Fwd: University PhD Dissertation Defense for Venkatesh H. R. Chembrolu

Venkatesh Chembrolu cvenky at stanford.edu
Sat Apr 19 18:24:21 PDT 2008

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Department of Applied Physics
University PhD Dissertation Defense

Time Resolved X-ray Imaging of Magnetization Dynamics in Spin 
Transfer Torque Devices

Venkatesh Hanumant Rao Chembrolu

Research Advisor: Professor Joachim Stohr

21 April 2008 @2:30 p.m.
McCullough Building, Room 335

The study of spin-dependent transport phenomena in ferromagnetic 
metals has gained a lot of interest amongst both theoretical and 
experimental physicists in the last two decades. In addition to being 
a rich field of study in terms of basic research, these
studies have given rise to, and are continuing to give rise to some 
very important technological applications. Discovery of the Giant 
Magneto Resistance (GMR) effect in 1988-89 revolutionized the read 
head technology for hard drives, thus enabling much higher storage 
density than before. The proposal and the subsequent experimental 
conformation of the existence of Spin Transfer Torque (STT) effect in 
ferromagnetic multilayers promises to be a good candidate for 
developing high density, non-volatile, magnetic random access memory 
(MRAM) and tunable DC driven microwave oscillators. The subject 
matter of study in the thesis work being presented is investigation 
of magnetization switching mechanisms in STT devices.

In the first part of this talk, I will present the results of a novel 
time resolved X-ray microscopy technique, developed to study 
magnetization dynamics at fast time scales (< 1 ns) and at short 
length scales (< 100nm). Experiments have been performed at the 
Advanced Light Source (ALS), Berkeley, and they show systematic size 
dependent trends in the switching behavior in STT devices. Samples 
with a lateral dimension of 100x180nm show vortex-driven switching, 
whereas smaller samples with a lateral dimension of 100x150nm do not 
switch by a vortex. Moreover, when a non-zero angle in introduced 
between the fixed and the free layers, vortex-driven switching 
becomes manifest in samples with smaller dimensions also. In the 
second part of the talk, I will briefly discuss the results of 
micro-magnetic simulation studies and also present a simple 
phenomenological model which explains the different switching 

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