Effect of Ti substitution on the magnetocaloric properties of HoAl2 for hydrogen liquefaction

Efficient hydrogen liquefaction remains a critical challenge for the development of a hydrogen-based energy economy. Magnetic refrigeration offers an energy efficient solid-state cooling technology capable of reaching 20 K, the temperature required for hydrogen liquefaction. Here, we investigate titanium (Ti) substitution on the non-magnetic aluminum sublattice of HoAl2, a comparatively unexplored approach for tuning cryogenic magnetocaloric performance. Ti substitution preserves the cubic C15 structure while inducing a systematic lattice expansion with increasing Ti content. Simultaneously, the Curie temperature reduces from 30.5 to 21 K, positioning the magnetocaloric response in the immediate vicinity of the hydrogen liquefaction temperature. Curie-Weiss analysis reveals that the Ho3+ effective magnetic moment remains essentially unchanged, while the Weiss temperature decreases monotonically, indicating an electron-count-driven weakening of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction rather than magnetic moment dilution. Arrott plot and universal-curve analyses confirm a second-order magnetic transition with negligible hysteresis across all compositions. Although the peak magnetic entropy change decreases with Ti substitution, broadening of the magnetic transition preserves the refrigerant capacity and expands the effective operating temperature range. The resulting compositionally tuneable Ho(Al1-xTiₓ)2 family spans a Curie temperature range relevant to both hydrogen pre-cooling and liquefaction stages. These findings establish non-magnetic sublattice engineering with abundant, non-critical elements as an effective strategy for tailoring cryogenic magnetocaloric materials and positions Ho(Al1-xTiₓ)2 as a promising material for hydrogen liquefaction applications in prop layered active-magnetic-regenerators.