Evidence for Magnetic Polarons in Hole-Doped Cobalt Perovskites
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1256-N04-02
Evidence for Magnetic Polarons in Hole-Doped Cobalt Perovskites A. Podlesnyak1, A. Furrer2, Th. Strässle2, E. Pomjakushina2,3, K. Conder3 and D.I. Khomskii4 1
Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge TN 37831, USA 2 Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 3 Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4 Institute of Physics II, University of Cologne, Zülpicher str. 77, Cologne 50937, Germany
ABSTRACT A substitution of La3+ by Sr2+ in LaCoO3 induces holes in the low-spin ground state of the Co ions, which behave like magnetic impurities with a very high spin value (13 PB per hole). In this work, using single-crystal neutron spectroscopy, we prove that the charges introduced by strontium doping do not remain localized at the cobalt sites. Instead, each hole not only creates Co4+ in low-spin state, but it also transforms the six nearest neighboring Co3+ ions to the intermediate-spin state thereby forming a magnetic seven-site (heptamer) polaron. Spin-state polarons behave like magnetic nanoparticles embedded in an insulating nonmagnetic matrix. Therefore, lightly doped La1-xSrxCoO3 is a natural analog to artificial structures composed of ferromagnetic particles in insulating matrices.
INTRODUCTION The hole-doped lanthanum cobalt oxides have been subject of growing interest in the past three decades for their rich temperature-concentration phase diagram. The ground state of parent LaCoO3 is nonmagnetic, corresponding to a low-spin (LS) state of Co3+ ions (S=0). With thermal excitations, first a crossover into a magnetic, but still insulating, state appears at about 80-120 K, followed by another crossover into a bad metallic magnetic state at T>500 K due to thermally induced spin-state crossover to higher spin states [1-10]. The temperature evolution of the Co3+ spin-states in LaCoO3 is due to the subtle balance between intra-atomic (Hund) exchange interactions EH and crystal-electric-ILHOGVSOLWWLQJǻcf. The first magnetic excited high-spin (HS) state in LaCoO3 is found at 10.3 meV (0.5-0.7 RIǻcf only) above the LS ground state [4,5], a very small energy compared to the values of EH DQGǻcf in 3d-oxides of 1.5-2 eV [6]. The delicate balance between EH DQGǻcf is easily shifted by carrier doping. Hole-doping of lanthanum cobalt oxides La1-xSrxCoO3 results in a spin-glass and superparamagnetic behavior (x0.18) [1114]. After substantial experimental [4,5,10,14-22] as well as theoretical [23-25] efforts in recent years, considerable progress has been achieved in understanding the physical properties of these compounds. It was shown that the electronic phase separation leading to formation of ferromagnetic (FM) clusters in a nonmagnetic matrix upon carrier injections plays a crucial role
in their magnetic and transport properties. The number and size of the FM clusters created via Sr doping increases with hole concentration x [10,13]. However, size e
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