Study on Spin-State Transitions in LaCoO_3

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跃迁的性质。LaCoO_3中的“自旋-态跃迁”一直是一个有争议的问题；两个宽磁异常，一个在100K附近，另一个在500K附近，都被解释为Co三价离子从低自旋态到高自旋态的共同的自旋态跃迁，总体上解释是不一致的。在这个项目中，不仅研究了LaCoO_3的磁性，而且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是LaCoO_3“自旋-态转变”的关键，LaCoO_3位于Mott跃迁边界附近。以下是澄清的内容。用中子衍射法和超声测量法分别在100K和500K附近观察到LaCoO_3异常晶格膨胀和晶格软化过程中的两个自旋态转变。这些晶格反常和…基于一个涉及Co原子的低自旋(LS；S=O)、中自旋(IS；S=1)和高自旋(HS；S=2)态的模型，在较宽的温度范围内成功地解释了Co磁矩的温度依赖关系。第一个自旋在100K附近是从LS到IS，第二个在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的能量与体积的依赖关系，并指出了500K相变中IS和HS态之间的能隙坍塌与电阻率的大幅下降之间的关系。该模型成功地解释了观测到的磁化率与压力的关系。我们还指出，在RCoO_3(R=稀土元素)中，LaCoO_3具有“两个自旋态转变”的独特特征。La_&lt；1-X&Gt；Sr_xCoO_3和LaCo_&lt；1-X&Gt；Ni_xO_3(X&lt；0.1)的磁性和输运性质在这两个体系中，Co原子在最低温度下处于IS态，其磁矩冻结为自旋玻璃态。400K以下的电阻率随取代量的增加而降低，但其温度依赖性仍为激活型。塞贝克效应表明，在这两个体系中，主导输运的载流子都是正极化子。La_xCoO_3的霍尔系数为正，LaCo_xT；1-X&GT；Ni_xO_3的霍尔系数为负。这一发现与未掺杂LaCoO_3的上述模型一致。LaCoO_3的光电子能谱、核磁共振和互自旋共振研究表明，用核磁共振测得的^&lt；59&gt；Co的光电子能谱和核弛豫速率随温度的变化与上述模型一致。我们发现，当温度降低到100K以下时，Mu的驰豫速率增加。为了理解Mu在LaCoO_3中所看到的自旋态转变，有必要进行更多的研究。

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