High-frequency giant magnetoresistance measurement to study the dynamics of a micron-scale spin-valve sensor

We report on the study of the magnetization dynamic of a single micrometric spin-valve element. by high-frequency giant magnetoresistance measurement. We have combined transport measurements in the microwave range and large band ferromagnetic resonance with microantennas. The spin valve is used as an in situ demodulator to probe the individual dynamics of each layer and the relative phase of their oscillations. We deduce the conditions for coupling the hard and free layer dynamics and show the influence of a static magnetic configuration on the sensitivity of spin valves in the microwave range. This work offers a different approach for studying the dynamics of single isolated micron-sized systems.

Microfluidic magnetic particles manipulation

Amar Rida

In this thesis work we propose new concepts of transporting and manipulating magnetic particles on microfluidic scale. The objective is to propose solutions to the problems related to the integration of the magnetic inductive components within a microfluidic network for magnetic particle manipulation. A second part of this work is the study of the physics of magnetic particle pattern formation when a suspension of the particles is subjected to an external magnetic field. The first concept that we propose is based on the dynamic motion of a selfassembled structure of ferromagnetic beads that are retained within a microfluidic flow using a local alternating magnetic field. We show that an alternating magnetic field induces a rotational motion of the magnetic particles, thereby strongly enhancing the fluid perfusion through the magnetic structure that behaves as a dynamic random porous medium. The result is a very strong particle-liquid interaction that can be controlled by adjusting the magnetic field frequency and amplitude, as well as the liquid flow rate. This fact is at the basis of very efficient liquid mixing. We anticipate that the intense interaction between the fluid and magnetic particles with functionalized surfaces holds large potential for the development of future bead-based assays. The second concept is the transport of magnetic particles on microfluidic scales over long-range distances using an array of simple planar coils placed in large static magnetic field. This magnetic field imposes a permanent magnetic moment to the microparticles, so that a very small magnetic field gradient of a simple planar coil is sufficient to displace the microparticles. We demonstrate that the repulsive and attractive magnetic forces generated by adjacent strongly overlapped coils, allow an efficient and well controlled magnetic particle displacement along one-dimensional or two dimensional paths. Finally, we study the dynamics of supraparticles structure (SPS) patterns of a ferromagnetic particle suspension in an alternating magnetic field. We show that the application of an alternating magnetic field to a ferromagnetic particle suspension results in a phase separation of the supraparticles structures (SPS) patterns into periodic structures of columns. Moreover, we investigate the dynamic motion, relative to a microfluidic flow, of such magnetic SPS. The latter are locally manipulated with an alternating magnetic field that is transverse to a microfluidic channel. We show that the SPS in the microchannel are composed of weakly aggregated and open columnar-like structures, allowing large particles surfaces to be in contact with the fluid flow.

EPFL2004

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 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