Epitaxial growth of half-metallic magnetic oxide thin films by pulsed laser deposition

Resumen

Great efforts have been directed towards the research field of Spintronics in the last years after the discovery of the Giant Magnetoresistance effect (GMR). An essential issue in this field is the obtention of magnetoresistive devices such as Magnetic Tunnel Junctions (MTJs) with half-metallic ferromagnetic materials as active electrodes at room temperature. Such materials have an energy gap at the Fermi level (EF ) for one spin direction, therefore, showing a spin polarization of 100%, which optimizes the output signal of the spintronic devices. In order to obtain devices fully operative at room temperature, materials with a high Curie temperature, TC, are required. However, as only a few oxides, semiconductors and metallic alloys are candidates to exhibit half-metallicity, finding new half-metallic materials is nowadays of great interest. Promising materials are, for example, magnetite and double perovskite oxides. In addition, the problem of the magnetization thermal fluctuation in high-density magnetic storage systems is of recent concern, and could be solved with materials with high magnetocrystalline anisotropy, such as the L10 - ordered FePt. The growth of thin films and heterostructures based on these materials is an issue of great interest. The epitaxial growth can be achieved by using suitable monocrystalline substrates with similar lattice parameters, by means of different deposition techniques such as laser ablation , sputtering, molecular beam epitaxy, or chemical deposition. The properties of the materials grown by means of these techniques can be tuned depending on different parameters such as the film-substrate lattice mismatch or the epitaxial growth coherence along the film thickness, which offers interesting possibilities. The objective of this Thesis has been the growth and characterization of thin films and heterostructures of half-metallic magnetic materials, in which the nanometric-scale phenomena are relevant, with potential applications in magnetoelectronics. Most of the systems studied have been produced by means of a Pulsed Laser Deposition (PLD) and sputtering system. PLD is a suitable deposition technique for the growth of high-quality oxide thin films, and as a PLD-sputtering system by Neocera Inc. company is available at the Instituto de Nanociencia de Aragón (INA) in Zaragoza (Spain) and fully operative since June 2005, this system will represent the fundamental instrument of the Thesis. This book is composed of five chapters, being the first one dedicated to the description of the experimental techniques used in this research, giving an special importance to the PLD technique. The second chapter is devoted to the study of magnetite epitaxial thin films. Firstly, the influence of the deposition conditions and thickness on the structural, magnetic and electrical transport properties of magnetite films grown on MgO substrates are studied. The magnetotransport measurements of the Fe3O4 thin films were widely and thoroughly studied, allowing the publication of four research papers. Finally, the origin of the enhanced magnetic moment found in magnetite ultra-thin films (t < 20 nm) was studied and identified, and as a consequence, a research article has been very recently published. The growth and complete characterization of double pervoskite Sr2CrReO6 (SCRO) thin films deposited on commercial SrTiO3 substrates is detailed in chapter three of this thesis. The obtained results were published in a research article, and a second paper is in process to be submitted. The fourth chapter deals with the growth and characterization of L10-ordered FePt epitaxial thin films, grown at the Instituto dei Materiali per l'Èlettronica ed il Magnetismo (IMEM), Consiglio Nazionale delle Ricerche (CNR) at Parma, during a short stay in the research center. Finally, the fifth chapter is devoted to the growth and thorough characterization of MgO (100) // Fe3O4 / MgO / FM heterostructures, where FM can be Fe3O4, Fe or FePt, in order to obtain adequate systems for devices with applications in magnetoelectronics. Finally, preliminary TMR results of MTJs obtained, by means of optical lithography, at the research center INESC-MN in Lisbon (Portugal) are presented.