Antiferromagnets with Anisotropic Spin-Spin Interactions: Stability of the Zero-Field Structure in an External Field

摘要

The transition from the polarized paramagnetic state to the antiferromagnetic phase in an appled external magnetic field has been investigated theoretically by linear spin-wave theory at T = 0. The authors' analysis applies to spins arranged in a lattice of cubic symmetry. In addition to the Zeeman term, the Hamiltonian consists of bilinear interactions. Antiferromagnetic transition to a state described by an ordering vector Q(vector) of softening of the corresponding spin-wave excitation in the paramagnetic phase. It is shown that the onset of antiferromagnetic order can be calculated by solving an eigenvalue problem. The smallest eigenvalue of the Fourier transformed 2 x 2 interaction matrix, which describes the spin-spin interactions in the plane perpendicular to B(vector), determines B(c), Q(vector), and the direction of the antiferromagnetic component. The general results were first applied to investigate the stability of the easy-axis type III antiferromagnetism of the FCC lattice. If the anisotropy is comparable to the isotropic next nearest-neighbor coupling, as in K2IrCl6, a high-field ordering vector, between the type I and III vectors, is predicted. As another application, the magnetic phase diagram of nuclear spins in copper was investigated.