Anomalous Magnetic Properties near the Spin-Flop Bicritical Point in Mn$_{2}A$S$_{4}$ ($A$ = Si and Ge)

The magnetic properties of the single crystalline Mn$_{2}A$S$_{4}$ ($A$ = Si and Ge) with an olivine structure, which are the uniaxially anisotropic antiferromagnets (the $b$-axis as an easy axis), were investigated. Near the N\'{e}el temperature, both compounds exhibit the contrastive magnetic responses along the $c$-axis, namely, the spontaneous weak ferromagnetism in $A$ = Si and the significant enhancement of the differential susceptibility (\textit{dM/dH}) under the small magnetic field in $A$ = Ge. When $A$ = Ge, we also observed the evolution of \textit{dM/dH} along the $a$-axis at low temperatures. We discuss these phenomena on the basis of the magnetic field-temperature ($H-T)$ phase diagram with the spin-flop bicritical point ($H_{BP}$, $T \quad _{BP})$. The role of the thermal or quantum fluctuation was stressed. (Ref.) K. Ohgushi and Y. Ueda, Phys. Rev. Lett. \textbf{95}, 217202 (2005)