Electrochemical deposition and characterization of Fe 3 O 4 films produced by the reduction of Fe(III)-triethanolamine
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In this paper, we demonstrate that films of magnetite, Fe3O4, can be deposited by the electrochemical reduction of a Fe(III)-triethanolamine complex in aqueous alkaline solution. The films were deposited with a columnar microstructure and a [100] preferred orientation on stainless steel substrates. In-plane electrical transport and magnetoresistance measurements were performed on the films after they were stripped off onto glass substrates. The resistance of the films was dependent on the oxygen partial pressure. We attribute the increase in resistance in O2 and the decrease in resistance in Ar to the oxidation and reduction of grain boundaries. The decrease in resistance in an Ar atmosphere exhibited first-order kinetics, with an activation energy of 0.2 eV. The temperature dependence of the resistance showed a linear dependence of log(R) versus T −1/2, consistent with tunneling across resistive grain boundaries. A room-temperature magnetoresistance of −6.5% was observed at a magnetic field of 9 T.
I. INTRODUCTION
Magnetite (Fe3O4) is a half-metallic metal oxide with the inverse spinel structure and space group Fd3m. Below the Curie temperature of 860 K, magnetite exhibits ferrimagnetism. The octahedral sites are shared by Fe3+ and Fe2+ ions, while the tetrahedral sites are occupied by Fe3+ ions. The moments from the Fe3+ ions cancel, yielding a net ferrimagnetism due to the moments on the Fe2+ sites. The material is a promising candidate for magnetic memory and spin-dependent transport devices because it has a calculated spin polarization of 100% at the Fermi level.1,2 A negative spin polarization for Fe3O4 has also been experimentally verified by spin polarized photoemission spectroscopy3,4 and by magnetoresistance measurements on Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions.5 Magnetoresistance devices have been produced based on Fe3O4 in which spin-polarized electrons are injected across thin tunnel barriers,6 resistive grain boundaries,7–9 antiphase boundaries,10 and nanocontacts.11 a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal 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/publications/jmr/policy.html. DOI: 10.1557/JMR.2006.0030 J. Mater. Res., Vol. 21, No. 1, Jan 2006
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Magnetite films have been deposited onto polycrystalline and single-crystal substrates using pulsed laser ablation,12–15 molecular beam epitaxy,16–18 and by oxidizing Fe thin films.19,20 Our interest is in the epitaxial electrodeposition of metal oxide thin films onto singlecrystal substrates.4,21–33 We have previously electrodeposited epitaxial films of Fe3O4 onto single-crystal Au substrates using anodic deposition.4,23 The method was a modification of the ferrite plating process developed by Abe and co-workers.9,34,35 Ansermet and co-workers have recently used the anodic deposition method to produce n
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