Crystal and magnetic structures of RbMnF4 and KMnF4 investigated by neutron powder diffraction: the relationship between structure and magnetic properties in the Mn3+ layered perovskites AMnF4 (A=Na, K, Rb, Cs)

Abstract

The crystal and magnetic structures of RbMnF4 and KMnF4 have been determined by neutron powder diffraction. The crystal symmetry of both compounds belongs to the layered perovskite structure and exhibits a pseudo-tetragonal unit cell, space group P21/a. The (MnF2F42/)- octahedra show a distortion induced by both steric and Jahn-Teller effects. They are also tilted by an angle which depends on the size of the alkali ion. KMnF4 orders as a noncollinear antiferromagnet below 5.2 +or- 0.1 K exhibiting four magnetic sublattices with an angle between the two spin directions of 17 degrees . RbMnF4 is a collinear antiferromagnet below 3.7 +or- 0.1 K. Interestingly enough and contrary to what is found for the K compound, there are two active irreducible representations in the magnetic structure of the Rb derivative. Moreover, the relationship between crystal structure and magnetic behaviour has been investigated in the AMnF4 (A=Na, K, Rb, Cs) series. The sign of the isotropic magnetic interaction is studied as a function of the superexchange angle Mn-F-Mn and the degree of distortion of the octahedra.