Transport phenomena and magnetism in nanostructures of lanthanum manganite-based oxide thin films

Resumen

Lanthanum manganite-based oxides conform an extensive family of compounds de- riving from LaMnO3: a perovskite with general formula ABO3. Its physical properties can be drastically modified by cation substitution. In this thesis we explore two compounds obtained by A-site and B-site substitutions. On the one hand, La2Co0.8Mn1.2O6 (LCMO), obtained by substitution of Mn by Co (B-site substitution), leads to a double perovskite structure with ferromagnetic insulating properties. On the other side, the La0.7Sr0.3MnO3 (LSMO) compound resulting from the partial substitution of La by Sr (A-site substitution) turns the material into a ferromagnetic half-metal. The development of techniques for the growth of oxide materials in the form of thin films ease their integration in semiconductors technology and enable the de- sign of micro and nanoscale devices with potential in spintronics and non-volatile memory applications. Nowadays, there is an increasing interest in the study of double perovskite thin films combining ferromagnetic and insulating properties due to their relatively high transition temperature and their integration on top of other perovskite oxides. The implementation of ferromagnetic insulators (FM-I) as substitutes of ferromagnetic metal-based devices can lead to much higher switching efficiency and less dissipative current transport. For instance, ultrathin films of (FM-I) might be used as spin-selective active tunneling barriers acting as spin-filters. Therefore, the main objective of the first part of this thesis has been to give a comprehensive understanding of LCMO physical properties and propose a guide to potential applications. Firstly, we present a detailed study of the growth of the LCMO films as well as their structural and magnetic characterization. We demonstrate that we can obtain high quality thin films, with transition temperatures up to 230 K and with good cationic order despite a certain degree of o -stoichiometry. Then, we analyze magnetic anisotropy effects, focusing on the following aspects: i) the study of the appearance of strong perpendicular magnetic anisotropy (PMA) with large coercive and anisotropy fields, and ii) the relation of magnetic anisotropy strength and sign with lattice mismatches. In particular, we show that PMA appears for the tensile strain case while compressive strain produces in-plane easy axis. We also give a more detailed understanding of the origin of magnetic anisotropy using a simple atomistic model based on rst-order perturbation theory calculations. We relate our predictions with X-ray magnetic circular dichroism (XMCD) experiments and evidence that magnetic anisotropy in LCMO has a magnetocrystalline origin due to the strong spin-orbit coupling of Co2+ ions. With the aim of integrating ferromagnetic insulating properties and PMA in a device, we have fabricated tunnel junctions using LCMO as a magnetically active barrier and have explored its spin-derived functionalities. We have found that the device provides high spin-filtering efficiency (of almost 100% of spin-polarization) as well as anisotropic sensing and memory functionalities. This is, the strong strain- induced PMA provokes a large difference between magnetoresistance curves measured with the magnetic field applied in the perpendicular or parallel directions, this phenomenon is the so called tunneling anisotropic magnetoresistance (TAMR). TAMR values as high as 20-30% have been found. Finally, we have proven that the device can be used as a magnetic memory as we can detect the existence of two non-volatile resistive states that switch depending on the direction of the magnetic field used to write them. The last part of thesis presents results focused on the A-substituted manganite, LSMO, thin films. We show that growth instabilities can lead to the formation of double-terminated surfaces. Indeed, deviations from the ideal growth behaviour constitute a way to obtain spontaneously formed nanostructures with modulated local functional properties at the surface. The transport properties and the composition of these films have been analyzed by making use of scanning probe techniques and space-resolved photoemission electron microscopy, which are surface-sensitive techniques suitable to characterize properties at the nanoscale of this type of systems.