Magnetic and neutron diffraction study on melilite-type oxides Sr ₂MGe ₂O ₇ (M = Mn, Co)

The crystal structures and magnetic properties of melilite-type oxides Sr ₂MGe ₂O ₇ (M = Mn, Co) were investigated. These compounds crystallize in the melilite structure with space group P4̄2 ₁m, in which the M and Ge ions occupy two kinds of tetrahedral sites in an ordered manner. The magnetic M ions form a square-planar lattice in the ab plane. Both compounds do not show the structural phase transition down to 2.5 K. The temperature dependence of magnetic susceptibility for Sr ₂MnGe ₂O ₇ shows a broad peak at ∼6.0 K because of a two-dimensional magnetic interaction between Mn ions in the ab plane. At 4.4 K, an antiferromagnetic transition was observed. The magnetic structure was determined by the neutron powder diffraction measurements at 2.5 K. It can be represented by the propagation vector k = (0, 0, 1/2), and the magnetic moments of Mn ²⁺ (3.99 μ _B) ions order antiferromagnetically in a collinear manner along the c axis. On the other hand, Sr ₂CoGe ₂O ₇ shows an antiferromagnetic transition at 6.5 K with divergence between zero-field-cooled and field-cooled susceptibilities. Its magnetic structure determined at 2.5 K has a magnetic propagation vector k = (0, 0, 0), and the ordered magnetic moment of Co ²⁺ (2.81 μ _B) adopts a collinear arrangement lying on the ab plane.