Abstract
Magnetic domain walls (MDWs) can move when driven by an applied magnetic field. This motion is important for numerous devices, including magnetic recording read/write heads, transformers and magnetic sensors. A magnetic film, with a sawtooth profile, localizes MDWs in discrete positions at the narrowest parts of the film. We propose a controllable way to move these domain walls between these discrete locations by applying magnetic field pulses. In our proposal, each applied magnetic pulse can produce an increment or step-motion for an MDW. This could be used as a shift register. A similarly patterned magnetic film attached to a large magnetic element at one end of the film operates as an XOR logic gate. The asymmetric sawtooth profile can be used as a ratchet resulting in either oscillating or running MDW motion, when driven by an ac magnetic field. Near a threshold drive (bistable point) separating these two dynamical regimes (oscillating and running MDW), a weak signal encoded in very weak oscillations of the external magnetic field drastically changes the velocity spectrum, greatly amplifying the mixing harmonics. This effect can be used either to amplify or shift the frequency of a weak signal.
Highlights
Controllable Magnetic domain walls (MDWs) step-motor or shift-registerUnder steady-state conditions, the MDWs in the film shown in figure 1 can be located only at certain discrete positions, corresponding to the narrowest parts of the patterned film (e.g., red dashed lines in figure 1)
The linear energy density E(x) of the film in the externally applied magnetic field He(t) along the y-direction can be written as x
Where x is the location of the domain wall, EW is its energy per unit area, l(x) is the variable film width, Ly is the constant film thickness, and M is the film magnetization
Summary
Under steady-state conditions, the MDWs in the film shown in figure 1 can be located only at certain discrete positions, corresponding to the narrowest parts of the patterned film (e.g., red dashed lines in figure 1) This would occur if the sawtooth profile were steep enough in order for the restoring force (per unit area) to overcome the coercive force, |l (X )| l(X ) > fp. For a rectangular pulse of the applied magnetic field, equation (3) can be integrated exactly This allows us to derive the times, τ+ and τ−, for the transitions of the domain wall from a minimum to the following maximum (τ+) and from a maximum to the following minimum (τ−): τ± =. It is important to emphasize the robustness of this controllable step motor: due to the discrete equilibrium positions of the domain wall, we can shift it an exact integer number of steps, even though the pulse duration is established within an accuracy of about (τ+ − τ−)
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