Magnetic multilayered nanowires by electrodeposition for spintronic applications
2013-08
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Magnetic multilayered nanowires by electrodeposition for spintronic applications
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2013-08
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Close-packed GMR arrays have recently drawn great attention because of their potential use in applications such as magnetic sensors and magnetic random access memory (MRAM). In this dissertation, Co/Cu multilayered nanowires were designed and engineered to meet these applications.All-metallic current perpendicular to the plane (CPP) giant magnetoresistive (GMR) layers were made within insulating matrices by direct growth to avoid sidewall damage that is caused by lithographical patterning in current vacuum-deposited devices. These insulating matrices were made of Anodic Aluminum Oxide (AAO) templates and were grown both as free-standing membranes and as integrated layers on Si, both with columnar nanopores with diameters of 10-500nm. The barrier layer, which is a thin oxide layer at the bottom of the nanopores, was completely removed in both cases and Co/Cu multilayered nanowires were successfully grown inside these nanopores by DC electrodeposition. The thicknesses, diameters, and growth conditions were then engineered to make competitive structures for magnetic recording read sensors and spin torque RAM.For read sensors, we synthesized 10nm-diameter nanowires composed of Co(15nm)/Cu(5nm)/Co(10nm) trilayers that had 30 Ω resistance and 19% magnetoresistance for disk drive read heads. In contrast to conventional read heads based on lithographically-produced magnetic tunnel junctions, these offer potentially easier fabrication and more than 100x lower resistance with commensurate reductions in heat production. These wires were measured to have resistivities of 5.4x10-8 Ω.m, only three times higher than bulk values for copper. The low resistance, yet high magnetoresistance, is due to smooth sidewalls from in situ templated chemical growth. Thus, unlike lithographically-etched sensors, these nanowire sensors will be capable of reading high density (2-10 Terabit/in2) bit patterned media, such as that produced by self-assembled block co-polymers.For STT-RAM applications, spin transfer torque switching of Co/Cu multilayered nanowires with perpendicular c-axis was observed without the need for external magnetic fields with switching current densities below 107 A/cm2. The switching current density varied with Cu interlayer thickness. When the Cu interlayers were thin, the samples exhibited one stable state (high resistance) at zero current which was explained by investigating interwire and intrawire dipole fields that were then confirmed by simple energy calculations. Structures with increasing the Cu spacer thicknesses required increased current densities to switch due to spin relaxation and also due to the switching of the effective magnetic anisotropy of the structures from parallel to perpendicular to the wire axes. This change in anistropy was also observed experimentally via magnetization hysteresis loops and confirmed by calculations of the effective demagnetizing fields for the layered structures. The ability to tune the demagnetizing field of these structures makes them very interesting candidates for future spin transfer torque RAM with multiple states per bit.
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University of Minnesota Ph.D. dissertation. August 2013. Major:Electrical Engineering. Advisor: Bethanie J. H. Stadler. 1 computer file (PDF); vii, 119 pages.
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Al-Maqablah, Mazin Mohammad. (2013). Magnetic multilayered nanowires by electrodeposition for spintronic applications. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/166924.
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