Spin-polarized transport in magnetic nanostructures

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Title: Spin-polarized transport in magnetic nanostructures
Author: O'Gorman, Brian Curtin
Abstract: Two of the principal phenomena observed and exploited in the field of spintronics are giant magnetoresistance (GMR ) and spin transfer torque (STT ) . With GMR , the resistance of a magnetic multilayer is affected by the relative orientation of its magnetic layers due to (electron ) spin dependent scattering . For the STT effect , a spin -polarized electric current is used to alter the magnetic state of a ferromagnet . Together , GMR and STT are at the foundation of numerous technologies , and they hold promise for many more applications . To achieve the high current densities ( ~10¹² A /m² ) that are necessary to observe STT effects , point contacts – constricted electrical pathways ( ~1–100 nm in diameter ) between conducting materials – are often used because of their small cross -sectional areas . In this sense , we have explored STT in bilayer magnetic nanopillars , where an electric current was used to induce precession of a ferromagnetic layer . This precessional state was detected as an increase in resistance of the device , akin to GMR . Temperature dependent measurements of the onset of precession shed light on the activation mechanism , but raised further questions about its detailed theory . Point contacts can also be used as local sources or detectors of electrons . In this context , we have observed transverse electron focusing (TEF ) in a single crystal of bismuth . TEF is a k -selective technique for studying electron scattering from within materials . Using lithographically fabricated point contacts , we have studied the temperature dependence of the relaxation time for ballistic electrons from 4 .2 to 100 K . These measurements indicated a transition between electron -electron dominated scattering at low temperatures and electron -phonon scattering as the Debye temperature was approached . We present preliminary work toward a TEF experiment to measure spin dependent scattering from a non -magnet /magnet interface . We also investigated spin wave propagation in thin , magnetic waveguide structures . At the boundary between the waveguide and continuous magnetic film , spin wave rays were found to radiate into the film , or to reflect and form standing waves in the waveguide . A circular defect in the waveguide was observed to cause diffraction of spin waves , generating an interference pattern of higher modes of oscillation .
URI: http : / /hdl .handle .net /2152 /ETD -UT -2009 -12 -550
Date: 2011-01-19


Spin-polarized transport in magnetic nanostructures. Doctoral dissertation, The University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /ETD -UT -2009 -12 -550 .

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