Magnetic First-Order Phase Transition and Anisotropy in Single-Crystal MnAs

The experimental and theoretical effects of magnetocrystalline anisotropy upon the magnetic first-order phase transition in the compound MnAs are here described. A 3°C orientation-dependent difference in the transition temperature is observed near 20 kOe for a single-crystal specimen. Anisotropy measurements using Shenker's method give anisotropy sums, ΣnnKn, varying from −11.9×106 ergs/cm3 at 77°K to −5.6× 106 ergs/cm3 at 314°K, where Kn are the anisotropy terms in the energy representation, EK=ΣnKn sin2nθ, that is appropriate for this hexagonal material. A separate method gives an anisotropy field of 18.3 kOe at 35°C. Data analysis indicates that three constants, K1, K2, and K3, are needed to describe the anisotropy. Their values at 35°C are −5.75, +1.5, and −1.15×106 ergs/cm3, respectively. Theoretical checks using the magnetic form of the Clausius-Clapeyron equation are in close accord with the experimental data and the determined constants.