Magnetic hysteresis loop measurement is the core characterization technique for evaluating the magnetic performance of magnetic fabrics—flexible textile composites embedded with ferromagnetic or ferrimagnetic particles. By tracing the complete B–H loop, this test quantifies key parameters such as saturation magnetization, remanence, coercivity, permeability, and hysteresis loss, providing essential data for material qualification, product design, and compliance with IEC, ASTM, and GB standards. This article explains the sample requirements, measured parameters, instrumentation, test procedures, and typical applications of hysteresis loop testing for magnetic textile materials.
Technical Overview
Magnetic fabrics are flexible textile composites in which magnetic functional phases—such as barium/strontium ferrite, iron oxide powders, carbonyl iron, or soft magnetic alloy flakes—are incorporated into fibers, coatings, or polymer-binder layers bonded to woven, knitted, or nonwoven substrates. These materials are increasingly used in electromagnetic interference (EMI) shielding, wearable inductive components, induction-heating textiles, magnetic closure systems, and geomagnetic-field-related applications. Because the magnetic fillers are dispersed in a non-magnetic textile matrix, the resulting macroscopic magnetic behavior differs significantly from that of bulk magnetic materials and must be characterized on the finished fabric itself.
The magnetic hysteresis loop (B–H loop) is the fundamental characterization curve describing how magnetic flux density (B) or magnetization (M) responds to an applied magnetic field strength (H) as the field is cycled between positive and negative saturation. The loop shape directly reveals the magnetic "hardness" or "softness" of the fabric, the energy dissipated per magnetization cycle, and the field levels required to fully magnetize or demagnetize the material. Measurement is typically performed using a DC hysteresisgraph with a ring (toroidal) specimen, or a vibrating sample magnetometer (VSM) for small coupon specimens, in accordance with test method principles established in the IEC 60404 series, ASTM A977/A977M, ASTM A341/A341M, and corresponding GB/T national standards.
Reliable hysteresis data depend strongly on specimen geometry, winding quality, demagnetization (degaussing) history, and environmental conditioning of the hygroscopic textile substrate. Accredited third-party laboratories follow standardized procedures for sampling, conditioning, calibration, and uncertainty evaluation so that reported parameters such as coercivity and remanence are traceable and comparable between production lots and suppliers.
Testing Samples
Flexible Magnetic Sheet Laminates on Textile Backing
These samples consist of a magnetizable compound layer (typically strontium ferrite or barium ferrite loaded at 75–90 wt% in a polymer binder) calendered or extruded onto a fabric carrier. They are the most common commercially available magnetic fabrics. Specimens are die-cut into rings (toroids) with a standardized ratio of outer to inner diameter (generally no greater than 1.4:1) to ensure uniform magnetization, or into small strips and discs for VSM analysis.
Magnetic Fiber Woven and Nonwoven Fabrics
Textiles produced from fibers containing embedded magnetic nanoparticles or from yarns twisted around magnetic wire cores. Because the magnetic loading is comparatively low and anisotropically distributed along the yarn direction, both in-plane and through-plane specimens should be prepared so that directional dependence of the hysteresis loop can be assessed.
Magnetic Coated and Printed Fabrics
Woven or knitted fabrics with magnetic pastes screen-printed, knife-coated, or spray-coated onto the surface. Coating thickness uniformity strongly influences measured magnetization; multiple specimens from different positions across the fabric width should be sampled to capture coating variability.
Specimen Preparation and Conditioning Requirements
- Ring specimens: typical outer diameter 45–55 mm, inner diameter 33–40 mm, stacked thickness 1–5 mm; edges deburred and free of delamination.
- VSM coupons: 2–5 mm squares or 3–5 mm discs, mass determined to 0.01 mg accuracy for magnetization normalization (emu/g or A·m²/kg).
- Conditioning: fabrics equilibrated at 20 ± 2 °C and 65 ± 4% relative humidity (ISO 139) for at least 24 hours before testing to stabilize moisture content.
- Demagnetization: all specimens thermally or AC-field demagnetized prior to measurement to erase prior magnetization history.
- Minimum sample quantity: at least three specimens per fabric lot to account for spatial non-uniformity of the magnetic filler.
Testing Items
The hysteresis loop measurement yields a complete set of DC magnetic characteristic parameters, including:
- Saturation magnetic flux density (Bs) or saturation magnetization (Ms) — the maximum magnetization attainable at the peak applied field.
- Remanence / residual magnetization (Br, Mr) — the magnetization remaining when the applied field returns to zero.
- Coercive field strength / coercivity (HcB, HcJ) — the reverse field required to reduce flux density or intrinsic magnetization to zero.
- Initial permeability (μi) and maximum permeability (μm) — derived from the virgin magnetization and commutation curves.
- Hysteresis loss (Wh) — the energy dissipated per unit volume per magnetization cycle, equal to the loop area.
- Squareness ratio (Mr/Ms) — an indicator of loop rectangularity relevant to switching and memory applications.
- Knee field and loop shape descriptors at specified peak fields (e.g., 10 kA/m, 100 kA/m, 800 kA/m) for application-specific acceptance criteria.
- Optional: temperature-dependent loops (−40 °C to +150 °C) to evaluate thermal stability of the magnetic fabric.
Testing Methods
Ring Specimen DC Hysteresisgraph Method
The preferred reference method for magnetically loaded fabrics follows the principle of IEC 60404-6 for soft magnetic materials. A primary (magnetizing) winding and a secondary (measuring) winding are uniformly wound around the toroidal fabric specimen, forming a closed magnetic circuit that eliminates demagnetizing-field corrections. The primary winding is driven by a programmable current source sweeping the field H between positive and negative peak values. The secondary induced voltage is integrated electronically to obtain flux density B. Hysteresis and commutation curves are recorded after cycling to technical saturation, and parameters (Bs, Br, Hc, μi, μm, Wh) are extracted from the stabilized loop. This method provides the lowest uncertainty for soft-magnetic fabric grades.
Vibrating Sample Magnetometer (VSM) Method
For small coupons, anisotropic fabrics, or research-level characterization, the VSM method is used. The specimen vibrates within a uniform DC field (electromagnet up to approximately 2 T, or superconducting magnet for higher fields) between pickup coils; the induced signal is proportional to the magnetic moment. Loop measurement per principles of ASTM A977/A977M and ASTM A341/A341M practice yields magnetization normalized to specimen mass. Demagnetization factors must be applied for non-ellipsoidal coupon geometry, and results should state the field orientation relative to the fabric plane.
SQUID and High-Sensitivity Magnetometry
Fabrics with low magnetic filler loading (below roughly 1 wt% magnetic phase) produce weak signals near the sensitivity limit of conventional VSM. Superconducting quantum interference device (SQUID) magnetometers provide detection sensitivity down to 10⁻⁸ emu, enabling loop measurement of lightly doped fibers, diamagnetic textile backgrounds, and trace magnetic contamination studies.
Procedure and Quality Control Highlights
- Instrument calibration with certified reference materials (e.g., nickel sphere for VSM moment calibration; mutual inductance standards for hysteresisgraph integrators).
- Field-sweep protocol: incremental field steps with dwell time sufficient to reach quasi-static conditions, avoiding rate-dependent eddy-current artifacts.
- Background subtraction of the sample holder and textile substrate diamagnetic/paramagnetic contribution.
- Repeatability verification: duplicate loops on the same specimen must agree within the stated measurement uncertainty.
- Reporting: loop plots, parameter table with units (A/m, T, A·m²/kg), test temperature, peak field, orientation, and specimen dimensions/mass.
Applications
Hysteresis loop data for magnetic fabrics directly support engineering decisions and regulatory compliance across multiple sectors:
- EMI shielding enclosures and conductive-magnetic garment liners: permeability and loss data feed shielding-effectiveness simulations at low frequencies where magnetic absorption dominates.
- Wearable inductors, NFC/RFID antennas, and wireless-charging textiles: soft-magnetic fabric layers with low coercivity and high permeability concentrate magnetic flux, and loop parameters determine efficiency and thermal loss.
- Induction-heating and thermotherapy fabrics: hysteresis loss per cycle quantifies heat generation under alternating fields, supporting safety and dosimetry calculations.
- Magnetic closure and fastening systems in apparel, luggage, and consumer goods: remanence and coercivity determine holding force and resistance to accidental demagnetization.
- Geomagnetic and low-frequency field management products: loop characterization validates the attenuation of slowly varying magnetic fields by high-permeability fabric laminates.
- Research and development: comparing ferrite particle grades, filler loadings, binder systems, and textile structures; incoming-material inspection and supplier qualification in serial production.
- Forensic and failure analysis: detecting degradation, oxidation of magnetic fillers, or demagnetization after thermal or mechanical stress exposure.
FAQ
What is the difference between measuring a magnetic fabric and a bulk magnetic material? Magnetic fabrics are heterogeneous composites: the magnetic phase is diluted in a non-magnetic matrix and distributed over a finite thickness. Measured magnetization is therefore reported per unit mass or normalized to magnetic filler content, and anisotropy between the fabric plane and thickness direction is often significant. Closed-circuit ring methods minimize geometry errors, while coupon methods require demagnetization-factor corrections.
Which standards govern the test? The IEC 60404 series defines methods for magnetic materials, including ring-specimen DC measurement of soft materials (IEC 60404-6); ASTM A977/A977M and ASTM A341/A341M cover hysteresisgraph and DC permeability methods; corresponding GB/T standards apply in the Chinese national framework. Textile conditioning follows ISO 139. Accredited third-party laboratories select and combine these frameworks appropriate to the fabric configuration.
How much sample is required? For the ring method, one or more toroids with approximately 1–5 mm stacked thickness; for VSM, coupons of a few millimeters with accurately known mass. Sampling several locations across the fabric roll is recommended because filler distribution is rarely perfectly uniform.
What accuracy can be expected? With calibrated instruments and proper specimen preparation, typical expanded uncertainties are within 2–3% for magnetization values and 2–5% for coercivity. Lower filler loadings and thinner specimens increase uncertainty due to weaker signals and geometry sensitivity.
Can measurements be performed at elevated or sub-zero temperatures? Yes. Temperature-controlled VSM and hysteresisgraph fixtures allow loops to be recorded from approximately −196 °C to several hundred degrees Celsius, which is valuable for automotive, aerospace, and outdoor textile applications where magnetic performance must remain stable across the service temperature range.
Why must specimens be demagnetized before testing? The hysteresis loop depends on magnetic history. Prior magnetization from manufacturing, transport, or handling shifts the apparent remanence and initial permeability. Standard practice requires AC or thermal demagnetization so that the virgin and commutation curves reflect the intrinsic material state.