MAGNETISM OF ALKALITETRAFLUOROMANGANATES(III) AMNF4 (A = K, RB, CS) -NEUTRON-DIFFRACTION, MOSSBAUER AND MAGNETIZATION INVESTIGATIONS

The magnetic properties of the d4 Jahn-Teller systems A(I)Mn(III)F4 wi th layered structures were investigated. Neutron diffraction on powder s of KMnF4 and RbMnF4 revealed different antiferromagnetic spin arrang ements below T(N) = 4.5 K and 2.3 K, respectively: for KMnF4 canted an tiparallel along a and b, for RbMnF4 parallel along a and antiparallel along b, in both cases parallel along c, the stacking direction of la yers. Mossbauer investigations on Fe-57 doped KMnF4 confirmed a spin o rientation approximately within the layer plane. A discussion is given of the contributions to the magnetic hyperfine field and the Mossbaue r linewidth in quasi-two-dimensional antiferromagnets with Ising aniso tropy due to thermal excitation of domain wall dynamics (solitons). Th e experimental data seem to confirm the predicted exponential temperat ure dependence of the linewidth. From magnetization measurements on po wders and a single crystal of KMnF4 the 2-d exchange energy and the ou t-of-plane and in-plane anisotropies could be extracted. In addition, from susceptibility measurements the exchange energies of NaMnF4, RbMn F4 and CsMnF4 were calculated. A linear dependence of these exchange e nergies (positive for ferromagnetic CsMnF4, negative for the other AMn F4 compounds) on the cos2 of the Mn-F-Mn bridge angle is observed and compared with the behaviour of the AFeF4 compounds which is also linea r but with reverse sign of the slope. The specific superexchange mecha nisms active in Jahn-Teller systems with antiferrodistortively ordered layers are suggested to be responsible for these findings.