Fast Dynamics Laboratory
Professor R. D. Kirby; Postdoctoral Research Associate (TBA)
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A critical aspect of materials for magnetic information storage is the understanding and characterization of the dynamical processes by which magnetization reversal occurs. The relevant time scales extend to the subnanosecond regime. Such studies are in their infancy, and so far have included only elemental or simple magnetic systems such as permalloy. The purpose of the W.M. Keck Fast Dynamics Laboratory is to extend such investigations to novel spin-logic structures and arrays, as well as a broad range of materials. |
A central component of the Keck Fast Dynamics Laboratory is a femtosecond laser
system which can be used to probe magnetization reversal and relaxation on subnanosecond
time scales. For most experiments, the magnetization state of the materials
or spin-logic structure will be probed by the Kerr magneto-optical effect, in
which the direction of the light polarization is changed upon reflection from
a magnetized surface. The magnitude and sense of the rotation is proportional
to the sample magnetization. A number of different kinds of experiments are
planned, including pump-probe, in which an initially intense laser pulse is
used to perturb the magnetic state of the system. The time evolution of the
magnetic state is then subsequently probed on fast time scales using less intense
laser pulses; fast magnetic field pulses can also be used to perturb the system,
and we will develop methods of doing this appropriate to our novel samples.
In addition to the femtosecond laser/parametric amplifier, the system requires
a digitizing oscilloscope, and optical components and mounts. We will use an
existing laser table and existing optical cryostats for measurements at low
temperature.
For Project 1,
the research includes NiFe dots prepared by Liou. Kirby will study the switching
speed and long term dynamics of the systems.
The magnetization reversal process in uniform films is reasonably well understood
for low magnetic anisotropy materials. In the low temperature limit, the reversal
process is well described by semiclassical models such as the finite element
Landau-Lifschitz-Gilbert model which takes into account interactions between
spins and includes phenomenological damping (which usually originates through
coupling between spin-waves and phonons). The situation is quite different for
nanoscale magnetic structures, especially if they are constructed of high magnetic
anisotropy materials. In the mesoscopic region, surface effects can predominate,
especially if the surface is rough. In the nanoscale region, the surface effects
become even more important, and quantum effects come into play. The interactions
between nanoelements (dipolar, exchange) can also strongly influence the time
scale of the reversal process. Finally, the effects of finite temperature can
be of great significance, with thermally assisted reversal dominating for most
high anisotropy materials at room temperature. Investigating these effects and
the interactions between magnetic nanoelements will be the primary focus of
this research.
For Project 2, a second central component of the Laboratory will be to measure fast conductance in junctions, spin valves, and other structures electronically, through magneto-resistance measurements. Such measurements have proved useful in spin-valve structures, and they will also be of great value in characterizing and understanding the properties of the nanojunctions proposed here. These measurements will make use of the fast digitizing oscilloscope, and require a picosecond pulse generator to provide the electrical excitation. This setup will require design and construction of a new probe for our existing high-field magneto-electronics cryostat.