Study on Spin-State Transitions in LaCoO_3

LaCoO_3自旋态跃迁的研究

• 批准号: 08640449
• 负责人: ASAI Kichizo
• 金额: $ 1.41万
• 依托单位: The University of Electro-Communications
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 1996
• 资助国家: 日本
• 起止时间: 1996 至 1997
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-08640449/
• 关键词: spin-state transitions; intermediate-spin state; magnetovolume effect; transport properties; carrier doping; strongly correlated electron system; lanthanum cobalt oxide; perovskite; 熱電能; ホール効果; 圧力効果; spin-state transition; magnetovolume effect

The purpose of the present project is to clarify the spin-states of Co ions and the nature of the transition between them in LaCoO_3. "Spin-state transition" in LaCoO_3 has been a controversial issue for a long time ; both of two broad magnetic anomalies, one near 100 K and a second near 500 K,have been interpreted by a common spin-state transition of Co trivalent ions from the low-spin to the high-spin state, and the interpretation is inconsistent as a whole. In this project, not only the magnetic but also the electronic transport and the lattice properties have been investigated since the interactions among spin, charge, and the lattice are crucial in the "Spin-state transition" in LaCoO_3 which locates near the Mott transiton boundary. The followings are clarified.1. Two spin-state transitions in LaCoO_3Anomalous lattice expansions and lattice softenings were observed near 100 K and 500 K by neutron-diffraction and ultrasonic measurements, respectively. These lattice anomalies and a … More lso the temperature dependence of the Co magnetic moments are successfully interpreted in a wide temperature range based on a model involving low-spin (LS ; S=O), intermediate-spin (IS ; S=1), and high-spin (HS ; S=2) states of Co atoms. The first spin transition, near 100K,is from LS to IS,and the second, near 500 K,is from IS to a mixed state of IS and HS.These studies reveal the volume dependence of the energy of each spin-state quantitatively, and also indicate the correlation between the large drop in resistivity and the collapse of the energy gap between IS and HS states in the 500 K transition. The proposed model successfully interprets the observed pressure dependence of the susceptibillity. We also point out that "the two spin-state transitions" is a unique feature of LaCoO_3 among RCoO_3 (R=rare earth element).2. Magnetic and Transport properties of La_<1-X>Sr_XCoO_3 and LaCo_<1-X>Ni_XO_3 (X<0.1)In both systems, Co atoms are in IS state down to the lowest temperature and their magnetic moments freeze into a spin-glass state. The resistivity below 400 K decreases by the substitution, although its temperature dependence remains to be the activation type. The Seebeck effect suggests that the carriers dominating the transport are positive polarons in both systems. However, the Hall coefficient is positive in La_<1-X>Sr_XCoO_3 but negative in LaCo_<1-X>Ni_XO_3. The magnetic-electronic transition near 500 K survives in the substituted samples. This finding is consistent with the model described above for undoped LaCoO_3.3.Phtoemission, NMR,and muSR studies of LaCoO_3The temperature dependences of the photoemission spectra and the nuclear relaxation rates of ^<59>Co measured by NMR are interpreted consistently with the model described above. We found an increase of the relaxation rates of mu^+ on lowering temperature below 100 K.A more investigation is neccessary to understand the spin-state transition which mu^+ sees in LaCoO_3. Less

本项目的目的是阐明LaCoO_3中Co离子的自旋态以及它们之间的跃迁性质，LaCoO_3中的“自旋态跃迁”长期以来一直是两个广泛磁性材料中的一个有争议的问题。异常现象，一个接近 100 K，第二个接近 500 K，已被解释为 Co 三价离子从低自旋态到高自旋态的常见自旋态跃迁，但解释不一致总的来说，在这个项目中，不仅研究了磁性，还研究了电子输运和晶格性质，因为自旋、电荷和晶格之间的相互作用对于位于附近的 LaCoO_3 的“自旋态转变”至关重要。澄清了以下问题： 1．LaCoO_3在100 K和100 K附近观察到了两种自旋态转变。分别通过中子衍射和超声波测量 500 K，这些晶格异常和 Co 磁矩的温度依赖性在涉及低自旋模型 (LS; S=O) 的宽温度范围内得到了成功解释。 ）、中间自旋态（IS；S=1）和高自旋态（HS；S=2）Co 原子的第一个自旋跃迁，在 100K 附近，是从 LS 到 LS。 IS，第二个接近 500 K，从 IS 到 IS 和 HS 的混合态。这些研究定量地揭示了每个自旋态能量的体积依赖性，并且还表明了电阻率大幅下降和500 K 跃迁中 IS 和 HS 态之间能隙的崩溃我们还指出，“两个自旋态跃迁”是所观察到的磁化率的独特特征。 RCoO_3中的LaCoO_3（R=稀土元素）。 2. La_<1-X>Sr_XCoO_3和LaCo_<1-X>Ni_XO_3（X<0.1）的磁性和输运性质在两个体系中，Co原子都处于IS状态最低温度和它们的磁矩冻结成自旋玻璃态。低于 400 K 的电阻率因取代而降低，尽管其温度依赖性仍然是激活。塞贝克效应表明，在两个系统中，主导传输的载流子都是正极化子，但是，La_<1-X>Sr_XCoO_3 中的霍尔系数为正，而 LaCo_<1-X>Ni_XO_3 中的霍尔系数为负。近 500 K 在取代的样品中保留下来，这一发现与上述未掺杂的 LaCoO_3.3 的模型一致。光发射、NMR 和 muSR。 LaCoO_3的研究通过NMR测量的光发射光谱和^ 59 Co的核弛豫率的温度依赖性与上述模型一致地解释我们发现当温度降低到100 K.A以下时μ^+的弛豫率增加。需要更多的研究来理解 mu^+ 在 LaCoO_3 Less 中看到的自旋态转变。