Remanence enhancement and coercivity in two-phase CoPt bulk magnets

Abstract

We have studied the effect of the atomic disorder–order transformation on remanence enhancement and coercivity in CoPt ingots by isothermal annealing at various temperatures well below the transformation point. The relative amount of the low-temperature hard-magnetic face-centered-tetragonal (fct) phase precipitated in the high-temperature magnetically soft face-centered-cubic (fcc) phase was determined by means of x-ray diffraction. As a function of annealing temperature, the mean grain size and the relative amount of the fct phase increase at the cost of the fcc phase. These changes were followed by means of magnetic measurements. We observed a continuous increase in coercivity with increasing annealing temperature, eventually reaching a maximum. The Kneller–Hawig model was used to explain the occurrence of remanence enhancement and the continuously changing degree of exchange coupling between the magnetically soft and hard phases.