Spin reorientation in easy-plane kagome ferromagnet Li ₉Cr ₃(P ₂O7) ₃(PO ₄) ₂

We report the successful growth and characterization of Li ₉Cr ₃(P ₂O 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 ₉Cr ₃(P ₂O 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 ₉Cr ₃(P ₂O 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 ₉Cr ₃(P ₂O 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 (BEC), the phase boundary scaling of field-induced ( B ∥ c) magnetic transition in Li ₉Cr ₃(P ₂O ₇) ₃(PO ₄) ₂ can be described as the quasi-2D magnon BEC, which has been observed in other ferromagnetic materials such as K ₂CuF ₄.