Evaluation of the effect of various mechanisms on the magnetoresistance of lanthanum manganites La 0.85 Sr 0.15 MnO 3 wi
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ISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Evaluation of the Effect of Various Mechanisms on the Magnetoresistance of Lanthanum Manganites La0.85Sr0.15MnO3 with Activation-Type Conductivity S. A. Gudin, M. I. Kurkin, E. A. Neifel’d, A. V. Korolev, N. N. Gapontseva, and N. A. Ugryumova Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620041 Russia e-mail: [email protected] Received July 10, 2015
Abstract—A method is proposed that allows one to divide the magnetoresistance (MR) observed in manganites into three mechanisms: dimensional, orientational, and magnetic. The first two mechanisms are associated with the stratification of a substance into ferromagnetic and nonferromagnetic phases, which significantly differ in electric resistivity. The dimensional mechanism of MR is attributed to the effect of a magnetic field on the size of magnetic inclusions. The orientational mechanism of MR is determined by the dependence of electric resistivity on the mutual orientation of the magnetizations of magnetic inclusions. The magnetic mechanism of MR is determined by the properties of the magnetization of a ferromagnet, in particular, by the Curie–Weiss singularity on the temperature dependence of magnetic susceptibility at the Curie point. This mechanism exists in homogeneous substances, although its value may depend on the magnetic properties of inhomogeneities. The method is developed for substances with activation-type conductivity and is applied to the analysis of MR of La0.85Sr0.15MnO3 manganite near the Curie point, where the MR attains its maximum. The dimensional mechanism turns out to be dominant in magnetic fields H greater than the saturation field Hs (H > Hs). The orientational, dimensional, and magnetic mechanisms have a comparable effect on the MR for H < Hs. The effect of the orientational mechanism on MR is relatively weak (does not exceed the third part of the total MR), although this mechanism determines the giant MR in multilayered metal films. The possibility of application of the method to the analysis of MR near the insulator–metal transition is analyzed. DOI: 10.1134/S1063776115120079
1. INTRODUCTION The dependence of the electric resistivity ρ on the magnetic field H (magnetoresistance (MR)) is one of the effects used in devices for converting magnetic signals into electrical signals. This effect is responsible for the interest in searching for substances with large values of the ratio (1) Δρ/ ρ = (ρ(0) − ρ(H ))/ ρ(H ). Today, the maximum values of Δρ/ρ ≫ 1 are observed in lanthanum manganites doped with bivalent ions of Sr, Ca, and Ba [1]. The MR of these compounds has been called colossal magnetoresistance (CMR). Various properties of such manganites were discussed in the surveys [2–7]. It is assumed that the CMR is attributed to the insulator–metal transition because such a transition is accompanied by a significant variation in ρ. Here the problem is how to ensure the effect of the field H on this transition. Such a possibility is a
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