Browsing by Subject "STT-RAM"
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Item Magnetic multilayered nanowires by electrodeposition for spintronic applications(2013-08) Al-Maqablah, Mazin MohammadClose-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.Item Spin Transfer Torque Induced Switching In Magnetic Tunnel Junction For Stt-Ram Application(2013-12) Zhao, HuiThis thesis describes experimental studies of the spin transfer torque induced switching in magnetic tunnel junctions (MTJ) for the application of spin transfer torque random access memory (STT-RAM). In the material development; the in-plane MTJ was optimized in order to meet the requirement of STT-RAM application. Perpendicular magnetocrystalline anisotropy was obtained in the L10 phase FePd designed for the top MTJ electrode and bottom MTJ electrode. Moreover, full CoFeB MTJ with interface perpendicular anisotropy was developed. An average of 48% reduction in the intrinsic critical current density was found by increasing the interface perpendicular anisotropy. Sub 200 ps ultrafast STT induced switching was also demonstrated in those CoFeB MTJs where the out-of-plane demagnetizing field was partially canceled by the interface perpendicular anisotropy. High J/Jc0 ratio and magnetization nucleation at the edge of free layer are possibly the two major factors that contribute to the ultrafast spin transfer torque switching. In the spin transfer torque (STT) induced switching study; systematic characterization of the probabilistic STT induced switching process was done. It includes the three STT induced switching modes, the switching energy, the switching speed and the high precision switching probability density function (PDF). The temperature dependent MTJ properties and STT switching distribution was also studied. Those results provided key parameters for the STT-RAM design. In the end, direct and compelling experimental evidence was provided to show the large dynamic energy barrier reduction induced by high frequency spin current excitations. The concept of magnetization logarithmic susceptibility was proposed to describe this dynamic effect. By comparing with the simulation results, the measured logarithmic susceptibility frequency response was used to reveal the magnetic properties of MTJs and understand the spin-transfer torque induced magnetization switching dynamics.