Published by the American Physical Society through the American Institute of Physics
Physical Review B, Condensed matter and materials physics vol:67 issue:1
The superexchange interaction between Yb3+ and Cr3+ ions in the mixed YbCrBr93- bioctahedral face-sharing dimer is quantitatively analyzed using a modified kinetic exchange theory, which is adapted to a realistic description of the electronic structure of lanthanide ions in solids. The general procedure of the calculation of the 4f-3d anisotropic exchange spin Hamiltonian is presented and applied to the YbCrBr93- dimer. The spin-Hamiltonian of the Yb3+-Cr3+ exchange interaction is found to be extremely anisotropic, H=J(z)S(Yb)(z)S(Cr)(z)+J(perpendicular to)((SYbSCrx)-S-x+(SYbSCry)-S-y), with the antiferromagnetic J(z) and ferromagnetic J(perpendicular to) parameters, where S-Yb(mu) and S-Cr(mu) (mu=x,y,z) are the components of the effective spin S-Yb=1/2 of the Yb3+ ion (corresponding to the ground Gamma(6) Kramers doublet) and the true spin S-Cr=3/2 of the Cr3+ ion, respectively. The calculated exchange parameters are quite consistent with the experimental data (J(z)=-5.16 cm(-1) and J(perpendicular to)=+4.19 cm(-1)) at reasonable values of the Yb-->Cr and Yb<--Cr charge transfer energies. The contributions to the J(z) and J(perpendicular to) exchange parameters from the individual states of the 4f(12)-3d(4) and 4f(14)-3d(2) charge transfer configurations are analyzed in detail and general regularities are established. Our results indicate that a very strong 4f-3d exchange anisotropy can appear even in the absence of the crystal-field anisotropy on the lanthanide ion.