Magnetic refrigeration, utilizing the magnetocaloric effect (MCE), offers a promising alternative to traditional gas-based methods, especially at ultralow temperatures. The MCE describes a material's temperature change in response to a varying magnetic field. Adiabatic demagnetization refrigeration, first demonstrated using Gd₂(SO₄)₃·8H₂O, leverages this effect to reach ultralow temperatures. Magnetic refrigeration finds applications in diverse fields like space-based astrophysical measurements (cooling bolometers), quantum computing, and superfluid helium production. Solid-state working materials are particularly advantageous for space applications due to their insensitivity to gravity. The development of efficient magnetic refrigeration hinges on identifying materials with large isothermal magnetic entropy change (ΔSm), large adiabatic temperature change (ΔTad), and low driving magnetic fields (ΔH). Gallium gadolinium garnet (GGG) has served as the benchmark material for commercial adiabatic demagnetization refrigerators operating near liquid-helium temperatures, achieving a maximum ΔSm of 38 Jkg⁻¹K⁻¹ under a 70 kOe field. However, GGG's practical applications are limited by the need for bulky and expensive superconducting magnets to generate such high fields, requiring substantial magnetic shielding, and resulting in large, inefficient systems. This necessitates the search for materials exhibiting excellent magnetocaloric performance under lower fields, ideally achievable with permanent magnets. Rare-earth (RE) elements, with their large intrinsic magnetic moments, are promising candidates. Lithium rare-earth fluorides (LiREF₄) with a tetragonal scheelite structure (where RE can be Gd, Tb, Dy, Ho, Er, Tm, or Yb) possess exotic low-temperature properties and high magnetocrystalline anisotropy due to crystal electric field effects. This study focuses on systematically investigating the magnetocaloric properties of LiREF₄ single crystals, aiming to identify superior alternatives to GGG for compact magnetic refrigeration.

The literature extensively explores low-temperature magnetocaloric materials. Early work established the potential of adiabatic demagnetization refrigeration. GGG emerged as a benchmark material for commercial applications due to its relatively high ΔSm, despite requiring high magnetic fields. However, the requirement for powerful superconducting magnets, associated shielding needs, and the resulting bulkiness hindered widespread adoption. Recent studies have investigated other low-temperature magnetocaloric materials, but many still require strong magnetic fields, limiting practical implementation. The unique properties of LiREF₄ compounds, particularly their high magnetocrystalline anisotropy, have been noted in previous research, but their full potential as magnetocaloric materials in ultralow-field applications remained largely unexplored before this study. The existing literature highlights the need for materials that perform well under readily available, lower magnetic fields.

The study involved the synthesis and characterization of LiREF₄ single crystals (RE = Gd, Tb, Dy, Ho, Er, Tm, Yb). Single-crystal alignment was crucial for measuring magnetic properties along both easy and hard axes. Zero-field cooled (ZFC) and field-cooled (FC) magnetization measurements were conducted to characterize magnetic ordering and reversibility. Isothermal magnetization curves were measured at various temperatures and magnetic fields up to 20 kOe along both crystallographic axes to determine the magnetocrystalline anisotropy. The isothermal magnetic entropy change (ΔSm) was calculated using the Maxwell relation from these magnetization data. The adiabatic temperature change (ΔTad) was directly measured using a pulsed magnetic field method, enabling the determination of the material's response under nearly adiabatic conditions. The authors carefully considered and controlled for experimental factors like thermal insulation to ensure accurate measurements of ΔTad.

LiREF₄ single crystals displayed significantly enhanced magnetocaloric properties compared to GGG. LiHoF₄ exhibited the most remarkable performance. Under a 5 kOe field (achievable with permanent magnets), LiHoF₄ demonstrated a record-high ΔSm of 16.7 Jkg⁻¹K⁻¹, substantially surpassing GGG's 1.0 Jkg⁻¹K⁻¹ under the same field. The ΔSm values for other LiREF₄ compounds under 5 kOe ranged from 0.04 Jkg⁻¹K⁻¹ (LiTmF₄) to 10.1 Jkg⁻¹K⁻¹ (LiTbF₄). At higher fields (10 and 20 kOe), LiGdF₄ showed the largest ΔSm, reaching 46.7 Jkg⁻¹K⁻¹ at 20 kOe. All LiREF₄ crystals exhibited excellent magnetic reversibility. The significant magnetocrystalline anisotropy led to substantial rotating magnetocaloric effects, with LiHoF₄ again demonstrating the highest performance (22.6 Jkg⁻¹K⁻¹ at 20 kOe). Direct measurements of ΔTad using pulsed magnetic fields confirmed the large temperature changes. LiHoF₄ reached a ΔTad of 2.38 K under a 5 kOe field at a base temperature of 1.6 K. The results demonstrate that the use of single crystals significantly enhances the magnetocaloric properties at lower fields, which is a significant improvement over the performance of polycrystalline samples. The observed trends correlated well with the magnetic nature (ferromagnetic or antiferromagnetic) of the different LiREF₄ compounds.

The exceptional performance of LiREF₄ single crystals addresses the limitations of current magnetic refrigeration technologies. The significantly larger ΔSm and ΔTad values compared to GGG allow for more compact devices, requiring less material to achieve a given cooling capacity. The ability to achieve near-saturation magnetocaloric performance under fields as low as 5 kOe eliminates the need for bulky and expensive superconducting magnets, replacing them with readily available permanent magnets. This simplification drastically reduces system size, cost, and complexity, particularly simplifying the magnetic shielding requirements. The high magnetocrystalline anisotropy of LiREF₄, while initially a challenge for polycrystalline samples, becomes an advantage when utilizing single crystals, enabling the exploitation of rotating MCEs to further enhance performance. This combination of factors points towards LiREF₄ single crystals as a promising next-generation working material for compact and efficient magnetic refrigeration near liquid-helium temperatures.

This study demonstrates the superior magnetocaloric performance of LiREF₄ single crystals, particularly LiHoF₄, for ultralow-field magnetic refrigeration. The record-high ΔSm values under low magnetic fields, combined with large ΔTad and excellent reversibility, offer a path towards compact and cost-effective magnetic refrigeration systems. Future research could focus on optimizing the material synthesis and device design to further enhance efficiency and explore applications beyond liquid-helium temperatures.

The study primarily focused on single-crystal samples. Further research is needed to investigate the scalability and reproducibility of these results in polycrystalline materials for large-scale applications. The long-term stability and durability of LiREF₄ under repeated magnetic field cycling also warrant investigation for practical applications. While the direct ΔTad measurements provided valuable insights, a more comprehensive analysis involving a wider range of temperatures and fields would be beneficial.