Japanese Journal of Applied Physics
Vol. 44, No. 9, 2005, pp. L 285–L 287
#2005 The Japan Society of Applied Physics
Direct Modification of Magnetic Domains in Co Nanostructures
by Atomic Force Microscope Lithography
Yasushi T AKEMURA, Satomi H AYASHI, Fuminori O KAZAKI, Tsutomu Y AMADA and Jun-ichi S HIRAKASHI1
Electrical and Computer Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
1
Electrical and Electronic System Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
(Received October 13, 2004; accepted December 27, 2004; published February 10, 2005)
The direct modifications of magnetic domain structures and magnetic anisotropy in Co-based nanostructures were studied. A
Co rectangular structure of 5.1 mm length 0.8 mm width was fabricated from a 15-nm-thick Co thin film deposited on a Pt
layer on a SiO2 /Si substrate by conventional processes of electron beam lithography and lift-off. The magnetic domain
structure of the Co rectangle was divided by atomic force microscope (AFM) nano-oxidation. It was found that the magnetic
domain structures could be controlled by AFM lithography. It was also proposed that the magnetic anisotropy could be
directly modified by the nanolithography technique. [DOI: 10.1143/JJAP.44.L285]
KEYWORDS: ferromagnetic thin film, nanostructure, nanolithography, atomic force microscope, Co thin film, magnetic domains,
magnetic anisotropy, magnetic force microscope
Lithography techniques using scanning probe microscopes have attracted much interest as novel tools for
fabricating well-defined nanoscale devices and materials.
Nanoscale fabrication techniques have been important in the
development of magnetic devices such as high-density
recording systems, memories and spin-related devices.
When a negative-biased voltage is applied to a conductive
atomic force microscope (AFM) cantilever, a metal thin film
is oxidized by an electrochemical reaction between the metal
and water in air. This anodic oxidation process is an
electrochemical reaction between metals and water in air. As
this fabrication technique does not require any pretreatments
such as resist coating, it is a useful tool for direct
modification of a metal surface.1–6) The nanostructures of
oxides fabricated by this technique can be used for potential
barriers in planar-type tunnel junctions,7) which have been
demonstrated to be single-electron devices operating at room
temperature.8)
We have reported the fabrication of nano-oxide structures
based on ferromagnetic metals9,10) and ferromagnetic planartype magnetic tunnel junctions.11) The Coulomb blockade
and single-electron charging effect were successfully observed in Ni-based double-tunnel junctions.12) In order to
develop these spin-related devices, the observation and
control of magnetic domain structures in ferromagnetic
nanostructures are important. In this study, the control of
magnetic domains in Co-based nanostructures by AFM
nanolithography is reported.
Co thin films of 15 nm thickness were deposited by an rf
sputtering method on thermally oxidized Si substrates.
Before the deposition of Co, a Pt bottom electrode of
10 nm thickness was deposited on the substrate in order to
conduct an electric current from the conductive AFM
cantilever to adjacent nanostructures as shown in Fig. 1.
The Co thin films exhibited no uniaxial magnetic anisotropy
and a coercive force of 20 Oe from a vibrating sample
magnetometer (VSM) measurement. Co-based rectangular
nanostructures of 5.1 mm length and 0.8 mm width were
fabricated by employing a conventional combination of
electron beam lithography and lift-off processes.
After the Co rectangles were magnetized along their
longitudinal axes by applying an external magnetic field,
(a)
AFM tip
Co oxide
Co dot
15nm
10nm
Pt
SiO2
Si
(c)
[nm]
(b)
20
10
0
100nm
200nm
Fig. 1. Schematic of nano-oxidation technique using atomic force microscope and structure of Co rectangle prepared on Pt layer on SiO2 /Si
substrate (a). AFM image (b) and height profile (c) of the Co oxide wire
fabricated on Co film are also shown.
magnetic domain structures under their remanent states were
observed by magnetic force microscopy (MFM). MFM was
performed in air at room temperature. In order to evaluate
the MFM images, a micromagnetic simulation was performed using the object-oriented micromagnetic framework
(OOMMF) available from the National Institute of Standards
and Technology.
The MFM image of the Co rectangle of 5.1 mm length 0.8 mm width exhibited a so-called S-shaped domain structure as shown in Fig. 2(a1). Figure 2(a2) is a schematic of
the magnetic domain structure supposed from Fig. 2(a1). Co
oxide nanowires were then fabricated across the Co
rectangle by the AFM nano-oxidation technique as shown
in Fig. 1(a). A conductive cantilever coated with Au was
used. The applied voltage for the cantilever was 4 V and
L 285
L 286
Jpn. J. Appl. Phys., Vol. 44, No. 9 (2005)
(a1)
1µ m
(a2)
(b1)
(c1)
(c2)
(b2)
(c1)
(c2)
Y. T AKEMURA et al.
by the arrows in Fig. 2(b1), the Co rectangle was separated
into three parts by two Co oxide nanowires and the magnetic
domain was also divided into three parts as shown by the
MFM image in Fig. 2(b1). The magnetic properties of the
Co oxide fabricated by this technique could not be measured,
because it was small and surrounded by the ferromagnet of
Co. Figure 2(b2) shows one example of the stable magnetic
domain structures obtained from the OOMMF simulation.
Figures 2(c1) and 2(c2) show the calculated results on the
magnetization configurations and MFM images of the dotted
square areas labeled by (c1) and (c2) in Fig. 2(b1),
respectively. These calculated domain structures agreed
with the observed image shown in Fig. 2(b1).
Figure 3(a) shows the MFM image of the Co rectangle
with two additionally drawn Co-oxide nanowires. The white
arrows indicate the positions of the additional nanowires. In
this case, the separated rectangles of Co have smaller aspect
ratios of length/width. Each separated rectangle has a larger
edged-domain area, which is well fitted by the calculations
as shown in Figs. 3(b) and 3(c). Figure 4(a) shows the MFM
image of the area specified by the white dotted line in
Fig. 3(a). The aspect ratio, length/width, of the isolated Co
rectangle is approximately 0.35. Because of its narrow width
and shape magnetic anisotropy, the rectangle is supposed to
be magnetized along the width direction as depicted by the
simulated magnetization configuration and MFM image
shown in Figs. 4(b) and 4(c), respectively. Although the
observed MFM image is not so clear, it is proposed that the
magnetic anisotropy in magnetic nanostructures can be
modified by AFM nanolithography.
In conclusion, a direct modification of magnetic domain
structures by AFM nano-oxidation was investigated. Magnetic domain structures of a Co rectangle were divided by
AFM nano-oxidation. It was found that AFM nanolithography can control the magnetic domain structure, which is
useful in the fabrication of spin-related devices.
Fig. 2. MFM images and magnetic domain structures of the Co
rectangular nanostructure of 5.1 mm length 0.8 mm width before (a1)/
(a2) and after (b1)/(b2) nano-oxidation, respectively. Black arrows
indicate the positions of nanowires of Co oxide fabricated by AFM.
Calculated results on magnetization configurations and MFM images of
square areas labeled by (c1) and (c2) in (b1) are also shown.
(a)
the current during the oxidation was of the order of 1 pA or
less.13,14) Details of an experiment on AFM nano-oxidation
including the applied voltage dependence on the size of
nano-oxides have been previously reported.10) It has also
been reported that the fabricated nanostructures had insulating properties from current-mapping images. This shows
that the nanostructures are oxides and that the metal coated
on the cantilever is not deposited on the sample surface. The
AFM image and height profile of the Co oxide wire are
shown in Figs. 1(b) and 1(c), respectively. The width of the
nanowire was approximately 100 nm. The thickness (height)
of the fabricated Co oxide nanostructure was 25–40 nm,
which suggested that the oxidation reached at least the
bottom of the 15-nm-thick Co film and that the Pt layer was
partially oxidized. The height profile indicated that the
cantilever did not scratch the top of the Co nano-oxide. After
drawing the nano-oxide wires whose positions are indicated
1µm
(b)
(c)
Fig. 3. MFM image of Co rectangular nanostructure of 5.1 mm length 0.8 mm width after nano-oxidation. Black and white arrows indicate the
positions of fabricated nano-wires of Co oxide. Calculated images of
magnetization configuration (b) and MFM (c) are also shown.
Jpn. J. Appl. Phys., Vol. 44, No. 9 (2005)
(a)
1 µm
(b)
(c)
Fig. 4. MFM image of Co rectangle corresponding to rectangular area
indicated by dotted line in Fig. 3(a). Calculated images of magnetization
configuration (b) and MFM image (c) are also shown.
Y. TAKEMURA et al.
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