Imaging of micro- and nanomagnetic strudures
nanomagnetic strudures
C. Konig 0
F. Kiendl 0
U Rudiger 0 1
G. Guntherodt 0
0 II. Physikalisches Institut, Aachen University , Aachen , Germany
1 Fachbereich Physik, Konstanz University , Konstanz , Germany
here is a great interest in understanding the micromagnetic T behavior of thin ferromagnetic layers, driven by their technological exploitation in magnetic field sensors and magnetic random access memory devices (MRAM). As a high storage density is a key goal in the design of such memory devices, recent research has been concentrating on the investigation of ferromagnetic elements with submicrometer lateral dimensions. For a magnetic storage cell to define a bit (0 or 1), it must have two stable remanent magnetization states that are uniformly magnetized, i.e. form a single magnetic domain, independent of its magnetic history. Apart from other powerful magnetic imaging techniques, we focus on two particular techniques to investigate the magnetic domain configurations of micro- and nanomagnetic structures: Magnetic Force Microscopy (MFM) [1,2] and Scanning NearField Optical Microscopy (SNOM) [3,4].
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MFM
The MFM detects the magnetic stray fields (blue arrows in Fig
ure 1) which are generated at the edges of magnetic elements
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europhysics news NOVEMBER/DECEMBER 2003
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(bits) or in domain walls. For this purpose a small magnetic nee
dle (tip) which is magnetized along the needle axis is attached to
the end of a cantilever. This tip is being attracted or repelled by the
magnetic stray fields pointing into or out of the surface of the
magnetic element (bit), respectively. The deflection of the can
tilever resulting from this dipole-dipole-interaction is detected by
a laser beam which is reflected at the upper side of the cantilever
and hits a four-segment detector. Figure 1 shows schematically
how the magnetic information is extracted from this signal. To
avoid a cross talk by "atomic force" information the MFM first
scans the topography.
The magnetic information is subsequently recorded by lifting
the tip to a height of approximately 90 nm and scanning along the
topographic profile obtained in the first scan. This method yields
the typical MFM images with "dark and bright contrast" depend
ing on direction and strength of the magnetic stray fields.
Figure 2 shows an MFM image of microstructured Fe( 11 0)
ellipsoid-shaped elements with lateral dimensions of 1.5 flm
500 nm. Starting with a 25 nm thick iron film with (110) orienta
tion these elements have been fabricated by using electron beam
lithography and argon ion etching [
6
]. After applying a magnetic
field of 1 Tesla perpendicular to the long axis of the elements - i.e.
along the magnetic hard axis - the MFM image exhibits three dif
ferent remanent magnetic states for zero magnetic field (see white
frame in Figure 2).
If the bright contrast represents an attractive interaction
between the tip and the stray fields the upper element in the white
frame shows a single domain state (or dipole state) with the mag
netization pointing to the right side as illustrated in the upper left
inset in Figure 2. According to this the magnetization of the
lower element in the white frame is pointing to the right. The ele
ment in the middle shows a less pronounced magnetic contrast.
This is an evidence for a multi domain state (or demagnetized
state). By forming such a state the stray fields are minimized by
dosing the magnetic flux within the element (see the upper right
inset in Figure 2).
SNOM
When a magnetic sample is illuminated with polarized light, it
rotates the polarization of the transmitted (Faraday effect) and
reflected (Kerr effect) light by an amount proportional to its mag
netization. Therefore, a pattern of differently magnetized
domains produces a contrast in a polarization-sensitive magneto
optical (MO) microscope. For this microscopy to work, neither a
vacuum nor atomically clean surfaces are required. It can be used
in an external magnetic field to probe the dependence of the
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domain pattern on the field. Also, unlike in MFM, there is no dan
ger that the domain pattern is altered by the imaging process
itself. Any optical microscopy, however, is limited in its resolution
by diffraction. Every illuminated spot on the sample produces its
own diffraction pattern, which cannot be distinguished from the
diffraction pattern of another spot located less than ")..,,/2 away,
where").." is the light's wavelength. Smaller structures therefore can
not be resolved.
Scanning near-field optical microscopy (SNOM) eliminates the
need to distinguish di (...truncated)