Browsing by Subject "Micromagnetic simulation"

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Shape-engineered ferromagnets and micromagnetic simulation techniques for spin-transfer-torque random access memory
(2018-05) Pramanik, Tanmoy; Banerjee, Sanjay; Register, Leonard F.; Lee, Jack C.; Sreenivasan, S. V.; Tsoi, Maxim
Spin-transfer-torque random access memory (STTRAM) has received great attention as a prospective universal memory due to high speed read and write capabilities, scalability to smaller technology nodes and non-volatile data retention. Two major factors that could limit the performance of large scale STTRAM arrays are the high switching current and the stochastic switching behavior. In this work, possible routes to mitigate these issues have been explored and new techniques have been proposed to estimate the reliability of the write process. Large area of the selection transistor required to support high switching current impacts the bit storage density of an STTRAM memory array. To increase the bit storage density, a multi-state STTRAM cell employing a cross-shaped ferromagnet was proposed previously. Here, the spin-transfer-torque (STT) driven mag-netization dynamics of the cross-shaped ferromagnet is revisited. As a low power alternative, voltage controlled magnetic anisotropy (VCMA) based writing scheme is studied. Trade-offs and limitations of the VCMA-induced switching over STT are also discussed. In the next part of this dissertation, magnetic properties and magnetization process of epitaxial chromium telluride thin films have been studied. Presence of strong perpendicular magnetic anisotropy in this material makes it an attractive choice for device applications. In this work, anisotropy energies of chromium telluride thin films have been estimated from magnetization measurements. The magnetization reversal process is then studied using analytical models as well as micromagnetic simulations. The last part of this work focuses on the write error rates (WER) of STTRAM. The stochastic write process of STTRAM at finite temperatures gives rise to write errors when a bit fails to switch within the duration of the write pulse. Ultra-low WER on the scale of 10⁻⁹ or less are desired for practical applications. Micromagnetic simulations are required to capture spatially-incoherent magnetization dynamics inside a ferromagnet, which may effect the WER. In this work, using the techniques of rare event enhancement, reliable calculation of WERs to 10⁻⁹ is demonstrated while keeping the computational effort to a minimum. Employing rare-event-enhanced micromagnetic simulations, WERs of both perpendicular and in-plane STTRAM bits are calculated and effects of spatially-incoherent excitations on the WER slopes are discussed.
Study of static spin distributions and dynamics of magnetic domain walls in soft magnetic nanostructures
(2013-05) Yang, Jusang; Erskine, James L.
The static and dynamic properties of spin distributions within domain walls(DWs) confined by Permalloy nanowire conduits are investigated by numerical simulations and high-speed magneto-optic polarimetry. Phase boundaries and critical points associated with DW spin distributions of various topologies are accurately determined using high-performance computing resources. Field-driven mobility curves that characterize DW propagation velocities in 20 nm thick nanowires are calculated with increasing the width of nanowires. Beyond the simple one-dimensional solution, the simulations reveal the four distinct dynamic modes. Oscillations of the field-driven DW velocity in Permalloy nanowires are observed above the Walker breakdown condition using high-speed magneto-optic polarimetry. A one-dimensional analytical model and numerical simulations of DW motion and spin dynamics are used to interpret the experimental data. Velocity oscillations are shown to be much more sensitive to properties of the DW guide structure (which also affect DW mobility) than the DW spin precessional frequency, which is a local property of the material. Transverse bias field effects on field-driven DW velocity are studied experimentally and numerically. DW velocities and spin configurations are determined as functions of longitudinal drive field, transverse bias field, and nanowire width. For a nanowire that supports vortex wall structures, factor of ten enhancements of the DW velocity are observed above the critical longitudinal drive-field (that marks the onset of oscillatory DW motion) when a transverse bias field is applied. The bias-field enhancement of DW velocity is explained by numerical simulations of the spin distribution and dynamics within a propagating DW that reveal dynamic stabilization of coupled vortex structures and suppression of oscillatory motion in the nanowire conduit resulting in uniform DW motion at high speed. Current-driven and current-assisted field-driven domain wall dynamics in ferromagnetic nanowires have thermal effects resulting from Joule heating, which make difficult to separate the spin-torque effects on DW displacements. To understand the thermal effects on DW dynamics, the temperature dependence of field-driven DW velocity is explored using high-bandwidth scanning Kerr polarimetry. Walker critical fields are decreased with increasing temperature and temperature-induced dynamic mode changes are observed. The results show that Joule heating effects are playing an important role in current-driven/current-assisted field-driven DW dynamics.