Magnetic states and spin-wave modes in single ferromagnetic nanotubes

In this thesis the electrical properties, magnetic states and spin wave resonances of individual magnetically hollow ferromagnetic nanotubes have been studied. They were prepared from the different materials Nickel (Ni), Permalloy (Py) and Cobalt-Iron-Boron (CoFeB), deposited as shells onto non-magnetic Gallium-Arsenide (GaAs) semiconductor nanowires via Atomic Layer Deposition (ALD), thermal evaporation and magnetron sputtering, respectively. The resulting nanotubes had lengths between 10 to 20 μm, diameters of 150 to 400 nm and tube walls (shells) which were 20 to 40nm thick. Structural analysis of the tubes by Transmission Electron Microscopy revealed a poly(nano)crystalline (Ni, Py) and amorphous (CoFeB) structure. Electrical transport experiments as a function of temperature revealed different transport mechanisms for each of the materials. Electron-phonon scattering dominated the temperature dependence of the resistivity in Ni, while a clear evidence for electron magnon scattering was observed in Py. Electron-electron interaction in granular and amorphous media was identified as the major contribution to the temperature dependence in CoFeB. The Anisotropic Magnetoresistance (AMR) ratios have been determined for all tubes and different temperatures. Ni nanotubes exhibited a large relative AMR effect of 1.4% at room temperature. The AMR measurements provided information about the magnetic configurations as well as the magnetization reversal mechanism. Indications for the formation of vortex segments in Ni tubes were found for the magnetization reversal when the magnetic field was perpendicular to the nanotube axis. In cooperation with the Poggio group in Basel, cantilever magnetometry has been used for the further characterization of the nanotube magnetization. The magnetization curves were compared to the AMR measurements and finite element method (FEM) micromagnetic simulations. The comparison between the experimental results and the simulations suggested that the roughness of Ni tubes gave rise to segmented magnetic switching. An almost perfect axial alignment of the remanent magnetization has been observed in Py and CoFeB nanotubes. The influence of the inhomogeneous internal field in transverse magnetic fields was investigated by simulation. The segment-wise alignment of spins with the field direction is argued to provoke characteristic kinks in the hysteresis curve and measured AMR effect. Magnetothermal spatial mapping experiments using the anomalous Nernst effect (ANE) complemented the magnetotransport experiments in cooperation with the group of Prof. Grundler in Munich. Here, first evidence of end-vortices entering the nanotube before reversal could be found. Electrically detected spin wave resonance experiments have been performed in cooperation with the group of Prof. Grundler on individual nanotubes. The detected voltage, generated by the spin rectification effect, revealed multiple resonances in the GHz frequency. The experimentally observed resonances were compared to calculated ones extracted from dynamic simulations. With this comparison, the signatures could be attributed to azimuthally confined spin-wave modes. The deduced dispersion relation suggested the quantization of exchange dominated spin waves in that resonance frequencies follow roughly a quadratic dependence on the wave vector.

Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

Magnetic thin films and magnetic nanostructures have become essential components of modern technological applications. Modern branches of magnetisms focus on spin-charge coupling (spintronics) and the collective excitation of spin waves in magnetically ordered materials (magnonics). In particular, spin waves represent a promising charge-free medium to encode and transmit information in the GHz frequency regime, relevant for microwaves technologies. The ever-increasing demand for compact and portable electronic and telecommunication devices leads to the need for further miniaturization and high integration density of their functional components. These aspects have prompted nanomagnetism to venture the third dimension. This development is also motivated by novel physical phenomena and functionalities predicted to arise from three-dimensional (3D) complex magnetic configurations. In order to advance the research in 3D spintronics and 3D magnonics, it will be essential to have control over the fabrication of 3D nanoarchitectures and to access the new properties of individual nanostructures emerging from new complex 3D spin-textures. These two challenges are addressed in this thesis. Ferromagnetic nanotubes (NTs) represent the ideal 3D system for the study of shape dependent static and dynamic magnetic properties. Their magnetic configurations, as well as the spin wave confinement and propagation inside them, can be engineered by changing their geometrical parameters. First, in order to fabricate 3D magnetic device architectures, we explored atomic layer deposition (ALD). The thickness and quality of the thin films deposited with ALD are irrespective of the geometry of the surface on which they are deposited. This characteristic makes it ideal to fabricate 3D nanomagnets by magnetically coating 3D nanotemplates, e.g. nanowires (NWs) in the case of NTs. In this thesis we propose ALD processes for the conformal coating by means of two ferromagnetic materials conventionally used in planar spintronics and magnonics systems: Ni and permalloy (Py, Ni80Fe20), the second material expected to have a lower spin wave damping. By identifying correlations between ALD process parameters and thin film properties we optimized the materials in terms of conformality, electrical resistivity, anisotropic magnetoresistance (AMR) effect, spin wave damping and, in the case of permalloy, also stoichiometry. The optimized materials were then transferred onto GaAs nanowires templates, obtaining ferromagnetic NTs with hexagonal cross sections. We achieved Ni NTs with the lowest resistivity ever reported in similar ALD-grown Ni systems and a large AMR effect. We observed the lowest spin waves damping for permalloy NTs, exhibithing a value comparable with permalloy thin films achieved with conventional planar deposition techniques. Second, we investigated magnetic states and spin waves confinement in nanotubes experimentally and via micromagnetic simulations. We combined magnetoresistance measurements with Brillouin light scattering spectroscopy (BLS) and micromagnetic simulations to study Ni and Py NTs. BLS measurements, performed while irradiating NTs with microwaves, show that these nanotubes form spin-wave nanocavities which impose discrete wave vectors and confine GHz microwave signals on the nanoscale. Our experimental results show that the confinement can be engineered by changing the NT's geometrical parameters and magnetic states.

EPFL2021

Effect of Heat Current on Magnetization Dynamics in Magnetic Insulators and Nanostructures

Francesco Antonio Vetrò

The term "spin caloritronics" defines a novel branch of spintronics that focuses on the interplay between electron spins with heat currents. In the frame of this research area, this thesis is aimed at investigating the effect of a heat current on magnetization dynamics in two different typologies of systems and materials: magnetic insulators and metallic nanostructures. In the first case we conduct studies on yttrium iron garnet (YIG) samples subjected to a temperature gradient. The irreversible thermodynamics of a continuous medium with magnetic dipoles predicts that a thermal gradient across a YIG slab, in the presence of magnetization waves, produces a magnetic field that is the magnetic analog of the well known Seebeck effect. This thermally induced field can influence the time evolution of the magnetization, in such a way that it is possible to modulate the relaxation of the precession when applying a heat current. We found evidence for such a magnetic Seebeck effect (MSE) by conducting transmission measurements in a thin slab of YIG subjected to an in-plane temperature gradient. We showed how the MSE can modulate the magnetic damping depending on the direction of the propagating magnetostatic modes with respect to the orientation of the temperature gradient. In the second part of the thesis we focus our investigation on metallic nanostructures subjected to a heat current. In a metal, the three-current model (current of entropy, of spin up and spin down electrons) predicts that a heat current induces a spin current which will then influence the magnetization dynamics like a charge-driven spin current would. Hence, we explore what has been called Thermal Spin Torque in electrodeposited Co / Cu / Co asymmetric spin valves placed in the middle of copper nanowires. These samples are fabricated by conventional electrodeposition technique in porous polycarbonate membranes using an original method that allows high frequency electrical measurements. We used a modulated laser to investigate the effect of a temperature gradient. We observed a heat-driven spin torque by measuring electrically the quasi-static magnetic response of a spin valve when subjected to the heat current, generated by two laser diodes heating the electrical contact at one end or the other of the nanowire. Analysing the variation in the resistance induced by a heat-driven spin torque, represented by peaks occurring in correspondence with the GMR transition, we found that a temperature difference of the order of 5 K is sufficient to produce sizeable torque in spin valves.

EPFL2015

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

This thesis is dedicated to the study of the magnetization reversal dynamics in compressively strained (Ga,Mn)As and differently functionalized (Ga,Mn)As materials. In the first part the domain wall dynamics of pure (Ga,Mn)As/GaAs materials is in the focus. The changes in the magnetic anisotropy energy landscape occurring as a function of temperature are monitored in detail by different experimental techniques. These measurements provide the necessary information for the identification of the temperature ranges corresponding to different magnetic anisotropy regimes. Knowing the biaxial anisotropy and uniaxial anisotropy dominated regimes the dependence of the domain wall dynamics on the magnetic anisotropy landscape is studied. Critical changes in domain wall alignment observed by Kerr microscopy are found upon changing from biaxial to uniaxial anisotropy. These changes could be partially attributed to the tendency of the system to minimize magnetic free poles at the domain boundaries. To complement the space resolved studies of the magnetization reversal, magnetic time relaxation effects are addressed. In particular the magnetic aftereffect in (Ga,Mn)As/GaAs is studied in detail. The irreversible aftereffect is evidenced as a critical reduction of the domain wall velocity with time under a constant applied magnetic field below coercivity. This time relaxation is tracked as a function of both time and magnetic field. The measurements show that the overall relaxation is composed of two relaxation processes acting in parallel at short and long time scales. By fitting these experimental results the values of the relaxation times of both relaxation processes were obtained. By this modeling also the values of the activation volumes for two independent (Ga,Mn)As materials are estimated. In the second part of this thesis the possibilities of tuning the electrical and magnetic properties of (Ga,Mn)As by volume and surface treatments are explored. As for the volume modification, oxygen species were incorporated into the (Ga,Mn)As films by means of exposure to an oxygen plasma. The incorporation of oxygen was evidenced by depth profile X-ray photoelectron spectroscopy. This treatment is found to weaken the ferromagnetism, visible as a reduction in the Curie temperature, and to hinder the electrical transport evidenced as an increase in the electrical resistance. In agreement with theories accounting for the hole mediated ferromagnetism in (Ga,Mn)As this behaviour was related to a hole compensation mechanism arising from the presence of oxygen impurities sitting in interstitial positions of the (Ga,Mn)As Zinc blende structure. In the last part, the modulation of the electrical and magnetic properties via surface functionalization is presented. The effects produced by the adsorption of molecular species on (Ga,Mn)As are similar to those found after oxygenation, namely a strong weakening of the magnetic properties and an increase in the values of the electrical resistance. However, in this case the distinctive feature is provided by the possibility of regulating the hole quenching effect by exploiting the additional degree of freedom provided by the chemical properties of the adsorbates. The adsorbate chosen for these experiments are dye molecules that absorb light in the visible range, in this way allowing for a light modulated interaction with the substrate. Using this concept is was possible to observe a modulation of the ferromagnetic transition temperature, the coercive field and the electrical resistance upon illumination. These observations provide a proof of principle for the realization of photo-sensitized (Ga,Mn)As devices where the magnetic properties can be regulated by light.

EPFL2011