Heterostructures based on L10-FePt for spintronics and magnetic recording.

This Thesis is focused on the design, growth and characterization of thin films and heterostructures, based on FePt, with potential applications in magnetic recording and spintronics, where the nanometric scale phenomena play a important role. With this aim, heterostructures with peculiar properties like high perpendicular anisotropy, exchange-spring effect, magnetoresistance and spin polarization have been developed. The studied systems have been produced mainly by means of physical deposition techniques, such as radio frequency sputtering and pulsed laser deposition. The structural, morphological and magnetic studies have been performed through a variety of techniques, allowing a constant feedback with the design and preparation of materials. This work has involved a considerable effort to reach a better understanding of various and innovative scientific aspects. The study of new nanocomposites recording media, based on FePt, has highlighted the dependence of the exchange-spring behavior on the soft magnetic layer properties. For example, varying its thickness and chemical com- position it is possible to change the effect of exchange interaction between hard and soft layers. Furthermore, the lattice mismatching with the substrate, which influences the morphology and chemical order of the nanostructure, can be exploited to tailor the magnetic behavior. Moreover, ion irradiation on L10-FePt thin film has been studied as possible technique to develop exchange-spring media with suitable characteristics closely defined by the irradiation parameters. The objective is to study innovative per- pendicular recording media that exploit the exchange-spring interaction between a hard and soft magnetic phase, in bilayers or graded systems, to increase the storage density in future hard disks. In spintronics, the original choice of ferromagnetic electrodes (i.e., L10-FePt and Fe3O4) and their magnetization configuration in the heterostructure for magnetic tunnel junctions (MTJs) has led to deepen the extrinsic properties effects on the electrical transport through the tunnel barrier and on tunnel magnetic resistance. The inverse tunneling effect, as expected for Fe3O4/MgO interface, has been studied as a function of film thickness and junction area. The perpendicular direction between the easy magnetization axes of the electrodes gives rise to a suitable configuration in spin-torque devices. The first part of my work (chapter 3) starts describing the optimization of thin films growth conditions, with emphasis on L10-FePt films grown on MgO and SrTiO3 substrates, studying the correlation among morphology, magnetism, crystalline order and substrate lattice mismatch. Chapter number 4 describes the next implementation of FePt layers with optimized properties, tailored interfaces and morphology in nanos- tructures for future recording media. Moreover, a numerical micromagnetic model is proposed to describe the reversal magnetization mechanism in these systems. The final chapter (number 5) is focused to the growth and characterization of heterostructures, based on L10-FePt, and their lithography process in order to obtain MTJs with new characteristics for application in spintronics. The first chapter presents some basic concepts of magnetism, in order to understand the underlying physics and a brief summary of the employed experimental techniques is given in the second chapter.