Magnetic ground state ofRBaCo2O5.5(R=Tb,Gd)compounds

Abstract

The oxygen-deficient perovskites $R{\mathrm{BaCo}}_{2}{\mathrm{O}}_{5.5}$ $(R=\mathrm{T}\mathrm{b},\mathrm{}\mathrm{Gd})$ 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 $[\ensuremath{\mu}\ensuremath{\approx}(0.05--0.1){\ensuremath{\mu}}_{B}]$ the high temperature effective magnetic moments are rather large $[{\ensuremath{\mu}}_{\mathrm{eff}}\ensuremath{\approx}(4.7--4.9){\ensuremath{\mu}}_{B}].$ We show in this paper that the paramagnetic behavior of R ions and ferrimagnetic ordering of high-spin ${\mathrm{Co}}^{+3}$ 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 $\mathrm{Dq}$ increases. However, only the high-spin high-spin antiferromagnetically ordered state is consistent with the experimental findings.