Atomic Assembly of Giant Magnetoresistive Multilayers
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Atomic Assembly of Giant Magnetoresistive Multilayers Haydn N.G. Wadley, Xiaowang Zhou and Robert A. Johnson Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903, U.S.A. ABSTRACT The emergence of metal multilayers that exhibit giant magnetoresistance (GMR) has led to new magnetic field sensors, and approaches for nonvolatile random access memories. Controlling the atomic scale structure across the many interfaces within these multilayers is central to improve the performance of these devices. However, the ability to manipulate atomic arrangements at this scale requires an understanding of the mechanisms that control heterometal film growth during vapor deposition. It is important to develop methods that enable prediction of the effects of deposition conditions upon this structure. Atomistic simulation approaches have been combined with deposition reactor models to achieve this. We have applied these approaches to analyze the atomic scale structure of sputter deposited CoFe/Cu/CoFe giant magnetoresistive multilayers similar to those used for magnetic field sensing. Significant intermixing is revealed at the CoFe-on-Cu interface, but not at the Cu-on-CoFe interface. Recent experiments verified these predictions. The insights provide a basis for the development of processes that inhibit thermally activated atomic diffusion while allowing the controlled use of the metal atom impact energy and inert gas ions to manipulate the structure of interfaces. BACKGROUND About 13 years ago Fe/Cr/Fe multilayers with nanoscale layer thickness were discovered to exhibit very large (5-25%) drops in their electrical resistance when a moderate magnetic field was applied [1]. These “giant” magnetoresistive (GMR) materials soon became the subject of very intensive research because they appeared well suited for sensing small magnetic fields. Many multilayer systems composed of ferromagnetic metal layers separated by nonferromagnetic conductive metal layers have since been found to possess this property. The most frequently studied systems include Co/Cu/Cu [2,3] and NiFe(Co)/Cu/NiFe(Co) [4,5]. Rapid progress has resulted in the development of sensors for the readheads of hard disk drives. This contributed to the very large recent increases in the hard drive storage capacity [6]. These sensor applications are driving the development of materials that possess larger magnetoresistance. Many groups are now also exploring the application of this technology to create magnetic random access memory (MRAM) [6]. This MRAM appears to offer nonvolatility in addition to a similar functionality to state of the art dynamic random access memory. Two device approaches are being explored. One utilizes all metal multilayers and exploits electron spin dependent scattering. The second seeks to exploit changes in the tunneling conductance across dielectric barrier sandwiched between magnetic multilayers [6]. Both the sensor and MRAM applications of GMR materials are greatly
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