Magnetic phase diagrams of the multicritical olivine Mn2SiS4Mn2SiS4 and Mn2GeS4Mn2GeS4

Using single crystal neutron diffraction, we studied the H(∥b)–TH(∥b)–T phase diagram of the antiferromagnetic olivine Mn2AS4(A=Si,Ge). We identified the two spin-flop phases at high fields that are separated by field-independent phase boundaries terminated at multicritical points. In the low temperature phase, the magnetic structures consist of antiferromagnetic Cz–Cz′ and ferromagnetic Fx′ components. In the high temperature phase, in contrast, they are replaced by Cx–Cx′ and Fz′. The absence of the anomalies in the DC magnetization data across the spin-reorientation transitions is discussed in terms of the Clausius–Clapeyron relation.