Direct comparison of domain wall behavior in permalloy nanowires patterned
by electron beam lithography and focused ion beam milling
M. A. Basith,
a)
S. McVitie, D. McGrouther, J. N. Chapman, and J. M. R. Weaver
School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom and School of
Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, United Kingdom
(Received 19 May 2011; accepted 15 August 2011; published online 18 October 2011)
Nominally identical permalloy nanowires, with widths down to 150 nm, were fabricated onto a
single-electron transparent Si
3
N
4
membrane using electron beam lithography (EBL) and focused
ion beam (FIB) milling. Transmission electron microscopy (TEM) experiments were performed to
compare the nanostructures produced by these two techniques in what we believe is the first direct
comparison of fabrication techniques for nominally identical nanowires. Both EBL and FIB meth-
ods produced high quality structures with edge roughness being of the order of the mean grain size
5–10 nm observed in the continuous films. However, significant grain growth was observed along
the edges of the FIB patterned nanowires. Lorentz TEM in situ imaging was carried out to compare
the magnetic behavior of the domain walls in the patterned nanowires with anti-notches present to
pin domain walls. The overall process of domain wall pinning and depinning at the anti-notches
showed consistent behavior between nanowires fabricated by the two methods with the FIB struc-
tures having slightly lower characteristic fields compared to the EBL wires. However, a significant
difference was observed in the formation of a vortex structure inside the anti-notches of the EBL
nanowires after depinning of the domain walls. No vortex structure was seen inside the anti-
notches of the FIB patterned nanowires. Results from micromagnetic simulations suggest that the
vortex structure inside the anti-notch can be suppressed if the saturation magnetization (M
s
)is
reduced along the nanowire edges. A reduction of M
s
along the wire edges may also be responsible
for a decrease in the domain wall depinning fields. Whereas the two fabrication methods show that
well-defined structures can be produced for the dimensions considered here, the differences in the
magnetic behavior for nominally identical structures may be an issue if such structures are to be
used as conduits for domain walls in potential memory and logic applications.
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C
2011 American
Institute of Physics. [doi:10.1063/1.3642966]
I. INTRODUCTION
Understanding and controlling magnetic domain wall
(DW) behavior in ferromagnetic nanowires is of fundamental
scientific interest and important for their potential applicabil-
ity in future spintronic devices such as magnetic logic
1
and
race-track memory.
2
Micromagnetic simulations,
3–5
magneto-
optic Kerr effect magnetometry,
6,7
off-axis electron hologra-
phy,
8
and magnetic imaging techniques
9,10
have contributed
useful information for a greater understanding of its proper-
ties. To ensure the reliable operation of devices, performance
variability must be reduced and high-quality nanofabrication
is extremely important. Previous reports
11,12
indicate that the
structural roughness at the edges of the nanostructures pro-
duced by fabrication processes affect the magnetic properties
of the nanostructures. Among a variety of nanofabrication
techniques, electron beam lithography (EBL) is widely used
for high-resolution submicron scale patterning of magnetic
materials. This technique requires a number of steps including
resist spinning, pattern exposure, metallization, removal of
resist from the sample surface, and lift-off of the residual
materials. In contrast, focused ion beam (FIB)-based fabrica-
tion by milling is essentially a one-step patterning process of
a continuous film and an excellent tool for rapid device proto-
typing.
9
Nevertheless, FIB has some disadvantages,
13
espe-
cially because of the heavy Ga
þ
used for the milling process.
These include radiation-induced damage and ion implanta-
tion. The extent to which the magnetic properties of nano-
structures are affected by the differences in physical
properties produced by patterning structures using EBL and
FIB techniques has not been studied extensively. Therefore,
in the present investigation, nominally identical permalloy
nanowires fabricated by these two nanofabrication techniques
are compared through characterization of their physical nano-
structure and magnetic behavior.
II. EXPERIMENTAL DETAILS
The permalloy (Ni
80
Fe
20
) film of 20-nm thickness was
deposited by thermal evaporation onto a single-electron
transparent Si
3
N
4
membrane for the fabrication of the nano-
wires. The structure of the fabricated nanowires is shown in
Fig. 1(a), which contains three 320-nm-wide and 140-nm-
high rectangular protrusions, referred to as anti-notches.
Two wire widths were chosen for study, 320 and 150 nm,
and both ends of the wire were connected to diamond-
shaped pads to allow control over the formation of DWs.
Nanostructures were fabricated first by an EBL/lift-off
a)
Electronic mail: m.basith@physics.gla.ac.uk.
0021-8979/2011/110(8)/083904/8/$30.00
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2011 American Institute of Physics110, 083904-1
JOURNAL OF APPLIED PHYSICS 110, 083904 (2011)