full paper - Institute of Problems of Mechanical Engineering
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Corresponding author: S.N. Barilo, e-mail: [email protected] Rev.Adv.Mater.Sci. 12 (2006) 33-45 © 2006 Advanced Study Center Co. Ltd. CRYSTAL GROWTH AND GIANT MAGNETORESISTANCE OF RARE EARTH LAYERED COBALTITES S. N. Barilo 1 , S.V. Shiryaev 1 , G.L. Bychkov 1 , A.S. Shestak 1 , Z.X. Zhou 2 , V. Hinkov 3 , V.P. Plakhty 4 , Yu. P. Chernenkov 4 , S.V. Gavrilov 4 , M. Baran 5 , R. Szymczak 5 , D. Sheptyakov 6 and H. Szymczak 5 1 Institute of Solid State and Semiconductor Physics, BAS, 17 P. Brovka st., Minsk 220072, Belarus 2 National High Magnetic Field Laboratory, 18 E. Paul Dirac Dr., Tallahassee, FL 32310, USA 3 Max-Planck Institut für Festkörperforschung, 1 Heisenberg str., D-70569 Stuttgart, Germany 4 Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia 5 Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, PL-02 668 Warsaw, Poland 6 Laboratory for Neutron Scattering, ETH Zürich & Paul Scherrer Institute, CH-5232, Villigen PSI, Switzerland Received: November 12, 2005 Abstract. A review on progress in crystal growth and study of layered cobaltites is presented. Single crystals of the LnBaCo 2 O 5+δ (Ln = Pr, Sm, Eu, Gd, Tb, Dy, Tb 0.9 Dy 0.1 ) family with the giant magnetoresistance (MR) effect were grown by the flux technique. Co-crystallization with the geometrically frustrated LnBaCo 4 O 7 phase was registered in a wide range of temperature. The details of structure of the as-grown and detwinned single crystals and their characterization by magnetic susceptibility, magnetoresistance, X-ray and neutron diffraction are discussed. A comparative study of the twinned and twins free EuBaCo 2 O 5+δ single crystals reveals a positive magnetoresistance effect ~ 500% at low temperature. 1. INTRODUCTION It was unambiguously shown that fascinating prop- erties of cobaltites can be tuned by chemical engi- neering to allow the observation of a spin-gap for- mation in the geometrically frustrated Kagomé lat- tice of the cobalt layers [1], the highly anisotropic giant magnetoresistance (MR) effect in a specific temperature range [2,3] and uncommon supercon- ductivity [4]. There are direct evidences that Ba- doped cobaltites LnBaCo 2 O 5+δ (Ln rare earth or yttrium, 0 ≤ δ ≤ 1) undergo spin, charge and orbital ordering when the oxygen content approaches δ = 0.50. Here exotic magnetic properties resulting in the highest amongst cobalt based oxides value of MR and a metal-insulator transition on the verge of Co 3+ spin state transition may be manifested, and often unusual valence states of cobalt are maintained. Taking in mind that Co 2+ , Co 3+ , and Co 4+ ions are all susceptible to become apparent in multiple spin states one can consider the LnBaCo 2 O 5+δ (Ln-112, 0 ≤ δ ≤ 1) family as a model system to investigate most of the possible electronic configurations of cobalt. In particular, for the nominal valence of co- balt 3+ at δ = 0.5 the Co 3+ ion has a non-magnetic, or low-spin ground state (LS, t e g g 2 6 0 ) as well as the intermediate-spin (IS, t e g g 2 5 1 ) and the high-spin (HS, t e g g 2 4 2 ) excited states. The Ln-112 phase at δ=0.5 has orthorhombic structure with the unit cell a 1 ≈a p , a 2 ≈2a p , a 3 ≈2a p , where a p is parameter of the pseudocubic perovskite cell, which is usually described by the Pmmm space
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