Magnetic phase diagrams of the multicritical olivine Mn₂ SiS₄ and Mn₂ GeS₄

Using single crystal neutron diffraction, we studied the H (∥ b) - T phase diagram of the antiferromagnetic olivine Mn₂ AS₄ (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 C_z - C_z^′ and ferromagnetic F_x^′ components. In the high temperature phase, in contrast, they are replaced by C_x - C_x^′ and F_z^′. The absence of the anomalies in the DC magnetization data across the spin-reorientation transitions is discussed in terms of the Clausius-Clapeyron relation.