Electron-nuclear double resonance of Mn 2+ in quadratic-layer fluorides. Nuclear quadrupole interaction and zero-point spin deviation

Abstract

The spin-hamiltonian parameters for Mn 2+ in K 2MgF 4, K 2ZnF 4, K 2CdF 4, Rb 2MgF 4, Rb 2ZnF 4 and Rb 2CdF 4 have been determined by means of ENDOR experiments at 4.2K. In addition the lattice constants have been measured, in order to enable us to study the influence of small changes in the distance between the fluorine and manganese ions on the nuclear-quadrupole and hyperfine interactions. Calculating the electric-field gradients on the basis of a point-charge model, a rather accurate description of the observed nuclear-quadrupole interactions is obtained. Also, the present results support the existence of a linear relation between the anisotropy in the hyperfine structure and the nuclear-quadrupole coupling proposed earlier. Furthermore it appears that the axial crystal-field parameter D is not simply proportional to terms linear or quadratic in the axial part of the crystal-field potential V ax. A relatively accurate value has been derived for the hyperfine-structure constant in concentrated K 2MnF 4 and Rb 2MnF 4. Together with experimental (literature) data on A〈S〉 in the antiferromagnetically ordered state this value leads to an experimental value for the zero-point spin deviation that agrees excellently with spin-wave theory. From the present data, together with results of ENDOR measurements on Mn 2+ in perovskite fluorides it is concluded that the supertransferred hyperfine interaction along a Mn-F-Mn path is a factor 2.25 (±30) larger than the values predicted theoretically by Owen et al. and Huang et al.