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⁻¹K⁻¹ (0.35 J cm⁻³K⁻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.

磁制冷利用了磁热效应，这是在材料中施加和去除磁场后的熵变，为制冷提供了除常规气体循环以外的另一种途径。尽管深入研究发现了显示出大磁热效应的大量磁性材料，但是对于许多化合物而言，这些性能仍然未知。为了探索这个未知空间中的新功能材料，机器学习被用作选择可能表现出大磁热效应材料的指南。通过这种方法，HoB ₂被分离出来并合成，并对其磁热性质进行了评估，从而导致实验发现40.1 J kg ^-1 K的巨大磁熵变^-1（0.35 J cm ^-3 K ^-1）对于居里温度为15 K的铁磁二阶相变附近的5 T场变化。这是迄今为止报道的最高值据我们所知，接近氢液化温度；因此，HoB ₂是非常适合氢液化和低温磁冷却应用的材料。