Structural and physical properties determined by measurements on large singlecrystals of the anisotropic ferromagnet MnBi are reported. The findings supportthe importance of magneto-elastic effects in this material. X-ray diffractionreveals a structural phase transition at the spin reorientation temperature$T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and uponcooling transforms the structure from hexagonal to orthorhombic. Heat capacitymeasurements show a thermal anomaly at the crystallographic transition, whichis suppressed rapidly by applied magnetic fields. Effects on the transport andanisotropic magnetic properties of the single crystals are also presented.Increasing anisotropy of the atomic displacement parameters for Bi withincreasing temperature above $T_{SR}$ is revealed by neutron diffractionmeasurements. It is likely that this is directly related to the anisotropicthermal expansion in MnBi, which plays a key role in the spin reorientation andmagnetocrystalline anisotropy. The identification of the true ground statecrystal structure reported here may be important for future experimental andtheoretical studies of this permanent magnet material, which have to date beenperformed and interpreted using only the high temperature structure.
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