Selective Resputtering-Induced Magnetic Anisotropy in High-Density Magneto-optic Media

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Perpendicular Magnetic Anisotropy: Amorphous rare earth-transition metal (a-RETM) films are used as materials in magneto-optic (MO) sensors and as media for high-density MO disks. They possess perpendicular magnetic anisotropy (PMA), a unique property that allows written bits of information to align perpendicular to the plane of the storage disk. This orientation allows much higher density of information per unit area. Conventional magnetic storage media (e.g., Zip and computer hard-drive media) have bits that lie in the disk plane and therefore occupy a larger amount of space.

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 magneto-optic storage. To this day, they remian the industry's mainstay material. For their discovery, these authors were awarded the 1995 National Medal of Technology.¹

Remarkably, although these materials have been used in commercial magneto-optic devices, the physical mechanism underlying their most important properties have never been made clear. In amorphous materials, unlike their more common crystalline cousins, atoms are disordered in their relative placement to each other. As such, a magnetic property that is traditionally determined by crystalline order, such as magnetic anisotropy energy that preferentially aligns the magnetization vector within a material, becomes very small. In the rare earth-containing alloys (e.g., a-TbFe), this property is often large and spontaneously aligns perpendicular to the film plane. Since the rare earth atom's shape, determined by its valence charge cloud, is nonspherical (for Tb it is more football-like), some form of local electrostatic anomaly had been proposed as the source of this property. In 1992, NRL researchers measured the presence of local atomic arrangements in a-TbFe and showed that they provide such an electrostatic anomaly and give rise to PMA via a crystal field interaction.² The anisotropic atomic structure is described as a statistical preference for like-atom pairs parallel to the film plane, with a corresponding preference for unlike pairs perpendicular to the plane. This preference was of the order of 5 to 8% from the ideal isotropic amorphous environment and is broadly referred to as a pair-order anisotropy (POA). Using a similar approach, we now focus our efforts to discover the growth mechanism responsible for such anisotropic atomic arrangements.

Dynamics of Film Growth in Sputter-Deposition: By examining the energy of the RF plasma used in magnetron sputtering of a-TbFe, and comparing this to the energy required to remove atoms from the growing film, deposition conditions are determined in which atoms are selectively removed from the growing film. The conditions for selective resputtering are defined in terms of a plasma energy envelope, where plasma energies between 34 eV <= E_Ar <= 65 eV result in the selective removal of one specie of adatom over another from the surface of the growing film, resulting in POA.

Correlation of Atomic Structure, Plasma Energy, and Magnetic Anisotropy Energy: Extended X-ray absorption fine structure (EXAFS) measurements were performed on several films grown by using different working gas pressures (Ar gas) and RF power. These growth conditions allow for the systematic change of the plasma energy to different regions of the energy envelope, thereby allowing for an increase POA and PMA. Figure 11 plots the inplane and out-of-plane atomic environments of Fe atoms for a subset of these samples as Fourier-transformed EXAFS data. Comparing the in-plane and out-of-plane structure clearly shows that the POA changes as a function of the RF power. In Fig. 12, the PMA is plotted with the POA as a function of Ar ion energy. A strong positive correlation exists between the Ar ion energy and both the PMA and the POA. The projection of this curve onto the three, two-dimensional planes indicates an exponential relationship between MA, POA, and Ar E, with a linear relationship between Ar E and POA.

After nearly three decades of research, both the source of PMA and the mechanism by which it is incorporated in sputtered films are now understood. This advancement will directly lead to improved processing of MO materials for sensor and media applications. Furthermore, the improved understanding of the growth dynamics of sputtered films will allow greater optimization of materials processing as well as improved understanding of the role of anisotropic atomic structure in a broad range of materials systems.

Acknowledgments: This work was performed in collaboration with Dr. Taras Pokhil of Seagate Technologies, Inc. (Minneapolis, MN). EXAFS measurements were performed using the Naval Research Laboratory-Synchrotron Radiation Consortium beamline X23B at the National Synchrotron Light Source (Brookhaven National Laboratory).

[Sponsored by NRL and ONR]