Spin/Orbital Modulation in Perovskite Manganite Superlattices
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*'**, and Y. TOKURA . *Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-0046, Japan "**Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8502, Japan ***Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
ABSTRACT A systematic study is presented for structural characterization and physical properties of two kinds of perovskite oxide superlattices composed of ferromagnetic (FM) and antiferromagnetic (AF) layers. Spin ordering structures is modulated in FM La0.6Sro.4 MnO 3/Gtype AF Lao. 6 Sro.4FeO 3 superlattices, whereas, ordering structures both in spin and orbital are modulated in Lao.6 SrO.4 MnO 3 /A-type AF La 0 .45Sr 0 .55MnO 3 along the growth directions. Large magnetoresistance subsists down to low temperature in La 0 .6 Sr 0 .4 MnO 3 /Lao. 6 Sr0 4 FeO 3 (F/G) superlattices as a result of recovery of ferromagnetism, which is once suppressed by spin frustration at the interface between FM and C-type AF layers. In contrast, the constituent layers in the Lao.sSrO. 4 MnO 3/La0. 45 Sro. 55MnO 3 (F/A) superlattices appear to keep their ground states due to the absence of spin frustration at the interface. Magnetoresistance is pronounced in this type of superlattices at low temperatures when the AF layer is very thin, indicating restoration of the electronic coupling between the neighboring FM layers which are otherwise decoupled by intervening A-type AF spin ordering and d_2_V2 orbital ordering in La 0 .45 Sro.55 MnO 3 layers. INTRODUCTION Perovskite type manganese oxides with chemical formula of RE1_-AE-,MnO 3 , where RE is trivalent rare earth element and AE is divalent alkaline earth element, show a variety of spin-charge coupled properties [1]. One of the most striking phenomena observed in doped manganites is the colossal magnetoresistance (CMR); a large drop of resistance by magnetic field is observed near the ferromagnetic (FM) transition temperature. The appearance of such an interesting phase can be explained in terms of subtle competition and compromise between the double-exchange interaction mediated with the conduction eg electron and the super-exchange interaction between local t2g electrons. In actual complex phase diagrams, not only the FM phase but also a variety of antiferromagnetic (AF) phases appear. This is partly due to the variety of coexistence of the AF superexchange interactions between the t2g spins and partly due to the electronic anisotropy arising from the orbital degree of freedom in the conduction eg electrons. The purpose of this report is to understand the physical properties of FM/AF artificial oxide superlattices where FM layer is Lao.6 Sr 0 .4 MnO 3 . Lai_,SrMnO 3 is a prototypical compound which shows CMR for the doping level of 0.15 < x < 0.5 [2]. FM transition takes place above 300 K at 0.3 < x < 0.5. La-_.SrMnO 3 is free from charge ordering phenomenon even when x = 0.50, unlike Prl_.Sr-MnO3 [3] or Ndl--SrxMnO 3 [4], but changes its ground state to an AF metal at x > 0.5 [5]. By combining with ot
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