Inverse giant magnetoresistance in granular Nd2Fe14B/α-Fe

Partially amorphous Nd-Fe-B ribbons containing 20 or 50 wt% Fe in excess of the nominal composition Nd2Fe14B with 2.9 wt% Cu and 9.7 wt% Zr (referred to Nd2Fe14B) were prepared by melt-spinning. After a heat treatment the material consists of a mixture of three parts: hard magnetic grains of the Nd2Fe14B phase with α-Fe grains and, in the case of 20 wt% excess Fe, small amounts of a metastable cubic phase, Nd2Fe23B3. Samples with 20 wt% Fe show a superposition of anisotropic magnetoresistance and giant magnetoresistance (GMR) effects, depending on structural changes caused by heat treatments at various annealing temperatures. For measurements with the applied field both parallel and perpendicular to the sample current an isotropic positive magnetoresistance ratio, MR = [R(H)-R(0)]/R(0), called inverse GMR has been found for samples with 50 wt% excess Fe.