Magnetic and electronic properties of La_1–xSr_xCoO₃ single crystals across the percolation metal-insulator transition

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H. M. Aarbogh,¹ J. Wu,¹ L. Wang,¹ H. Zheng,² J. F. Mitchell,² and C. Leighton¹
¹Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA
²Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

Received 9 June 2006; published 12 October 2006

Lightlydoped La_1–xSr_xCoO₃ single crystals (i.e., x0.15) have been intensively studiedprimarily due to the occurrence of spin-state transitions and magnetoelectronicphase separation. We report here the electronic transport and magneticproperties of more heavily doped crystals (0.15<x<0.30) with compositions clusteredaround the percolation metal-insulator transition (MIT). While the magnetic orderingtemperature and saturation magnetization increase gradually as x is increasedabove 0.17, the resistivity and coercivity show drastic decreases inthe range 0.17<x<0.20, due to the percolation and coalescence ofthe metallic ferromagnetic (FM) clusters that form in a semiconductingnon-FM matrix at low doping. The doping dependence of thecoercivity can be qualitatively understood in terms of the thermalstability of isolated FM clusters and the formation of domainsin percolated networks. The magnetoresistance shows a smooth crossover fromthe intergranular giant magnetoresistance (GMR)-type behavior that is dominant atlow x (due to spin-dependent transport between FM clusters) tothe conventional negative magnetoresistance occurring in the vicinity of theordering temperature at high x. These two mechanisms coexist inthe range 0.17<x<0.20, providing further evidence of the spatial coexistenceof insulating and metallic phases. The data suggest that althoughmagnetoelectronic phase separation certainly occurs, it may be active overa smaller doping range than in polycrystals. At a compositionbetween x=0.20 and 0.30 the signatures of phase coexistence inthe magnetotransport disappear. Interestingly, this is coincident with a transitionin the high-temperature (above the Curie point) transport from semiconducting-liketo metallic-like. The largest "colossal magnetoresistance-type" negative magnetoresistance effects onthe metallic side of the MIT are ~20% in La_1–xSr_xCoO₃,smaller than the equivalent manganites, due to the absence ofclose competition with a strongly insulating phase. The incompatibility ofsome of the relevant spin states and crystal symmetries witha static, coherent, Jahn-Teller distortion, and the absence of long-rangeantiferromagnetic order in the La_1–xSr_xCoO₃ phase diagram are likely keyfactors in this regard.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.74.134408
DOI:	10.1103/PhysRevB.74.134408
PACS:	75.47.Gk; 71.30.+h; 72.15.Gd 75.47.Gk Colossal magnetoresistance 71.30.+h Metal–insulator transitions and other electronic transitions 72.15.Gd Galvanomagnetic and other magnetotransport effects (metals/alloys) YEAR: 2006
KEYWORDS:	lanthanum compounds, strontium compounds, metal-insulator transition, percolation, phase separation, magnetic transitions, coercive force, ferromagnetic materials, giant magnetoresistance, Jahn-Teller effect

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