Temperature evolution of coercive field and thermal relaxation effects in nanocrystalline FeNbB alloys

The temperature dependence of the coercive field and thermal magnetic creep at elevated temperatures are studied in nanocrystalline Fe_80.5Nb₇B_12.5. A magnetic hardening near the Curie temperature of the amorphous matrix, T_c(am), is followed by a decrease of the coercive field due to the thermal relaxation of the magnetization above T_c(am). Our analysis of the logarithmic-like creep above T_c(am) reveals activation volumes which are temperature independent and involve many grains. This indicates collective effects mediated by dipolar grain-grain interactions.