Observations of Short-Range Order by Optical Birefringence in Quadratic-Layer Antiferromagnets K₂NiF₄, Rb₂NiF₄, Ba₂NiF₆ and K₂CoF₄

Abstract

By measuring optical birefringences the behaviour of short-range order has been studied on quadratic-layer quasi-Heisenberg antiferromagnets: K₂NiF₄, Rb₂NiF₄, Ba₂NiF₆ and K₂CoF₄. An explicit form of the spin-dependent birefringence for these systems is given, which is proportional to the nearest-neighbouring correlation functions ⟨S_jS_l⟩. It is shown that with increasing Ising-like character the high tempetature persistence of ⟨S_jS_l⟩ is reduced. In the case of K₂CoF₄, where the Ising-like anisotropy is comparable to the isotropic exchange field, a symmetric and logarithmic anomaly in the behaviour of ⟨S_jS_l⟩ has been observed for |T−T_N|⁄T_N<7.5·10⁻² around the Néel point.