Mobility Measurement in Nanowires Based on Magnetic Field-Induced Current Splitting Method in H-Shape Devices

This work investigates a new method to measure mobility in nanowires. Based on a simple analytical approach and numerical simulations, we bring evidence that the traditional technique of Hall voltage measurement in low dimensional structures such as nanowires may generate large errors, while being challenging from a technological aspect. Here, we propose to extract the drift mobility in nanowires by measuring a variation of the electric current due to the presence of a magnetic field, in a specific nanowire network topology. This method overcomes the limitations inherent to the standard Hall effect technique and might open the way to a more precise and simple measurement of mobility in nanowires, still a matter of intensive research.

Spin dependent thermoelectric effects in magnetic nanostructures

Santiago Serrano Guisan

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.

EPFL2006

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

Growth and optical, electrical and mechanical characterization of zinc oxide nanowires

Martial Duchamp

The topic of this dissertation is embedded into the new-born field of inorganic nanowires. The research was focused on zinc oxide (ZnO) nanowires, in particular, which besides posing fundamental questions in physics, promise broad range of applications. ZnO nanowires were grown by chemical-vapour-deposition (CVD) using the vapour-liquid-solid (VLS) mechanism with gold particles as catalysts. Oxygen was introduced in the argon carrier gas and zinc was carboreduced from the ZnO/C mixture. We produced micrometer long ZnO nanowires and their diameter was controlled by the size of the gold catalyst particles. The diameter range of our ZnO nanowires is between 30 to 100nm. After growth, the alloyed particles were found at the top of the nanowires. ZnO nanowires are composed of single crystals of ZnO in the wurtzite phase. Growth direction is defined by their epitaxial relation with the substrate. From photoluminescence (PL) and cathodoluminescence (CL) characterizations, a deep impurity energy level at 2.3eV was found in a band gap of 3.3 eV, which could be related to oxygen off-stoichiometry. PL measurements showed that this energy level does not depend on the oxygen concentration used during the growth. Scanning-Tunnelling-Microscopy-based CL (STM-CL) measurements were made on single ZnO nanowires and showed that this energy level is likely to come from curved nanowires. Electrical measurements were performed on single nanowires, by using dielectrophoresis to place individual ZnO nanowires at a predefined location. E-beam lithography was used to deposit four metallic contacts on top of individual micrometer long ZnO nanowires. In order to avoid oxidation and the Shockley effect, we used chromium nitride (CrN) as electrical contacts. By measuring the temperature dependence on the resistance we extracted the activation energy of 2eV for charge transport, possibly arising from the same impurity level, as measured in PL. In the field-effect-transistor (FET) configuration, by using PMMA as dielectric layer and Cr as gate electrode, we extracted the charge carrier mobility of an individual ZnO nanowire as being 9.5 cm2/(Vs). In order to discover the origin of the energy level in the band gap, we measured the relaxation time of the current, after UV-illumination was switched off, and we found out that the surface states have significant effects. The elastic properties of individual ZnO nanowires were studied as a function of diameter and point defect/dislocation concentration. Using dielectrophoresis, nanowires were placed over a hole of a microfabricated Si3N4 membrane and, from bending measurements using an atomic force microscope (AFM) the Young's modulus was extracted. The diameter dependence of the Young's modulus showed a maximal value for a diameter of 50nm. Annealing under argon atmosphere showed an improvement of the Young's modulus which is explained by the removal of the defects/dislocations left during the growth. For comparison, the elastic properties of ZnS nanotubes produced by atomic layer deposition (ALD) were also measured. A clear dependence of the Young's modulus on the diameter and the wall thickness were found and attributed to the variation of the crystal structure of the nanotubes.

EPFL2010