Complex Oxide Heterostructures for Spin Electronics

Abstract

The direct manipulation of spin currents with no charges involved along with the delicate interplay between charge, spin and orbital degrees of freedom in materials and structures has attracted a renewed interest in the last few years. Technologically speaking, one of the most attractive features of pure spin currents is the disappearance of Joule heating, which is directly linked to the amazing problem of heat dissipation in today’s microelectronic devices. As such, the generation, manipulation, and detection of pure spin currents is still one of the major challenges in spintronics nowadays. In this regard, the charge-to-spin current interconversion processes play a very relevant role and having the set of materials and structures that maximize these processes is of paramount importance. In this Thesis, the generation, manipulation, and detection of pure spin currents in ferromagnetic/normal metal bilayers is addressed. The use of manganese-based complex oxide perovskites is especially relevant: complex oxide materials are very appealing from the technological viewpoint due to their inherent multifunctional nature. The capabilities and versatility of ferromagnetic resonance (FMR) spectroscopy for disclosing the crucial magnetodynamical parameters of our samples are foreseen by first studying the archetypal bilayer composed by Ni80Fe20/Pt. Simultaneously, since the generation of pure spin currents by means of spin pumping is also triggered by microwave absorption during FMR, the effective generation of pure spin currents is demonstrated via ISHE in ferromagnetic/normal metal bilayers. The generated pure spin current is converted to a transverse charge current (via ISHE) in the high spin-orbit coupling (SOC) Pt capping layer and detected as a transverse voltage difference across the film. In this Thesis, the dual nature of FMR spectroscopy is highlighted and a novel method for suppressing parasitic voltage contributions to the overall ISHE voltage signal is presented. Finally, in order to gain insight into the behavior of these parasitic effects, a numerical study based on our actual experimental system is performed. The suitability as spin current injectors/detectors of three different complex oxide heterostructures, namely La0.92MnO3 (LMO)/Pt, La2/3Sr1/3MnO3 (LSMO)/Pt and LSMO/SrIrO3 (SIO), is also investigated. The latter is especially relevant in the context of this Thesis for it is the only all-oxide complex structure evaluated. Unlike any other film studied, LMO films were prepared by polymer assisted deposition. In this regard, there has been some concerns regarding the suitability of chemically deposited thin films for challenging applications which require microstructural quality and sharp interfaces, as in spintronics. Nevertheless, the structural high-quality and efficient spin injection capabilities of chemically deposited LMO films for a wide range of temperatures are demonstrated. Similarly, the spin injection performance of sputtered LSMO films in LSMO/Pt bilayer systems is shown. In a second step, a material with presumably large SOC, such as SIO, is introduced as spin detector, and the spin injection/detection processes in the all-oxide heterostructure LSMO/SIO are addressed. In this case, it is demonstrated the promising role of SIO as an efficient spin-to-charge converter, thus opening the door to the development of high-quality spin-to-charge conversion devices based on all-oxide heterostructures. Finally, this Thesis also focuses on the measurement of a rather different spintronic effect: spin Hall magnetoresistance (SMR). In this magnetoresistive effect, a pure spin current is generated via spin Hall effect (SHE) in Pt and directed towards the La2CoMnO6 (LCMO)/Pt interface. The interaction between this spin current and the magnetization of the ferromagnetic insulating thin LCMO film is at the heart of SMR. The experimental observations exemplify the exceptional role played by the interface. Additionally, a theoretical model is employed for interpreting the results and it is inferred that the magnetic state of the uppermost layers of the LCMO film are actually magnetically decoupled from the rest of the film, exhibiting a behavior compatible with a 2D Heisenberg ferromagnet, with no long-range magnetic order nor transition temperature.

La manipulació directa de corrents d'espí sense necessitat de càrregues juntament amb l'entrelligament entre els diferents graus de llibertat – de càrrega, d'espí i orbital – en materials i estructures ha esdevingut un tema de gran interès. Tecnològicament, una de les característiques més atractives dels corrents purs d'espí és la desaparició de l'escalfament per efecte Joule, el qual està directament connectat al gran problema de la dissipació de calor present als aparells electrònics actuals. Com a resultat, la generació, manipulació i detecció de corrents purs d'espí és encara un dels grans reptes de l'espintrònica. Així mateix, els processos d'interconversió entre càrregues i espins juguen un paper molt rellevant, i el fet de tenir els materials i les estructures que maximitzen aquests processos és del tot crucial.

El primer bloc experimental es concentra en l'ús de microones per a la generació de corrents purs d'espí mitjançant el bombeig d'espins en bicapes ferromagnet/metall normal. Aquests corrents són detectats mesurant el voltatge induït per l'efecte Hall d'espí invers (ISHE). En particular, un nou mètode per a suprimir els senyals paràsits presents al voltatge mesurat és utilitzat en la coneguda bicapa Ni80Fe20/Pt. Els resultats obtinguts són recolzats amb càlculs numèrics basats en l'estructura. A continuació, la capacitat de generació i injecció de corrents d'espí a les bicapes La0.92MnO3/Pt, La0.67Sr0.33MnO3/Pt, i La0.67Sr0.33MnO3/SrIrO3 és demostrada de forma anàloga. En aquest cas, i) es demostra la capacitat per a usar capes crescudes per mètodes químics (La0.92MnO3) en aplicacions espintròniques que requereixen alta qualitat estructural; i ii) es demostra la capacitat i eficiència del SrIrO3 per al seu ús en processos d'interconversió espí-càrrega. Aquest resultat obre la porta al desenvolupament d’aparells d’alta qualitat per a l’espintrònica basats en heteroestructures d’òxids complexos.

Finalment, el segon bloc experimental se centra en mesures de magnetoresistència Hall d'espí (SMR) en bicapes formades per La2CoMnO6/Pt. En aquest cas, és evidenciat el paper crucial jugat per la interfície. Mitjançant l'aplicació d'un model teòric, es demostra que l'estat magnètic superficial de la capa La2CoMnO6 és diferent i està desconnectat de la resta de la capa: aquest, en comptes de ser ferromagnètic, es comporta com un ferromagnet 2D de Heisenberg, el qual no posseeix ordre magnètic de llarg abast ni temperatura de Curie.