Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys

TL;DR

This paper reports the discovery of a giant inverse magnetocaloric effect in Ni-Mn-Sn alloys, where applying a magnetic field cools the sample, driven by a martensitic phase transformation affecting magnetic interactions.

Contribution

It demonstrates a giant inverse MCE in Ni-Mn-Sn alloys caused by a martensitic phase transition, a novel finding in ferromagnetic materials.

Findings

Inverse MCE causes cooling upon magnetic field application.

Giant entropy change comparable to giant MCE materials.

Phase transformation alters magnetic exchange interactions.

Abstract

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can be exploited also for room temperature refrigeration by using recently discovered giant MCE materials. In this letter, we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions due to the change in the lattice parameters.