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Magnetic Excitations Induced by Surface Acoustic Waves and Spin-Polarized Currents

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Universitat de Barcelona. Facultat de Física

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[eng] The goal of this thesis is to explore and control the magnetization dynamics on magnetic multilayered thin films through two different techniques: the application of strain and spin- polarized currents, which represent lower-power consumption approaches to the control of magnetization dynamics compared with conventional techniques.

The ferromagnetic materials with nanometric thickness used in this thesis are magnetic materials widely used in research. Aside the purely scientific interest, these materials are potentially applicable in telecommunications or technologies for storing and transmitting information at high speeds.

1.	Magnetization Dynamics Induced by the Application of Oscillating Strain

The first part of the thesis studies the magnetization dynamics induced by the application of dynamic strain on the magnetic material. The strain deforms the magnetic material and induces a change in the direction and intensity of the magnetic anisotropy. Therefore, the magnetic states are affected by this variation and align with the new direction of magnetic anisotropy inducing dynamics in the magnetization.

The main result of the first part of the thesis is the simultaneous time- and space-resolved observation of both the piezoelectric voltage wave associated to the SAW and the induced magnetization excitations on the ferromagnetic thin film of Nickel (Ni). We have found that manipulation of magnetization states in ferromagnetic thin films with SAWs is possible at the picosecond scale with efficiencies as high as for the static case. In Chapter 3 we have studied Ni nanostructures whose magnetization dynamics are governed by the intrinsic configuration of the magnetic domains and by their orientation with respect to the SAW- induced strain resulting in considerable delays between strain and magnetization. In Chapter 4 we have studied extended Ni thin film, on which SAWs induce spin waves that propagate millimeter distances and have a rotation amplitude of about 25 deg.

2.	Magnetization Dynamics Induced by the Spin-Polarized Current

The second part of the thesis studies the magnetization dynamics induced by the application of spin-polarized current through the magnetic material that exchanges magnetic moment with the magnetic spins of the electrons in the current. The current density has to be high to induce dynamics on the magnetization (~106-107 A/cm2) and this results in a reduction of the diameter of the electrical contact 50-200 nm.

The main results of the second part of the thesis are related with the stability and the nucleation process of magnetic solitons. On the one hand, we have showed that magnetic solitons can exhibit multiple stable states, which are tunable with current or magnetic field. We also have correlated the existence of unstable states with an increment of low- frequency noise. Using simulations, we have identified the low-frequency spectra with the existence of drift resonances and we have observed that any asymmetry on the effective magnetic field suffered by the magnetic soliton can leads to drift resonances. On the other hand, we have experimentally observed that the processes of nucleation and annihilation of magnetic solitons have different intrinsic times, and using simulations we have identified a waiting time associated with the creation process, which make it a longer than annihilation. We also have studied, using micromagnetic simulations, the initial magnetization states that lead to the nucleation of topological and non-topological magnetic solitons.

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[spa] La tesis gira en torno al estudio de la dinámica de la magnetización en capas y multicapas delgadas ferromagnéticas. Sin embargo, los sistemas estudiados son diversos y pueden clasificarse por la técnica utilizada para la excitación de la dinámica de la magnetización. Este hecho queda plasmado en la estructura de la tesis que consta de una introducción general, Capítulo 1, y luego de dos partes independientes y separadas, a su vez, en varios capítulos. El orden en la exposición de los resultados pretende seguir una linea lógica para su compresión. Como contrapartida, los resultados son presentados sin seguir un orden cronológico.

La primera parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de tensión dinámicamente sobre el material magnético, que al deformarlo induce en él un cambio en la dirección e intensidad de la anisotropía magnética. Por lo tanto, los estados magnéticos se ven afectados por esta variación y cambian para alinearse con la nueva dirección de anisotropía magnética induciendo dinámica en la magnetización.

La segunda parte de la tesis estudia la dinámica de la magnetización inducida por la aplicación de corriente polarizada a través del material magnético que intercambia momento magnético con los espines magnéticos de los electrones de la corriente. Para que esta transferencia de momento magnético sea efectiva la densidad de corriente ha de ser elevada (~106-107 A/cm2) y para conseguirla se reduce hasta los 50-200 nm el diámetro del contacto eléctrico.

Los materiales ferromagnéticos con grosor nanométrico usados en esta tesis son materiales magnéticos usados ampliamente en la investigación. Aparte del interés puramente científico, estos materiales son potencialmente aplicables en telecomunicaciones o tecnologías del almacenaje y transmisión de información a altas velocidades.

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Magnetic Excitations Induced by Surface Acoustic Waves and Spin-Polarized Currents