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IEC TR 62797: International Comparison of Magnetic Moment Measurements Using VSM and SQUID Magnetometers
Understanding the reproducibility and uncertainty of magnetic measurements across laboratories for hard ferrites and magnetic tape samples
Introduction to IEC TR 62797
IEC/TR 62797:2013 is a Technical Report that presents the results of an international inter-laboratory comparison of magnetic moment measurements. Coordinated by INRIM (Italy) and Hannam University (Korea), the study involved ten laboratories worldwide using Vibrating Sample Magnetometers (VSM) and Superconducting Quantum Interference Device (SQUID) magnetometers. The primary objective was to verify the degree of reproducibility of the VSM method as a prerequisite for developing a future IEC measurement standard for magnetic moment determination.
The VSM is widely used in industrial and research laboratories due to its sensitivity, ruggedness, and relative simplicity. However, it is not an absolute method — it requires calibration using a reference sample (typically a pure nickel sphere). This intercomparison exercise was designed to quantify just how reproducible VSM results are across different laboratories, instruments, and operators, thereby establishing confidence limits for the method.
Two key facts about VSM: (1) It is an open-circuit method, making it unsuitable for measuring the magnetization curve of soft magnetic materials. (2) Its application to soft materials is generally restricted to saturation magnetization determination.
Test Samples and Measured Quantities
The study used two distinct sample types to cover different magnetic measurement challenges:
| Sample Type | Specifications | Quantities Measured |
|---|---|---|
| Isotropic hard ferrite (HF-Iso1, HF-Iso2) | Spherical, ~74.5 mg, density 4950 kg/m³ | J₈₀₀ₖ, Jᵣ, HcJ, HcB, (BH)ₘₐₓ |
| Anisotropic hard ferrite (HF-Aniso1, HF-Aniso2) | Spherical, ~73.3 mg, density 4870 kg/m³ | J₈₀₀ₖ, Jᵣ, HcJ, HcB, (BH)ₘₐₓ |
| Magnetic tape (1A, 2A) | Disk, d=3 mm, ~1.25 mg | m₄₀₀ₖ, mᵣ, S=mᵣ/m₄₀₀ₖ, HcJ |
Each sample was circulated among participating laboratories, with INRIM performing measurements both at the beginning and end of the exercise to detect any sample degradation. A slight mass decrease of 0.2-0.3% was observed in ferrite samples over the course of the study, which was incorporated into the overall measurement uncertainty.
The magnetic tape samples proved fragile — Tape 1B and Tape 2B were damaged during circulation and their data had to be excluded from the analysis. This highlights the challenge of using delicate reference materials in inter-laboratory comparisons.
Key Results and Reproducibility Findings
The study produced valuable quantitative data on measurement reproducibility across laboratories. For anisotropic hard ferrites, the relative standard deviations around the unweighted mean were impressively low: HcJ ~1.0%, HcB ~0.9%, and J₈₀₀ₖ ~0.8%. Isotropic ferrites showed somewhat higher scatter, particularly for (BH)ₘₐₓ (~6.2%), reflecting the greater sensitivity of this derived quantity to alignment and measurement conditions.
Two laboratories used SQUID magnetometers (PTB and NPL) alongside the VSM users. The inclusion of SQUID data provided an interesting cross-method perspective, though the primary focus remained on VSM reproducibility. The study also investigated temperature effects, finding that the coercive field HcJ changes at approximately +0.2%/°C, while HcB shows a more complex dependence with coefficients ranging from -1.5% to +0.5%/°C depending on material type.
For anisotropic hard ferrites, the maximum energy product (BH)ₘₐₓ showed only ~1.2% relative standard deviation — excellent reproducibility that supports the use of VSM as a reliable quality control tool in permanent magnet manufacturing.
Implications for Standardization and Industry Practice
The results of IEC TR 62797 provide the technical foundation needed to proceed with a formal IEC measurement standard for VSM-based magnetic moment determination. The study demonstrated that with proper calibration, temperature control (23±1°C), and careful sample alignment, VSM measurements can achieve reproducibility sufficient for both quality assurance and research applications. The data also highlighted critical factors that laboratories must control: sample orientation relative to the applied field (a ±5° misalignment can reduce remanence by ~1% in anisotropic samples), demagnetization correction for spherical samples (Nd = 1/3), and accurate temperature monitoring near the sample region.
FAQs
Q: What is the main difference between VSM and SQUID magnetometers?
A: VSMs measure magnetic moment by vibrating the sample in a uniform magnetic field and detecting the induced voltage in pickup coils. SQUID magnetometers use superconducting quantum interference devices for much higher sensitivity, suitable for very small or weakly magnetic samples. Both methods were compared in this study.
A: VSMs measure magnetic moment by vibrating the sample in a uniform magnetic field and detecting the induced voltage in pickup coils. SQUID magnetometers use superconducting quantum interference devices for much higher sensitivity, suitable for very small or weakly magnetic samples. Both methods were compared in this study.
Q: Can the results of this report be used for uncertainty budgeting in my laboratory?
A: Yes. The relative standard deviations reported (e.g., ~1% for HcJ in hard ferrites) provide realistic benchmarks for measurement uncertainty that laboratories can use in their uncertainty budgets per ISO/IEC Guide 98-3 (GUM).
A: Yes. The relative standard deviations reported (e.g., ~1% for HcJ in hard ferrites) provide realistic benchmarks for measurement uncertainty that laboratories can use in their uncertainty budgets per ISO/IEC Guide 98-3 (GUM).
Q: Why were tape samples included alongside hard ferrites?
A: Magnetic tapes represent a different measurement regime — thin-film magnetic materials with very small magnetic moments. They test the sensitivity limits of VSM instruments and provide data on reproducibility at the low end of the measurement range.
A: Magnetic tapes represent a different measurement regime — thin-film magnetic materials with very small magnetic moments. They test the sensitivity limits of VSM instruments and provide data on reproducibility at the low end of the measurement range.
Q: What temperature control is required for reliable VSM measurements?
A: The prescribed measuring temperature is 23°C ± 1°C in the region occupied by the sample and sensing coils. Labs measuring at different temperatures should apply correction coefficients determined during the study.
A: The prescribed measuring temperature is 23°C ± 1°C in the region occupied by the sample and sensing coils. Labs measuring at different temperatures should apply correction coefficients determined during the study.
