Andrew Kunz

Field induced domain wall collisions in thin magnetic nanowires

In a two-dimensional magnetic nanowire, it is possible to engineer collisions between two domain walls put into motion by an externally applied field. We show that the topological defects that define the domain wall can be controlled to allow for both domain wall annihilation and preservation during the collisions as long as the wire remains thin. The preservation process can be used to release pinned domain walls from notches with small applied fields.

Enhancing domain wall speed in nanowires with transverse magnetic fields

Dynamic micromagnetic simulation studies have been completed to observe the motion of a domain wall in a magnetic nanowire in an effort to increase the field-driven domain wall speed. Previous studies have shown that the wire dimensions place a cap on the maximum speed attainable by a domain wall when driven by a magnetic field placed along the direction of the nanowire. Here we present data showing a significant increase in the maximum speed of a domain wall due to the addition of a magnetic field placed perpendicular to the longitudinal driving field. The results are expressed in terms of the relative alignment of the transverse field direction with respect to the direction of the magnetic moments within the domain wall. In particular, when the transverse field is parallel to the magnetic moments within the domain wall, the velocity of the wall varies linearly with the strength of the transverse field increasing by up to 20%. Further examination of the domain wall structure shows that the length of the ...

Fast domain wall motion in nanostripes with out-of-plane fields

Controlling domain wall motion is important due to the impact on the viability of proposed nanowire devices. One hurdle is slow domain wall speed when driven by fields greater than the Walker field due to nucleation of vortices in the wall. We present simulation results detailing the dynamics of these vortices including the nucleation and subsequent fast ejection of the vortex core leading to fast domain wall speeds. The ejection is due to the reversal of the core moments by an out-of-plane field. The technique can be used to produce domain walls of known orientation, independent of the initial state.

Dependence of Domain Wall Structure for Low Field Injection into Magnetic Nanowires

Micromagnetic simulation is used to model the injection of a domain wall into a magnetic nanowire with field strengths less than the so-called Walker field. This ensures fast, reliable motion of the wall [N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974)]. When the wire is located at the edge of a small injecting disk, a bias field used to control the orientation of the domain wall can reduce the pinning potential of the structure. The low field injection is explained by a simple model, which relies on the topological nature of a domain wall. The technique can quickly inject multiple domain walls with a known magnetic structure.

Simulating the Maximum Domain Wall Speed in a Magnetic Nanowire

The dynamics of domain wall motion in permalloy nanowires have been simulated utilizing the Landau-Lifshitz-Gilbert (LLG) equation of motion. The simulation results are presented in terms of the domain wall speed for ranges of the Gilbert damping parameter alpha and nanowire width. The maximum domain wall speed is independent of alpha. The speed of the domain wall can be increased by increasing the nanowire width, but this lowers the critical field. For applied fields below the critical field, the wall moves uniformly along the wire and the speed of the wall increases with increases in the driving field. This behavior is consistent with current analytic models; however, the models overestimate both the value of the domain wall speed and the critical field

Simulated domain wall dynamics in magnetic nanowires

The simulated domain wall dynamics in rectangular 10nm thick, 2000nm long Permalloy wires of varying width is presented. In the absence of an applied field the static domain wall length is found to be linearly dependent to the width of the nanowire. As magnetic fields of increasing strength are applied along the wire’s long axis, the domain wall motion changes from a uniform reversal to a steplike reversal. The onset of the stepping motion leads to a decrease in the domain wall speed. By continuing to increase the field it is possible to decrease the time between steps increasing the domain wall speed. The critical field associated with the crossover from uniform to nonuniform reversal decreases as the wire width increases.

Dynamic Notch Pinning Fields for Domain Walls in Ferromagnetic Nanowires

Artificial defects such as notches and antinotches are often attached to magnetic nanowires to serve as trapping (pinning) sites for domain walls. The magnetic field necessary to release (depin) the trapped domain wall from the notch depends on the type, geometric shape, and dimensions of the defect but is typically quite large. Conversely we show here that for some notches and antinotches there exists a much smaller driving field for which a moving domain wall will travel past the defect without becoming trapped. This dynamic pinning field also depends on the type, geometric shape and defect dimensions. Micromagnetic simulation is used to investigate both the static and dynamic pinning fields and their relation to the topologic structure of the domain wall.

Antivortex dynamics in magnetic nanostripes

In a thin magnetic nanostripe, an antivortex nucleates inside a moving domain wall when driven by an in-plane magnetic field greater than the so-called Walker field. The nucleated antivortex must cross the width of the nanostripe before the domain wall can propagate again, leading to low average domain wall speeds. A large out-of-plane magnetic field, applied perpendicularly to the plane of the nanostripe, inhibits the nucleation of the antivortex leading to fast domain wall speeds for all in-plane driving fields. We present micromagnetic simulation results relating the antivortex dynamics to the strength of the out-of-plane field. An asymmetry in the motion is observed which depends on the alignment of the antivortex core magnetic moments to the direction of the out-of-plane field. The size of the core is directly related to its crossing speed, both depending on the strength of the perpendicular field and the alignment of the core moments and direction of the out-of-plane field.

Normal mode mixing and ferromagnetic resonance linewidth

Summary form only given. A recent work by McMichael et. al. described a repulsion between interacting ferromagnetic resonance (FMR) modes when only two such modes are strongly coupled with the two magnon model. Mode repulsion is consistent with diagonalizing a two mode system but it is not a realistic model for most physical systems. The two magnon model is an application of perturbation theory used to describe scattering from in FMR between k=0, the uniform precession mode, to other k/spl ne/0 spin wave modes. In this work we take a different approach valid for large perturbations by diagonalizing the full Hamiltonian including interactions between all spin wave modes, as well as looking at other modes than just the uniform precession mode.

Application of local transverse fields for domain wall control in ferromagnetic nanowire arrays

In ferromagnetic nanowire arrays, where each wire contains multiple domain walls, it will be necessary to select an individual domain wall (DW) to move. In the field driven DW case, the field is typically applied globally affecting all of the domain walls in the system. We present micromagnetic simulation results demonstrating selectivity and control of an individual DW in such an array of nanowires using a combination of global and locally generated magnetic fields. Arranging the orientation of the local field allows for selectivity of a specific DW and its controllable movement to a new location.