High-efficiency magnetic refrigeration using holmium

Magnetic refrigeration (MR) is a method of cooling matter using a magnetic field. Traditionally, it has been studied for use in refrigeration near room temperature; however, recently MR research has also focused on a target temperature as low as 20 K for hydrogen liquefaction. Most research to date has employed high magnetic fields (at least 5 T) to obtain a large entropy change, which requires a superconducting magnet and, therefore, incurs a large energy cost. Here we propose an alternative highly efficient cooling technique in which small magnetic field changes, Delta mu H-0 <= 0.4 T, can obtain a cooling efficiency of-Delta S-M/Delta mu H-0=32 J kg(-1)K(-1)T(-1), which is one order of magnitude higher than what has been achieved using typical magnetocaloric materials. Our method uses holmium, which exhibits a steep magnetization change with varying temperature and magnetic field. The proposed technique can be implemented using permanent magnets, making it a suitable alternative to conventional gas compression-based cooling for hydrogen liquefaction. Magnetic refrigeration offers a promising alternative to gas cycle cooling; however, it is typically hampered by the need for large magnetic fields. Here, the authors demonstrate that holmium can exhibit a large magnetic caloric effect while requiring only small magnetic fields.