Birefringence Measurements on Substitutionally Random Two-Dimensional Antiferromagnets K₂Ni_1−xCo_xF₄

Abstract

Birefringence study of the nearest-neighbour spin correlation function was performed on the quadratic layer substitutionally random antiferromagnet K₂Ni_1−xCo_xF₄ (0<x<1). The system is found to exhibit two-dimensional Ising critical behaviour within the nearly same reduced temperature region as the pure K₂CoF₄. The Néel temperature has a convex dependence on the substitutional concentration x with a maximum. The phase diagram of this mixture is discussed by introducing a model Hamiltonian.