  Using Lorentz transmission electron microscopy we investigate the behavior of
domain walls pinned at non-topographic defects in Cr(3 nm)/Permalloy(10
nm)/Cr(5 nm) nanowires of width 500 nm. The pinning sites consist of linear
defects where magnetic properties are modified by a Ga ion probe with diameter
~ 10 nm using a focused ion beam microscope. We study the detailed change of
the modified region (which is on the scale of the focused ion spot) using
electron energy loss spectroscopy and differential phase contrast imaging on an
aberration (Cs) corrected scanning transmission electron microscope. The signal
variation observed indicates that the region modified by the irradiation
corresponds to ~ 40-50 nm despite the ion probe size of only 10 nm. Employing
the Fresnel mode of Lorentz transmission electron microscopy, we show that it
is possible to control the domain wall structure and its depinning strength not
only via the irradiation dose but also the line orientation.
