Title: First order magneto-structural transition and magnetocaloric effect in MnNiGe$_{0.9}$Ga$_{0.1}$,
Abstract: The first order magneto-structural transition ($T_t\simeq95$ K) and
magnetocaloric effect in MnNiGe$_{0.9}$Ga$_{0.1}$ are studied via powder x-ray
diffraction and magnetization measurements. Temperature dependent x-ray
diffraction measurements reveal that the magneto-structural transition remains
incomplete down to 23 K, resulting in a coexistence of antiferromagnetic and
ferromagnetic phases at low temperatures. The fraction of the high temperature
Ni$_2$In-type hexagonal ferromagnetic and low temperature TiNiSi-type
orthorhombic antiferromagnetic phases is estimated to be $\sim 40\%$ and $\sim
60\%$, respectively at 23 K. The ferromagnetic phase fraction increases with
increasing field which is found to be in non-equilibrium state and gives rise
to a weak re-entrant transition while warming under field-cooled condition. It
shows a large inverse magnetocaloric effect across the magneto-structural
transition and a conventional magnetocaloric effect across the second order
paramagnetic to ferromagnetic transition. The relative cooling power which
characterizes the performance of a magnetic refrigerant material is found to be
reasonably high compared to the other reported magnetocaloric alloys.