Coercivity and its thermal dependence in microsized magnetic particles: Influence of grain boundaries

Fe_73.5Si_13.5B₉Nb₃Cu₁ powder particles have been obtained by gas atomization. Magnetization curves and coercivity were studied for particles ranging in size up to 1000 μm. The overall magnetic behavior of such material is a consequence of compositional heterogeneity of the microstructure as a whole. Anomalous temperature variation of coercivity (H_c) (i.e., a decrease in H_c with decreasing temperature) together with a decrease of saturation magnetization has been observed for less than 25 μm size. The origin of this behavior has been ascribed to metastable FeCu and FeNbSi phases in combination with an Fe-rich one. Making magnetic powders with coercive fields of the order of mOe remains a challenge for researchers. Our experiment has allowed us, at low temperature, achieving a coercive field of 9 Oe, much lower than those observed so far in this type of materials. This behaviour has been related with a FeCu phase present on grain boundaries.