Neutron-scattering study of the randomly mixed antiferromagnet with competing anisotropies K2CoxFe1-xF4

Abstract
The phase diagram of the two-dimensional (2D) randomly mixed antiferromagnet with orthogonal competing anisotropies K2CoxFe1-xF4 has been examined with elastic neutron scattering, in particular for x=0.27. For this composition the system supports long-range order of the axial spin components in 3D below a regular second-order transition at TN=64.4+or-0.6K. The transverse components also order below TN, but in 2D and over limited distances only. Below TL=27+or-2K the transverse components acquire 3D order by reorientation, establishing an oblique antiferromagnetic phase in four domains. The lower transition is first order.