Magnetic ground state of RBaCo2O5.5 (R=Tb, Gd) compounds -: art. no. 094436

The oxygen-deficient perovskites RBaCo2O5.5 (R = Tb, Gd) unveil similar magnetic properties; the magnetization [M(T)] demonstrates a sharp cusp near the magnetic transition temperature T-C and a 1/T dependence at low temperature, while the remnant magnetic moments at zero temperature are small [muapproximate to(0.05-0.1)mu(B)] the high temperature effective magnetic moments are rather large [mu(eff)approximate to(4.7-4.9)mu(B)]. We show in this paper that the paramagnetic behavior of R ions and ferrimagnetic ordering of high-spin Co+3 ions on both octahedron and pyramid sites can account for all above experimental observations. Our conclusion is based on the calculation within the unrestricted Hartree-Fock approximation and the real-space recursion method. Among the large number of spin and charge ordered states we considered, only the high-spin high-spin antiferromagnetically ordered state, the high-spin intermediate-spin antiferromagnetically ordered state, and the low-spin intermediate-spin ferromagnetically ordered state may become the ground state of the system as Dq increases. However, only the high-spin high-spin antiferromagnetically ordered state is consistent with the experimental findings.