TEMPERATURE-DEPENDENCE OF COERCIVITY IN MNBI

In contrast to most ferromagnetic materials, the low-temperature phase of MnBi exhibits an increased coercivity, H(c), with temperature. mu0H(c) has a value of 0.2 T at room temperature, and rises dramatically to a maximum value of 1.9 T at 550 K. In the temperature region near its maximum value, H(c) is much larger than that of Nd-Fe-B and has a very-low-temperature coefficient. For this reason, MnBi has a great potential as a permanent magnet material at high temperatures. To describe the temperature dependence of H(c), we develop a hybrid domain-wall pinning model which combines Hilzinger and Kronmuller's scaling theory for an anisotropic domain wall with Gaunt's theory of thermal activation. The hybrid model gives an excellent fit to the temperature dependence of H(c) and provides good estimates for the domain-wall energy and thickness over the temperature range studied.