NRL Researchers Resolve 28-Year-Old Mystery

10/4/2001 - 32-01r
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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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