Magnetic properties of the ordered double perovskite Sr ₂MnTeO₆

The ordered double perovskite Sr₂MnTeO₆ was prepared by the freeze-drying method. The nuclear and magnetic structures were determined by both X-ray and neutron (D2B and D1B) powder diffraction. A distorted, double perovskite structure type described by the monoclinic P12₁/n1 space group is observed at room temperature and down to 4 K. The room temperature EPR spectrum shows an isotropic signal centered at a g value of 1.998 indicating that manganese ions are in the +2 oxidation state. At low temperature the EPR signal broadens as the exchange and dipolar interactions between Mn²⁺ cations increase. Below 25 K no signal is observed, indicating that the sample is magnetically ordered. The dc magnetic susceptibility shows the existence of antiferromagnetic interactions with an ordering temperature around 20 K. A sharp λ-type anomaly observed at 19 K in the specific heat curve and refinement of low-temperature neutron diffraction data confirm the presence of a three-dimensional, antiferromagnetic ordered structure with an ordering temperature of ca. 23 K. The propagation vector of the magnetic structure is k = [0,0,0], yielding a type I antiferromagnetic ordering between Mn²⁺ (S = 5/2) cations. At 2 K, each manganese ion is antiferromagnetically aligned to 8 out of 12 nearest-neighbors at an approximate distance of 5.6 Å. This magnetic structure can be viewed as being composed of ferromagnetic arrangement of Mn²⁺ moments in the ab plane, coupled antiferromagnetically along the c axis.