  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
Ni50-xCoxMn31.5Ga18.5 (x = 9 and 9.7). From direct MCE measurements for
Ni41Co9Mn31.5Ga18.5 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 Ni41Co9Mn31.5Ga18.5 and Ni45Co5Mn36.7In13.3 revealed
that the magnetic-field-induced austenite phase of Ni41Co9Mn31.5Ga18.5 is more
unstable than that of Ni45Co5Mn36.7In13.3 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.
