X-Ray Microscopic Imaging
of Magnetic Domains
Using X-Ray Magnetic Circular Dichroism∗
P. Fischer1 , T. Eimüller1 , G. Schütz1 , G. Schmahl2 , P. Guttmann2 ,
D. Raasch3
1
2
Univ. Augsburg, Exp. Phys. II, Memmingerstr. 6, D-86135 Augsburg, Germany
Univ. Göttingen, Forschungseinrichtung Röntgenphysik, Geiststr. 11, D-37073
Göttingen, Germany
3
Philips Res. Labs, Weisshausstr. 2, D-52066 Aachen, Germany
Abstract. The combination of a transmission X-ray microscope based
on the zone plate technique providing a spatial resolution of ≈ 30nm
with the effect of X-ray magnetic circular dichroism, which gives a huge
contrast in the absorption mode is a new method to visualize in a quantitative and element-selective manner magnetic domains. We report on
the study of the variation of the shape and the magnetization of domains
in an applied magnetic field in a magneto-optical GdFe layered system
with a lateral resolution of 50nm.
1
Introduction
The magnetism of systems of reduced dimensionality (surfaces, thin films, multilayers, etc.) is attracting both scientific interest and technical importance. The
available techniques to prepare layers with defined structures and to characterise their morphology allows new insights into the correlation of the complex
microstructure with macroscopic properties, as e.g. the occurence of giant magnetoresistance (GMR), oscillatory exchange effects and especially the magnetic
anisotropy perpendicular to the surface.
One important issue in understanding the micromagnetic performance is the
imaging of domains, wall structures and their dynamics driven by an applied
external field. Static domain properties can be visualized with very high spatial
resolution down to several nm by powerful methods, as e.g. Scanning Electron
Microscopy with Polarization Analysis (SEMPA), Lorentz microscopy and Magnetic Force Microscopy (MFM) which, however, are inherent sensitive to the
surface or restricted to very thin layered magnetic structures. The dynamics
of the domains as a function of an external magnetic field can be studied by
Kerr-microscopy but with the resolution limit of optical microscopy.
Here we report on a development of a novel domain imaging technique using
the transmission X-ray microscope (TXM) at BESSY I in combination with a
contrast enhancement via X-ray magnetic circular dichroism (X-MCD). First
results on a GdFe alloy are presented, where the hysteresis properties of the
?
This work is funded by BMBF proj. no. 05 621 WAA and 05 644 MGA.
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P. Fischer et al.
domain pattern with a spatial resolution of 50nm and their long term variation
have been studied in real time transmission mode.
2
Principle of Dichroic Contrast
X-ray magnetic circular dichroism (X-MCD), i.e. the dependence of the corelevel absorption coefficient of circular polarized photons on the
sample magnetization is a powerful X-ray spectroscopic method to study the
magnetic characteristics of the electronic structure[1, 2].
Its physical origin is closely related to the polar magneto-optical Kerr effect.
However, since in the X-MCD the initial state is a well defined core level state, XMCD features an element-sensitivity and according to dipole selection rules also
a symmetry-selectivity. Due to angular momentum conservation and spin-orbit
interaction in the initial state the absorption of a circular polarised photon in an
unpolarised initial state causes a photoelectron to exhibit an expectation value of
both the spin and the orbital momentum projected onto the photon propagation
direction. Hence X-MCD spectroscopy allows to determine with high relative
accuracy (< 10−2 µB ) in appropriate cases as Fe,Co,Ni local magnetic moments
separated into spin and orbital contributions invoking magneto-optical sum rules
[3, 4].
The magnetic contrast can be described as the energy-dependent deviation
∆µ(E) of the absorption coefficient relative to the polarization averaged absorption coefficient µ|i> (E). It can be expressed by
σc
∆µ
(E) =
(E)(m̂ · êz )Pc
µ|i>
σ|i>
(1)
where (m̂ · êz ) denotes the projection of the normalized magnetic moment m̂ =
m
|m| onto the propagation direction (êz ) of the photons with a degree of circular
polarization Pc . The magnetic absorption cross section normalized to the polarc
ization averaged atomic cross section σσ|i>
reaches at the maximum of the Fe
σc
metal L3 edge σ|2p > (E = 706eV ) ≈ 23% [5]. Following Eq.(1) the value of
∆µ
µ|i>
3/2
is a sensitive measure of m̂ · êz , if Pc is known. Taking into account the
value of the absolute magnetic moment in the pure Fe-metal of |m| = 2.2µB
the observable experimental quantity reflects the absolute value of the spatial
magnetization distribution.
Hence the dichroic effect in transmission can serve as a quantitative magnetic
contrast suited even for multicomponent systems in spectroscopic x-ray imaging
techniques which employ the absorption in the vicinity of an absorption edge.
Previous attempts reported in literature have succesfully combined the magnetic dichroism in photoemission [6, 7] to image magnetic domains with a resolution of some µm.
X-Ray Microscopic Imaging of Magnetic Domains
3
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Experimental Details
The sample investigated was a binary Gd27.7 Fe72.3 system with a Curie temperature of Tc ∼ 510K. The amorphous film with thickness h = 59 ± 1nm was
prepared by coevaporation from two electron-gun sources onto a 325nm polyimid foil as substrate and embedded between 10 − 15nm Al layers for chemical
protection. The out-of-plane coercivity Hc was determined both by Faraday effect, VSM and Kerr magnetometry measurements and in-situ with the X-ray
microscope yielding ∼ 6(2)mT with a shift of the hysteresis loop of ∼ 8(2)mT ,
which can be attributed to remaining magnetically hard regions.
Polychromatic X-Radiation
Monochromator
Pinhole
Mask
B-Field
Object
Image
Field
CCD
Camera
Image
Micro Zone Plate
Circular Polarized Light
Condensor Zone Plate
Fig. 1. The X-ray optical set-up of the TXM at BESSY I.
The x-ray optical set-up of the x-ray microscope (Fig. 1) which is based on the
zone-plate technique is described in more detail in [8, 9]. It consists of a condensor
zone-plate (D = 9mm) and a pinhole (d = 20µm), acting together as a linear
monochromator with the monochromaticity given by λ/∆λ = D/2d = 225.
Furthermore a microzone plate as a high resolution x-ray objective generates
a magnified image of the object in the image field with a spatial resolution of
about 50nm, where it is recorded with a slow scan CCD camera with a thinned,
backside illuminated CCD chip (DQE ≈ 70%) [10]. The total extinction of the
radiation in the target I/I0 for the GdFe system at a photon energy of E =
706eV was determined to 90% with a contribution due to Fe L3 absorption
of 70%. The magnetic images were taken at the Fe L3 -edge (≈ 706eV ) with
exposure times for the x-ray images shown in Figs.2–6 of 1 − 3s. The upper part
of the condensor was masked so that the sample was illuminated by circular
polarized X-rays with a degree of circular polarization of about 60%. Magnetic
fields up to 80mT with the field direction pointing parallel/antiparallel to the
photon beam propagation direction could be applied onto the sample.
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P. Fischer et al.
Results and Discussion
While at photon energies below the Fe L3 absorption edge the spatial intensity distribution is homogenous, significant structures appeared at E = 706eV
as demonstrated in Figs. 2 and 3. The applied external magnetic fields were
12mT and 25mT below −Hc , resp. The sign of the magnetic field corresponds
to the magnetization consistent with the dichroic effect. Since the photons are
left-handed the net magnetization of the darker/lighter shaded domains point
into/out of the paper plane, i.e. antiparallel/parallel to the photon beam direction.
Fig. 2. Image of magnetic domains
(field of view 7 × 7µm2 ) at a magnetic
field 12mT below −Hc .
Fig. 3. Image of magnetic domains
(field of view 7 × 7µm2 ) at a magnetic
field close to the saturation.
The domains in Fig. 2 appear with irregular spotted patterns of average
widths of the white (light) domains of D ∼ 0.5µm corresponding to a reduced
domain size of D/h ∼ 10. These serpentine-like structures tend to more wormlike ones with increasing field as seen in Fig. 3. The width of these worms of
100nm remain relatively stable by approaching the saturation field only a partly
shortening of the tails with magnetic after-effects of some minutes are observed.
Then they collapse suddenly at a field larger than 25mT below −Hc . Going back
from the negative high field limit the state of the saturation (homogenous black
(dark) picture) remains until the value of −Hc is reached. Then suddenly within
0.2mT the irregular patterns are back again. A series of selected images within a
complete hysteresis loop are shown in Fig.6. Figs. 2 and 3, which have been taken
in different hysteresis loops, demonstrate that there is no evidence for domain
pinning as the locations of the domains especially the worm like structures are
completely irreproducable although the global characteristic of the pattern does
not change.
0.00
0.00
-0.05
-0.05
(µ−µsat)x
(µ−µsat)x
X-Ray Microscopic Imaging of Magnetic Domains
-0.10
-0.10
-0.15
-0.15
-0.20
-0.20
12
13
14
(µm)
15
16
Fig. 4. Absorption scan across the region indicated in Fig.2.
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9.5
10.0
(µm)
10.5
Fig. 5. Absorption scan across the region indicated in Fig.3.
In Fig. 4 the absorption profile (µ − µsat ) along the line marked in Fig. 2 is
shown. Hereby the intensity pattern of Fig. 2 is normalized to those for complete
magnetization corresponding to fully dark image, where the maximum absorption µsat is achieved. An analysis of the wall profiles proves that their widths is
significantly smaller than the experimental resolution of 50nm [11].
p This is in excellent agreement with the estimation of the wall width δdw = π A/Ku ∼ 30nm
depending on the exchange constant A = 3.1 ± 0.8 · 10−12 J/m, which has
been determined by domain expanding and collapsing experiments [12], and the
anisotropy constant Ku ≈ 0.33 · 105 J/m3 , which has been determined on reference glass substrates using a torque magnetometer. This demonstrates that the
micromagnetic performance of the sample prepared on the polymere substrate
does not differ much from the sample prepared on the glass substrate.
The difference of the absorption for reversed domain magnetization in Fig.4
yields values of ∼ 17%. Taking into account only the Fe L3 absorption and
the finite value of degree of circular polarization, the strengths of the dichroic
effects corresponds to m̂ · êz = 1 in eq.(1) proofing the complete alignment of
the magnetic Fe moments onto the photon beam direction, i.e. perpendicular to
the plane of the target.
5
Outlook and Conclusion
The combination of X-MCD with TXM allows to image magnetic domains with a
resolution of 50nm (improvable to better than 20nm and to study the dynamics
of the formation of magnetic domains in real-time and applied external field. The
bulk-sensitivity and element-selectivity of this technique enables to investigate
the concentration dependent magnetization distributions and the quantitative
analysis of the magnetization distribution. The separation of spin and orbital
moment distributions seems to be feasible at spin-orbit correlated absorption
edges providing a unique method to learn more about the role of orbital polarization and spin-orbit effects on the micromagnetic performance especially for
thin layers.
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P. Fischer et al.
Fig. 6. Series of pictures (field of view diameter 16.9µm) taken at different applied
magnetic field strengths within a hysteresis loop. The dark and light regions correspond
to different orientations of the local magnetization.
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