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.