Random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets

In this study, the random magnetic anisotropy in isotropic nanocrystalline composite permanent magnets was investigated by means of numerical simulations. The magnetization reversal of randomly oriented hard-soft exchange-coupled grains was simulated using the Landau–Lifshitz–Gilbert equation in which the magnetization in a particular grain is assumed to align in the same direction (single-spin model). The calculations show that the energy product (BH)_max of nanocomposite magnets has a peak value 50 MGOe at 6 nm. It is about 80% of that of single-crystal Nd₂Fe₁₄B. The coercivity H_C exhibits a steep decrease toward smaller grain size, which is the result of the suppression of the random magnetic anisotropy by exchange interaction. Therefore, in isotropic nanocomposite magnets, the enhancement of energy product is limited by the suppression of the random magnetic anisotropy by exchange interaction.