Spin dependent thermoelectric effects in magnetic nanostructures

The aim of this thesis work was to obtain a deeper understanding of spin-dependent transport by the study of mixed effects of heat and charge currents in magnetic nanostructures. Indeed, even thermodynamics establish a relationship between both currents, no previous works were performed in this sense up to now. In order to do this, an original experimental setup was developed which allows to carry out a novel transport measurement called the thermogalvanic voltage (TGV). It measures the AC voltage response of the sample driven by an oscillatory variation of the temperature induced by a chopped laser beam focused on the nanostructure under a direct current flow. The main advantage of this new experimental setup is that it can be used to perform three different transport measurements: resistance, thermoelectrical power and TGV measurements. Furthermore, the well-defined junctions of our magnetic nanowires, in addition to the lock-in detection of signals, allows to obtain a better quality of thermoelectrical measurements than those observed by other groups. Different kind of magnetic nanowires like ferromagnetic/non-magnetic multilayers and ferromagnetic homogeneous nanowires were studied by these three different transport measurements. All these nanowires were 6μm in length and 50nm in diameter. All TGV measurements reveal a linear dependence with currents. This behaviour can be interpreted in terms of adiabatic conductivity for homogeneous nanowires and in terms of Peltier effect for multilayer nanowires produced in ferromagnetic/non-magnetic interfaces. Furthermore, the magnetic field dependence of TGV (MTGV) cannot be accounted for by GMR and MTEP effects. Therefore, a novel spin-dependent mechanism is invoked in such measurements. A simple phenomenological model, ascribed in the framework of thermodynamics of irreversible processes, was developed to explain this effect. It allows to identify this phenomena to a novel spin-dependent variable: The asymmetry of spin-mixing mechanisms (ΔL). This variable will be related to the difference of transition rates between both spin channels (τ-1+- and τ-1-+) developed in ferromagnetic material due to electron-magnon interactions. A large magnetic field dependence of TGV signals were observed in magnetic granular systems. Moreover, MTGV shows a different dependence from GMR on magnetic field, grain size and temperature. This effect is identified as a spin-mixing mechanism produced by the electron spin precession about the magnetic grain moments.

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

Search for nuclear magnetic resonance evidence of spin accumulation

Aurore Rudolf

Developments towards an understanding of the nature of conductance at the interface between two different metallic layers – ferromagnetic and non magnetic – as well as the discovery of giant magnetoresistance have stirred attention from both the scientific community as well as the electronics industry, with the prospect of using the spin of electrons in fast, nano-sized electronic devices. The spin polarization obtained by driving a current through a magnetic nanostructure is referred to as "spin accumulation". In this thesis work, we use 59Co zero-field nuclear magnetic resonance (NMR) under an additional current in order to probe spin accumulation. To the best of our knowledge, no local measurement of spin accumulation has yet been reported. NMR was chosen as the most appropriate technique for this study, as it is a local probe of the electronic structure and its dynamics. In the first part of this thesis, we present the main characteristics of NMR in ferromagnets. We also discuss the effect expected in the NMR due to spin accumulation. In the second part, we describe and characterize the different samples that we investigated during this study: granular samples, multilayered nanowires and micro-pillars. The third part is devoted to the description of 59Co NMR lineshapes obtained for different samples. In addition, we measured the spin-spin and spin-lattice relaxation times in the Co/Cu granular samples. This study is of tremendous importance to choosing the best candidate sample for NMR experiments under additional current. Finally, we used a method of electrically excited and electrically detected ferromagnetic nuclear resonance (EDFNR) developed during the course of this work. This technique presents several advantages over standard FNR: neither coil nor resonant circuit is needed and, overall, EDFNR opens the possibility of studying samples with much fewer nuclei than is achievable with standard nuclear resonance methods. This could provide an important development to decrease the current needed for the same current density in the samples, which is of great interest due to the smaller magnetic field created by the reduced current. EDFNR was applied to measure the nuclear resonance in the samples under an additional applied current. The current densities achieved were high enough to induce measurable effects in the EDFNR spectra, which are interpreted in the final part.

EPFL2009

Spin mixing processes in magnetic nanostructures detected by thermoelectric measurements

Jean-Philippe Ansermet, Giulia Di Domenicantonio, Santiago Serrano Guisan

Spin-dependent transport properties of magnetic nanostructures have been investigated by means of magneto-thermogalvanic voltage measurements: the ac voltage response to an ac temperature oscillation is measured for various magnetic nanostructures under dc current. The samples studied include Co/Cu multilayered nanowires, homogeneous Ni nanowires and cobalt clusters embedded in copper films. The magnetic field dependence of this signal is always larger than the magneto-resistance (MR) and may be anisotropic even when the MR is not. A thermodynamic argument introduces spin mixing as the main process measured by this novel thermoelectric measurement technique. This effect is not observed in magnetite as can be justified by the absence of an accessible second spin channel.