MAGNETOSTRUCTURAL PHASE TRANSITIONS IN La_1XSr_XMnO₃ WITH CONTROLLED CARRIER DENSITY. Atsushi Asamitsu¹ and Yoshinori Tokura^{1,2 1}Joint Research Center for Atom Technology, Tsukuba 305, Japan ²University of Tokyo, Tokyo 113, Japan

Magnetic field-induced structural phase transitions as well as thermally induced ones between the orthorhombic (O) and rhombohedral (R) structures have been investigated for perovskite-type manganese oxides, La_1-xSr_xMnO₃, with finely controlled carrier density (x=0.160, 0.170 and 0.180). In x=0.170 crystal, the composition of which is tuned so that the structural transition temperature TS (~285K) is located close to the ferromagnetic transition temperature TC (~264K). As a result, novel magnetostructural effects have been observed: the TS is lowered by more than 50K with application of an external magnetic field of 7T. We determined the structural phase diagram in the field-temperature plane from measurements of the lattice striction. Utilizing the structural phase diagram, we can switch the crystal structure, reversibly or irreversibly, between the O- and R-phases by applying magnetic field at a fixed temperature. Such a large magnetostructural effect arises from the mutual coupling among the transfer interaction of doped e_g carriers, the local spin moment of t_2g electrons, and the lattice distortion. In the case of La_1-xSr_xMnO₃ crystal, the transfer interaction of e_g carriers that is responsible for the ferromagnetic double-exchange interaction and hence the induced magnetization M is larger in the R-phase than in the O-phase in a magnetic field. Therefore the gain of the free energy by Zeeman term, -M.H, can drive the field-induced structural transition from the O- to R-phase. Thermodynamical aspects of these phenomena are understood semi-quantitatively in terms of the Landau free energy with coupled order parameters, the magnetization M and the lattice distortion Q.

In x=0.160 or 0.180, on the other hand, the T_S's are not changed conspicuously up to 7T, since their T_S and T_C differ considerably, either T_S>>T_C or T_S<<T_C, and hence the effective coupling between M and Q is reduced.