Enhanced magnetoresistance of La 2/3 Ca 1/3 MnO 3 /Sr 2 FeMoO 6 core/cell nanocrystalline composites synthesized by poly
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Hua Yang State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, People’s Republic of China
Qing Zhao Department of Physics, School of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
Changqing Jin and Richeng Yua) Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China (Received 6 September 2009; accepted 10 November 2009)
This paper reports that the homogeneous nanocrystalline LCMO(core)/SFMO(shell) and SFMO(core)/LCMO(shell) series composites are successively synthesized using polymer-network sol-gel method. With the increase of SFMO content in the composites, the remanence magnetization Mr increases while the coercivity Hc decreases. This fact indicates that the ferromagnetic phase boosts up. Moreover, the LFMR (1 T) of the composites succeeds the preponderances of both SFMO and LCMO; i.e., the magnetoresistance (MR) value increases from 300 to 5 K and keeps a high level. In particular, the MR value of the LS-8 composite reaches 55% at 5 K and 7 T.
I. INTRODUCTION
For many years, more and more attention has been paid to the Mn3+/Mn4+ mixed-valence state compounds RE1xAExMnO3 with the perovskite structure, where RE is a trivalent rare-earth cation and AE is a divalent alkaline-earth element, because of the existence of their extraordinary colossal magnetoresistance (CMR) phenomenon.1–4 Typically, considerable experimental and theoretical efforts have been made for a doped lanthanum-based manganite La0.67Ca0.33MnO3 (LCMO), whose prominent CMR effect has been quantified within the framework of the Zenner’s double-exchange interaction5 between Mn3+ (t2g3eg1) and Mn4+ (t2g3). In addition, from a laboratory viewpoint,2 the CMR behavior of LCMO could be optimized by changing the grain size into a nanometer scale. However, current studies of LCMO generally show that potential industrial applications meet obstacles since the observation of so-called CMR is only around the Curie temperature (Tc), and the large CMR value occurs at high external magnetic fields up to several teslas.6 To overcome the limitations of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0053 J. Mater. Res., Vol. 25, No. 2, Feb 2010
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narrow temperature range and large applied magnetic field, Hwang et al.7 discovered low-field intergranular magnetoresistance (IMR), as a result of Fermi-level electrons being fully spin-polarized for tunneling junctions across insulating grain boundaries (GB). Subsequently, in 1998 Kobayashi et al.8 reported on a sort of half-metallic ferromagnet Sr2FeMoO6 (SFMO) with an ordered double perovskite structure, which possesses a wide magnetoresistance (MR) temperature range. However, the previous works9,10 proved that the intrinsical influence of nanometer grain size in polycrystalline SFMO modifies remarkably its intrinsic
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