Project 2: Nanoscale Spin-Transport Systems
Professors B. Doudin (Coordinator), P. A. Dowben, E. Tsymbal, S.-H. Liou; Drs. A. Sokolov, K. Belachtchenko; GRA L. Rosa, C. Ilie, URA P. Jacobson
This work is important for the development of a new generation
of devices incorporating electronic spins into existing semiconductor technology,
providing nonvolatility, increased processing speed, decreased power consumption
and increased integration densities.
The aims:
- obtain new information about the physics governing electronic properties
of magnetic heterosystems at the nanoscale
- test new designs and ideas for novel spintronic devices.
We are using original synthetic methods to make nanometer-sized magnetic materials
expected to have enhanced spin transport properties.
Understanding and utilizing spin polarization and
transport through nanoscale ordered and disordered interfaces are major goals
for future spin-electronics technologies. A recent development is room-temperature
tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs). Advances
in the fabrication of MTJs, have demonstrated that TMR has tremendous potential
for future magnetic sensors and magnetic random-access memories (MRAM). Several
industrial laboratories, such as IBM, Motorola, Seagate, and Hewlett-Packard
are currently pushing ahead vigorously in the field of MTJs. The potential of
these devices could be similar to GMR devices, provided that a deep understanding
of the underlying physical mechanisms that control MTJs is achieved. In order
to be competitive, future generations of magnetic sensors and memory elements
must be on a length scale of 10 nm or smaller. Such small dimensions will involve
new physical processes that govern the magnetoresistive phenomena. A typical
example is the ballistic conductance of magnetic nanojunctions. Experiments
performed on atomic-size contacts fabricated from Ni nanowires have shown that
the magnitude of magnetoresistance can be as high as 3000% at room temperature.
Moreover, very recent experiments indicated that by manipulating the contact
shape and structure electrochemically it is possible to achieve any desired
value of magnetoresistance. The physical mechanism causing this phenomenon is,
at present, unknown.
The magnetic configuration in a device can alternatively be controlled by the
electric current, if the torque transferred by a spin-polarized current on the
magnetic nanoparticle is large enough to switch its magnetic orientation. Furthermore,
if the device is made of particles of different volumes, one can change the
relative orientation of the particles by changing the polarity of the current.
The critical parameter is the current density, which means that the necessary
critical electric current decreases like the area of the device. Spin transfer
becomes a parameter of importance when the device size becomes smaller than
100 nm, and is the dominant factor for sub-10 nm dimensions. More experiments
in this size range are therefore of primary importance for future devices.
Our goal is to combine experimental and theoretical expertise to obtain sufficient
insight into the properties of nanojunction materials to be able to design them
with specific properties. We are using original synthetic methods to make magnetic
materials of nanometer sizes, for which enhanced ballistic spin-transport properties
are expected. We are combining spin-resolved photoemission and spin-resolved
inverse photoemission with electron-transport investigations for gaining access
into the surface electronic states and their spin dependence. Our goal is to
obtain new information about the physics governing electronic properties of
magnetic heterosystems at the nanoscale, as well as to test new designs and
ideas for spintronics devices of tomorrow.
2.2. Spin Transfer in Tunnel Junctions