Theoretical investigations of the local structure distortion and the spin Hamiltonian parameters of Cr 3+ ions at tetragonal charge-compensation defect CrF 5O site in Cr 3+: KMgF 3 crystals

Abstract

The local structure distortion and the spin Hamiltonian (SH) parameters, including the zero-field splitting (ZFS) parameter D and the Zeeman g-factors g ‖ and g ⊥, are theoretically investigated by means of complete diagonalization method (CDM) and the microscopic spin Hamiltonian theory for tetragonal charge compensation CrF 5O defect center in Cr 3+:KMgF 3 crystals. The superposition model (SPM) calculations are carried out to provide the crystal field (CF) parameters. This investigation reveals that the replacement of O 2− for F − and its induced lattice relaxation Δ 1(O 2−) combined with an inward relaxation of the nearest five fluorine Δ 2(F −) give rise to a strong tetragonal crystal field, which in turn results in the large ZFS and large anisotropic g-factor Δg. The experimental SH parameters D and Δg can be reproduced well by assuming that O 2− moves towards the central ion Cr 3+ by Δ 1(O 2−)=0.172R 0 and the five F − ions towards the central ion Cr 3+ by Δ 2(F −)=0.022 R 0. Our approach takes into account the spin–orbit (SO) interaction as well as the spin–spin (SS), spin–other-orbit (SOO), and orbit–orbit (OO) interactions omitted in previous studies. This shows that although the SO interaction is the most important one, the contributions to the SH parameters from other three magnetic interactions are appreciable and should not be omitted, especially for the ZFS parameter D.