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

The metal-insulator $(M$-$I)$ phase transitions relevant to charge ordering (CO) have been investigated for perovskite-type ${(\mathrm{N}\mathrm{d}}_{1-y}{\mathrm{Sm}}_y{)}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_3$ $(0\mathrm{}<~y<~1)$ crystals, in which the one-electron bandwidth $\mathrm{}\left(W\right)\mathrm{}$ is systematically controlled by varying the averaged ionic radius of the $A$ site and by application of quasihydrostatic pressure $\mathrm{}\left(P\right)\mathrm{}$. Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex $M-I$ phase diagrams with temperature $\mathrm{}\left(T\right)\mathrm{}$ and $W$ $(y$ and/or $P)$ as the parameters. The $M-I$ phase boundaries are associated with critically $W$- 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 $y>~0.5$. 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.