Specific heat anomalies inLa1−xSrxMnO3(0.12⩽x⩽0.2)

Abstract

By performing specific heat measurements for a series of the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ samples ($x=0.12$, 0.135, 0.155, 0.185, and 0.2) several intricacies of the phase diagram have been clarified. For all the samples, specific heat was measured over the temperature range from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}380\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in zero magnetic field and in the field of $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, on heating and on cooling. Additionally, the temperature dependence of specific heat of the $x=0.155$ sample was studied by a heat-flow method in magnetic fields up to $13\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We show that the phase transition observed below the Curie temperature for the compositions $0.155\ensuremath{\leqslant}x\ensuremath{\leqslant}0.17$ should be interpreted as a structural Jahn-Teller transition as reported in Dabrowski et al. [Phys. Rev. B 60, 7006 (1999)] and not as a charge ordering transition [Liu et al. Phys. Rev. B 64, 144414 (2001)]. Moreover, we show that the Jahn-Teller transition is of the first-order for all the compositions studied and no evolution of its order from the second to the first in the range $0.11\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14$, which was suggested in earlier papers [Liu et al., Phys. Rev. B 64, 144414 (2001)], occurs. Two main parameters characterizing magnitude of the magnetocaloric effect, i.e., the adiabatic change in temperature and the isothermal change in entropy under influence of the external magnetic field, were determined as functions of temperature for all the samples. They were found to be, respectively, $\ensuremath{\sim}4$ and $\ensuremath{\sim}2$ times smaller than those for gadolinium.