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.

具有磁与结构强耦合转变的材料可展现巨磁热效应，该效应在开发节能环保型制冷设备、热泵及热磁发电机方面具有重要应用价值。然而，亦存在一类尚未发现磁结构转变，但仍可展现出显著磁热效应的材料。近期研究将MnB提出为这类化合物之一，其在居里温度（570 K）下可产生大磁热效应，在利用热磁发电回收低品位余热领域颇具应用前景。与之形成鲜明对比的是，本研究表明同结构的FeB虽展现出极为相似的磁有序特性，却并非高效的磁热材料。温度与磁场依赖的衍射研究揭示，MnB中存在显著的磁弹性耦合，且这种耦合并不伴随磁结构转变；而FeB中并未观察到此类现象。此外，MnB中的磁相变被证实为微妙的一级相变，尽管其行为与其他具有一级相变的磁热材料存在明显差异。基于密度泛函理论（Density Functional Theory）的电子结构计算表明，MnB中的磁弹性行为源于Mn磁矩形成与B-B键合之间的竞争。