Elsevier

Materials Letters

Volume 61, Issue 3, February 2007, Pages 767-769
Materials Letters

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

https://doi.org/10.1016/j.matlet.2006.05.057Get rights and content

Abstract

The results of the structure, electrical transport and magnetoresistance of a ferromagnet–ferroelectric-type La0.67Ca0.33MnO3 (LCMO)/BaTiO3 composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO3. The insulator-metal transition temperature (TIM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO3. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO3. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO3 and the associated magnetoelectric coupling effect.

Introduction

Perovskite based rare-earth manganites of the type Ln1−xAx MnO3 (Ln is rare earth, A is a divalent cation) have attracted much attention as they exhibit a colossal magnetoresistance (CMR) effect [1]. However, the intrinsic CMR effect in the perovskite manganites is only triggered within a narrow temperature range around the ferromagnetic transition at high magnetic fields of several teslas, which restrains its use for practical applications. Recently growing attention is being paid to polycrystalline manganites in which the grain boundary effects dramatically modify their physical properties [2]. An attractive feature for the polycrystalline manganites is a large MR at very a low magnetic field over a wide temperature range below TIM. Various attempts have been made to enhance the low field MR through controlling the grain boundary effect by forming composites of the CMR oxides with secondary phases such as an insulating oxide, a magnetic material, a metal, or with other CMR oxides [3], [4], [5], [6], [7]. However, composites composed of ferromagnetic and ferroelectric materials have rarely been investigated.

Recently interests have been renewed in multiferroic materials in which ferromagnetism and ferroelectricity coexist [8]. Wu et al. [9] reported a large electroresistance in LCMO with PbZr0.2Ti0.8O3(PZT)-ferroelectric gate. Hong et al. [10] reported that the ferromagnetic Curie temperature of ultrathin LSMO films was shifted to 35 K reversibly using the polarization field of the ferroelectric oxide PZT in a field effect structure. A recent study of the artificial superlattices composed of ferromagnetic and ferroelectric showed that MR can be enhanced due to the ferroelectric spacer layer and the associated magnetoelectric coupling [11], [12].

In this letter, we have synthesized La0.67Ca0.33MnO3/BaTiO3 composites by the sol-gel method and have characterized them for electrical transport and magnetoresistance properties. The TIM value increases from 185 to 230 K and MR is enhanced over the whole temperature range. These transport properties could be attributed to the introduction of BaTiO3 and the associated magnetoelectric coupling.

Section snippets

Experiment

The (1  x)LCMO/xBaTiO3 samples with x = 0, 1, 3, 5, 10 and 30 mol% were prepared by three steps. Firstly, the LCMO and BaTiO3 nanopowders were prepared by the sol-gel method [13], [14] taking stoichiometric amounts of La2O3, CaCO3, MnCO3, Ba(OH)2·8H2O and C16H36O4Ti, each of 99% purity, as starting materials. The powders were calcinated at 1100 °C and 1000 °C respectively for 12 h to form a perovskite phase. Secondly, appropriate amounts of LCMO and BaTiO3 nanopowders were mixed, ground and a

Result and discussion

Fig. 1 shows the XRD patterns at room temperature for pure LCMO, pure BaTiO3 and (1  x)LCMO/xBaTiO3 composites where x = 0.1. As one can see in the composites, small peaks related to BaTiO3 are clearly observed. This suggests that the BaTiO3 and LCMO are coexistent in composites.

Fig. 2 shows the electrical transport in (1  x)LCMO/xBaTiO3 composites. The BaTiO3 concentration dependence of TIM and the peak resistivity are shown in Fig. 3. The TIM value increases from 185 K for pure LCMO to 230 K for

Conclusion

We have synthesized LCMO/BaTiO3 composites by the sol-gel method. In addition to the shift of the TIM value to a higher temperature, the peak resistivity decreases with increasing content of BaTiO3, which could be explained by the magnetoelectric coupling effect induced by the introduction of BaTiO3. The low field MR is enhanced and it is triggered at the higher temperature in the LCMO/BaTiO3 composites, which suggests a kind of possible method for obtaining grain boundary magnetoresistance at

Acknowledgement

This work was supported by the National Science Foundation of China (Grant nos. 10374032 and 10574049).

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