Magnetostructural Coupling Drives Magnetocaloric Behavior: The Case of MnB versus FeB

Materials with strongly coupled magnetic and structural transitions can display a giant magnetocaloric effect, which is of interest in the design of energy-efficient and environmentally friendly refrigerators, heat pumps, and thermomagnetic generators. There also exist, however, a class of materials with no known magnetostructural transition that nevertheless show remarkable magnetocaloric effects. MnB has been recently suggested as such a compound, displaying a large magnetocaloric effect at its Curie temperature (570 K) showing promise in recovering low-grade waste heat using thermomagnetic generation. In contrast, we show that isostructural FeB displays very similar magnetic ordering characteristics, but is not an effective magnetocaloric. Temperature- and field-dependent diffraction studies reveal dramatic magnetoelastic coupling in MnB, which exists without a magnetostructural transition. No such behavior is seen in FeB. Furthermore, the magnetic transition in MnB is shown to be subtly first-order, albeit with distinct behavior from that displayed by other magnetocalorics with first-order transitions. Density functional theory-based electronic structure calculations point to the magnetoelastic behavior in MnB as arising from a competition between Mn moment formation and B–B bonding.