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Magnetic refrigeration materials containing rare-earth ions are promising for hydrogen liquefaction and energy storage applications. Here, the role of crystal-field level splitting on magnetic entropy change is systematically investigated, comparing mean-field calculations with neutron scattering experiments in HoB2.

Recently, a massive magnetocaloric effect near the liquefaction temperature of hydrogen has been reported in the ferromagnetic material HoB2. Here we investigate the effects of Dy substitution in the magnetocaloric properties of Ho1-xDyxB2 alloys (x = 0, 0.3, 0.5, 0.7, 1.0). We find that the Curie temperature (TC) gradually increases upon Dy substitution, while the magnitude of the magnetic entropy change |ΔSM| and adiabatic temperature change ΔTad showed a gradual decrease. On the other hand, due to the presence of successive transitions in these alloys, the peak height of the above magnetocaloric properties tends to be kept in a wide temperature range, leading to a relatively robust figure of merit in a wide temperature span. These alloys could be interesting candidates for magnetic refrigeration in the temperature range of 10–60 K.

We have prepared a set of polycrystalline samples of La 0.8 Sr 0.2 Co 1 − x Al x O 3 ( 0 ≤ x ≤ 0.2 ), and have measured the magnetization as functions of temperature and magnetic field. We find that the average spin number per Co ion ( S Co ) evaluated from the room-temperature susceptibility is around 1.2⁻1.3 and independent of x. However, we further find that S Co evaluated from the saturation magnetization at 2 K is around 0.3⁻0.7, and decreases dramatically with x. This naturally indicates that a significant fraction of the Co 3 + ions experience a spin-state crossover from the intermediate- to low-spin state with decreasing temperature in the Al-substituted samples. This spin-state crossover also explains the resistivity and the thermopower consistently. In particular, we find that the thermopower is anomalously enhanced by the Al substitution, which can be consistently explained in terms of an extended Heikes formula.

We report the fabrication of ErAl2 magnetocaloric wires by a powder-in-tube (PIT) method and the evaluation of magnetic entropy change through magnetization measurements. The magnetic entropy change of ErAl2 PIT wires exhibits similar behavior to the bulk counterpart, while its magnitude is reduced by the decrease in the volume fraction of ErAl2 due to the surrounding non-magnetic sheaths. We find that another effect reduces the magnetic entropy change of the ErAl2 PIT wires around a ferromagnetic transition temperature, and discuss its possible origin in terms of a correlation between magnetic properties of ErAl2 and mechanical properties of sheath material. [ABSTRACT FROM AUTHOR]

Stimulated by a recent report of a giant magnetocaloric effect in HoB2 found via machine-learning predictions, we have explored the magnetocaloric properties of a related compound ErB2 that has remained the last ferromagnetic material among the rare-earth diboride (REB2) family with unreported magnetic entropy change $|{\rm \Delta} {S_M}|$. The evaluated $|{\rm \Delta} {S_M}|$ at field change of 5 T in ErB2 turned out to be as high as 26.1 J kg−1 K−1 around the ferromagnetic transition (${T_C}$) of 14 K. In this series, HoB2 is found to be the material with the largest $|{\rm \Delta} {S_M}|$ as the model predicted, while the predicted values showed a deviation with a systematic error compared to the experimental values. Through a coalition analysis using SHAP, we explore how this rare-earth dependence and the deviation in the prediction are deduced in the model. We further discuss how SHAP analysis can be useful in clarifying favorable combinations of constituent atoms through the machine-learned model with compositional descriptors. This analysis helps us to perform materials design with aid of machine learning of materials data.