IEEE Magnetics Society

Distinguished Lecturers for 2001-2002

Advanced Magnetic Materials and Transducers:

Enabling Factors for the Digital Storage Explosion

Shan X. Wang

Stanford University

A

DVANCED MAGNETIC MATERIALS and transducers are critical building blocks in numerous electromagnetic devices such as magnetic hard disk drives. They are essential for the explosive growth in the storage capacity of hard drives at least by two orders of magnitude in the 1990s. However, conventional magnetic information storage technology is approaching the perceived superparamagnetic limit at which the stored bits may self-erase in less than ten years. This requires new magnetic materials and transducers, and even a departure from the old paradigm of magnetic storage technology.

One approach is to use new magnetic media with high anisotropy, but this requires write heads to deliver more intense magnetic field, which in turn requires higher saturation magnetization of the soft magnetic material used in write heads. As an example, the talk will describe films of a new soft magnetic material based on Fe-Co-N with a saturation magnetization of 24 kG, exceeding that of any currently available soft magnetic material, with a superior permeability of over 1000 up to 1.2 GHz. The films have a hard-axis coercivity of 0.6 Oe and an in-plane uniaxial anisotropy. They are very promising for extending the superparamagnetic limit in magnetic recording while achieving a data rate of over 2.4 Gbit/s, as well as for applications in gigahertz integrated inductors and other electromagnetic devices. The soft magnetism of Fe-Co-N films will be discussed based on their microstructures, stress, magnetostriction, and magnetic ripple structures. In addition, sub-nanosecond spin-dynamic data of these materials are of great interest and will be presented.

Rapid development in giant magnetoresistive materials and novel spin-dependent devices has enabled read heads to detect ever-smaller bits written in hard disk drives. In search of new magnetoresistive materials, we encounter many interesting scientific questions. As an example, the talk will describe work on electron specular reflection and specular spin valves using an in-situ resistance and magnetoresistance probe and semiclassical transport models.

Shan Wang (S'88-M'94) received the B.S. degree in physics from the University of Science and Technology of China in 1986, the M.S. in physics from Iowa State University in 1988, and the Ph.D. in electrical and computer engineering from Carnegie Mellon University in 1993.

He is an associate professor in the Department of Materials Science and Engineering and the Department of Electrical Engineering at Stanford University. He is also associated with the Center for Research on Information Storage Materials (CRISM) and the Geballe Laboratory for Advanced Materials. He was a Frederick Terman Faculty Fellow at Stanford University (1994-1997). His current research interests include magnetoresistive materials and spin electronics, magnetic inductive heads and soft magnetic materials, and magnetic recording physics. He has published over 60 papers on these subjects. He is co-author, with Alex Taratorin, of Magnetic Information Storage Technology (Academic Press).

Prof. Wang served as a member of IEEE Magnetics Society Administrative Committee (1998-2000) and chair of the Santa Clara Valley Chapter of the IEEE Magnetics Society (1999-2000).

Contact: Prof. Shan X. Wang, Geballe Laboratory for Advanced Materials, McCullough Building, 476 Lomita Mall, Stanford University, Stanford, CA 94305-4045; telephone: 650-723-8671; fax: 650-736-1984; e-mail: sxwang@ee.stanford.edu.

Advanced Magnetic Materials:

Development and Micromagnetics

Josef Fidler

Vienna University of Technology

T

HE INCREASING INFORMATION density in magnetic recording, the miniaturization in magnetic devices, the trend towards nanocrystalline magnetic materials, and the improved availability of large scale computer power are the main reasons why micromagnetic modeling has been developing extremely rapidly. Nanofabrication, offering unprecedented capabilities in the manipulation of material structures and properties, opens new opportunities for engineering innovative magnetic materials and devices and for developing ultra-high-density magnetic storage and magnetic microsensors.

Hard magnetic materials have become key components in information and transportation technologies, machines, sensors, and many other systems. The increase in the operating temperature of motors, generators, and other electronic devices will lead to an improvement in their efficiency.

A key problem encountered in the improvement and development of advanced magnetic materials is the influence of the real microstructure on the magnetization reversal process. Besides micromagnetic simulations, both advanced microstructural characterization and magnetic measurement techniques with high spatial and temporal resolution are necessary. Computational micromagnetism leads to a deeper understanding of hysteresis effects at an intermediate length scale between magnetic domains and atomic distances by visualization of the magnetization reversal process.

The numerical solution of Brown's equations can be effectively performed using finite-element and related methods that easily handle complex microstructures and take into account the long-range magnetostatic interactions and short-range exchange coupling between the grains. Dynamic finite-element simulations successfully predict the influence of microstructural features like grain size, particle shape, intergranular phases, and surface irregularities on the magnetic properties. Theoretical limits for remanence, coercive field, switching behavior at short time scales, and other properties have successfully been calculated for a large number of materials.

The lecture will review the physics and the recent development of advanced magnetic materials. Topics will include the switching dynamics of patterned mesoscopic and nanoscopic elements including the thermal activation process; the remanence enhancement in exchange-coupled, nanocrystalline magnets; the nucleation field of highest energy density magnets; and the domain wall pinning in magnets for high temperature applications. In particular, the influence of the granular microstructure of the materials on their magnetization reversal processes will be illustrated with experimental data and numerical results. Emphasis will be given to the limits and trends of the micromagnetic simulations.

Josef Fidler (M'82) received the Dipl.-Ing. degree in physics in 1973 and the Dr. Techn. degree in 1976 from the Vienna University of Technology, Austria.

In 1982 he became a Lecturer (Dozent) and in 1991 a Professor in physics at the Vienna University of Technology. His main research interests are the relations between the microstructure and the properties of magnetic materials and the application of computational micromagetics to magnetization processes. He established the Working Group on Magnetic Materials and Micromagnetism at the Institute of Applied and Technical Physics. He has published over 180 papers on magnetic materials, especially on high coercivity magnets, electron microscopy, and numerical micromagnetism.

Dr. Fidler is member of the German Physical Society, the Austrian Society of Electron Microscopy, and the Materials Research Society.

Contact: Prof. Dr. J. Fidler, Vienna University of Technology, Institute of Applied and Technical Physics, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria; telephone: 43 1 58801 13714; fax: 43 1 58801 13798; e-mail: fidler@tuwien.ac.at; http://atp6000.tuwien.ac.at/MAGNET/.

Ferromagnetic Resonance Force Microscopy:

Probing Ferromagnets at the Micrometer Level

Philip E. Wigen

Ohio State University

W

ITH THE EVOLUTION of fabrication methods to produce materials and devices with nanoscale dimensions, there is a need for the development of new techniques to characterize the materials of miniaturized devices at these scales. In the field of magnetic materials, such innovative devices include spintronic elements and submicrometer memory storage elements. Magnetic resonance force microscopy (MRFM) is a new technique with a projected sensitivity sufficient to enable single spin detection (electron or nuclear) at atomic resolution. It combines the principles of magnetic resonance with those of scanned-probe force detection to detect the spin resonance through mechanical, rather than inductive, means. MRFM achieves high sensitivity by means of a mechanical resonator that sensitively detects the force between a small probe magnet mounted on the resonantor and the precessing spin moment in the sample.

Ferromagnetic resonance force microscopy (FMRFM) is a variant of MRFM developed for the investigation of microscopic ferromagnets. Ferromagnetic systems pose unique challenges for microscopic imaging due to the strong interactions between the moments which causes the resonances to be non-local excitations. In the present stage of development, FMRFM is able to probe the spatial features and the relaxation properties of excited modes at the micrometer level. FMRFM takes advantage of the strength of the magnetic field of the microscopic probe magnet to determine three distinct regimes of interaction with the local ferromagnetic moment: (1) the weak field limit, where ferromagnetic dynamics are solely determined by the sample dimensions and internal energies, (2) an intermediate regime, where the local perturbation of the probe field alters the intensities of the ferromagnetic modes but not their resonant frequencies, and (3) the strong interaction limit, where the ferromagnetic resonance mode is entirely determined by the probe field and independent of the sample geometry. Examples of FMRFM applied to each of these regimes will be demonstrated and discussed in this lecture.

Philip Wigen (M'90) received the B.S. degree in chemistry from Pacific Lutheran University in 1955 and the Ph.D. in Physics from Michigan State University in 1960. He was a research scientist with the Lockheed Research Laboratories in Palo Alto, California, from 1960 to 1965 where he initiated his work on ferromagnetic resonance in magnetic metal films.

In 1965 he joined the physics faculty of the Ohio State University where he continued his work on the dynamical properties of ferromagnetic materials including ferromagnetic resonance, magnetic domain wall resonance, and chaos in magnetic systems. His recent work in magnetic resonance force microscopy has been pursued in collaboration with Prof. Roukes' group at California Institute of Technology, where he holds a visiting professorship, and Prof. Hammel's group at Los Alamos National Laboratory. Prof. Wigen is a fellow of the American Physical Society.

Contact: Prof. Philip Wigen, Department of Physics, Ohio State University, 174 West 18th Ave., Columbus, OH 43210, USA; telephone: 614-292-7439; fax: 614-292-7557; e-mail: wigen.1@osu.edu.