Physical Properties of Oxygen Composition Controlled La 1-x Sr x MnO y Single Crystals

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Physical Properties of Oxygen Composition Controlled La1-xSrxMnOy Single Crystals Yuui Yokota, Jun-ichi Shimoyama, Tetsuro Ogata, Shigeru Horii, and Kohji Kishio Applied Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

ABSTRACT We have investigated the effect of high-temperature post-annealing and control of excess oxygen for La1-xSrxMnOy (0.05 ≤ x ≤ 0.2) single crystals. Thin plate-like single crystals prepared by careful cutting and polishing enabled the control of excess oxygen for the single crystals by post-annealing for relatively short times at various temperatures. Post-annealed crystals with y ~ 3.00 exhibited higher TC than that of as-grown crystals due to homogenization of cation and oxygen compositions and reduction of local lattice strains. With an increase of y, TC systematically increased for all Sr compositions. Competition between an increase of mean valence of manganese and generation of cation vacancy determines the TC of La1-xSrxMnOy crystals. INTRODUCTION The La1-xSrxMnO3 perovskite exhibiting colossal magnetoresistance (CMR) effect around ferromagnetic temperature TC (x ≥ 0.1) have attracted great attention of an application for next generation of magnetoresistance head in recent several years [1]. Mean valence of manganese can be arbitrarily controlled by substituting Sr2+ for La3+ in the undoped parent compound LaMnO3 and the La1-xSrxMnO3 show a very rich phase diagram [2, 3] result from competitions between double exchange interaction, superexchange interaction, Jahn-Teller distortion on MnO6 octahedron and orbital ordering. An increase of x corresponds to an increase of mean valence of manganese i.e., an increase of Mn4+ concentration. With increasing x, magnetic and electrical characters change from an antiferromagnetic insulator to a ferromagnetic insulator at x ~ 0.1 and to a ferromagnetic metal at x ~ 0.16, while TC systematically increases from 145 K for x = 0.1 to 371 K for x = 0.4 [2]. In addition, rhombohedral phase at room temperature changes to orthorhombic phase for x > 0.16 at low temperatures and the structural transition temperature TS systematically deceases with an increase of x. The La1-xSrxMnOy was well known to have large oxygen nonstoichiometry [4] and generation of cation vacancies of La and Mn was generally accepted in the excess oxygen composition [5]. Although there are several papers reporting the effects of excess oxygen on crystal structure, magnetic properties and electrical transport for La1-xSrxMnOy polycrystalline samples [6] and thin films [7], such studies for La1-xSrxMnOy single crystals are very few because of difficulty in controlling oxygen content. In order to elucidate the intrinsic effects of excess oxygen on various physical properties of La1-xSrxMnOy, investigations for single crystals free from effects of grain boundaries and extrinsic lattice distortion are essential. Since oxygen diffusion accompanying generation of cation vacancies is relatively slow in this system, achievement of homogeneous and equi