Spin Dimer Analysis of the Magnetic Structures of Ba_3Cr_2O_8,Ba_3Mn_2O_8,Na_4FeO_4.and Ba_2CoO_4 with a Three-Dimensional Network of Isolated MO_4 (M = Cr,Mn,Fe,Co) Tetrahedra

摘要

The spin exchange interactions of the magnetic oxides Ba_3Cr_2O_8,Ba_3Mn_2O_8,Na_4FeO_4,and Ba_2CoO_4 with a three-dimensional network of isolated MO_4 (M = Cr,Mn,Fe,Co) tetrahedra were examined by performing spin dimer analysis on the basis of tight-binding electronic structure calculations.Although the shortest O-O distances between adjacent MO_4 tetrahedra are longer than the van der Waals distance,our analysis shows that the super-superexchange interactions between adjacent MO_4 tetrahedra are substantial and determine the magnetic structures of these oxides.In agreement with experiment,our analysis predicts a weakly interacting isolated AFM dimer model for both Ba_3Cr_2O_8 and Ba_3Mn_2O_8,the (0.0,0.5,0.0) magnetic superstructure for Na_4FeO_4,the (0.5,0.0,0.5) magnetic superstructure for Ba_2CoO_4,and the presence of magnetic frustration in Ba_2CoO_4.The comparison of the intra-and interdimer spin exchange interactions of Ba_3Cr_2O_8 and Ba_3Mn_2O_8 indicates that orbital ordering should be present in Ba_3Cr_2O_8.