Magnetic interactions and reversal behavior ofNd2Fe14Bparticles diluted in a Nd matrix

Abstract

Reversible magnetization measurements, micromagnetic modeling, the temperature dependence of coercivity, and magnetic viscosity measurements have been used to clarify the magnetic reversal mechanism of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ particles contained in a Nd matrix. The coercivity was observed to increase markedly as the dilution of the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ phase was increased. The increase in coercivity was accompanied by a change in the reversal mechanism. In the least dilute samples, domain wall motion involving several grains governed by intergrain interactions was active. In the most dilute samples nonuniform reversal of individual grains was dominant, reversal occurring particle by particle and resembling the behavior of isolated Stoner-Wohlfarth particles. The value of the coercivity in the most dilute sample was in excellent agreement with micromagnetic modeling results for isolated particles when the effect of thermal activation of magnetization reversal was accounted for. Despite the single particle reversal mechanism of the most dilute samples, a linear dependence of coercivity on packing fraction was not observed. This is attributed to a clustering of the grains in the samples and changes in grain shape with composition. In all samples, regardless of dilution, the initial magnetic state after thermal demagnetization was found to be one in which a substantial proportion of grains were in a multidomain state. However, micromagnetic simulations for isolated particles of similar shape to those in the most dilute sample showed that the single domain state is the lowest energy state. It is concluded that thermal demagnetization can result in the system remaining in a local metastable state and not the global energy minimum. Micromagnetic calculations showed that one or more domain walls can arise in a grain during thermal demagnetization and that magnetostatic effects provide a significant energy barrier in zero field to the removal of a domain wall once it is formed.