Specific heat and phase diagram of heavily doped La 1 − x Sr x MnO 3 ( 0.45 ⩽ x ⩽ 1.0 )
Specific heat and phase diagram of heavily doped La 1 − x Sr x MnO 3 ( 0.45 ⩽ x ⩽ 1.0 )
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DOI：
10.1103/physrevb.72.224429
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发表时间：
2005-12
影响因子：
3.7
通讯作者：
A. Szewczyk;M. Gutowska;B. Dabrowski
中科院分区：
文献类型：
--
作者：
A. Szewczyk;M. Gutowska;B. Dabrowski
测量了锶含量较高 ($x=1.0$, 0.9, 0.7, 0.55, 0.45) 和较低 $(x=0.0)$ 的化学计量 ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ 样品的比热，从 3 到 393 K，在零磁场和 7 T 磁场中加热和冷却时。特定相变的温度和顺序已确定，并且相图鲜为人知的部分 $1\ensuremath{\geqslant}xg0.6$ 已被研究。研究发现，从反铁磁相（$x=0.0$, 0.7, 1.0）或铁磁相（$x=0.45$, 0.55）到顺磁相的相变，当它们是纯磁相变时，是二阶相变；当它们伴随着结构相变时，是一阶相变，例如，从$C$型反铁磁相到顺磁相的转变与从四方到立方结构，以 $x=0.9$ 组成。从$A$-和$C$-型反铁磁相到顺磁态的转变比从$G$-型构型的转变受磁场的影响更强。这种行为归因于 $A$ 和 $C$ 配置中铁磁有序最近邻的存在以及 $C$ 配置的可能的准一维特征，这也通过该相中比热的定量不同的临界行为表明。从$A$型反铁磁相到铁磁相的一级转变，加上从四方结构到斜方结构的转变，发生在$x=0.55$样品中，并伴随着$\ensuremath{\delta}$形比热异常，被发现是最非常规的。它在磁场的作用下强烈地向较低温度移动（在 9 T 时通过 $\ensuremath{\sim}33\phantom{\rule{0.3em}{0ex}}\mathrm{K}$），而比热异常的形状没有发生实质性变化。补充磁化强度研究揭示了在这种转变附近存在有趣的磁畴结构效应。通过分析比热的温度依赖性，确定了表征磁热效应的两个主要参数：熵的等温变化和温度的绝热变化。在后一个转变附近，对于磁场从0至7T。
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.