Spin-flop multicritical phenomena in Mn₂ AS₄ (A = Si and Ge)

Magnetic properties of Mn₂ AS₄ (A = Si and Ge) with the olivine structure are elucidated by depicting the phase diagram associated with a spin-flop transition. Both compounds show a sudden decrease of the spin-flop field near the Néel temperature, which gives rise to anomalous magnetic responses along the c-axis: a spontaneous weak ferromagnetism in A = Si and a significant enhancement of the differential susceptibility under the small magnetic field in A = Ge. We also discuss the nature of two types of unusual multicritical points in the phase diagrams.