Effect of disorder on the thermodynamic phase transition in La0.70Ca0.30MnO3
Effect of disorder on the thermodynamic phase transition in La0.70Ca0.30MnO3
复制标题
无序对La0.70Ca0.30MnO3热力学相变的影响
DOI：
10.1103/physrevb.75.012412
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发表时间：
2007
影响因子：
3.7
通讯作者：
R. F. Jardim
中科院分区：
文献类型：
--
作者：
J. A. Souza;J. Neumeier;R. F. Jardim
钙钛矿锰氧化物中的巨磁电阻CMR效应源于晶格、电荷和自旋自由度之间的强烈相互作用。1在典型化合物La 0.70 A 0.30 MnO 3中，较小的A离子减小了平均Mn-O-Mn键角，这削弱了铁磁双交换DE相互作用。2当小的稀有金属-用Y等稀土离子取代La。3 YMnO 3是一种超交换SE反铁磁物质，TN=70 K，具有六方晶体结构，在900 K以下具有铁电行为，和受抑磁性。在La 0.70 A 0.30 MnO 3中，1.019 A的4 Y离子半径5取代1.160 A的La离子半径将导致DE和SE相互作用之间的显著局部扭曲和竞争，这也可能促进磁晶格内的无序和挫折。在某些情况下，可能会产生类似自旋玻璃的状态，或者挫折的水平可能足够高，以防止建立长程磁序。6事实上，一些研究表明，锰氧化物中的铁磁FM转变可以通过掺杂Y，Ga，Gd，7 -9虽然Y取代的La0.70Ca0.30MnO3的磁性和输运性质已经得到了很好的研究，恒压下热容CP和热膨胀受到较少关注。在这里，我们表明，15%的Y取代La抑制CP在铁磁转变温度TC的异常。与CP数据相反，高分辨率的热膨胀测量揭示了TC体积的明显异常。仔细比较CP的未掺杂和掺杂的样品揭示了一个非常广泛的功能在La0.55Y0.15Ca0.30MnO3。此外，热力学分析表明，在TC的CP峰应该是小而宽的;这一分析正确地预测了一个非常大的压力效应TC以及。结果进行了讨论，在局部应变场引起的较小的Y离子，DE和SE之间的相互作用，从而导致一个不均匀的铁磁状态组成的FM和倾斜FM区域所产生的竞争。通过溶胶-凝胶法制备了La0.70−xYxCa0.30MnO3 x=0.0和0.15的多晶样品，该方法提供了比标准固态反应更好的化学均匀性，更小的粒度和更高的密度（理论密度的90%）。将干燥的凝胶在1000 °C和1100 °C下在空气中热处理30小时，随后压制成粒料，并在1200 °C下在空气中进行最终热处理30小时。Rietveld细化空间群Pnma的X射线粉末衍射数据证实了单相的性质。使用Quantum Design PPMS测量热容。采用熔凝石英电容式热膨胀仪测量线性热膨胀，灵敏度为0.1A。用SQUID磁强计测量了零场冷却ZFC和场冷却FC条件下的交流和直流磁化强度MT、H、P。对于流体静压测量，将Cu-Be夹式单元放置在磁力计中，其中正戊烷-异戊醇50：50混合物作为压力介质。用放置在具有样品的单元中的超导锡压力计来确定压力。x=0.0和0.15的样品的直流磁化ZFC和FC与温度的关系如图1所示。在5 T和5 K下由曲线确定的饱和磁矩分别为3.6和3.3 B（x=0和0.15）。这些略小于预期的3.7 B（Mn 3 +/Mn 4+离子的比例为70：30），但表明两种样品均显示铁磁有序。x=0.15的样品具有较小的磁矩，这表明倾斜的FM排列。当x=0时，T_c =250 K，H= 0.1T.钇的掺杂使TC在降温和升温时分别下降到90 K和100 K，在5 T时滞后消失。对于x=0，没有观察到这种大的热滞后。已知La0.55Y0.15Ca0.30MnO3表现出
The colossal magnetoresistance CMR effect in perovskite manganese oxides results from the strong interplay between lattice, charge, and spin degrees of freedom.1 In the canonical compound La0.70A0.30MnO3, smaller A ions reduce the average Mn-O-Mn bond angle, which weakens the ferromagnetic double-exchange DE interaction.2 A similar effect occurs when small rare-earth ions such as Y are substituted for La.3 YMnO3 is a superexchange SE antiferromagnet TN=70 K with hexagonal crystal structure, ferroelectric behavior below 900 K , and frustrated magnetism.4 Y ionic radius5 of 1.019 A substitution for La ionic radius of 1.160 A in La0.70A0.30MnO3 will lead to significant local distortions and competition between DE and SE interactions, which might also promote disorder and frustration within the magnetic lattice. In some cases a spin-glass-like state may result or the level of frustration could be high enough to prevent the establishment of long-range magnetic order.6 Indeed, several studies indicate that the ferromagnetic FM transition in manganese oxides can be suppressed through doping of Y, Ga, Gd, or Dy leading to a cluster-glass state.7–9 Although the magnetic and transport properties of Y-substituted La0.70Ca0.30MnO3 have been well studied, heat capacity at constant pressure CP and thermal expansion have received less attention. Herein we show that substitution of 15% Y for La suppresses the anomaly in CP at the ferromagnetic transition temperature TC. In contrast with the CP data, high-resolution thermal-expansion measurements reveal a distinct anomaly in the volume at TC. Careful comparison of CP of the undoped and doped samples reveals a very broad feature in La0.55Y0.15Ca0.30MnO3. Furthermore, thermodynamic analysis indicates that the peak in CP at TC should be small and broad; this analysis correctly predicts an extremely large pressure effect on TC as well. The results are discussed in terms of local strain fields induced by the smaller Y ions, and the resulting competition between DE and SE interactions, which leads to an inhomogeneous ferromagnetic state composed of FM and canted-FM regions. Polycrystalline samples of La0.70−xYxCa0.30MnO3 x=0.0 and 0.15 were prepared through a sol-gel method, which provides better chemical homogeneity, smaller particle size, and higher density 90% of theoretical density than a standard solid-state reaction. The dried gel was heat treated at 1000 °C and 1100 °C in air for 30 h, subsequently pressed into pellets, and subjected to a final heat treatment at 1200 °C in air for 30 h. Rietveld refinement space group Pnma of x-ray powder-diffraction data confirmed the single-phase nature. Heat capacity was measured using a Quantum Design PPMS. A fused quartz capacitive dilatometer was used to measure the linear thermal expansion with a sensitivity in l of 0.1 A. ac and dc magnetization M T ,H , P under zero-field cooling ZFC and field cooling FC , was measured in a SQUID magnetometer. For hydrostatic pressure measurements, a Cu-Be clamp-type cell was placed in the magnetometer with n-pentane-isoamyl alcohol 50:50 mixture as a pressure medium. Pressure was determined with a superconducting tin manometer placed in the cell with the sample. The dc magnetization ZFC and FC versus temperature of the samples with x=0.0 and 0.15 is shown in Fig. 1. The saturated magnetic moments, determined from the curves at 5 T and 5 K, were found to be 3.6 and 3.3 B for x=0 and 0.15, respectively. These are slightly smaller than expected 3.7 B for a 70:30 ratio of Mn3+/Mn4+ ions but indicate that both samples display ferromagnetic order. The sample with x=0.15 has a smaller magnetic moment, which suggests a canted FM alignment. For x=0, TC=250 K at H=0.1 T. Yttrium doping causes a strong decrease of TC to 90 K and 100 K upon cooling and warming, respectively; the hysteresis disappears at 5 T. This large thermal hysteresis was not observed for x=0. La0.55Y0.15Ca0.30MnO3 is known to exhibit