Role of boundaries on low-field magnetotransport properties of La 0.7 Sr 0.3 MnO 3 -based nanocomposite thin films
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en-Fong Tsai and Qing Su Materials Science and Engineering Program, Texas A & M University, College Station, Texas 77843
Quanxi Jia Center for Integrated Nanotechnologies (CINT), Division of Materials Physics and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Judith L. MacManus-Driscoll Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom (Received 16 November 2012; accepted 29 March 2013)
The effects of boundaries such as grain boundaries and phase boundaries on low-field magnetoresistance (LFMR) have been investigated in single-phase lanthanum strontium manganates, in this case La0.7Sr0.3MnO3 (LSMO) and LSMO: zinc oxide (ZnO) nanocomposite thin films. In the pure LSMO films with similar grain size, it is found that the LFMR increases as the grain misorientation factor (b) increases. The LFMR in the nanocomposite films is greatly enhanced, as compared with single-phase films, due to the reduced grain size, and increased phase boundary (PB) and b effects. The composition study shows that the LFMR can be dramatically enhanced when the secondary phase content approaches the percolation threshold. The increased b and secondary phase concentration reduce the cross-section of electron conduction paths and favor the formation of the quasi-one-dimensional transport channels. Our results demonstrate that the reduction of cross-section of the electron conduction paths by tuning the grain orientation and secondary phase composition is necessary for enhancing LFMR effect.
I. INTRODUCTION
In nanostructured thin film materials, boundaries play a significant role in determining the electrical, optical, thermal and magnetic properties.1–3 Especially, the extrinsic low-field magnetoresistance (LFMR) effect, induced by either spin-polarized tunneling or spin-dependent scattering at the grain boundaries (GBs) of manganite bulks and thin films, has stimulated considerable interest in the past decades.3–6 The GBs, decoupling the neighboring ferromagnetic (FM) grains and serving as energy barriers for the tunneling, are the key for the enhanced LFMR effect.7–11 Doping of a secondary phase at the boundaries is an effective strategy to further increase the LFMR properties because doping not only creates additional artificial GBs but also further increases the height of tunneling barriers due to the magnetic decoupling of FM phases.12–18 a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2013.89 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
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The enhancement of the spin-dependent tunneling and scattering at the boundaries is considered responsible for the increased LFMR effect.19–22 There is no doubt that boundaries play critical roles in the LFMR
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