MAGNETOSTRUCTURAL PHASE-TRANSITIONS IN LA1-XSRXMNO3 WITH CONTROLLED CARRIER DENSITY

Magnetic-field-induced structural phase transitions between the orthor hombic and rhombohedral structures have but a investigated for single crystals of perovskite-type manganese oxides, La1-xSrxMnO3, with finel y controlled carrier density (x = 0.16-0.18). In x = 0.170 crystal, wh ose composition is tuned so that the structure transition temperature T-S is located close to the Curie temperature T-C, T-S decreases drast ically from 280 K at zero field down to 220 K at 7 T. Such a large mag netostructural effect arises from the mutual coupling among the kineti c energy of doped e(g) carriers, the local-spin moment of t(2g) electr ons, and the lattice degree of freedom. We have determined the structu ral phase diagram in the plane of temperature and magnetic field. For x = 0.16 or 0.18 crystals, on the other hand, the T-S's that differ co nsiderably from respective T-C's have not been changed conspicuously u p to 7 T. Thermodynamical aspects of the interesting magnetostructural phenomena are argued in terms of the free energy with coupled order p arameters, the magnetization M, and the lattice distortion Q.