YbCo2: Large magnetic entropy change per volume in Yb-based metallic magnetic refrigerants for sub-Kelvin temperature

A Yb-based intermetallic compound YbCo2 exhibiting a magnetic-field-induced order is known to show a giant specific heat divided by temperature, C/T∼ 6.5 J/K2 mol, around 0.3 K. We investigate the potential of this substance as the magnetic refrigerants by measuring the thermodynamic properties down to 0.1 K. The entropy change per volume by applying magnetic field of 3 T is found to be −ΔSM∼ 0.15 J/K cm3 around 1 K. This value is particularly large in the Yb-based metallic magnetic refrigerants to provide the sub-Kelvin temperature. In addition, we demonstrate that a 2.2 g sample is cooled down to 0.26 K by the adiabatic demagnetization refrigeration from 1.8 K and 12 T. This cooling performance is attributed to the ground state, at zero field, locating around the boundary between the field-induced ordered phase and the paramagnetic one. Our study clarifies the utility of the sub-Kelvin magnetic refrigerants, which combine the large −ΔSM and the high thermal conductivity of metal.