Theory of Magnetic Excitations in Random Antiferromagnets. III. Spin Waves in Mixtures with Competing Spin Anisotropies

Abstract

Magnetic excitations in an antiferromagnetic mixture with competing spin anisotropies are studied by using a theory developed in I and II. Calculations are made of the dynamic susceptibility of K₂Fe_xMn_1−xF₄ and Co_xFe_1−xCl₂2H₂O in which the oblique antiferromagnetic (OAF) phase is found in a limited range of x, and of Co_xNi_1−xCl₂6H₂O in which the OAF phase is found in the entire range of x. It is shown that, in spite of the difference of the spin directions of two species of magnetic ions in the steady state, properties of spin waves in the OAF phase are very similar to those in an antiferromagnetic mixture in which constituent magnetic ions have a common easy axis. The energy gap is finite in the OAF phase as in the antiferromagnetic phase, and it vanishes at the phase boundary between the OAF and antiferromagnetic phases. The latter result may be used to determine the phase boundary. A comment is given on the T_N−x curve of a mixture with competing spin anisotropies.