It has long been known that materials with coupled first-order magnetic and structural phase transitions can show giant magnetocaloric effects (changes in temperature and entropy when placed in a magnetic field). Such compounds have been proposed as active materials in a new generation of environmentally friendly refrigerators and heat pumps. However, the hysteresis and mechanical instability associated with cycling through the first-order magnetostructural transition poses engineering challenges to the adoption of these materials. On the other hand, certain compounds with completely reversible second-order magnetic transitions also show large magnetocaloric effects. We have performed high-throughput DFT calculations to search for ferromagnets with large magnetostructural coupling, and have shown that such coupling is the driving feature of good magnetocalorics, whether or not a first-order magnetostructural transition is observed. In order to understand the chemical origins of coupling between magnetism and atomic structure, we have used detailed synchrotron and neutron diffraction experiments, as well as DFT calculations, to look at the temperature and magnetic-field evolution of features in several materials with first- and second-order transitions, including MnB, AlFe2B2, and MnAs. We find a diversity of coupling mechanisms, but a unifying theme is that competition between magnetism and bonding drives coupling. Structures which cannot simultaneously optimize magnetic moment and bonding are caught in a balance between the two. This competition can drive a large magnetocaloric effect, as well as a variety of other multifunctional properties.

How magnetism and structure couple in magnetocaloric materials

2019 Spring National Meeting

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