Specific heat and phase diagram of heavily dopedLa1−xSrxMnO3(0.45⩽x⩽1.0)

Abstract

Specific heat of stoichiometric ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ samples with large ($x=1.0$, 0.9, 0.7, 0.55, 0.45) and low $(x=0.0)$ strontium contents was measured from 3 to 393 K, on heating and on cooling, in zero magnetic field and in the field of 7 T. The temperatures and the orders of particular phase transitions have been determined and the poorly known part of the phase diagram, $1\ensuremath{\geqslant}xg0.6$, has been investigated. The phase transitions from the antiferromagnetic ($x=0.0$, 0.7, 1.0) or ferromagnetic ($x=0.45$, 0.55) to the paramagnetic phase were found to be of the second order, when they were purely magnetic transitions, and of the first order, when they were accompanied by the structural transitions, as for example, the transition from the $C$-type antiferromagnetic to the paramagnetic phase coupled with the transformation from the tetragonal to the cubic structure, occurring in the $x=0.9$ composition. The transitions from the $A$- and $C$-type antiferromagnetic phases to the paramagnetic state were influenced stronger by the magnetic field than the transitions from the $G$-type configuration. This behavior was attributed to the presence of ferromagneticaly ordered nearest neighbors within the $A$ and $C$ configurations and to the presumable quasi one dimensional character of the $C$ configuration, indicated also by quantitatively different critical behavior of the specific heat in this phase. The first order transition from the $A$-type antiferromagnetic to the ferromagnetic phase, coupled with the transformation from the tetragonal to the orthorhombic structure, occurring in the $x=0.55$ sample and accompanied by the $\ensuremath{\delta}$-shaped specific heat anomaly was found to be the most unconventional. It was strongly shifted towards lower temperatures by magnetic field (by $\ensuremath{\sim}33\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at 9 T) without substantial change in the shape of the specific heat anomaly. Supplementary magnetization studies revealed the presence of interesting domain structure effects near this transition. By analysis of the temperature dependences of specific heat, two main parameters characterizing the magnetocaloric effect, isothermal change in entropy and adiabatic change in temperature, have been determined. Near the latter transition they reach $3.4\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕(\mathrm{kg}\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $\ensuremath{-}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for the change in magnetic field from 0 to 7 T.