Ultralow-field magnetocaloric materials for compact magnetic refrigeration

Magnetic refrigeration around the liquid-helium temperature plays a critical role in many technological sectors. Even if gallium gadolinium garnet (GGG) has been regarded as the benchmark, its application is highly limited by the small magnetic entropy changes, the requirement of superconducting magnets, and the large device sizes. Here, we report that LiREF4 (RE = rare earth) single crystals exhibit significantly superior magnetocaloric performance levels to commercial GGG. Under a small magnetic field of 5 kOe, which can be easily achieved by a permanent magnet, the magnetic entropy change reaches a record-high value of 16.7 J kg−1 K−1 in LiHoF4 in contrast to the value of 1.0 J kg−1 K−1 in GGG. The combination of small driving fields, large entropy changes, and excellent thermal and/or magnetic reversibility enables this series to be employed as the ideal working material for compact magnetic refrigeration around the liquid-helium temperature. Compact and sustainable magnetic refrigeration technology can achieve unprecedented performance using lithium rare earth fluorides. For over a century, researchers have realized that magnetic fields can heat up or cool down other magnets thanks to magnetic entropy, the thermodynamic energy released when spins align or de-align. Finding magnets with sufficient thermal response for refrigeration has been a long-standing challenge. Now, Peng Liu from the University of Science and Technology of China in Shenyang and colleagues report that lithium holmium fluorides (LiHoF4) show record-setting magnetic entropy changes around liquid-helium temperatures, about 16 times larger than those of commercial magnetic refrigeration crystals. The entire chemical family of lithium rare earth fluorides measured by the team showed remarkable magnetic entropy changes under very small driving magnetic fields. The single crystals of lithium rare earth fluorides exhibit remarkable magnetocaloric performance with a record-high entropy change of 16.73 J kg-1 K-1 achieved under a very small magnetic field of 5 kOe.