  Magnetic refrigeration exploits the magnetocaloric effect which is the
entropy change upon application and removal of magnetic fields in materials,
providing an alternate path for refrigeration other than the conventional gas
cycles. While intensive research has uncovered a vast number of magnetic
materials which exhibits large magnetocaloric effect, these properties for a
large number of compounds still remain unknown. To explore new functional
materials in this unknown space, machine learning is used as a guide for
selecting materials which could exhibit large magnetocaloric effect. By this
approach, HoB$_{2}$ is singled out, synthesized and its magnetocaloric
properties are evaluated, leading to the experimental discovery of gigantic
magnetic entropy change 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 our knowledge, near the hydrogen liquefaction temperature thus it is
a highly suitable material for hydrogen liquefaction and low temperature
magnetic cooling applications.
