SPIN-CHARGE-LATTICE COUPLED PHASE-TRANSITIONS IN BANDWIDTH-CONTROLLEDSYSTEMS - (ND,SM)(1 2)SR1/2MNO3/

The metal-insulator (M-I) phase transitions relevant to charge orderin g (GO) have been investigated for perovskite-type (Nd1-ySmy)(1/2)Sr1/2 MnO3 (0 less than or equal to y less than or equal to 1) crystals, in which the one-electron bandwidth (W) is systematically controlled by v arying the averaged ionic radius of the A site and by application of q uasihydrostatic pressure (P). Competition between the ferromagnetic do uble exchange and the antiferromagnetic CO interactions give rise to c omplex M-I phase diagrams with temperature (T) and W (y and/or P) as t he parameters. The M-I phase boundaries are associated with critically Rr-and T-dependent hystereses, which result in unique appearance of t he metastable state. We have demonstrated the pressure-induced phase t ransition from the metastable ferromagnetic metal to the thermodynamic ally stable charge-ordered insulator for the y = 0.875 crystal locatin g near the critical M-I phase boundary. With decrease of W, the CO ins tability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (T-c) and enhance s the electron-lattice coupling. Consequently, the lattice-coupled fir st-order I-M transition is observed at T-c in the small-W region of y greater than or equal to 0.5. It was found that application of magneti c field also induces the phase transition from the insulator with anti ferromagnetic spin correlations to the ferromagnetic metal, which is a ccompanied by lattice-structural change.