Transverse field-induced nucleation pad switching modes during domain wall injection Page: 1 of 14
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Transverse Field-Induced Nucleation Pad Switching Modes During Domain Wall Injection
M. T. Bryan', P. W. Fry2, T. Schrefl', M. R. J. Gibbs', D. A. Allwood", M.-Y. Im3, P. Fischer3,
'Department of Engineering Materials, University of Sheffield, Sheffield S13JD, United Kingdom
2 Centre for Nanoscience and Technology, University of Sheffield, Sheffield S3 7HQ, United Kingdom
3 LBNL/CXRO, 1 Cyclotron Road, Berkeley, California 94720, USA
We have used magnetic transmission X-ray microscopy (M-TXM) to image in-field
magnetization configurations of patterned Ni80Fe20 domain wall 'injection pads' and attached planar
nanowires. Comparison with micromagnetic simulations suggests that the evolution of magnetic
domains in rectangular injection pads depends on the relative orientation of closure domains in the
remanent state. The magnetization reversal pathway is also altered by the inclusion of transverse
magnetic fields. These different modes explain previous results of domain wall injection into
nanowires. Even more striking was the observation of domain walls injecting halfway across the
width of wider (>400 nm wide) wires but over wire lengths of several micrometers. These extended
Neel walls can interact with adjacent nanowires and cause a switching in the side of the wire
undergoing reversal as the domain wall continues to expand.
Magnetization reversal in patterned ferromagnetic nanowires usually occurs via domain wall
nucleation and propagation from one end (or both ends) of the wire.'2 The switching of the
nanowire can be significantly reduced if a large, magnetically soft pad is fabricated on one of the
wire ends.3-' These 'nucleation pads' reverse at lower fields than a typical isolated nanowire and go
on to introduce a domain wall to the wire from the attached wire end. Domain walls nucleated in a
pad often become pinned at the pad/wire junction, 3,4,6,7 so cannot be introduced into the wire until
a critical 'injection' field is reached. Once this occurs, the domain wall sweeps through the wire,
reversing its magnetization. The injection field is designed to be lower than the nucleation field
without a pad so that devices can be tested without unwanted domain wall nucleation, or device
breakdown, occurring.8" Nucleation pads are, therefore, a simple and convenient way of
introducing domain walls with a clearly defined propagation direction to sometimes complex devices
at low fields. Carefully designed nucleation pads can also be used to control the chirality of injected
vortex domain walls." Nucleation pads vary widely in dimensions and shape, with squares, circles,
ellipses, rectangles and several irregular shapes being investigated.3-''" More than one remanent
magnetic domain configuration, or mode, can be observed within a pad after successive domain wall
injections, with each mode resulting in a particular injection field."
Injection pads are frequently used as part of nanowire devices and experimental structures.
Magnetic-field-driven shift register memory can include an injection pad to write data4 while those
attached to nanowire spiral turn sensors act as both a source and sink of domain walls.5 Both of
these devices use two-dimensional wire circuits and therefore require the use of orthogonal in-plane
magnetic fields to drive domain walls through wires of different orientations. These bi-axial fields
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Bryan, M. T.; Fry, P. W.; Schrefl, T.; Gibbs, M. R. J.; Allwood, D. A.; Im, M.-Y. et al. Transverse field-induced nucleation pad switching modes during domain wall injection, article, March 12, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1014610/m1/1/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.