Observation of inverse magnetocaloric effect in magnetic-field-induced austenite phase of Heusler alloys Ni50−xCoxMn31.5Ga18.5 (x=9 and 9.7)

The magnetocaloric effect (MCE), magnetization, specific heat, and magnetostriction measurements were performed in both pulsed and steady high magnetic fields to investigate the magnetocaloric properties of Heusler alloys ${\mathrm{Ni}}_{50\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ $(x=9 \mathrm{and} 9.7)$. From direct MCE measurements for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ up to 56 T, a steep temperature drop was observed for magnetic-field-induced martensitic transformation (MFIMT), designated as inverse MCE. Remarkably, this inverse MCE is apparent not only with MFIMT, but also in the magnetic-field-induced austenite phase. Specific heat measurements under steady high magnetic fields revealed that the magnetic field variation of the electronic entropy plays a dominant role in the unconventional magnetocaloric properties of these materials. First-principles based calculations performed for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ and ${\mathrm{Ni}}_{45}{\mathrm{Co}}_{5}{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ revealed that the magnetic-field-induced austenite phase of ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ is more unstable than that of ${\mathrm{Ni}}_{45}{\mathrm{Co}}_{5}{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ and that it is sensitive to slight tetragonal distortion. We conclude that the inverse MCE in the magnetic-field-induced austenite phase is realized by marked change in the electronic entropy through tetragonal distortion induced by the externally applied magnetic field.