Stochastic domain wall depinning in permalloy nanowires with various types of notches

J Wu,J Wang,H L Yang,W S Zhao,Q F Zhan,Z Li,N Lei,W Zhang,J Du,B You,H Q Tu,L J Wei,Y Gao,Y B Xu

doi:10.1063/1.4973647

J Wu, J Wang + Show 12 more

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https://doi.org/10.1063/1.4973647

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Abstract

Stochastic phenomena in magnetic nanowires based on domain wall (DW) motion is scientifically important thus to understand and control such behaviors are very meaningful. Here we report on the investigation of pinning and depinning of DWs in permalloy nanowires with six types of longitudinally asymmetric notches using focused magneto-optic Kerr effect (FMOKE) magnetometer and magnetic force microscopy (MFM). The hysteresis loops obtained by FMOKE indicate the generation of one or two distinct depinning fields by creating one notch close to the edge of the nanowires, in comparison multiple depinning processes occur in the nanowires with two identical notches symmetrically placed along the transverse direction, indicating more remarkable stochastic DW depinning phenomena. The MFM images verify the existence of DW in each type of nanowires and the DW sizes in the latter kind of nanowires are generally larger than those in the former ones. These observations can be explained by considering the thermal perturbation and edge or surface roughness effects in nanowires.

Highlights

Precise and effective control of the domain wall (DW) motion in ferromagnetic nanostructures can have important impacts on developing generation magnetic devices, including magnetic memory[1] and logic components.[2,3] Propagation and depinning of DW in magnetic nanowires can be achieved using external magnetic field[4,5] or spin-polarized current.[6,7,8] In these nanostructures, artificial defects, such as notches or protrusions[9,10,11,12] can become potential pinning sites for DW trapping
By comparing the focused magneto-optic Kerr effect (FMOKE) and magnetic force microscopy (MFM) results in these nanowires, we show the influence of the transverse asymmetry of the notch on the DW depinning behavior and identify which type of notch is most suitable to suppress the stochasticity of DW depinning
When Ha decreases from maximum to zero and increases in magnitude along the negative direction, a DW initially nucleated at the elliptic portion propagates along the left arm and is pinned at the notch to create the first jump at Ha = -68 Oe

Summary

Introduction

Precise and effective control of the domain wall (DW) motion in ferromagnetic nanostructures can have important impacts on developing generation magnetic devices, including magnetic memory[1] and logic components.[2,3] Propagation and depinning of DW in magnetic nanowires can be achieved using external magnetic field[4,5] or spin-polarized current.[6,7,8] In these nanostructures, artificial defects, such as notches or protrusions[9,10,11,12] can become potential pinning sites for DW trapping. Two basic types of DWs13,14 are identified, namely, the vortex and transverse DWs. The vortex DW (VDW) is energetically favored in the thicker and wider magnetic nanowires,[2,4] while the transverse DW (TDW) is preferred in thinner and narrower magnetic nanowires,[3] a result of the competition between the exchange energy and the demagnetizing energy.

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