PUB - Publikationen an der Universität Bielefeld

This thesis presents a novel spintronic design of a nano-sized high frequency oscillator, which is a cutting edge physics research field with a huge potential for future applications in the areas of mobile telecommunication and information- as well as in biomedical-technologies. In this device, a direct current flow through a nanocontact generates a spin polarized current in a spin-valve multilayer, which exerts a spin-transfer torque (STT) on the magnetic moments of a free magnetic layer. This torque can either cause the moments to switch their orientation or to drive them into a harmonic precession. The latter periodically modulates the voltage drop at the giant magnetoresistance (GMR) spin-valve multilayer patterned mesa in the sub-GHz frequency regime. Applications for the presented STT driven oscillator concept can be identified for example, as nano-sized high frequency source, as fast and non-volatile computer memory cell, or as nano-sized magnetic field sensor. The system provides clear advantages over the existing technological concepts of voltage controlled oscillators in terms of size, performance and dynamics. This work in particular investigates the spin-torque-related dynamics of nonuniform magnetic vortex-like states in spin-valve nanocontacts, employing an unpinned artificial antiferromagnet as polarizer and amorphous CoFeB as free layer. Subgigahertz spectra are obtained for nanocontact radii of 40 to 130 nm. Low critical current density and reversibility of the dynamic spectra with respect to the current are obtained and compared to numerical and analytical models. The oscillation frequency, spectral power density and linewidth depend on the in-plane magnetic field and reach extreme values (i.e. maxima and minima) close to the free layer magnetization switching field. For certain field and current regimes metastable dynamic states are clearly demonstrated. The influence of the magnetic free layer moment and of the nanocontact size on these spintorque related phenomena are investigated. Different GMR multilayers are fabricated and subsequently optimized for magnetotransport performance. In this work three particular architecture types are presented, which show both current induced switching of the magnetic moments and, in certain cases, sub-GHz excitation dynamics related to magnetic vortex precession. The latter could only be observed for multilayers with a low magnetic moment amorphous CoFeB free layer and only for contact radii below 100 nm. The current and frequency dependence of the oscillation on the point contact radius and magnetic eld is investigated and discussed with regard to a potential magnetic field sensor application in the nano scale. Additionally, multiple excitation modes are shown to exist at zero or small magnetic fields as a result of a highly inhomogeneous metastable magnetic state below the nanocontact. The results of this thesis clearly demonstrate the feasibility of a nano-sized oscillation source and provided a detailed study of the key fabrication-, measurement- and performance-factors. Further, a numerical and analytical description of the basic physics phenomena is presented.