2@3 __ lii!B !&ji ELSEVIER 6 October 1997 Physics Letters A 234 (1997) 384-390 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE PHYSICS LETTERS A Electrical transport properties of bulk La, _xBa,CoO, at low temperature A. Barman, M. Ghosh, SK. De, S. Chatterjee zyxwvutsrqponmlkjihgfedcbaZYXWVU Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Materials Science, Indian Association for the Cultivation of Science, Jadaupur. Calcutta 700 032, India Received 7 February 1997; revised manuscript received 17 June 1997; accepted for publication 15 July 1997 Communicated by A. Lagendijk zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Abstract The resistivity and magnetoresistance of La,_,Ba,CoO, (0.1 5,~ I 0.4) systems have been investigated in the temperature range 1.8-300 K and magnetic fields up to 8 T. Electrical conduction is dominated by the spin state transition of Co ions and thermally activated electrons above 50 K (30 K) for x = 0.1 (z 0.2) and hopping electrons below 30 K for x 2 0.2. A large magnetoresistance has been observed for low doping concentration and it has been interpreted by the double exchange model. 0 1997 Elsevier Science B.V. 1. Introduction Since the recent discovery of giant magnetoresis- tance (GMR) [l-4] in La, _.A.MnO, (A = Ca, Sr, Ba, Pb), the perovskite oxides have drawn the atten- tion of researchers. In La,_,A,MnO, systems cre- ation of mixed valency of Mn (Mn3+/Mn4’) by divalent cation substitution causes profound effects on crystal structure, spin arrangement and on the transport properties. The double exchange (DE) mechanism 1.5-71 is usually used to interpret both the negative magnetoresistance and the ferromagnetic metallic behaviour in these materials. Recently, Mil- lis et al. [8] have shown that GMR cannot be ex- plained alone by DE interaction and they suggested that a strong electron-phonon coupling due in part to a Jahn-Teller distortion should be included. Besides the MnO, perovskites, the COO, com- pounds have also been investigated due to the obser- vation of similar properties [9- 141. The parent com- pounds LaMnO, and LaCoO, are insulators and their anomalous electric and magnetic properties are governed by the 3d orbitals of Mn and Co. The electronic structure of LaCoO, is more interesting than LaMnO,. In LaCoO,, the crystal field splitting between the t,, and eg states and the Hund coupling energy is comparable which causes a temperature dependent spin state transition between the high spin state t&e: (S = 2) and low spin state t&e: (S = 0) states of Co ions. Such a spin state transition causes an insulator to metal transition at temperature T > 570 K [15] in LaCoO,. The substitution of a divalent ion such as Sr for La (La, _,Sr,CoO,) produces the mixed valency of Co (Co3+/Co4+) ion which gives rise to interesting electrical and magnetic properties. Both the DE interaction and spin state transition mainly depend on the Co-O bond length [ 161. We report in this work low temperature electrical resis- tivity and magnetoresistance measurement of bulk La,-, Ba,CoO, (x = 0.1-0.4) in the temperature range 1.8-300 K and in presence of magnetic fields varying between 0 and 8 tesla. 037%9601/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO375-9601(97)00594-X