Magnetic entropy changes for the rotating magnetocaloric effect in RB4 (R = Gd, Tb, Dy, Ho, Er, and Tm)

Abstract

Rare-earth tetraboride compound, RB4 (R = Gd, Tb, Dy, Ho, Er, and Tm), is, to our best knowledge, an interesting system, which contains both magnetic ordering and geometrical frustration and their strong interaction in a material. The interaction results in consecutive magnetic transitions at T=TN1 and TN2 (TN1>TN2) and strongly anisotropic magnetic configuration in an ordered state. The study on the anisotropic entropy change (ΔSRM) is systematically performed for the single crystals of RB4 (R = Gd, Tb, Dy, Ho, Er, and Tm). It is found that the negative entropy change at T>TN1 follows conventional field dependence and the positive entropy change at T<TN1 follows anomalous field dependence. At T>TN1, ErB4 (TN1=15.5K) and TmB4 (TN1=11.7K) show ΔSRM values for H=50kOe of ≈11.6 J/(K⋅kg) and ≈10.2 J/(K⋅kg) and corresponding relative cooling power, RCPR, values of ≈237.1 J/kg and ≈225.9 J/kg, respectively. Interestingly, DyB4 (TN1=20.5K) shows a large ΔSRM value of ≈18.8 J/(K⋅kg) near T=TN2(=13.0K) and an RCPR value of ≈208.39 J/kg for H=50KOe. The ΔSRM and RCPR values are large enough for the compounds to be utilized for RMCE applications. In particular, ΔSRM≈18.8 J/(K⋅kg) is very close to the largest reported value of a single crystal operating in the temperature range of 10K≤T≤20K. In addition, HoB4, ErB4, and TmB4 are also found to be effective for RMCE at T<TN1, especially in low field cooling, e.g., H<50kOe. This work suggests that RB4 compounds are suitable materials for RMCE cooling over a wide temperature range both above and below the antiferromagnetic Néel temperature. In particular, the anisotropic entropy change of DyB4 near T=TN2 would be quite useful for RMCE applications, e.g., hydrogen liquefaction, because it occurs within a limited temperature range with large ΔSRM and RCPR values. © 2023 Elsevier B.V.