Phase competition driven temperature broadening of colossal magnetoresistance in La0.815Sr0.185MnO3
Introduction
The phenomenon of colossal magnetoresistance (CMR) when it was discovered in doped manganites A1−xBxMnO3 (A = La3+, Nd3+, Pr3+ and B = Ca2+, Ba2+, Sr2+) in 1993 [1] held great promise for making sensitive magnetic devices and enhanced magnetic storage. However, the intense research activity that followed has led to widespread agreement that the CMR arises as a result of competing phases and the ensuing intrinsic nano-microscale inhomogeneity [2]. While this has unearthed a fertile area of basic research, it seems to have dissuaded applied researchers. The present report is an account of our ongoing efforts to manipulate the competing phases and inhomogeneity to extract desirable device-worthy features.
An important criterion for making a magnetosensitive device is that the observed MR should not vary appreciably with temperature over the range of ‘room temperatures’ observed around the globe. Since the MR in manganites exhibits a peak across the concomitant paramagnetic–ferromagnetic and insulator–metal transitions, it is a strong function of temperature. To the best of our knowledge, there is only one report that has addressed the problem of broadening the temperature dependence of MR by using a heterostructure of fused polycrystalline samples of (La1−yPry)0.7Ca0.3MnO3 [3].
We report preliminary results of magnetotransport and magnetic susceptibility studies on TiO2: La0.875Sr0.185MnO3 mixtures in an attempt to address this issue. La0.815Sr0.185MnO3 was chosen because of its proximity to the ferromagnetic metal (FMM)–ferromagnetic insulator (FMI) phase boundary (making it susceptible to disorder induced phase competition) in the phase diagram but with a high Curie temperature (294 K).
Section snippets
Experiment
Polycrystalline La0.815Sr0.185MnO3 (LSMO185) samples were prepared by conventional solid-state reaction route. The powder obtained was mixed with TiO2 in the mole ratio of 1: x where x = 0, 0.01, 0.03 and pressed into pellets and sintered again in air at 1100 °C for 3–4 h. AC susceptibility measurements were performed using a Sumitomo AC susceptometer (Sumitomo Heavy Electric Co., Japan). Magnetotransport measurements using the four probe method in a 1 T electromagnet.
Results and discussion
The temperature dependence of resistance for the three samples (x = 0, 0.01 and 0.03) is shown in Fig. 1. It is seen that the resistivity peak observed at 313 K for LSMO185 decreases to 288 K for (x = 0.01)TiO:LSMO185 sample and to 227 K for the (0.03)TiO:LSMO185 sample. The temperature dependence of the AC susceptibility for the three samples is shown in Fig. 2. The paramagnetic–ferromagnetic transition temperature (TC) decreases from 294 K for LSMO185 to 263 and 234 K for (0.01)TiO:LSMO185 and
Acknowledgments
The authors would like to thank N. Rama, N. Sivaramakrishnan, J. Arout Chelvane, J. Sarala Shanti, M. Kottaisami, C. Krishnamoorthy for help with the measurements and useful discussions.
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Present address: Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea.