/A596M. This method delivers the highest accuracy for soft magnetic materials and is suitable for laminated alloys, ferrites, amorphous and nanocrystalline ribbons, and powder cores.

Permeameter and Hysteresisgraph Method (IEC 60404-4 / ASTM A341)

Bar or rod specimens are clamped between pole pieces of a magnetically closed circuit. The tangential field at the specimen surface is sensed with a Hall probe or a compensated coaxial coil, and flux is collected through a search coil wound directly on the specimen or on a fixture. This approach is used when specimens cannot be made into rings, including bars of silicon steel, soft magnetic alloys, and some permanent magnet materials prior to full loop tracing.

Epstein Frame Method (AC)

For electrical steel sheet and strip, the 25 cm Epstein frame per IEC 60404-2 subjects the stacked strips to a controlled sinusoidal flux waveform. The measurement yields the AC magnetization curve and specific total loss versus peak flux density; the peak magnetizing field is derived from the primary current. This method underpins commercial grading of electrical steels.

Magnetometer Methods for Bulk Materials

Vibrating sample magnetometers and extraction-type magnetometers measure the magnetic moment of bulk or small specimens as a function of applied field generated by an electromagnet or superconducting solenoid. The initial curve is obtained after demagnetizing the sample, with corrections for the demagnetizing field applied when the specimen geometry is open-circuit. This route is common for permanent magnet characterization and research-grade studies.

Test Procedure and Quality Control

• Demagnetize the specimen by thermal demagnetization or by a slowly decaying alternating field (dynamic demagnetization), then allow stabilization.
• Establish the zero reference of the fluxmeter with the search coil shorted or compensated, and record any air-flux compensation constant.
• Increase the field in defined increments, allowing settling time at each step, and record the corresponding flux density.
• Compute μ = B/H at each point, verify repeatability by a second traverse after re-demagnetization, and plot the curve.
• Control temperature, winding uniformity, and integrator drift, which dominate the measurement uncertainty (typically a few percent for permeability values).

Applications

Initial magnetization curve data are used across the entire life cycle of magnetic components, from material development to design verification and incoming inspection:

• Transformer and inductor core design: selecting working flux density below the knee of the curve and matching permeability to inductance targets.
• Electrical steel qualification: grading of grain-oriented and non-oriented laminations for motors, generators, and distribution transformers.
• Soft magnetic alloys: characterization of silicon steel, nickel-iron (permalloy-type) and cobalt-iron alloys for relays, actuators, and magnetic shielding.
• Amorphous and nanocrystalline ribbons: optimization of annealing treatments and verification of high-permeability performance for common-mode chokes and current transformers.
• Ferrites and powder composites: datasheet generation for switch-mode power supply cores, inductors, and soft magnetic composite parts.
• Permanent magnet production: establishing the virgin-curve segment required for full hysteresis characterization and for magnetizing fixture design.
• Research and failure analysis: detecting changes in texture, impurity content, or stress state through shifts in permeability and saturation behavior.

FAQ

What is the difference between the initial magnetization curve and the normal magnetization curve?

The initial magnetization curve is traced once from a demagnetized state as the field rises monotonically to saturation. The normal (commutation) magnetization curve is constructed from the tips of a family of symmetric hysteresis loops measured at increasing field amplitudes. The two curves are close in most soft magnetic materials but differ slightly because loop tip values contain the history of prior cycling.

Why must the specimen be demagnetized before measurement?

Any remanent magnetization shifts the starting point away from the true origin, biasing the low-field portion of the curve and producing erroneous initial permeability. Dynamic demagnetization with a decaying alternating field or thermal demagnetization above the Curie temperature is therefore a mandatory first step in all standard procedures.

Are DC and AC initial magnetization curves the same?

No. Quasi-static (DC) measurement yields the intrinsic magnetization behavior free of eddy current effects. AC measurements at 50/60 Hz or higher include eddy-current shielding and loss-related phase shifts, so the apparent permeability is lower and depends on frequency, waveform, and lamination thickness. Standards such as IEC 60404-2 and IEC 60404-6 define the applicable conditions for each case.

Which standards govern initial magnetization curve determination?

Principal references include IEC 60404-4 (permeameter method), IEC 60404-6 (ring specimen method), IEC 60404-2 (Epstein frame for electrical steel), ASTM A341/A341M and ASTM A596/A596M (DC permeameter and ring methods), and GB/T 3656 for DC properties of soft magnetic materials. Tests can be performed in-house or by accredited third-party laboratories operating under these frameworks.

What measurement uncertainty can be expected?

With careful demagnetization, calibrated fluxmeters, and correct air-flux compensation, flux density values are typically determined within about 1–2%, while permeability values, especially the initial permeability, carry uncertainties in the range of 2–5% owing to sensitivity to residual stress, specimen history, and zero drift.