Magnetic structure of Sr₂MnO_3.5

Magnetic behavior of Sr₂MnO_3.5 has been studied by measuring the magnetic susceptibility χ, high-field magnetization and specific heat C. The spin structure has also been studied by means of the powder neutron diffraction. The system has the two-dimensional linkage of corner-sharing MnO₅ pyramids derived by regularly reducing the oxygen atoms from the MnO₂ layers of Sr₂MnO₄. The oxygen shared by two neighboring pyramids belongs, in one case, to the basal planes of both pyramids, or in another case, to the basal plane of one pyramid and the apex of another one. Due to the existence of these two kinds of the bridging oxygens, it provides a quite characteristic example of moment systems with ferromagnetic (F) clusters of four Mn³⁺ moments, which are interacting with the neighboring clusters in the antiferromagnetic (AF) way. The results of the studies are as follows. The Neel temperature T_N is 126 K. The exchange interactions corresponding to the two kinds of the bridging oxygens are estimated by the high temperature expansion method to be -50.9K (AF) and 18.7K (F), where the signs are in agreement with our expectation. The magnetic structure is consistent with the picture of interacting clusters, though the F-interaction is weaker than the AF one. It is collinear AF one with the moments along the b axis.