Electrical and magnetic properties of non-stoichiometric (La0.8Sr0.2)1−xMnO3 perovskites

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Abstract

The structure, transport and magnetic properties of (La0.8Sr0.2)1−xMnO3 (0≤x≤0.30) polycrystalline perovskite manganites have been investigated. For all the samples the Curie temperatures, Tc, remain nearly unchanged (329±3 K). Resistivity versus temperature curves for the samples show a double-peak behavior. A significant magnetoresistance (MR) effect and different temperature dependences of the MR ratios of the samples are observed. The shapes of the MR–T curves of the samples can be adjusted by changing x. For the x=0.30 sample, a nearly constant MR ratio of (9.5±0.5)% is obtained over the temperature range from 205 to 328 K.

Introduction

The rare-earth manganite RMnO3 is an antiferromagnetic insulator, and presents a perovskite structure. When the trivalent rare-earth ion R in RMnO3 is replaced or partially substituted by a divalent alkaline-earth metal ion A, the compounds with the nominal composition R1−xAxMnO3 formed show a metal–insulator and ferromagnetic–paramagnetic transition at a certain temperature (Tp), and a colossal magnetoresistance (CMR) effect [1], [2], [3] due to the double-exchange (DE) ferromagnetic interactions between Mn3+ and Mn4+ [4], and the electron–phonon interaction arising from the distortions of MnO6 octahedra [5]. These special physical properties have been making them important targets of fundamental or applied research, especially in the field of magnetic recording and magnetic sensors, in the past ten years.

In fact, if the R content in A sites of a perovskite structure for RMnO3 is deficient, the coexistence of Mn3+ and Mn4+ and, thus, the CMR effect also will occur. So far, it has been found that the compounds La1−xMnO3, bulk materials and thin films, also have a significant CMR effect [6], [7], [8], [9]. However, analogous studies on the effect of the A-site deficiency on the CMR and magnetic properties of the non-stoichiometric (R1−yAy)1−xMnO3 system have not been reported. In addition, from the point of view of application, for example, for magnetic field sensors, it is required that the magnetoresistance ratio MR versus temperature T curve in a certain temperature range remains unchanged, especially near room temperature. But experiments show that it is hard to achieve this for most of the R1−xAxMnO3 compounds, because the MR ratio often has a maximal value near Tc. Therefore, improving the temperature stability of the MR ratio of these manganites over a wider temperature range is still an important task. In this work, we investigate the magnetic and transport properties of the nominal compounds (La0.8Sr0.2)1−xMnO3 (0≤x≤0.30).

Section snippets

Experimental procedure

A series of nominal (La0.8Sr0.2)1−xMnO3 (0≤x≤0.30) polycrystalline samples were prepared by the standard solid-state reaction method. Appropriate amounts of La2O3, SrCO3 and Mn3O4 powders were mixed and ground, and then pressed into discs. After being prefired in air at 1000 C for 15 h, the discs were broken and ground again, pressed into pellets under a pressure of 300 MPa, and sintered in air at 1200 C for 20 h. The phase purity of the sample was checked by means of powder x-ray diffraction

Results and discussion

The room temperature XRD patterns of the powdered Mn3O4 and (La0.8Sr0.2)1−xMnO3 (x=0.03, 0.15, and 0.30) samples are shown in Fig. 1(a). As can be seen from this figure, diffraction peaks corresponding to impurity phases occur significantly for all the (La0.8Sr0.2)1−xMnO3 samples, except the perovskite peaks for the rhombohedral structure. There are five obvious peaks for 2θ=28.96, 32.36, 36.06, 50.86 and 59.82 respectively in the XRD patterns for x=0.03, 0.15 and 0.30 samples; however,

Summary

In summary, the nominal (La0.8Sr0.2)1−xMnO3 (x≥0.03) system is one of diphasic compounds. The ρ(T) curves of these samples not only show a metal–insulator transition and paramagnetic–ferromagnetic transition, but also present double-peak behavior. All the samples (0.03x0.30) exhibit nearly the same Curie temperature, while the resistivity peaks at Tm well below Tc become more and more prominent with increasing value of x. At the same time, the resistivity peak near Tc declines gradually. The

Acknowledgements

This work was supported by the Natural Science Foundation of Anhui Education Office (No. 2006KJ270B) and the Master’s Scientific Research Foundation of Anhui Institute of Architecture & Industry (No. 2004-057).

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