Initial magnetization curve and hardening mechanism in rapidly quenched NdFeB magnets

Abstract

Rapidly quenched NdFeB alloys were prepared with varying grain sizes well below and above the critical size for single domain particles. The contributions of the single and multi-domain particles to the initial magnetization curve are analyzed. By changing the quenching speed, i.e., the average grain size, the shape of the initial magnetization curve changes characteristically. The volume fraction of single domain grains is determined from the initial magnetization curves. From scanning electron micrographs the grain size distribution is evaluated and the critical size for single domain particles in the bulk material is deduced from a quantitative analysis of the magnetization curves. It reaches twice the value of the theoretical value for isolated spherical particles. A low temperature treatment following the thermal demagnetization lowers the initial susceptibility in underquenched ribbons as well as in an MQIII magnet. This effect reflects the irreversibility of the transition from the multi to the single domain particle state during the cooling. The temperature dependence of the single domain particle size is deduced from the initial magnetization curves of low temperature treated samples. It is shown that these experimental results are consistently explained assuming the nucleation mechanism to apply for rapidly quenched NdFeB magnets. The results are compared to the behaviour of hard ferrites.