Ami E. Berkowitz - US grants

An MRG is to be established which attempts to develop a fundamental understanding at the microscopic level of magnetic media. The program involves study of both model magnetic media as well as real materials utilized in industry, and attempts to make the link between these data. The group will conduct both experimental and theoretical investigation of microstructure and magnetic properties of thin films and particulate. The goal of the project is the complete microscopic description of observed macroscopic properties such as hysteresis and magnetization time decay in materials used for information storage in tape and disk recording.

This work involves a balanced theoretical and experimental program designed to relate the microscopic properties of collections of magnetic particles to their macroscopic response to a magnetic field. This fundamental research has potential for application in the magnetic recording industry. The research group receives substantial matching funds from the Center for Magnetic Recording Research at UC San Diego. The theoretical work applies finite element analytical methods, as well as an integral equation approach, to micromagnetic behavior. A wide variety of experimental methods are employed. They range from modern nano-scale lithographic methods to an innovative use of the classical Millikan oil-drop technique. The latter is used to levitate a magnetic particle in an oil bath and measure its magnetic moment. Further research is devoted to the important problem of inter-particle interactions across contact asperities. This topic is being investigated by high resolution electron microscopy with collaborators at UC Berkeley.

This project on nanocrystalline materials has three primary objectives. First, the spark erosion technique for producing nanoparticles of a very wide range of materials will be investigated and optimized for maximum yield of the desired particle sizes and compositions. Second, a variety of nanoparticle systems representing superconducting, magnetic and structural properties will be extensively characterized. Third, the consolidation of the nanoparticles into macroscopic samples retaining the unusual properties of the precursors, utilizing dynamic extrusion and shock consolidation as well as conventional processing methods, will be implemented. Using liquid dielectics, the spark erosion technique has been shown to yield high purity particles of metals, alloys, oxides, carbides, and semiconductors in sizes from nanometers to micrometers at significant production rates. Sparks erosion may be carried out also in ultrapure gases. This approach increases the performance of this method in the synthesis, treatment and subsequent handling of nanoparticles. The characterization of various types of nanoparticles produced will involve diffraction (x-ray, electron, neutron), high resolution TEM, surface analysis, as well as methods particularly suited to specific systems such as optical and Mossbauer spectroscopy, transport measurements and magnetic properties. Research is planned to demonstrate and extend the capabilities of the spark erosion method to produce economically significant quantities of various types of nanoparticles, as well as to investigate some of the unique properties of these systems in particulate and consolidated form.

The Materials Research Science and Engineering Center (MRSEC) at the University of California - San Diego undertakes research related to the performance of magnetic recording materials. The research program is organized through a single interdisciplinary research group. The research group is focused on the experimental and theoretical analysis of hysteresis, dynamics, and spin transport in magnetic particles and thin films. The goal of the group is to provide a quantitative microscopic description of the phenomena underlying the observed macroscopic hysteresis properties of these magnetic systems. The MRSEC is closely associated with the Center for Magnetic Recording Materials (CMRM), which is fully funded by industry. In addition the MRSEC will support a broad range of interactions and collaboration with industry and an expanded range of educational outreach programs. The MRSEC currently supports 11 senior investigators, 4 postdoctoral research associates, 2 technical staff members, 11 graduate students, and 5 undergraduates. The MRSEC is co-directed by Professors Ami E. Berkowitz and Sheldon Schultz.

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Berkowitz

The goal of this SGER program is to determine the feasibility of using a novel technique, spark erosion, to provide the particles for a material with strong technological potential, high-energy product 'exchange-spring' permanent magnets (ESPMs). ESPMs are duplex alloys of magnetically 'hard' and 'soft' phases in which the required length scales for the two phases are in the several tens of nm range. These length scales must be well controlled for optimum performance and this has been a principal obstacle to achieving a satisfactory production method. The spark erosion approach controls the sizes of the phases by precipitating them within particles of suitable diameters and microstructure. The idea is to produce amorphous particles of the appropriate composition and diameter such that, after annealing to the equilibrium duplex crystalline state, the length scales of the two phases are constrained by the particle size and initial microstructure as optimum for ESPM. Spark erosion is an under-appreciated, but extremely versatile and economical particle production method. The particle synthesis method is on firmer ground than the control of particle size/microstructure constraining equilibrium phase dimensions, since a large variety of powders have been produced by this method. Hot pressing is the primary method for preparing the final product, high-density bulk magnets with dimensions of several cm. A second approach for preparing the bulk magnets is thermal spraying, and this technique will be pursued if warranted by the results of the hot pressing. Both of these methods should benefit from the initial amorphous state expected for the spark-eroded particles since the equilibrium crystalline duplex structure can be developed during or after consolidation. The conceptual approach, although reasonable, is currently unproven. Particle production by spark erosion without contaminants seems feasible, but must be demonstrated. In addition, the particle synthesis method is a novel and economical one, which deserves attention. It is a technique that has great potential for a wide range of applications. Finally, there is new materials science to be clarified by examining the influence of particle size on precipitate dimensions.
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The concept for exchange-spring permanent magnets (ESPM) has been available since 1991, and it has been intensively pursued because ESPM offer the promise of permanent magnets with higher energy products than currently available, at a much-reduced cost for the rare-earth materials used. The permanent magnet market is huge, and this has inspired a great deal of very useful processing and modeling work on ESPM. However, no bulk processing technique is available at present.
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