Machine-learning-guided discovery of the gigantic magnetocaloric effect in HoB2 near the hydrogen liquefaction temperature

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.

Effect of Dy substitution in the giant magnetocaloric properties of HoB2

“…The Curie temperatures that are defined by the peak position in ∂M/∂T curves, are marked by the T C arrows, showing a systematic increase with Dy content. On the other hand, a second magnetic transition marked by T * that is observed at lower temperatures, which is also observed at HoB 2 at T * = 11 K [ 5 ] and DyB 2 at T * = 15 K [ 12 ], seems to be almost unchanged by partial substitution of Dy. The origin of T* was attributed to a possible spin-reorientation mechanism [ 12 ], however, the nature of this transition is still unknown and its investigation is outside the scope of this work.…”

“…curves by first calculating the zero-field entropy from specific heat data (not shown here) and then we obtain the field-dependent entropy by subtracting the values of |Δ S M | to the zero-field entropy, in the same manner as Refs. [ 5 , 19 ]. For the x = 0, we used the previously reported data of Ref [ 5 ]., and for the x = 1 sample, we used the previously reported zero-field specific heat data of Ref [ 20 ].…”

“…[ 5 , 19 ]. For the x = 0, we used the previously reported data of Ref [ 5 ]., and for the x = 1 sample, we used the previously reported zero-field specific heat data of Ref [ 20 ]. Then, the Δ T ad s is estimated by taking the horizontal difference between the entropy curves under zero field and final field [ 21 ] and the results are shown in Figure 4(k-o) .…”

“…Let us compare the magnetocaloric properties in Ho 1-x Dy x B 2 with those of representative materials that often show prominent magnetocaloric effect with transition temperatures ranging up to 77 K, based on Figure S5 of Ref [ 5 ]. Here we consider Δ S M , Δ T ad , and Temperature averaged Entropy Curve (TEC) as a practical figure of merit proposed earlier [ 23 ].…”

“…Recently, our group has unveiled a giant magnetocaloric effect of |Δ S M MAX | = 0.35 J cm −3 K −1 (40.1 J kg −1 K −1 ) in the vicinity of a ferromagnetic transition at the Curie temperature ( T C ) of 15 K for a field change of μ 0 Δ H = 5 T in HoB 2 [ 5 ]. Due to the closeness of its T C to the liquefaction point of hydrogen (20.3 K), this material became an attractive candidate for use in low-temperature magnetic refrigeration applications focused on the liquefaction stage of hydrogen.…”