Magnetoresistance, temporal evolution, and relaxation of the electrical resistivity in the re-entrant semiconducting La
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Magnetoresistance, temporal evolution, and relaxation of the electrical resistivity in the re-entrant semiconducting La0.80 Ba0.20 CoO3 perovskite R. D. S´anchez,a) J. Mira, and J. Rivas Dpto. F´ısica Aplicada, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain
M. P. Breijo and M. A. Se˜nar´ıs-Rodr´ıguezb) Dpto. Qu´ımica Fundamental e Industrial, Universidad de A Coru˜na, 15071 A Coru˜na, Spain (Received 26 January 1998; accepted 12 February 1999)
We report here a study on the electrical and magnetic properties of La12x Bax CoO3 in the re-entrant semiconducting region (x 0.20). We find that in this material: (i) the insulator-metal-insulator sequence is unstable and evolves toward a purely semiconducting behavior; the initial r versus T curve can be reinstated upon appropriate annealing treatments; (ii) there are relaxation effects that can be seen by changing the polarity of the electrodes; (iii) there is a negative magnetoresistance Dryr , 2–3%, for a field as low as 9 kOe, especially at the metal-insulating transition temperatures; and (iv) there are important fluctuations in the electrical resistivity. Taking into account these experimental observations, we can interpret this material as an inhomogeneous system where two thermodynamic phases, one semiconducting and the other metallic and ferromagnetic, coexist, although they are crystallographically indistinguishable.
I. INTRODUCTION
Early studies on the magnetic and electrical properties of magnetic semiconductors such as EuSe and EuTe showed that their particular properties can be described as a consequence of their heterogeneity due to a two-phase segregation. This segregation forms ferromagnetic (FM) microregions (called ferrons, giant quasimolecules, or magnetic polarons) within an antiferromagnetic (AF, T , TN ) or paramagnetic (PM, T . TN ) phase.1 This idea was used by some authors to explain the peculiar transport properties in the copper oxide superconductors.1,2 More recently, it has been used to interpret the colossal magnetoresistance observed in manganese perovskites near the Curie temperature.3–5 Another perovskite system where a phase segregation has been reported is La12x Srx CoO3 . The extreme member of the series with x 0, LaCoO3 , shows peculiar magnetic and transport properties due to a thermally induced spin transition from low-spin Co(III) st26 e0 d to mostly high-spin Co31 st24 e2 d.6–8 The complete evolution and interpretation of both its electrical and magnetic properties as a function of temperature have been recently revised.9 Upon Sr doping, very noticeable changes take place in the material10 : while LaCoO3 shows high resistivity a)
Permanent address: Centro At´omico de Bariloche – Carrera de Investigador del CONICET (Argentina). b) Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
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and antiferromagnetic exchange interactions, the materials La12
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