Magnetocaloric properties of holmium for hydrogen liquefaction: Single-crystal vs polycrystalline behavior

Single-crystal and polycrystalline holmium were systematically investigated through magnetization measurements and direct magnetocaloric characterization in the temperature range relevant for hydrogen liquefaction. The single crystal exhibits a pronounced magnetic anisotropy, which strongly affects its magnetocaloric response. The largest magnetocaloric effect (MCE) is obtained when the magnetic field is applied within the basal plane, where sharp field-induced transitions lead to large entropy and adiabatic temperature changes. However, these transitions are first-order and are accompanied by thermal hysteresis, reducing the reversible component of the effect. At moderate magnetic fields (≈1 T), the MCE is confined to a limited temperature interval, while larger fields are required to extend its useful range. In contrast, polycrystalline Ho displays an averaged response over all crystallographic orientations, resulting in a reduced but still significant MCE with improved operational stability. The polycrystal retains up to ∼85% of the peak performance of the single crystal at high fields, while differences become more pronounced at lower fields. Additionally, the reduced macroscopic anisotropy mitigates anisotropy-induced torques and mechanical loads, which are critical factors for device design. Overall, this work provides a comprehensive comparison between single-crystal and polycrystalline holmium, highlighting the trade-off between maximum performance and practical applicability, and supporting the potential of polycrystalline Ho for magnetocaloric hydrogen liquefaction.