Magnetostructural phase transitions in ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_3$ with controlled carrier density

Magnetic-field-induced structural phase transitions between the orthorhombic and rhombohedral structures have been investigated for single crystals of perovskite-type manganese oxides, ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_3$, with finely controlled carrier density (x=0.16–0.18). In x=0.170 crystal, whose composition is tuned so that the structural transition temperature ${\mathit{T}}_{\mathit{S}}$ is located close to the Curie temperature ${\mathit{T}}_{\mathit{C}}$, the ${\mathit{T}}_{\mathit{S}}$ decreases drastically from 280 K at zero field down to 220 K at 7 T. Such a large magnetostructural effect arises from the mutual coupling among the kinetic energy of doped ${\mathit{e}}_{\mathit{g}}$ carriers, the local-spin moment of ${\mathit{t}}_{2\mathit{g}}$ electrons, and the lattice degree of freedom. We have determined the structural phase diagram in the plane of temperature and magnetic field. For x=0.16 or 0.18 crystals, on the other hand, the ${\mathit{T}}_{\mathit{S}}$’s that differ considerably from respective ${\mathit{T}}_{\mathit{C}}$’s have not been changed conspicuously up to 7 T. Thermodynamical aspects of the interesting magnetostructural phenomena are argued in terms of the free energy with coupled order parameters, the magnetization M, and the lattice distortion Q. © 1996 The American Physical Society.