Spin reorientation in easy-plane kagome ferromagnet Li9Cr3(P(2)O7)(3)(PO4)(2)

We report the successful growth and characterization of Li9Cr3(P2O7)(3)(PO4)(2) single crystal, and investigate its magnetic properties under external magnetic fields via magnetization and heat capacity measurements. Our study reveals that Li9Cr3(P2O7)(3)(PO4)(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, Li9Cr3(P2O7)(3)(PO4)(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 Li9Cr3(P2O7)(3)(PO4)(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 (BEC), the phase boundary scaling of field-induced (B parallel to c) magnetic transition in Li9Cr3(P2O7)(3)(PO4)(2) can be described as the quasi-2D magnon BEC, which has been observed in other ferromagnetic materials such as K2CuF4.