Collapse of the charge-ordering state at high magnetic fields in the rare-earth manganite $Pr_{0.63}Ca_{0.37}MnO_3$

We have investigated the specific heat and resistivity of a single crystal of $Pr_{0.63}Ca_{0.37}MnO_3$ around the charge ordering (CO) transition temperature, $T_{CO}$, in the presence of high magnetic fields $(\leq 12 T)$ which can melt the charge-ordered state. At low magnetic fields $(\leq 10 T)$, the manganite transforms from a chargedisordered paramagnetic insulating (PI) state to a charge-ordered insulating (COI) state as the temperature is lowered. The COI state becomes unstable beyond a threshold magnetic field and melts to a ferromagnetic metallic phase (FMM). This occurs for $T < T_{CO}$. However, above a critical field $\mu_0$ $H_\rho ^*$, the sample shows the onset of a metallic phase for $T > T_{CO}$ and the COI transition occurs from a metallic phase. The onset temperature of the high-field metallic behavior decreases with an increase in the field and above a field $\mu_0 H^*$, the COI transition does not occur and the CO state ceases to occur at all T. The entropy change involved in the CO transition, $\Delta S_{CO} \approx 1.6 J /mol \hspace {2mm}K $at 0 T, decreases with increasing field and eventually vanishes for a field $\mu_0H^*$. The collapse of the CO state above $\mu_0H^*$ is thus associated with a collapse of the entropy that stabilizes the CO state