Magnetic Entropy Changes in Rare-Earth Tetraborides (RB₄, R = Dy, Ho, and Tm)

The RB₄ (R = Gd, Tb, Dy, Ho, and Tm) compound presents an intriguing system with simultaneous magnetic ordering and geometrical frustration, leading to a strong interaction within the matarial. The interaction manifests in consecutive magnetic transitions at temperatures <i>T</i> = <i>T</i>_N1 and <i>T</i>_N2 (where <i>T</i>_N1 &gt; <i>T</i>_N2) and results in a highly anisotropic magnetic configuration when in an ordered state. In this study, we systematically investigate the anisotropic entropy chnage (ΔS_AN) of single crystals of RB₄ (R = Dy, Ho, and Tm). Our findings reveal distinct behavior: the negative entropy change above <i>T</i>_N1 follows conventional field-dependence, while a positive entropy change below <i>T</i>_N1 follows anomalous field-dependence. Notably, ErB₄ (<i>T</i>_N1 = 15.5 K) and TmB₄ (<i>T</i>_N1 = 11.7 K) demonstrate ΙΔS_ANΙ values of ≈ 11.6 J/(K kg) and 10.2 J/(K kg), respectively, at <i>T</i> &gt; <i>T</i>_N1 and for <i>H</i> = 50 kOe, accompanied by relative cooling power (RCP) values of ≈ 237.1 J/kg and 225.9 J/kg. Interestingly, DyB₄ (<i>T</i>_N1 = 20.5 K) exchibits a significant ΙΔS_ANΙ value of 18.8 J/(K kg) near <i>T</i> = <i>T</i>_N2 (= 13.0 K) and an RCP value of ≈ 208.39 J/kg for <i>H</i> = 50 kOe. These ΙΔS_ANΙ and RCP values demonstrate the potential of these comounds for applications in refrigeration using rotational magnetocaloric effect (RMCE). Additionally, HoB₄, ErB₄ and TmB₄ exchibit also promising behavior for RMCE even at temperatures of <i>T</i> &lt; <i>T</i>_N1and particularly during low-field cooling (<i>H</i> &lt; 50 kOe). This work highlights the suitability of RB₄ compounds as effective materials for RMCE cooling, covering a broad temperature range above and below the antiferromagnetic Néel temperature. Specifically, the anisotropic entropy change observed in DyB₄ near <i>T</i>= <i>T</i>_N2 holds considerable potential for RMCE applications, such as hydrogen liquefaction due to its occurrence which a limited temperture range, accompanied by large ΙΔS_ANΙ and RCP values.