Abstract
Magnetic refrigeration exploits the magnetocaloric effect, which is the entropy change upon the application and removal of magnetic fields in materials, providing an alternate path for refrigeration other than conventional gas cycles. While intensive research has uncovered a vast number of magnetic materials that exhibit a large magnetocaloric effect, these properties remain unknown for a substantial number of compounds. To explore new functional materials in this unknown space, machine learning is used as a guide for selecting materials that could exhibit a large magnetocaloric effect. By this approach, HoB2 is singled out and synthesized, and its magnetocaloric properties are evaluated, leading to the experimental discovery of a gigantic magnetic entropy change of 40.1 J kg−1 K−1 (0.35 J cm−3 K−1) for a field change of 5 T in the vicinity of a ferromagnetic second-order phase transition with a Curie temperature of 15 K. This is the highest value reported so far, to the best of our knowledge, near the hydrogen liquefaction temperature; thus, HoB2 is a highly suitable material for hydrogen liquefaction and low-temperature magnetic cooling applications.
Highlights
1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Introduction The magnetocaloric effect (MCE) is a promising approach for environmentally friendly cooling, as it does not depend on the use of hazardous or greenhouse gases[1,2,3,4] while being, in principle, able to attain a higher thermodynamic cycle efficiency[1,5,6], where this cycle makes use of the magnetic entropy change (ΔSM) and the adiabatic temperature change (ΔTad) through the application/ removal of a magnetic field in a material
We started with the screening of magnetocaloric relevant papers from the MagneticMaterials[28] database and gathered the reported MCE properties from a total of 219 different journal titles contained therein, mostly focusing on the reported peak values of | ΔSM | for a given field change (μ0ΔH) of a given material composition, combining these data with the data available in a recent review[2] by Franco et al To remove any possible duplicates in the final dataset, the materials that contained more than one value of | ΔSM | for a given μ0ΔH had their values averaged, and this average was used as the final value
Inset: Lower field range (T = 5 K). c M–T curves for a wide range of different applied fields. d Magnetic entropy change calculated from the M–T curves shown in c using Eq (1)
Summary
The magnetocaloric effect (MCE) is a promising approach for environmentally friendly cooling, as it does not depend on the use of hazardous or greenhouse gases[1,2,3,4] while being, in principle, able to attain a higher thermodynamic cycle efficiency[1,5,6], where this cycle makes use of the magnetic entropy change (ΔSM) and the adiabatic temperature change (ΔTad) through the application/ removal of a magnetic field in a material. Since large values of ΔSM are usually achieved near the magnetic transition temperature (Tmag), the working temperature range is confined around the Tmag of the material. The remarkable discovery of giant MCEs near room temperature in families of materials such as Gd5Si2Ge210, La(Fe,Si)1311, and MnFeP1–xAsx[3] shifted the main focus of research into finding and tuning new materials, such as NiMnIn Heusler alloys[12], working around this temperature range due to its potential economic and environmental impact[2]. It has been shown that MCE-based refrigerator prototypes are highly appropriate for this task[5]. In this context, the discovery of Castro et al NPG Asia Materials (2020) 12:35
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