Antiferromagnetic ordering and critical behavior induced giant magnetocaloric effect in distorted kagome lattice Gd₃BWO₉

We synthesize the high-quality Gd₃BWO₉ single crystal and investigate its low-temperature magnetic and thermodynamic properties. Below _TN=1.08 K, the anisotropic behavior of magnetic susceptibilities reveals that the Gd³⁺ moments exhibit the dominant antiferromagnetic coupling along the c axis, while displaying a ferromagnetic arrangement in kagome plane. With pronounced magnetic frustration, in adiabatic demagnetization refrigeration experiments starting from initial conditions of 9 T and 2 K, Gd₃BWO₉ polycrystal reaches a minimum temperature of 0.151 K, significantly lower than its _TN. Due to the high density of Gd³⁺ ions (S=7/2), the maximum magnetic entropy change reaches over 50 J kg⁻¹K⁻¹ under fields up to 7 T in Gd₃BWO₉, nearly 1.5 times as large as commercial sub-kelvin magnetic coolant Gd₃Ga5O₁₂. The H-T phase diagram of Gd₃BWO₉ under H∥c exhibits field-induced critical behavior near the phase boundaries. This observation aligns with the theoretical scenario in which a quantum critical point acts as the endpoint of a line of classical second-order phase transitions. Such behavior suggests the importance of further investigations into the divergence of the magnetic Grüneisen parameter in the vicinity of critical field at ultralow temperatures.