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Preparation and Characterization of the Single-Layered Cobaltate La2-xCaxCoO4 D. Fuchs,1 M. Merz1, R. Schneider,1 and H. v. Löhneysen1,2 1

Karlsruher Institut für Technologie, Institut für Festkörperphysik, 76021 Karlsruhe, Germany

2

Karlsruher Institut für Technologie, Physikalisches Institut, 76131 Karlsruhe, Germany

ABSTRACT Polycrystalline samples of the single-layered cobaltate La2-xCaxCoO4 were prepared in a wide doping range of 0 ” x ” 1.5. Structural properties were characterized at room temperature. The orthorhombic distorted structure of the mother compound La2CoO4 changes to a tetragonal structure for x = 0.5 and then becomes orthorhombic again for x > 0.5. The magnetic properties were investigated in the temperature range from 5 K ” T ” 300 K. With increasing hole-doping a successive decrease of antiferromagnetic exchange is observed for x ” 0.5 whereas an increase of ferromagnetic exchange evolves for x • 0.5. . INTRODUCTION In La2MO4 (M = Co, Ni, Cu), tilt distortions of the MO6 octahedra, metal –insulator transitions, charge segregation instabilities and spin-glass phases are strongly affected by chemical composition and pressure and hence can be modulated rather easily. The parent compound La2CoO4 shows antiferromagnetic ordering in the low-temperature orthorhombic (LTO) distorted structure with a Néel temperature of TN = 275 K [1]. Hole-doping with Sr suppresses the long-range antiferromagnetic order and lifts the orthorhombic distortion [2]. La2-xSrxCoO4 has also been extensively studied with respect to a spin-state transition. The spindegree of freedom found in the cobaltates can lead to different spin-states, i. e., low-spin (LS), intermediate-spin (IS) and high-spin (HS) state, depending on the delicate balance between the crystal-field splitting of the Co 3d t2g and eg states and the Hund`s rule coupling [3,4]. Ferromagnetic correlations are observed for x > 0.5 where the maximum of TC = 250 K is observed for the end member Sr2CoO4 [5]. In contrast to the doping with Sr, Ca-substituted La2-xCaxCoO4 has been investigated only in a narrow concentration range of 0.3 ” x ” 0.8 by Horigane et al.[6 -8]. To shed more light on the magnetic phase diagram of La2-xCaxCoO4 we have prepared polycrystalline samples in a wide concentration range (0 ” x ” 1.5). Roomtemperature structural properties as well as magnetic properties are presented. EXPERIMENT Polycrystalline samples of the composition La2-xCaxCoO4 with x = 0, 0.5, 1, and 1.5 were prepared by standard solid-state reaction using La2O3, CaCO3 and CoO powders with a purity • 99.9% as starting materials. First, the powders were mixed in stoichiometric composition and calcinated at 950° C for 24 hours. Before sintering, the powders were milled in a ball-mill for 30 minutes. The first sintering step was carried out at 1150°C for 40 hours under ambient

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conditions. After the first sintering step, the powders were grinded again and pressed with 5 t/ cm2 to pellets with a diameter of 12 mm. Since the melting temperature of the composition obviousl