Fluctuations de l'aimantation induites par des courants de haute densité dans des vannes de spin électrodéposées

Copper wires each one containing a pseudo-spin valve of Co/Cu/Co were produced by electrodeposition inside nanoporous polycarbonate membranes. These wires have a diameter of 40 nm and a length of 6000 nm of which only 50 nm relates to the spin valve. In order to be able to measure the magnetic behavior of a single wire, the entire deposition was carried out in a Co/Cu multibath and then electrical contact of a single wire was done with a special equipment using a pure Copper bath. Knowing that such wire has a resistance of the order of 500 Ω, with a Giant Magneto-Resistance ratio of about 0.2 %, one needs a sufficiently sensitive measuring apparatus be able to detect such variations. After having carried out a magnetoresistive characterization, we used a set-up built with a Wheatstone bridge in which the sample is integrated. The resistance variation associated with the magnetization reversal being weak, we were very concerned with the sample quality during these three years of research. As high current densities are injected, a behavior of "Two Level Fluctuation" appeared between parallel and antiparallel states. Such phenomena can be treated only by statistics. Therefore, several thousands of measurements were taken in order to be able to understand the statistical behaviors of the current induced magnetization switching. To better understand the mechanisms of the reversal, we studied its dynamics with a model of double potential well by measuring the average time of residence in parallel (respectively in antiparallel) state as a function of current and applied field for different amplitudes and signs. From our analyses, it arises clearly that the dynamics of the magnetization reversal induced by a high current density, in pseudo-spin valves, could be formulated in terms of Spin-Transfer-Torque. We show that the agreement with this model is remarkable.

Current Induced Magnetization Dynamics in Electrodeposited Nanostructures

Elena Murè

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.

EPFL2010

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

Current-induced magnetization switching in pseudo spin-valves

Jean-Philippe Ansermet, Santiago Serrano Guisan

The spin transfer torque model applied in the context of a Fokker-Planck analysis (Li and Zhang 2004 Phys. Rev. B 69 134416) is shown to account for a complete set of statistical data for switching times obtained with pseudo spin-valves (Fabian et al 2003 Phys. Rev. Lett. 91 257209). Current densities of the order of 10(7) A cm(-2) injected in Co/Cu/Co bilayers electrodeposited in nanoporous membranes gave rise to magnetization switching. Statistics could be accumulated on one single nanowire at a time: the field at which the average residence times in parallel and antiparallel configurations were equal, these times as a function of current, and the ratio of the times as a function of current and field.