Spin reorientation in easy-plane kagome ferromagnet Li₉Cr₃(P₂O7)₃(PO₄)₂

Abstract

We report the successful growth and characterization of Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ single crystal, and investigate its magnetic properties under external magnetic fields via magnetization and heat capacity measurements. Our study reveals that Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ is an easy-plane kagome ferromagnet with S = 3/2, as evidenced by the Curie-Weiss temperature of 6 K which implies a ferromagnetic exchange coupling in the material. Under zero magnetic field, Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ undergoes a magnetic transition at T C = 2.7 K from a paramagnetic state to a ferromagnetically ordered state with the magnetic moment lying in the kagome plane. By applying a c-axis directional magnetic field to rotate the spin alignment from the kagome plane to the c-axis, we observe a reduction in the magnetic transition temperature as the field is increased. We construct a magnetic phase diagram as a function of temperature and magnetic field applied parallel to the c-axis of Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ and find that the phase boundary is linear over a certain temperature range. Regarding that theoretically, the field-induced phase transition of the spin reorientation in the easy-plane ferromagnet can be viewed as the ferromagnetic magnon Bose-Einstein condensation, the phase boundary scaling of field-induced (B||C) magnetic transition in Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ can be described as the quasi-2D magnon BEC, which has been observed in other ferromagnetic materials such as K2CuF4.