Researchers at NRL and Seagate
Technologies,
Minneapolis, MN, have resolved a 28-year-old mystery
concerning
the materials that are used for information storage
in
high-density magnetooptic (MO) disks. These materials, amorphous
rare earth transition metal films, all have the property known
as. The scientists from NRL and Seagate Technologies have discovered
the source of perpendicular magnetic anisotropy in these
materials.
In
perpendicular magnetic anisotropy,
written bits of information
align perpendicular to the plane
of the storage disk. This
orientation allows for a much higher
density of information per
unit area of disk. Conventional magnetic
storage media, such as
computer hard drive media, rely upon bits
that lie in the disk
plane and therefore occupy a larger amount
of
space.
This class
of materials, amorphous
rare earth-transition metal films, were
discovered in 1973 by
IBM researchers P. Chaudhari, J.J. Cuomo,
and R.J. Gambino. These
materials ushered in the modern era of
high-density magnetooptic
storage and remain to this day the
industry's mainstay material.
For their discovery, these
authors were awarded the 1995 National
Medal of
Technology.
Remarkably, although these materials
have been
used in MO disks, the physical mechanism underlying
their most
important properties has never been made clear."This
problem has been at the forefront of this technology for the
past 28 years," explains NRL researcher, Dr. Vince Harris.
It is this mechanism that scientists from NRL and Seagate
Technologies
have discovered.
In amorphous materials, unlike
their
more common crystalline cousins, atoms are disordered in
their
placement with respect to each other. As such, magnetic
properties that are traditionally determined by crystalline order,
become very small. In the rare earth containing alloys, this
property is often large and spontaneously aligns perpendicular
to the film plane. Since the rare earth atom's shape is
non-spherical,
some form of local electrostatic irregularity
had been proposed
as the source of this property.
In 1992, a research team led
by
Dr. Harris of NRL using the National Synchrotron Light Source
at Brookhaven National Laboratory, Upton, NY, measured the presence
of local atomic arrangements that could provide such an
irregularity.
Working with Dr. Taras Pokhil of Seagate
Technologies, they
have revealed how such arrangements
form.
In a series
of experiments performed
over the past 8 years, Drs. Harris and
Pokhil examined the energy
of the plasma used in radio
frequency magnetron sputtering of
such materials and compared
this to the energy required to remove
atoms from the growing
film. They determined that for some deposition
conditions,
atoms are selectively removed from the growing film.
This
selective removal, or results in anisotropic atomic arrangements
that provide the local anisotropic electrostatic field that acts
on the rare earth ions.
Ironically, the IBM researchers
proposed a similar
model as early as 1973, but without the advances
made in
synchrotron radiation sources and characterization techniques
the model was never substantiated.
Using extended x-ray absorption
fine
structure spectroscopy at the National Synchrotron Light
Source, Drs. Harris and Pokhil showed that the density of such
anisotropic atomic arrangements scale with the magnetic anisotropy
energy and the plasma energy that maximizes the selective
resputtering.
After three decades of experimental and
theoretical research,
both the source of perpendicular magnetic
anisotropy and the
mechanism by which it is incorporated in
sputtered films are
now understood.
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