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

本项目的目的是澄清Co离子在LaCoO_3中的自旋态和它们之间跃迁的性质。LaCoO_3的“自旋态跃迁”一直是一个有争议的问题；在100k附近和500k附近，用Co三价离子从低自旋到高自旋的共同自旋态跃迁解释了两个广泛的磁异常，但解释总体上是不一致的。由于在Mott跃迁边界附近的LaCoO_3中，自旋、电荷和晶格之间的相互作用是“自旋态跃迁”的关键，因此本项目不仅研究了磁性，还研究了电子输运和晶格性质。澄清如下：在100 K和500 K附近，通过中子衍射和超声测量分别观察到lacoo_3的两个自旋态转变，反常晶格膨胀和晶格软化。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这些晶格异常和Co磁矩的温度依赖性。第一次自旋跃迁发生在100K附近，是从LS到is，第二次自旋跃迁发生在500k附近，是从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，也表明了500 K跃迁中电阻率的大幅下降与IS态和HS态之间能量间隙的缩小之间的相关性。该模型成功地解释了观察到的压力依赖性。我们还指出，“双自旋态跃迁”是LaCoO_3在RCoO_3 （R=稀土元素）中所特有的特征。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这两种体系中，Co原子在最低温度下都处于IS态，其磁矩冻结成自旋玻璃态。400k以下的电阻率因取代而降低，但其对温度的依赖仍然是活化型的。塞贝克效应表明在两个系统中主导输运的载流子都是正极化子。而在La_<1-X>Sr_XCoO_3中，霍尔系数为正，而在LaCo_<1-X>Ni_XO_3中，霍尔系数为负。在取代的样品中，500k附近的磁电跃迁仍然存在。这一发现与上述未掺杂LaCoO_3.3的模型一致。lacoo3的光发射、核磁共振和muSR研究3 .用核磁共振测量的^<59>Co的光发射光谱和核弛豫率的温度依赖性与上述模型一致。我们发现，当温度降低到100k以下时，mu^+的弛豫速率增加。要理解mu^+在LaCoO_3中看到的自旋态跃迁，还需要进一步的研究。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）。

K.Asai, A.Yoneda, O.Yokokura, J.M.Tranquada, G.Shirane, and K.Kohn: "Two Spin-State transitions in LaCoO_3" J.Phys.Soc.Jpn. 67, no.1. 290-296 (1998)

K.Asai、A.Yoneda、O.Yokokura、J.M.Tranquada、G.Shirane 和 K.Kohn：“LaCoO_3 中的两个自旋态跃迁”J.Phys.Soc.Jpn。