Giant magnetocaloric effect of the K3Gd5(PO4)6 compound at ultra-low temperature

Magnetic refrigerants possessing large low-field magnetic entropy changes (−ΔS_M) in the sub-Kelvin temperature region are urgently desired for adiabatic demagnetization refrigeration (ADR). Here, we report the large low-field magnetocaloric effect of K₃Gd₅(PO₄)₆, a phosphate compound based on gadolinium, which experiences an antiferromagnetic transition at a Néel temperature of 0.64 K, while the dominant magnetic interaction is ferromagnetic. Under a field change of 0–1 T, the maximum value of magnetic entropy change (−ΔS^max_M) is 25.4 J kg⁻¹ K⁻¹ below 2 K. Upon quasi-adiabatic demagnetization measurement, the quasi-adiabatic demagnetization curves show a dip, leading to the lowest temperature to occur at a nonzero applied field. Furthermore, a temperature as low as 510 mK is obtained with an initial condition of 4 T and 4 K, which is much lower than the temperature that GGG can reach under the same initial condition (about 880 mK). The above results make K₃Gd₅(PO₄)₆ a promising magnetic refrigerant in the sub-Kelvin temperature region.