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A new technique of magnetic imaging on a spin-stand [Mayergoyz et al., J. Appl. Phys. 87, 6824 (2000)] is further developed and extensively tested. The results of successful imaging of digital patterns overwritten with misregistration ranging from 0.3 to 0.07 μm are reported. The results are compared with magnetic force microscopy (MFM) images and the conclusion is reached that the spin-stand imaging technique can provide (at least) the same level of resolution and accuracy as the MFM imaging technique. © 2001 American Institute of Physics.
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Spin-stand imaging of overwritten data and its comparison with magnetic
force microscopy
I. D. Mayergoyza) and C. Tse
Electrical and Computer Engineering Department, University of Maryland, College Park, Maryland 20742
C. Krafft
Laboratory for Physical Sciences, College Park, Maryland 20740
R. D. Gomez
Electrical and Computer Engineering Department, University of Maryland, College Park, Maryland 20742
A new technique of magnetic imaging on a spin-stand Mayergoyz et al., J. Appl. Phys. 87, 6824
2000兲兴 is further developed and extensively tested. The results of successful imaging of digital
patterns overwritten with misregistration ranging from 0.3 to 0.07
m are reported. The results are
compared with magnetic force microscopy MFMimages and the conclusion is reached that the
spin-stand imaging technique can provide at leastthe same level of resolution and accuracy as the
MFM imaging technique. © 2001 American Institute of Physics. DOI: 10.1063/1.1359233
It has long been recognized that the imaging of magne-
tization patterns recorded on hard drive disks is a source of
valuable information that may enhance our understanding of
recording processes and assist in the design of new recording
systems. The magnetization imaging is routinely performed
by using magnetic force microscopy MFM.1It has been
realized that MFM has the following intrinsic limitations: 1
low rate of image acquisition, 2special requirements for
the preparation of the sample to be imaged, and 3virtual
impracticality of fast accumulation of numerous images of
the same target area in order to increase the signal-to-noise
ratio.
Recently, a new technique of magnetic imaging on a
spin-stand has been developed.2In this technique, raw image
acquisition is performed by scanning a target area of a hard
drive disk by a magnetoresistive MRhead in the along- and
cross-track directions. Scanning in the along-track direction
is realized due to the rotation of the disk, while scanning in
the cross-track direction is achieved by using very small and
accurately controlled radial displacements of the head. As a
result of this scanning mechanism, the spin-stand imaging
technique has the following advantages over conventional
MFM imaging: a high rate of image acquisition, increase in
the signal-to-noise ratio due to multiple imaging of the same
target area, and performance of imaging under similar con-
ditions as in conventional hard disk drives. However, due to
the nonlocalized nature of the magnetoresistive head in the
cross-track direction, the collected images can be quite dis-
torted. In addition, the collected images are scalar in nature,
while magnetization distributions are vector fields. For these
reasons, the collected images must be treated as raw images,
and image reconstruction is needed in order to retrieve the
actual magnetization distributions from the raw images. The
image reconstruction technique is based on the response
function characterization of the MR reading element and can
be described as follows.
The position of the scanning MR element can be identi-
fied by the xand ycoordinates of its center. The recorded
magnetization distribution can be characterized by the
equivalent distribution of virtual magnetic charges
m(x,y):
mx,y⫽⫺
0hdivM,1
where his the thickness of the recording media and it is
tacitly assumed that the recorded magnetization is uniform
over the media thickness and, for this reason, divMhas the
meaning of ‘‘surface’’ divergence.
The distribution of virtual magnetic charges is equiva-
lent in the sense that they create the same magnetic field as
the actual magnetization distribution. This magnetic field
causes the signal collected by the MR element. This signal
can be viewed as the superposition of the signals due to the
elementary magnetic charges distributed over the disk sur-
face. The last assertion can be mathematically expressed as
follows:
Sx,y
冕冕
Rxx,yy
mx,ydxdy.2
Here S(x,y) is the signal of the MR element, while
R(xx,yy) can be interpreted as the response function
of the MR element. This function has the physical meaning
of the signal induced in the MR element at position (x,y)by
the point unit magnetic charge located at position (x,y).
In order to experimentally determine the response func-
tion, an isolated sharp transition is first written. This transi-
tion is then trimmed by using dc erasure on both sides of the
same track. As a result, a ‘‘tiny’’ isolated spot of magnetic
charges is written that can be viewed as an approximation to
a point charge. The MR reading element can now be used to
measure the signal as a function of relative position with
respect to the recorded ‘‘point’’ magnetic charge. This signal
can be interpreted as a scaled version of R(xx,yy).
An example of the spin-stand measurement of the response
function of the MR element is shown in the left plot of Fig.
1. To get the information about geometric dimensions of the
aElectronic mail: isaak@eng.umd.edu
JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001
67720021-8979/2001/89(11)/6772/3/$18.00 © 2001 American Institute of Physics
recorded tiny spots of magnetic charges, the magnetic force
microscope was used. An example of the MFM image of the
‘‘point’’ magnetic charge is shown in the right plot of Fig. 1.
It is worthwhile to mention that we were able to record and
measure the response function for such tiny spots of mag-
netic charges that their counterpart MFM images were found
to be elusive and could not be clearly observed.
Having determined the response function, formula 2
can be viewed as a convolution integral equation that relates
the raw image S(x,y) to the distribution of virtual magnetic
charges
m(x,y) which in turn is related to the divM
through Eq. 1. There is no way to reconstruct the actual
vectorial field of recorded magnetization by using only for-
mulas 1and 2. To circumvent this difficulty, we have
used the known fact3–5 that only the curl-free component of
magnetization can be retrieved from MR measurements. This
is because the curl-free component of magnetization distri-
bution is the field producing part of the total magnetization
distribution. For this reason, only this component is sensed
by the MR element. The last statement can be best illustrated
by an example of a dc erased track. In this example, there
exists nonzero magnetization within the track, however, this
FIG. 2. ColorReconstructed image of F6 overwritten by F9: 0.15
m
misregistration.
FIG. 3. ColorReconstructed image of F6 overwritten by F9: 0.09
m
misregistration.
FIG. 4. ColorReconstructed image of F6 overwritten by F9: 0.07
m
misregistration.
FIG. 1. ColorSpin-stand image of a head response function of a MR
element left plotand a MFM image of a tiny spot of magnetic charges
right plot.
6773J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Mayergoyz
et al.
track does not produce any magnetic field. This is because
the magnetization within the track is divergence-free but not
curl-free.
Thus Mxand Myin formula 1are meant to satisfy the
additional equation:
curlzM
My
x
Mx
y0. 3
Now, by using Eqs. 13, the measured response
function Rof the MR reading element, and the Fourier trans-
form technique, the curl-free component distribution of mag-
netization can be fully retrieved from the scalar raw image
S(x,y). The mathematical details of the reconstruction tech-
nique can be found in our previous work.2
The imaging technique described above has been imple-
mented and extensively tested using a Guzik model 1701 MP
spin-stand. The main emphasis has been on imaging of edge
areas of tracks overwritten with small misregistrations rang-
ing from 0.3 to 0.07
m. In our experiments, giant magne-
toresistive GMRheads produced by ALPSwith write
widths of 1.1
m and read widths of 0.7
m have been used.
First, F6 patterns hexadecimal F611110110 in binary no-
tationwere recorded and then they were overwritten by F9
patterns hexadecimal F911111001 in binary notation
with controlled misregistrations ranging from 0.3 to 0.07
m. The overwritten tracks were scanned and the collected
raw images were reconstructed. Figures 2, 3, and 4 show the
reconstructed images of F6 patterns overwritten by F9 pat-
terns with misregistration of 0.15, 0.09, and 0.07
m, respec-
tively. These figures show only the Mxalong-trackcompo-
nent of magnetizations. In order to emphasize the binary
nature of the patterns, the color contrast of the images of Mx
has been deliberately saturated. The artifact of this saturation
is that noise has also been enhanced, as can be seen from
Figs. 2, 3, and 4.
To assess the accuracy and the resolution of the de-
scribed spin-stand imaging technique, extensive comparison
of this technique with MFM imaging has been carried out.
The sample results of this comparison are shown in Figs. 5
and 6 for misregistrations of 0.3 and 0.15
m, respectively.
Any such comparison, however, should be carried out in the
context that MFM images represent magnetic charges of the
patterns while the reconstructed spin-stand images are the
magnetization distributions of the patterns. Still, it remains
apparent from Fig. 6 that remnants of the overwritten F6
patterns are barely visible on the MFM image, while on the
spin-stand image see Fig. 2these remnants are well-
pronounced with many interesting details. This comparison
suggests that the developed spin-stand imaging technique has
at least the same level of resolution and accuracy as the
MFM imaging technique and it is clearly superior to the
latter as far as the rate and conditions of image acquisition
are concerned.
1D. Rugar, H. J. Mamin, P. Guethner, S. E. Lambert, J. E. Stern, I. Fadyen,
and T. Yogi, J. Appl. Phys. 68,11691990.
2I. D. Mayergoyz, C. Serpico, C. Krafft, and C. Tse, J. Appl. Phys. 87,
6824 2000.
3R. Madabhushi, R. D. Gomez, E. R. Burke, and I. D. Mayergoyz, IEEE
Trans. Magn. 32, 4147 1996.
4I. A. Beardsley, IEEE Trans. Magn. 25, 671 1989.
5I. D. Mayergoyz, A. A. Adly, R. D. Gomez, and E. R. Burke, J. Appl.
Phys. 73, 5799 1993.
FIG. 5. ColorMFM image of F6 overwritten by F9: 0.3
m misregistra-
tion. FIG. 6. ColorMFM image of F6 overwritten by F9: 0.15
m misregistra-
tion.
6774 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Mayergoyz
et al.
... 10,11 Spin-stand magnetoresistive microscopy is another magnetic imaging technique, which utilizes a conventional magnetic read/write head (RWH) of a hard disk drive (HDD) as a sensor and a spinning disk as the sample. 12,13 However, the requirement for laminar air flow during sample measurement, a) Author to whom correspondence should be addressed. Electronic mail: dmitriy.mitin@physik.uni-augsburg.de and the restriction to samples that can be rotated at a high angular velocities under the head, adds utilization complexity that limits this technique to samples with extremely flat surfaces. ...
... Please note that the obtained values are representing the full system resolution, including errors from write pole fringing fields and grain size distribution of the media. 12,13 Ultimately, the spatial magnetic imaging resolution of the SMRM is limited by the physical dimensions of the TMR sensor. ...
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