Spin-charge-lattice coupled phase transitions in bandwidth-controlled systems: (Nd,Sm)(1/2)Sr1/2MnO3

The metal-insulator (M-I) phase transitions relevant to charge ordering (GO) have been investigated for perovskite-type (Nd1-ySmy)(1/2)Sr1/2MnO3 (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 varying the averaged ionic radius of the A site and by application of quasihydrostatic pressure (P). Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex M-I phase diagrams with temperature (T) and W (y and/or P) as the parameters. The M-I phase boundaries are associated with critically Rr-and T-dependent hystereses, which result in unique appearance of the metastable state. We have demonstrated the pressure-induced phase transition from the metastable ferromagnetic metal to the thermodynamically stable charge-ordered insulator for the y = 0.875 crystal locating near the critical M-I phase boundary. With decrease of W, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (T-c) and enhances the electron-lattice coupling. Consequently, the lattice-coupled first-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 magnetic field also induces the phase transition from the insulator with antiferromagnetic spin correlations to the ferromagnetic metal, which is accompanied by lattice-structural change.