Effect of geometry on magnetic domain structure in Ni wires with perpendicular anisotropy:
A magnetic force microscopy study
S. H. Lee, F. Q. Zhu, C. L. Chien, and N. Marković
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
Received 28 January 2008; revised manuscript received 21 March 2008; published 18 April 2008
We investigated the magnetic domain structure of thermally evaporated nickel wires with perpendicular
anisotropy as a function of width and geometry. Magnetic force microscope images revealed the presence of
stripe domains, which tended to orient themselves either perpendicular or parallel to the edges of the wires.
This is in agreement with the result of the minimization of the total magnetic energy of a wire near an edge,
which predicts the minima of energy in these two particular cases. The general orientation of the stripes in
wider wires can be manipulated by using an in-plane external field, but the stripe orientation in the vicinity of
the edge stays unaffected. A rough edge forces the stripe domains to orient themselves perpendicular to the
edge, rather than parallel to it. In narrow wires, the stripe domains are parallel to the edge, and the width of the
domain increases as the width of the sample is decreased in order to fit an integer number of domains in the
wire.
DOI: 10.1103/PhysRevB.77.132408 PACS numbers: 75.60.Ch, 75.47.Np, 75.75.+a
The magnetic domain structure in nanometer-scale mag-
netic elements strongly depends on the size and shape of the
elements.
1
The magnetostatic energies associated with the
edges of the sample become very important in small
samples,
2–5
which allow their magnetic properties to be con-
trolled by the geometry of the sample.
6–11
While most mag-
netic thin films have in-plane magnetic anisotropy due to the
preponderance of the shape anisotropy, certain thin films
have the less common perpendicular magnetic anisotropy
PMA,
12
such as thin films of Fe on Cu100,
13
Co / Pt
multilayers,
14
epitaxially grown magnetic thin films,
15
and
thick single Ni crystals.
16
The control of the magnetic prop-
erties of materials with PMA is particularly relevant to per-
pendicular magnetic recording. There are only a handful of
studies on the effects of geometry
17–20
in materials with
PMA. A few recent studies focused on the dynamics close to
the edges.
21,22
As is typically found for systems with PMA,
these films are characterized by stripe domains, which reflect
the periodic change of magnetization direction throughout
the sample.
23
In this work, we study the effect of geometry
on the orientation of magnetic stripe domains in thermally
evaporated Ni wires. We find that the size and the orientation
of the stripe domains depend on the lateral size of the wire,
due to the fact that the edge effects start to dominate as the
width of the wire is decreased.
Ni wires of various widths were prepared by using stan-
dard electron-beam lithography. The width of the wires
ranged from 200 nm to 6 m. Some Ni wires were depos-
ited by electron-beam evaporation with a base pressure of
2 10
-8
Torr and show similar results. The Ni wires were
examined by using the magnetic force microscope MFM
immediately after fabrication to determine the magnetic do-
main structure in their as-prepared state. MFM images were
taken by using a Nanoscope III multimode atomic force mi-
croscope from Digital Instruments. A Veeco microetched sili-
con probe tip was magnetized along the tip axis by using a
permanent magnet and used in the vibrating-lift mode. The
tip was kept at a height between 30 and 50 nm above the
surface of the sample. The MFM image of a set of wires of
eight different widths is shown in Fig. 1. It is evident that the
magnetic domains form stripes of dark and bright regions of
opposite magnetizations. The stripe domains are typically
observed in systems in which the magnetization is perpen-
dicular to the plane of the substrate. Perpendicular anisotropy
has been confirmed by the magnetometry measurements in
thermally evaporated nickel films of similar thicknesses.
Figure 1 also seems to suggest that the orientation of the
stripe domains depends on the width of the wire. In the wid-
est wire Fig. 1a, the stripes are randomly oriented and
meander in the plane of the film. As the width of the wire is
decreased, the stripe domains tend to orient themselves per-
pendicular to the long edge of the wire Figs. 1b–1d. As
the width of the wire becomes comparable to the domain
width, the stripes start to turn increasingly parallel to the
edge of the wire Figs. 1e–1g. The thinnest wire Fig.
1h appears to contain only one domain.
The general orientation of the stripe domains can be un-
derstood by a closer examination of the stripes near the edges
of the samples. Very close to the edge, the stripes are ori-
ented either perpendicular or parallel to the edge, regardless
of the stripe orientation in the bulk of the film. This behavior
FIG. 1. Color online Magnetic force microscope images of
thermally evaporated Ni wires showing stripe domains in their as-
prepared state. The general orientation of the stripe domains
changes as the width of the wire changes. a w =6 m, b w
=2 m, c w =1 m, d w =800 nm, e w =600 nm, f w
=400 nm, g w =300 nm, and h w =200 nm.
PHYSICAL REVIEW B 77, 132408 2008
1098-0121/2008/7713/1324084 ©2008 The American Physical Society 132408-1