Lattice vibrations boost demagnetization entropy in shape memory alloy

TL;DR

This study reveals that lattice vibrations significantly enhance demagnetization entropy in a magnetocaloric alloy, with inelastic neutron scattering showing anomalous phonon behavior linked to magnetic transitions.

Contribution

It demonstrates the role of lattice dynamics in magnetocaloric effects, supported by experimental measurements and first-principle calculations, highlighting a novel entropy mechanism.

Findings

Anomalous increase in phonon entropy across Curie temperature

Abrupt softening of transverse optic phonon

Strong coupling between lattice distortions and magnetic excitations

Abstract

Magnetocaloric (MC) materials present an avenue for chemical-free, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material Ni45Co5Mn36.6In13.4. Upon heating across the Curie Temperature (TC), the material exhibits an anomalous increase in phonon entropy of 0.22 +/- 0.04 kB/atom, which is ten times larger than expected from conventional thermal expansion. This transition is accompanied by an abrupt softening of the transverse optic phonon. We present first-principle calculations showing a strong coupling between lattice distortions and magnetic excitations.