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离子的自旋态及其之间跃迁的性质。LaCoO_3中的“自旋态转变”一直是一个有争议的问题，在100 K附近和500 K附近的两个宽磁异常都被解释为Co三价离子从低自旋态到高自旋态的共同自旋态转变，但这种解释总体上是不一致的。由于自旋、电荷和晶格之间的相互作用在LaCoO_3中Mott相变边界附近的“自旋-态转变”中起着关键作用，因此本项目不仅研究了LaCoO_3的磁性，而且研究了LaCoO_3的电子输运和晶格性质。以下是澄清。1.用中子衍射和超声波测量分别在100 K和500 K附近观察到LaCoO_3的两个自旋态转变.这些晶格异常和 ...更多信息 基于Co原子的低自旋（LS ; S= 0）、中自旋（IS ; S=1）和高自旋（HS ; S=2）态模型，在较宽的温度范围内成功地解释了Co磁矩的温度依赖性.第一个自旋跃迁发生在100 K附近，是从LS到IS，第二个自旋跃迁发生在500 K附近，是从IS到IS和HS的混合态.这些研究定量地揭示了各自旋态能量的体积依赖性，也表明了500 K跃迁中电阻率的大幅度下降与IS和HS态之间能隙的崩塌之间的相关性.所提出的模型成功地解释了所观察到的压力依赖性的可伸缩性。指出了LaCoO_3在RCoO_3（R=稀土元素）中所特有的“两个自旋态跃迁”. La_<1-X>Sr_XCoO_3和LaCo_<1-X>Ni_XO_3（X&lt;0.1）的磁性和输运性质在这两个体系中，Co原子到最低温度都处于IS态，磁矩冻结成自旋玻璃态。低于400 K的电阻率下降的替代，但其温度依赖性仍然是激活型。Seebeck效应表明，在这两个系统中，主导输运的载流子都是正极化子。而La_Sr_XCoO_3的霍尔系数为正<1-X>，LaCo_Ni_XO_3的霍尔系数为负<1-X>。在500 K附近的磁-电子转变的替代样品中生存。3. LaCoO_3的光电子能谱、核磁共振和μ SR研究用核磁共振测量的~ 2Co的光电子能谱和核弛豫速率随温度<59>的变化与上述模型一致。我们发现，当温度低于100 K时，μ ^+的弛豫速率增加。进一步的研究对于理解μ ^+在LaCoO_3中所看到的自旋态转变是必要的。少

项目成果

K.Asai, O.Yokokura, M.Suzuki, T.Naka, T.Matsumoto, H.Takahashi, N.Mori, and K.Kohn: "Pressure Dependence of the 100 K Spin-State transition in LaCoO_3" J.Phys.Soc.Jpn. 66, no.4. 967-970 (1997)

K.Asai、O.Yokokura、M.Suzuki、T.Naka、T.Matsumoto、H.Takahashi、N.Mori 和 K.Kohn：“LaCoO_3 中 100 K 自旋态转变的压力依赖性”J.Phys

S.Murata, S.Isida, M.Suzuki, Y.Kobayashi, K.Asai, and K.Kohn: "Elastic anomalies with the spin-state transitions in LaCoO_3" Physica B. in press.

S.Murata、S.Isida、M.Suzuki、Y.Kobayashi、K.Asai 和 K.Kohn：“LaCoO_3 中自旋态转变的弹性异常”Physica B. 正在出版。

S.Murata: "Elastic anomalies with the spin-state transitions in LaCoO_3" Physica B. (in press).

S.Murata：“LaCoO_3 中自旋态转变的弹性异常”Physica B.（出版中）。

K.Asai: "Pressure Dependence of the 100 K Spin-State transition in LaCoO_3" J.Phys.Soc.Jpn. 66・4. 967-970 (1997)

K.Asai：“LaCoO_3 中 100 K 自旋态转变的压力依赖性”J.Phys.Soc.Jpn 66・4（1997）。

Kichizo Asai: "Pressure Dependence of the look Spin-State Transition in LaCoO_3" Journal of the Physical Society of Japan. 66.4. 967-970 (1997)

Kichizo Asai：“LaCoO_3 中外观自旋态转变的压力依赖性”日本物理学会杂志。