Evolution of noncollinear magnetism in magnetocaloric MnPtGa

Abstract

MnPtGa crystallizes in the hexagonal ${\mathrm{Ni}}_{2}\mathrm{In}$ structure type in space group $P{6}_{3}/mmc$ and has been reported to display a ferromagnetic Curie temperature near 220K. Here we find a transition near ${T}_{C}=236$ K to a ferromagnetic state, albeit with a reduced moment from what is expected for collinear ordering. The peak magnetocaloric entropy change was determined to be $\mathrm{\ensuremath{\Delta}}{S}_{M}=\ensuremath{-}1.9\phantom{\rule{0.28em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for an applied magnetic field of $H=5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ at the ferromagnetic ordering temperature. Magnetostructural coupling manifests as a change in the slope of the thermal expansion coefficients of the $c$ lattice parameter near ${T}_{C}$, with a negative spontaneous volume magnetostriction; $\ensuremath{\omega}=\ensuremath{-}300$ ppm at 190K. Neutron powder diffraction studies of the magnetic ground state reveal an evolution in complexity as temperature decreases: from a ferromagnet, to a canted antiferromagnet, to the eventual formation of a spin-density-wave state at low temperatures.