Current Induced Magnetization Dynamics in Electrodeposited Nanostructures

In this thesis we make use of the Spin Transfer Torque effect from a continuous microwave current to induce and study the spin dynamics of an individual sub-100 nm nanostructure. The idea that an electrical current can carry a spin angular momentum was introduced in the 1980 with the advent of Spintronics. In 1996, Slonczewski and Berger predicted that, when flowing through a metal ferromagnet, such a spin polarized current can exert a torque on the magnetization. This effect is known as Spin transfer Torque (STT) and implies the possibility to manipulate the magnetization by means of an electrical current. One of the characteristics of the STT effect is that it allows for the magnetization dynamics to be excited and detected electrically. This makes the STT a perfect tool for the investigation of spin dynamics at the nano-scale. Indeed, the intrinsic local character of transport measurements permits a single nanostructure to be addressed and studied. This thesis is devoted to the study of the STT effect induced by a microwave current (AC STT). Our experiment focuses on two main aspects: the intrinsic dynamics of the magnetization and the role of a microwave current in assisting the magnetization reversal. We perform our measurements on Co/Cu nanowires electrodeposited in nanoporous polycarbonate templates. The electrical contact to a nanostructure is realized thanks to a home-made sample-holder, without the use of any lithographic technique. In the first part of the thesis a continuous microwave current is used to both drive and probe the magnetization dynamics of a pseudo spin valve nanostructure. Our measurements show that the magnetization dynamics in such a structure is extremely complex and arises from the excitation of both magnetic layers in the pseudo spin valve. In the dynamical spectrum we identify the fundamental modes of both magnetic layers, as confirmed by macrospin simulations. Higher frequency modes are attributed to spatially non-uniform spin wave excitations, in agreement with that observed by others in arrays of nanoelements. These results validate our technique for making samples and contacting them for AC STT-induced dynamical studies. The second part of this work is aimed at testing the effect of AC STT on the static switching field of magnetic nanoelements. The magnetoresistive curve of a pseudo spin valve structure is recorded with and without an additional microwave current. Our results prove the effectiveness of AC STT in assisting the magnetization reversal. We measured a reduction of the switching field up to 80 mT by injecting a microwave current of about 100 µA. This experiment suggests that the range of sensitivity of magnetoresistive devices could be tuned by simply injecting a continuous microwave current. This point is particularly interesting for the prospect of technological applications such as magnetic sensors.

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

Thermal Spin Transfer Torque and Transverse Spin Relaxation in Spin Valves

Haiming Yu

Spin caloritronics, i.e., the addition of thermal effects to the electrical and magnetic properties of nanostructures, has recently seen a rapid development. It has been predicted that a heat current can exert a spin torque on the magnetization in a nanostructure, analogous to the well-known spin-transfer torque induced by an electrical current. In this thesis, I provide the experimental evidence for this effect in spin valves, showing the switching field change with heat current. I present measurements of the second harmonic voltage response of Co-Cu-Co pseudo-spin-valves deposited in the middle of Cu nanowires. I exploit the quasi-1D nature of the nanostructures to generate a heat current by way of asymmetric Joule heating in the Co layers. Both the magnitude of the second harmonic response of the spin valve and the field value of the maximum response are found to be dependent on the heat current. Both effects show that the magnetization dynamics of the pseudo-spin-valves is influenced by the heat current. Thus, the data provide a quantitative estimate of the thermal spin torque exerted on the magnetization of the Co layers. In the last chapter, I measured the second harmonic response to an oscillating current crossing an exchange-biased spin valve as a function of the angle between the magnetization vectors of the fixed and free layers. This signal characterizes the linear response of the magnetization to a torque. A diffusive model is shown to predict that this angular dependence is quite sensitive to the value of the transverse spin diffusion length λJ , i.e. the decay length of the spin polarization which is perpendicular to the magnetization. Our observations imply a value of about 6 nm for CoFe.

EPFL2011

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.