Structural phase transition of KMnF3

Abstract

The crystallographic and magnetic transitions of KMnF₃ have been studied by X-ray diffraction and measurements of magnetic susceptibility. It is found that the uniaxial antiferromagnetic transition coincides with the crystallographic transition, caused by the condensation of the M ₃-type soft phonon, at 88 K. It is also confirmed that the transition at 83 K induces a weak ferromagnetic component by spin canting and a large rearrangement of crystallographic domains. Diffuse scattering is observed at the reduced reciprocal lattice points, M(½ ½ 0) _p and X(0 0 ½) _p above 88 K, in addition to the R point, (½ ½ ½) _p , above 186 K. The transitions at 88 K and 83 K are interpreted in terms of magneto elastic coupling, induced by superexchange between Mn⁺⁺ spins and the tilting of the MnF₆ octahedra related to the M ₃ rotational mode.