University PhD dissertation Defense for John Paul Strachan

John Paul johnpaul at stanford.edu
Wed May 9 09:56:16 PDT 2007


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*DEPARTMENT OF APPLIED PHYSICS*
*UNIVERSITY PhD DISSERTATION DEFENSE*
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*Time-Resolved X-Ray Imaging of Spin Transfer Torque in Magnetic 
Nanostructures*
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*John Paul Strachan*
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*Research Advisor: Professor Joachim Stöhr*
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* Thursday 10 May, 2007*
*10:30 A.M.*
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*Center for Integrated Systems, Room 101*
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*ABSTRACT*
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*Spin-torque (or spin transfer torque) is a novel phenomenon involving 
the transfer of angular momentum from a spin-polarized current to a 
ferromagnet. There is much excitement in the use of this effect for 
developing non-volatile, high density magnetic RAM, as well as for DC 
current-driven microwave oscillators.  Indeed, steady-state precessional 
modes as well as full magnetization reversal of nanoscale magnetic 
elements driven by spin-torque have been observed.  These observations 
have been via giant magneto-resistance measurements, using a reference 
"fixed" magnetic layer, which also serves as the spin-polarizer.  Given 
the experimental challenges in probing thin, buried nanomagnets, the 
detailed magnetic configuration of the element has remained unknown. 

I describe a high resolution, time-resolved x-ray microscopy technique 
to image directly the nanomagnet during the switching process.  Motion 
pictures with 200 ps time resolution and 35 nm spatial resolution reveal 
that the process is based on the transient formation of a vortex 
configuration.  A vortex is a magnetic pattern analogous to the wind 
pattern of a hurricane.  The vortex moves across the magnetic element, 
driven by the spin-current and leaving behind a switched magnetization 
in its wake.  A physical understanding of this novel mechanism is 
discussed, as well as the dependence on sample size and shape.  It is 
seen that other switching mechanisms may dominate for smaller length 
scales.*


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