Ph.D. Oral Examination--Hyunsoo Yang

Hyunsoo Yang skier75 at stanford.edu
Wed Jan 4 23:02:27 PST 2006


Special University Ph.D. Oral Examination

"Metal spintronics: Tunneling spectroscopy in junctions with magnetic and
superconducting electrodes"

Hyunsoo Yang
Department of Electrical Engineering
Advisor:  Professor James S. Harris / Dr. Stuart Parkin

3:00 PM, Monday, Jan 9, 2006  Packard 202
(Refreshments served at 2:45 PM)


Recent advances in generating, manipulating and detecting spin-polarized
electrons and their electrical current make possible entirely new classes
of spin-based sensor, logic and storage devices. An important such device
is the magnetic tunnel junction (MTJ) which has been under intensive study
in recent years: important applications include nonvolatile memory cells
for high performance magnetic random access memory (MRAMs), and magnetic
field sensors for high density hard disk drive read heads. Many aspects of
the tunneling magnetoresistance (TMR) phenomenon are poorly understood
although it is clear that the fundamental origin of TMR is the
spin-polarization of the tunneling current. Thus, the measurement of the
magnitude and sign of the tunneling spin polarization (TSP) is very
important to help the further understanding of TMR.

Recently, an extremely high TMR value, of up to 350% at room temperature,
has been reported in practical MTJ devices. These MTJs are fabricated with
highly oriented crystalline MgO(100) tunnel barriers by straightforward
magnetron sputter deposition at room temperature. In parallel with this
observation, we report extremely high TSP values exceeding 90% from
CoFe/MgO tunnel spin injectors (using an aluminum superconducting electrode
as a spin detector at 250mK). These TSP values rival the highest
polarization values previously reported using exotic half-metallic oxide
ferromagnets.

The spin polarization of electrons extracted from ferromagnetic films can be
probed by a variety of techniques including photoemission, point contact
Andreev reflection, and superconducting tunneling spectroscopy (STS).
Amongst these techniques, STS is perhaps the most relevant with respect to
TMR but until now all measurements have been made with Al superconducting
films which have low superconducting transition temperatures (Tc) so that
the measurements must be made at temperatures below 400mK. We demonstrate
the use of superconducting electrodes formed from NbN which has a much
higher Tc (~16K) than Al.  The use of NbN allows measurements of TSP at
higher temperatures above 1K.

We have observed the phenomenon of Kondo-assisted tunneling in planar
magnetic tunnel junctions for the first time. We demonstrate not only an
increased conductance at low bias (due to the Kondo resonance) but also
show that the tunneling magnetoresistance is quenched in the Kondo regime.
The Kondo effect may be a useful means of detecting and possibly
manipulating the spins of individual electrons in nanodots.




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