Engineering Magnetic Domain-Wall Structure in Permalloy Nanowires

Using a focused-ion-beam microscope, we create nontopographic features that provide controlled modification of domain-wall structure, size, and pinning strength in 500-nm-wide nanowires composed from Cr(3 nm)/permalloy(10 nm)/Cr(5 nm). The pinning sites consist of linear defects where magnetic properties are modified by a Ga+-ion probe of diameter similar to 10 nm. Detailed studies of the structural, chemical, and magnetic changes induced by the irradiation, which show the modified region to be similar to 40-50 nm wide, are performed using scanning-transmission-electron-microscopy modes of bright-field imaging, electron-energy-loss spectroscopy, and differential-phase-contrast imaging on an aberration corrected (Cs) instrument. The Fresnel mode of Lorentz-transmission-electron microscopy is used for studies of domain-wall behavior, where we observe changes in depinning strength and structure with irradiation dose and line orientation. We present an understanding of this behavior based upon micromagnetic simulation of the irradiated defects and their effect on the energy terms for the domain walls.