Systematic motion of magnetic domain walls in notched nanowires under ultrashort current pulses

Abstract

The precise manipulation of transverse magnetic domain walls in finite/infinite nanowires with artificial defects under the influence of very short spin-polarized current pulses is investigated. We show that for a classical 3d ferromagnet material like Nickel, the exact positioning of the domain walls at room temperature is possible only for pulses with very short rise and fall time that move the domain wall reliably to nearest neighboring pinning position. The influence of the shape of the current pulse and of the transient effects on the phase diagram current-pulse length are discussed. We show that large transient effects appear even when α=β, below a critical value, due to the domain wall distortion caused by the current pulse shape and the presence of the notches. The transient effects can oppose or amplify the spin-transfer torque (STT), depending on the ratio β/α. This enlarges the physical comprehension of the DW motion under STT and opens the route to the DW displacement in both directions with unipolar currents.

Spin and Charge Pumping by a Steady or Pulse-Current-Driven Magnetic Domain Wall: A Self-Consistent Multiscale Time-Dependent Quantum-Classical Hybrid Approach

“…Thus, too large transient inertial displacement will be detrimental for racetrack memory operation. The origin of transient inertial displacement is deformation of the DW leading to delayed response at the current onset and at the end of the current pulse, which then requires to tune the duration [38,[69][70][71] and the shape (i.e., its rise and fall time) [72] of the pulse. The experiments [69][70][71] and classical micromagnetic simulations [38,70,72] typically employ short ∼ ns pulses, which generate higher DW velocities than longer ∼ µs pulses due to easier depinning by an additional force on the DW during the pulse rise time or by a small mean distance between pinning centers.…”

“…Therefore, using NEGF+LLG approach precludes taking into account self-consistent feedback [55,62] where the dynamics of M i (t) leads to pumped spin currents which, in turn, can exert additional torque and timeretarded damping (with microscopically [63,64] rather than phenomenologically [65,66] determined memory kernel) on M i (t) thereby modifying its dynamics. Finally, time-dependent quantum treatment of electrons is required to describe pulse-current-induced dynamics of M i (t) which is of paramount importance in basic research experiments [8] and, e.g., racetrack memory applications [17,18] where usage of current pulses [67] or their trains [68] reduces threshold current density to move the DW while precise control of the DW position can be achieved by tailoring pulse duration and shape [38,[69][70][71][72].…”

Analytical description of the topological interaction between magnetic domain walls in nanowires

“…In the results shown below, an antivortex appears only in a few points in the upper right quadrant of the micromagnetic phase diagrams (detailed in ref. 44) where the symbols are missing, and does not influence our results.…”

“…Each DW sits in a potential well created by the notches [42,43]. The form of the pinning potential was determined from micromagnetic simulations and is presented elsewhere [44] (harmonic at the notches and sinusoidal between them).…”

“…The analytical diagrams are compared with the micromagnetic ones (24×31 points). As previously determined [44], the range of the current amplitude was chosen (≤10 A/µm 2 ) as to to have only viscous motion (no precession) for the pulse duration used ( 1.5ns), which is on the same order of magnitude with access or reading/writing time in possible magnetic memories based on DWs. At high current amplitude or longer pulse duration, an antivortex appears when a DW depins from a notch [50,51].…”