Monte Carlo analysis for finite-temperature magnetism of Nd2Fe14B permanent magnet

We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare-earth intermetallic compound, Nd2Fe14B. The constrained Monte Carlo method is applied to a Nd2Fe14B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants K-m(A) (m = 1,2,4) at finite temperatures. Subsequently, it is found that the temperature dependence of K-1(A) deviates from the Callen-Callen law, K-1(A) (T) alpha M(T) 3, even above room temperature, T-R similar to 300 K, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. It is also found that the exponent n in the external magnetic field H-ext response of barrier height F-B = F-B(0) (1 - H-ext/H-0)n is less than 2 in the low-temperature region below T-R, whereas n approaches 2 when T > T-R, indicating the presence of Stoner-Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the K-1(A) term in the T >T-R region.