Collapse of the charge-ordering state at high magnetic fields in the rare-earth manganite Pr 0.63 Ca 0.37 Mn O 3

We have investigated the specific heat and resistivity of a single crystal of ${\mathrm{Pr}}_{0.63}{\mathrm{Ca}}_{0.37}\mathrm{Mn}{\mathrm{O}}_{3}$ around the charge ordering (CO) transition temperature, ${T}_{\mathit{CO}}$, in the presence of high magnetic fields $(\ensuremath{\leqslant}12\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ which can melt the charge-ordered state. At low magnetic fields $(\ensuremath{\leqslant}10\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, the manganite transforms from a charge-disordered 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 $Tl{T}_{\mathit{CO}}$. However, above a critical field ${\ensuremath{\mu}}_{0}{H}_{\ensuremath{\rho}}^{*}$, the sample shows the onset of a metallic phase for $Tg{T}_{\mathit{CO}}$ 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 ${\ensuremath{\mu}}_{0}{H}^{*}$, the COI transition does not occur and the CO state ceases to occur at all $T$. The entropy change involved in the CO transition, $\ensuremath{\Delta}{S}_{\mathit{CO}}\ensuremath{\approx}1.6\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ at $0\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, decreases with increasing field and eventually vanishes for a field ${\ensuremath{\mu}}_{0}{H}^{*}$. The collapse of the CO state above ${\ensuremath{\mu}}_{0}{H}^{*}$ is thus associated with a collapse of the entropy that stabilizes the CO state.