KOHN Kei-ichirou Waseda Univ., Dep.of Physics Professor, 理工学部, 教授 (40063656)
SUZUKI Masaru Univ.Electro-commun., Assoc.Prof., 電気通信学部, 助教授 (20196869)
YAMADA Nobuyoshi Univ.Electro-commun., Assoc.Prof., 電気通信学部, 助教授 (40017405)
KOBAYASHI Yoshihiko Univ.Electro-commun., Assist.Prof., 電気通信学部, 助手 (60293122)
|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1997 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1996 : ¥1,500,000 (Direct Cost : ¥1,500,000)
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_3
Anomalous 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
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_3
The 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