Field-driven and spin-transfer-torque-driven domain-wall dynamics in permalloy micro-/nano-structures

Abstract

This dissertation explores magnetic-field- and electric-current-driven domain-wall motion in thin-film-based magnetic microstructures. Conventional thin-film growth and microstructure fabrication techniques including electron-beam lithography and focused ion beam milling are used to fabricate nanometer-scale one-dimensional and two-dimensional magnetic structures that support magnetic domains (regions of different magnetization orientation separated by domain walls). A high-spatial resolution, hightemporal resolution technique for measuring the field- or current- driven dynamics of the domain walls, based on the magneto-optic Kerr effect, is developed and used to study the wall dynamics. Field-driven domain-wall motion at slow magnetic field sweep rates is dominated by Barkhausen jumps, the discontinuous random movement of domain-wall displacements. The experiments described represent one of the first successful attempts to extend the study of Barkhausen effects into the two-dimensional region. The experiments successfully probe velocity distributions, jump amplitude distributions, and attempt to address issues that pertain to the universal exponents that describe the scaling behavior of Barkhausen jump distribution function including effects of dimensionality and sweep-rate effects on the exponents. A novel dual-beam magneto-optical experiment is performed on thin-film microstructure that probes negative Barkhausen jumps (jumps that oppose the direction favored by Zeeman energy driving the magnetic reversal). A new mechanism for negative Barkhausen jumps is proposed that accounts for the observed effects. Domain-wall motion driven by (spin-polarized) electric current is studied in nanoscale thin-film based wires. The experiments address issues pertaining to the basic mechanisms responsible for current-driven domain-wall motion, which are believed to be the adiabatic spin-torque mechanism and non-adiabatic mechanisms. The experiments described are the first true time-resolved measurements of current-driven displacements, and the results reveal new information about the stochastic properties of current-driven domain wall displacements. The results also provide information on domain-wall velocities and spin-flip efficiencies that address issues related to spin-torque mechanisms.