IEEE Magnetics Society Distinguished Lecturers for 2003

IEEE Magnetics Society
Distinguished Lecturers for 2003

Wall Watching: The Progress of Domains in Small Elements

John Chapman

University of Glasgow


An understanding of magnetization processes is of direct interest to physicists and is crucial for developing high performance magnetic devices. The domain structure, and the way it changes under the influence of a magnetic field, depends not only on basic material parameters but also on the physical shape and size of the magnetic material. Thus, quite different domain configurations are found in bulk materials, thin films, and small magnetic elements made from the same material. The same is true of domain walls, whose structure can change markedly as one or more of the dimensions of the material under investigation moves into the sub-micrometer regime. Given the extreme miniaturization that occurs in magnetic storage and sensing devices, as detailed a knowledge as possible of the magnetization configuration in small elements is essential.

For many years, the Lorentz imaging mode of transmission electron microscopy (TEM) has yielded high resolution magnetic images of domains and walls in magnetic films and elements. Since only a modest amount can be learned from a single image of an element, however, recent advances -- whereby in situ magnetizing capabilities within the TEM have been enhanced -- have made a considerable impact.

In this talk I will illustrate the radical changes that occur as the dimensions of magnetic elements are reduced from a few micrometers to tens of nanometers. While size is a very important parameter, the detailed shape can also exert a major role, and changes here offer a way of tailoring properties to meet specific requirements. Other important influences are coupling between layers (if the element is formed from a magnetic multilayer) and the nature of the substrate. It is hoped that many of the images, as well as revealing in a very direct way how the magnetization process proceeds, will appeal to the aesthetics of the audience.

John Chapman received both the M.A. degree in Natural Sciences and the Ph.D. degree from the University of Cambridge, United Kingdom, in 1973.

Following a Research Fellowship at Fitzwilliam College, Cambridge, he became a Lecturer at the University of Glasgow in the Department of Physics and Astronomy. Promotion to readership in 1984 and full professorship in 1988 followed; currently he is Head of Department. Professor Chapmanís main research interest concerns the characterization, development, and application of advanced functional materials. Overall his aim is to gain understanding at a microscopic level of how various physical properties relate to material nanostructure and how the former can be improved by the ways in which materials are grown and processed. He studies magnetic materials extensively, with particular emphasis on magnetic nanostructures and multilayer films. Much of his work uses electron microscopy and related analytical techniques. He has co-authored about 250 papers.

In 1991 Professor Chapman was elected a Fellow of the Royal Society of Edinburgh. He is also a Fellow of the Institute of Physics and of the Royal Microscopical Society.

Contact: Prof. John N. Chapman, Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, U.K.; telephone: +44 141 330 4462; fax: +44 141 330 4464; e-mail: j.chapman@physics.gla.ac.uk

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Characterization of Magnetic Recording Channels:

A Historical Perspective

Thomas D. Howell

San Jose State University

The design of advanced signal processing systems for recovering data stored on magnetic media requires an accurate understanding of the input/output characteristics of the storage system. The designer must be able to predict the output resulting from an arbitrary input in order to select the optimum set of signals to represent the data. He or she should also know the statistical properties of the noise and the types of distortion affecting the storage and readback processes.

Early systems used simple models of channel behavior. As densities increased and signal processing schemes became more complex, more sophisticated models were needed. It is interesting to observe how effects once considered negligible became important, and conversely, how dominant distortions, once understood, became part of the expected signal and hence of negligible importance as disturbances.

In this lecture I will examine selected developments from the history of magnetic recording channel characterization. I will discuss the changing roles of intersymbol interference and nonlinear transition shift, along with some of the techniques used to measure and model them. Magnetic recording systems continue to evolve at a rapid pace; the lessons learned from history often help speed progress and avoid future pitfalls.

Thomas D. Howell (M'81, SM'89) received the B.S. degree in mathematics from the California Institute of Technology, Pasadena, CA, in 1973 and the Ph.D. degree in computer science from Cornell University, Ithaca, NY, in 1976.

He became a Lecturer in computer science and electrical engineering at San Jose State University, CA, in 2002. From 1977 to 1990 he was a research staff member in the IBM Research Division at their San Jose, Zurich, and Almaden centers, where he conducted research on the application of advanced signal processing techniques to magnetic recording channels. After joining Quantum Corporation in 1990, he managed advanced engineering groups in a variety of areas and helped introduce new technologies including digital channels, magnetoresistive and giant magnetoresistive heads into the company's products. He held a number of positions, ending as Vice President of Research. He served on the board of directors of the National Storage Industry Consortium and on industrial advisory councils at several university research centers during the 1990s.

Dr. Howell served as an editor of the IEEE Transactions on Magnetics (1997-2000) and chaired The Magnetic Recording Conference (2000).

Contact: Dr. Thomas D. Howell, Department of Computer Science, San Jose State University, One Washington Square, San Jose, CA 95192; telephone: +1 408 924 7171; fax: +1 408 924 5080; e-mail: t.howell@ieee.org

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Thermal Magnetization Noise and Fluctuation-Dissipation in

Magnetoresistive Heads, Sensors, and Ferromagnetic Thin-Film Devices

Neil Smith

IBM Almaden Research Center, IBM Corporation

Continuing technological development of giant magnetoresistive (GMR) spin-valve materials and devices, and tunneling magnetoresistive (TMR) sensors, has been largely driven by ever-increasing demands for greater areal storage density and data transfer rates for hard-disk drives. These technological demands will require future GMR (or TMR) materials with increasing MR coefficients DR/R >> 10%, and read-head/sensor dimensions at and below the scale of 100 nm. In this regime, the sensor's intrinsic electrical noise can be exceeded by resistance noise arising from thermally-induced magnetization fluctuations ("mag-noise") in the very thin, magnetically soft, ferromagnetic sensing layers of the MR read head. This mag-noise contribution scales as P ∑ (DR/R)2ci 2/V (where P is the input power, ci is the sensorís internal magnetic susceptibility, and V is the sensor volume), whereas the signal power similarly scales as P. (DR/R)2c e2 (where ce is the external field susceptibility).

Hence, mag-noise serves as a fundamental limit on GMR sensor signal-to-noise ratio that does not substantially improve with further increases in DR/R or sensitivity c, but which can become more severely limiting as sensor volume decreases.

In addition to its technological implications, observation of mag-noise in sub-micrometer MR sensors provides a relatively simple electrical measurement to study basic damping properties and loss mechanisms in the constituent ultra-thin ferromagnetic films. This can include geometric finite-size effects in very small (100 nm) structures not easily probed by traditional ferromagnetic resonance experiments. The basic relationships between intrinsic magnetic damping and measured thermal magnetization fluctuations can be described by application of the fluctuation-dissipation theorem.

In this talk I will offer a brief tutorial on the fluctuation-dissipation theorem and how it may be properly employed to quantitatively model the mag-noise amplitude and spectrum observed in MR sensors. I will review some recent measurements of mag-noise in MR devices, compare experimental with model expectations, and offer scaling projections of magnetic noise vs. sensor size. In addition, I will discuss how fluctuation-dissipation arguments can discriminate between alternative phenomenological damping models in ways not obvious using traditional uniform magnetization descriptions of damped ferromagnetic resonance, and conclude with a brief consideration of excess damping contributions from inhomogeneity and finite-size effects.

Neil Smith received the S.B. degree in physics from the Massachusetts Institute of Technology, Cambridge, in 1977, and the Ph.D. degree in physics, also from MIT, in 1983.

He joined the Eastman Kodak Company in 1984 and worked in the Magnetic Heads Division of Kodak Research Labs, San Diego, CA, until 1998. His work there primarily involved the physics of magnetic recording of magnetic tape heads and systems, with particular emphasis on the development of magnetoresistive read heads and very high sensitivity anisotropic and giant magnetoresistance magnetic field sensors. In 1998 he joined the IBM corporation, working in the Recording Heads Group at the IBM Almaden Research Center, San Jose, CA. At IBM he has concentrated on both write and read head technology for hard-disk drives, including research on the basic physical and technological limits of read heads for ultra-high disk storage densities. He has recently conducted some of the first investigations into fundamental signal-to-noise limits of magnetoresistive read heads due to thermally induced magnetization fluctuations.

Contact: Neil Smith, IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099; telephone: +1 408 927 2808; fax: +1 408 927 3010; email: neils@almaden.ibm.com

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