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

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 = T-N1 and T-N2 (T-N1 > T-N2) and strongly anisotropic magnetic configuration in an ordered state. The study on the anisotropic entropy change (Delta S-RM) 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 > T-N1 follows conventional field dependence and the positive entropy change at T < T-N1 follows anomalous field dependence. At T > T-N1, ErB4 (T-N1 = 15.5K) and TmB4 (T-N1 = 11.7K) show |Delta SRM| values for H = 50kOe of approximate to 11.6 J/(K center dot kg) and approximate to 10.2 J/(K center dot kg) and corresponding relative cooling power, RCPR, values of approximate to 237.1 J/kg and approximate to 225.9 J/kg, respectively. Interestingly, DyB4 (T-N1 = 20.5K) shows a large vertical bar Delta SRM vertical bar value of approximate to 18.8 J/(K center dot kg) near T = T-N2(= 13.0K) and an RCPR value of approximate to 208.39 J/kg for H = 50KOe. The vertical bar Delta SRM vertical bar and RCPR values are large enough for the compounds to be utilized for RMCE applications. In particular, |Delta SRM| approximate to 18.8 J/(K center dot kg) is very close to the largest reported value of a single crystal operating in the temperature range of 10 K < T < 20 K. In addition, HoB4, ErB4, and TmB4 are also found to be effective for RMCE at T < T-N1, 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 Ne ' el temperature. In particular, the anisotropic entropy change of DyB4 near T = T-N2 would be quite useful for RMCE applications, e.g., hydrogen liquefaction, because it occurs within a limited temperature range with large Delta SRM and RCPR values.