Spin state transition in Ca-doped Na0.7CoO2 with the nominal Co valence below 3.16 |
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| Affiliation: | 1. Frontier Research Groups, Toyota Central R&D Laboratories Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan;2. TRIUMF, CIAR and Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada, V6T 1Z1;3. TRIUMF, Vancouver, BC, Canada, V6T 2A3;4. Department of Applied Physics, Nagoya University, Nagoya, Aichi 464-8603, Japan;5. Department of Applied Physics, Tohoku University, Sendai, Miyagi 980-8579, Japan;1. School of Metallurgy and Environment, Central South University, Changsha 410083, PR China;2. Citic Dameng Mining Industries Limited, Nanning 530028, PR China;3. Qing Hai Kuai Lv High-tech Co., Ltd, Qinghai 810000, PR China;1. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan;2. Research Institute for Energy Conservation, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, 305-8568, Japan;3. AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Kashiwa, Chiba, 277-8565, Japan;4. Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki, 305-0047, Japan;5. PRESTO, Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi, Saitama, 332-0012, Japan;6. Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, 305-0801, Japan;7. Synchrotron Radiation Research Organization, The University of Tokyo, Bunkyo-ku, Tokyo, 113-8586, Japan;1. Jiangxi University of Science and Technology, Ganzhou, China;2. Sustainable Minerals Institute, University of Queensland, St Lucia, Brisbane, 4072, Australia;1. Dept. of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada;2. Dept. of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada |
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| Abstract: | In order to clarify the nature of the transition around 300 K in Ca-doped Na0.7CoO2, the magnetism of Na0.7CayCoO2 with y=0.035 and 0.07 was investigated in a positive muon spin rotation and relaxation (μ+SR) experiment. Transverse field μ+SR measurements showed that the spin state of the Co ions in Na0.7Ca0.07CoO2 changes at around 300 K; i.e. the exponential relaxation rate vs. T curve exhibited a large maximum around 300 K with an accompanying small peak in the muonic Knight shift, whereas no changes were found in the asymmetry, similar to [Ca2CoO3]0.62[CoO2] at around 400 K.Although the spin-density-wave (SDW) state exists for NaxCoO2 with x≥0.75 at low temperatures, zero-field μ+SR spectra in the Ca-doped samples showed no muon spin precessions down to 1.8 K but only fast relaxations indicating disorder. This is probably because the Ca2+ ions in the Na planes alter the charge and/or spin distribution in the CoO2 planes. As a result, the SDW order is hindered, as the nominal Co valence is decreased below 3.16. |
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