Magnetic and Magnetotransport Properties of Misfit Cobaltite Ca3Co4O9

The spin-entropy contribution to thermopower in misfit cobaltate Ca3Co3.93O9+δ

J. Hejtmánek, Z. Jirák, and J. Šebek Institute of Physics of ASCR, Na Slovance 2, 182 21 Praha 8, Czech Republic

Abstract The Ca3Co3.93O9+δ misfit cobaltates with extreme oxygen contents (δ ≈ 0.10 and 0.30) were explored by measurements of their thermal and electrical properties.. The studied cobaltates emerge as composite systems both in the structural and electronic sense. In particular, the hole-like charge carriers in the framework of low-spin Co3+ ions in CoO2 layers show a broad spectrum of behaviours - from complete localization in the form of low-spin Co4+ ions up to holes of small or large polaron characters. The attention is given to magnetic field effects on the electrical and thermal transport, with main focus to the thermoelectric power. The latter experiments, performed up to 140 kOe, evidence a huge suppression of Seebeck coefficient in external magnetic field. The suppression of Seebeck coefficient gradually increases with decreasing temperature and, finally, the saturation is observed when temperatures are low enough. To understand the results, the thermopower is treated in terms of purely thermodynamic Kelvin formula, and so-called Spin liquid model is evoked, providing a reason for extra spin entropy contribution not occurring explicitly for Fermi liquids. Experimentally determined values of the spin-entropy contribution to the low - temperature Seebeck coefficient are unprecedentedly large and make at zero field up to 50% of the theoretical limit

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kB ln 2 = 60 µV/K. qe

I. Introduction The sodium cobaltates NaxCoO2 and related misfit-layered compounds are generally considered as promising systems for thermoelectric applications with respect to their high Seebeck coefficient, good electrical conductivity, low toxicity and chemical stability at elevated temperatures [1, 2]. As the crystal structure is concerned, these systems have in common hexagonal layers of edge shared CoO6 octahedra, in which electric transport is carried out. These CoO2 layers alternate with planes of variable content of Na+ ions or, in the “misfits”, there are interpolating blocks of two-, three-, or fourplane oxide layers of rock-salt (RS) type. In particular, the commonly known misfit system of approximate composition Ca3Co4O9 contains a block formed by the CoO plane sandwiched by two CaO planes. The lattice parameters of the CoO2 layers and Ca2CoO3 triple RS blocks are incommensurate in the basal b-direction. In the block compositions, the appropriate chemical formula at full oxygen stoichiometry is [Ca2CoO3]RS.[CoO2]1.62, yielding Ca3Co3.93O9.36 after summation and additional renormalization. Lot of the experimental and theoretical work has been done on the NaxCoO2 system – see e. g. [3]. The compound with full sodium content x=1 is a band insulator with Co3+ ions in the low-spin (LS) state. The removal of some sodium is charge compensated by partial emptying of the t2g based valence band, resulting in essentially metallic properties of systems with 0.5