Soft surface magnons and the first-order magnetic phase transitions in antiferromagnetic hematite

A theoretical study is made of the mechanism of the field-induced first-order phase transitions and the Morin transition in hematite ($\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$) at low temperatures. The field-induced transitions between the spin-flop and the antiferromagnetic phase may take place at the critical field of equal free energies, without a softening of three-dimensional magnons. Spin-flop nuclei, which are small regions of about ${10}^2$ to ${10}^3$ layers of spins turning nearly 90°, can be generated at smaller fields by the softening of surface magnons. These spin-flop nuclei grow very slowly with increasing field but catastrophically expand across the three-dimensional material as the critical field is approached. Above the critical field the system is virtually in a spin-flop phase, although a few antiferromagnetic nuclei, where the spins are almost antiferromagnetically aligned, are maintained near the surfaces by the applied field (or plus strong positive surface anisotropy fields). In decreasing field, the transition from the spin-flop to the antiferromagnetic phase occurs at the same critical field, i.e., there is no hysteresis. In this case the transition is triggered by a sudden expansion of antiferromagnetic nuclei. The Morin transition, which may take place at the critical temperature of equal free energies, may also be accounted for by the magnetic transition nuclei generated by the surface effect.