International comparison of measurements of hard magnets with the Vibrating Sample Magnetometer

We present the results of an international comparison regarding the magnetic properties of hard magnets, measured using the Vibrating Sample Magnetometer. The comparison involved eight different international laboratories, plus two laboratories using the SQUID magnetometer. It was directed, in particular, at the determination of the hysteresis loop parameters of hard ferrite and magnetic tape samples. It was carried out in the framework of the activity of the IEC TC68 Working Group 2. The statistical analysis of the obtained results was performed according to standard rules (e. g. ISO and EURAMET guidelines). The standard deviation around the mean of the laboratories best estimates is found to be s similar to 1% in anisotropic ferrites for all the measured quantities: coercive fields H-cJ and H-cB, high-field polarization J(800) (k), polarization at remanence J(r), maximum energy product (BH)(max). Isotropic ferrites show larger dispersion of the best estimates of J(r) and H-cB (similar to 3%) and (BH)(max) (similar to 6%). For the quantities measured in the magnetic tapes (magnetic moment under applied field H = 400 kA/m m(400) (k) and at remanence mr, squareness S = m(400) (k)/m(r), H-cJ)(s) is found to fluctuate around 2%. In all cases, the relative expanded uncertainty of the reference value (weighted mean), associated with a 95% confidence level, is at most +/- 1.5%. c