This article is one of three papers to be presented exclusively on the web as part of the June 2000 JOM-e the electronic supplement to JOM.
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The following article appears as part of JOM-e, 52 (6) (2000),
http://www.tms.org/pubs/journals/JOM/0006/Parsey/Parsey-0006.html

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Materials for Magnetic Memory: Commentary

Magnetic Materials in Electronic Applications

John M. Parsey, Jr.

Editor's Note: Materials for magnetic memory is the topic of this month's installment of JOM-e, the journal's electronic supplement. The articles that are referenced here appear only on the JOM web site and include hypertext enhancement. The titles and addresses of the articles appear in the issue's table of contents in both the print and on-line versions.

During the past decade there have been many important advances in electronics, semiconductor-material epitaxy, photonic devices, thin-film metallurgy, and nondestructive characterization techniques for assessing these new classes of materials. This issue of JOM-e looks at an interesting new area of activities undertaken by members of the TMS Electronic, Magnetic, & Photonic Materials Division: magnetic-film materials for advanced electronic devices.

Thin films of magnetic materials can be used for high-speed read/write heads in disk memory devices or as permanent memory for computer applications. Devices such as these retain the state of the memory cell when power is turned off, in contrast to the volatile memory in a standard dynamic RAM device. Also, such memory should consume a negligible amount of power when in operation, in contrast to the steady drain of semiconductor dynamic random access memory (DRAM).

The utility of this new class of magnetic materials is founded in the giant magnetoresistive (GMR) effect, identified in the late 1980s by Babich and coworkers and by Sato et al. (see the references in both of the articles in this issue). By layering conductors with domain-oriented layers of magnetic thin films, a large resistance change can be induced in the conductor. This can be understood from the interaction of electric and magnetic fields in current transport (the "right-hand rule") and Maxwell's equations. The GMR effect produces a relatively large signal when correctly incorporated in a layered device structure, which makes it very attractive for commercial applications. The understanding of the GMR effect has led to several device constructs, but absolute signal levels are still quite small, which hinders processing, testing, and robustness in applications.

Recent findings have shown that by modifying the magnetic materials, layer structure, and chemical makeup, substantially higher performance structures can be created that overcome the limitations of simple GMR-based devices. These devices are based on NiFe thin films and are characterized as tunneling devices, where a current is forced through a nominally nonconducting material, such as a very thin oxide layer, while interacting with magnetic fields of varying orientations.

Presented here are two contributions from both industrial and academic viewpoints that discuss deposition methods for magnetic tunnel junction (MTJ) and GMR films, the fabrication of MTJ devices, and characterization methods for the extremely thin nanostructured magnetic film materials. Some of the critical layers in an MTJ are on the order of 1-2 nm, which requires very precise and powerful characterization tools for assessment.

The first paper comes from an advanced technology research group at Motorola Physical Sciences Research Laboratory, investigating MTJ materials for high-speed magnetic random-access memory (MRAM) devices. Slaughter et al. present an analysis of a multilayer stack of magnetic thin films coupled with a dielectric tunneling layer. In their structure, current flow is modulated through the very precisely controlled dielectric layer by tunneling processes. They have realized even higher responsivity in their MTJ devices than those reported from GMR devices. The key feature in their work is the interaction of a fixed-orientation magnetic layer with a variable-orientation magnetic layer that produces the 0 or 1 nature for the memory cell. The authors describe some of the issues in creating and fabricating devices and characterizing the thin layers of materials by nondestructive methods.

One of the interesting points of the article is controlling the deposition of such thin layers uniformly over large areas (the reader is invited to review some of the past issues of JOM for nondestructive thin-film characterization methods and large-scale epitaxy of semiconductor materials). Another issue is pinning or fixing the orientation of one of the two magnetic films in the MTJ device. This is an interesting materials science problem involving domain pinning, long-studied in magnet materials, but an absolute necessity for the memory cell function. They note that film processing and the pinning effect are closely coupled, necessitating a low-thermal-budget processing and fabrication sequence. Slaughter et al. note that their materials are very well suited for MRAM and magnetic-sensor applications, but are not yet performing at a level necessary for read/write heads in disk memory devices. They propose that a very simple MTJ cell connected to a complementary metal-oxide semiconductor transistor would function as a memory element for fast MRAM applications. Their initial device results were quite encouraging, demonstrating an access time of ~14 ns in a 256 2 memory cell configuration.

The paper by Keavney and Falco looks at the characterization of GMR-type materials by optical methods. They note that many standard characterization techniques cannot be applied because of detection limits or the penetration capability of the probe. However, several approaches are available that utilize the interaction of light with magnetic materials. They describe the magneto-optical Kerr effect (polarization-rotation) for polarized light interacting with a material containing magnetic atoms (e.g., iron, chromium, or nickel). This approach is phenomenally sensitive, capable of detecting materials properties in films of 1-2 nm, which is in the range needed for MTJ or other GMR-type devices. A limitation is noted in the ability to penetrate thick films.

Another approach involves probing the magnetic film, again with polarized light, for inelastic interactions. This method is known as spin-wave brillouin light scattering. A high-intensity laser is focused on the material, and the backscattered light is analyzed for bulk and surface/interface magnetic spin effects. By careful analysis and modeling, the magnetic properties of the films may be extracted.

X-ray absorption is a third approach for dissecting the magnetic thin-film materials. Magnetic circular dichroism allows the unique detection of core-related spins in magnetic atoms. The authors comment on three approaches as they relate to the characterization of thin magnetic films: photocurrent (absorption); fluorescence yield (emission); and total transmission intensity (the net signal passed through the sample).

The use of these new magnetic materials systems is still in the R&D laboratory environment, but the prospects for commercial development are exciting. Many companies manufacturing computer disk drives or other mass-storage devices (such as huge, semipermanent, memory banks) are deeply involved in the development of thin-film magnetic devices for read/write and storage applications. If the practical aspects of film deposition, processing, and device fabrication can be resolved, such devices could be manufactured early in the new millennium.

John M. Parsey, Jr., is the section manager for advanced materials in the Digital DNA Laboratory of Motorola SPS. He is the advisor to JOM from the Electronic Materials Committee of the Electronic, Magnetic, & Photonic Materials Division of TMS.


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