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Magnetocaloric effect in manganites

机译：锰矿中的磁热效应

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The magnetocaloric effect (MCE) in La_(1-x)Sr_xMnO _3, Sm_(0.55)Sr_(0.45)MnO_3, and PrBaMn _2O_6 compounds is studied. The maximum values of MCE (ΔT_(max)) determined by a direct method in the second and third compositions and in La_(0.9)Sr_(0.1)MnO_3 are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (T_C) and the Néel temperature (T_N) range. The negative contribution of the antiferromagnetic (AFM) part of a sample in the La_(1-x)Sr_xMnO_3 system at 0.1 ≤ x ≤ 0.3 decreases ΔT_(max) and changes the ΔT(T) curve shape, shifting its maximum 20-40 K above T_C. Lower values of ΔT_(max) are detected in the range T_C = 130-142 K in polycrystalline and single-crystal Sm_(0.55)Sr _(0.45)MnO_3 samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O-Mn bonds and, thus, increases the volume of CE-type AFM clusters), the maximum in the temperature dependence of MCE is located at T_N(243 K) for CE-type AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = TN. The PrBaMn_2O_6 composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering T _C. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.
机译：研究了La_（1-x）Sr_xMnO_3，Sm_（0.55）Sr_（0.45）MnO_3和PrBaMn _2O_6化合物的磁热效应（MCE）。发现通过直接方法在第二和第三组成中以及在La_（0.9）Sr_（0.1）MnO_3中通过直接方法确定的MCE的最大值（ΔT_（max））远低于根据磁性成分的变化计算得出的值居里温度（T_C）和Néel温度（T_N）范围内的熵。 La_（1-x）Sr_xMnO_3系统中样品的反铁磁（AFM）部分在0.1≤x≤0.3处的负贡献会降低ΔT_（max）并改变ΔT（T）曲线形状，从而将其最大值移动20-40 T_C上方的K。在空气中冷却的多晶和单晶Sm_（0.55）Sr_（0.45）MnO_3样品中，在T_C = 130-142 K范围内检测到较低的ΔT_（max）值。如果将此类样品在氧气气氛中冷却（还原断裂的Mn-O-Mn键，从而增加CE型AFM簇的体积），则MCE的温度依赖性最大值位于T_N（243 K）用于CE型AFM群集。在MCE测量期间施加到样品的磁场将这些簇转换为铁磁（FM）状态，并且两种簇在T = TN时分解。 PrBaMn_2O_6成分在231 K处经历AFM-FM转变，其MCE的温度依赖性在T = 234 K时具有极小的最小值，其中MCE为负，并且覆盖了T _C的最大值。两个极值处的MCE的绝对值都比根据磁熵的变化所计算出的绝对值低几倍。这些现象由这些锰矿中存在磁性异质FM-AFM状态来解释。

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