Magnetic microstructures and their dynamics studied

by X-ray microscopy

P. Fischer *, D.-H. Kim, B. Kang, W. Chao, E.H. Anderson

E.O. Lawrence Berkeley National Laboratory, Center for X-ray Optics, 1 Cyclotron Road, Berkeley, CA 94720, USA

Abstract

Full-field soft X-ray microscopy in combination with X-ray magnetic circular dichroism as contrast mechanism is a powerful technique to

image with elemental specificity magnetic nanostructures and multilayered thin films at high lateral resolution down to 15 nm by using Fresnel

zone plates as X-ray optical elements. Magnetization reversal phenomena on a microscopic level are studied by recording the images in varying

external magnetic fields. Local spin dynamics at a time resolution below 100 ps can be addressed by engaging a stroboscopic pump-and-probe

scheme taking into account the time pattern of synchrotron storage rings. Characteristic features of magnetic soft X-ray microscopy are reviewed

and an outlook into future perspectives with regard to increased lateral and temporal resolution is given.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: X-ray microscopy; Magnetization reversal; Spin dynamics; X-ray optics; X-ray magnetic circular dichroism

1. Introduction

Magnetic microstructures of low dimensional magnetic

systems is currently a crucial issue both from a fundamental

physical point of view but moreover in a technologically

relevant context. Particularly magnetic thin layers, multi-

layered films, and nanostructures are subject to intensive

studies of fundamental aspects of magnetism since they exhibit

intriguing properties like interlayer exchange coupling (Grun-

berg et al., 1986), the origin of magnetic perpendicular

anisotropies, spin injection, etc. To illustrate the technological

relevance, the giant magneto-resistance effect (Baibich et al.,

1988) found in layered magnetic thin films could be

implemented into read-head technology shortly after its

discovery, thus enabling a further miniaturization of the device

and a concomitant increase of storage density. Similar trends

refer to current developments of magnetic sensors, actuators

and magnetic storage devices, where an increase of perform-

ance and versatility is achieved by tailoring specific elemental

components and decreasing the dimensionality. Laterally

patterned magnetic elements in the sub-micrometer range are

promising candidates in the field of magnetic random access

memory (MRAM) technologies, where in addition to the

0968-4328/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.micron.2005.10.005

* Corresponding author. Tel. C1 510 486 7052; fax: C1 510 486 4550.

E-mail address: pjfischer@lbl.gov (P. Fischer).

charge of the electron the spin and its transport properties will

play a dominant role (Prinz, 1998). The preparation of such

elements benefits from the availability of sophisticated

techniques like e.g. e-beam lithography originally developed

in semiconductor physics.

In addition to the static behavior and the morphology of

micromagnetic structures their fast dynamics is receiving

particular interest recently. Although the time scale span from

years which are of importance, e.g. for reliability of magnetic

storage media, scientifically the more relevant and challenging

time scale is currently extending from the sub-nanosecond

regime where precessional and relaxation phenomena of

magnetic nanosystems take place down to the femtosecond

regime, which corresponds to the time scale of exchange

interactions.

Though there is a huge variety of thin film magnetic

systems, common to almost all of them is that they are mostly

composed either of 3d transition metals, like Fe, Co, Ni, and Cu

or, in combination with rare-earth elements, like Gd, Tb or Dy.

The systems of interest comprise either single thin (less than

100 nm) magnetic films or stacks of several different thin

layers, thus tailoring the functionality via the different

magnetic coupling between each layers.

Since imaging magnetic microstructure on a nanometer

spatial and sub-nanosecond time scale is an outstanding

challenge, consequently, an abundance of powerful imaging

techniques have been established so far. They can be classified

according to the probes that they use. There are electron

microscopies, like scanning electron microscopy with

Micron 37 (2006) 296–300

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P. Fischer et al. / Micron 37 (2006) 296–300 297

polarization analysis (SEMPA), transmission electron micro-

scopies acting as Lorentz microscopies, spin polarized

scanning tunneling microscopies (SP-STM) or the photo-

electron emission microscopy (PEEM), optical microscopies

like scanning near field optical microscopies (SNOM) or Kerr

microscopies and probe microscopies such as magnetic force

microscopy (MFM).

To meet the challenges mentioned above magnetic imaging

techniques have to provide a high lateral and time resolution,

high sensitivity in combination with large magnetic contrast

and elemental specificity and the ability to record the

microstructures in varying sample environment such as

external magnetic fields and temperatures.

In this review, we describe recent advances with magnetic

full-field soft X-ray transmission microscopy (M-TXM) which

fulfils most of these criteria. It combines X-ray magnetic

circular dichroism (X-MCD) with a high-resolution trans-

mission X-ray microscope (Fischer et al., 1996, 2001) thus

offering high lateral and temporal resolution for images with a

large field of view and elemental selectivity. Selected examples

obtained with the soft X-ray microscope XM-1 located at the

Advanced Light Source in Berkeley to study magnetization

reversal processes and spin dynamics in magnetic nanostruc-

tures and thin film systems will be presented.

2. Experimental details

The work described here has been performed with the full-

field soft X-ray microscope located at beamline 6.1.2 at the

Advanced Light Source in Berkeley, CA. Its schematic optical

setup is shown in Fig. 1.

Polychromatic X-radiation from the bending magnet of the

synchrotron ring is focused onto the sample through a

condenser zone plate (CZP) lens. The present CZP has a

diameter of 9 mm, an outer zone width of 55 nm, and 41,000

zones. Due to the chromatic aberrations of zone plates the CZP

and a pinhole (typically a squared Si3N4 membrane with a side

length between 10 and 20 mm) near the sample plane (typically

Fig. 1. Experimental setup of the full-field soft X-ray transmission microscope at

nanomagnetism, materials and environmental science and biology.

250mm from the sample plane) form a linear monochromator.

The illumination energy can be changed in the range between

250 and 900 eV with a measured spectral resolution of E/DEZ700 by mechanically shifting the distance between the

condenser and the pinhole/sample.

The radiation passing through the sample is projected

through the micro zone plate (MZP) onto a CCD camera. The

outermost zone width of the MZP determines largely the lateral

resolution obtained with such X-ray optics. Recent advance-

ments in the nanotechnology to prepare such Fresnel optics by

e-beam lithography techniques has pushed the resolution limits

down to 15 nm (Chao et al., 2005). The CCD camera is a

2048!2048 pixel array which is back-thinned and back-

illuminated. It has a quantum efficiency of approximately 60–

70% in the range of energies that the microscope operates.

The MZP acts as an X-ray lens and generates a typically

2000-fold magnified image into the image field. The field of

view of the microscope is typically 10 mm in diameter. A

preorientation of the sample’s positions and focus can be

achieved with a custom Zeiss Axioplan visible light

microscope which is mutually indexed with the sample stage

of XM-1. The X–Y position accuracy is typically 2 mm over a

3 mm field with a focal accuracy of 1 mm.

To image magnetic domains X-ray magnetic circular

dichroism (X-MCD) is used as magnetic contrast mechanism.

Two modifications have to be applied. In ferromagnetic

systems the photoabsorption cross-section for circularly

polarized X-rays depends in the vicinity of element specific

binding energies of inner core atomic levels corresponding to

L2 and L3 absorption edges strongly on the relative orientation

between the projection of the magnetization onto the photon

propagation direction and the helicity of the photons.

Circularly polarized X-radiation is provided at a bending

magnet by the off-orbit emitted radiation with an estimated

degree of circular polarization of about 50–60%.

Since the degree of polarization is reversed for radiation

emitted above and below the orbital plane (Fig. 2) this allows

one to modulate the magnetic contrast and thus to reduce

the Advanced Light Source in Berkeley, CA that is used for applications in

Fig. 2. Degree of circular polarization versus vertical distance to the orbital

plane of the storage derived from MTXM images recorded with a 2 mm wide

aperture in front of the CZP positioned at various vertical distances to the

orbital plane (Kang et al., 2005).

Fig. 3. M-TXM images of an amorphous GdFe layer (59 nm thickness). (a)

Magnetic domain structure at the Fe L3 edge, (b) at the Gd M5 edge. The bar

corresponds to 1 mm (Fischer, 2003).

P. Fischer et al. / Micron 37 (2006) 296–300298

non-magnetic background contributions to the images (Kang et

al., 2005). Additionally, it could be used in the future to set up a

lock-in scheme for time-resolved studies.

Magnetic fields of in principle any strength and direction

can be applied to the sample thus allowing to record images

within complete hysteresis loops. So far at the XM-1 the

solenoids provide magnetic fields up to several hundred kA/m.

To image in-plane magnetized domains, which are the most

favorable configurations for magnetic systems of low

dimensionality a sample stage to tilt the sample up to 308

relative to the photon propagation direction is available at the

XM-1.

To study the magnetization dynamics on a sub-nanosecond

time scale the inherent pulsed time structure of the storage ring

is used in a stroboscopic pump-and-probe scheme. The pump is

an electronic pulse with a rise time at about 100 ps that is

launched into a microcoil or into striplines so as to generate a

short magnetic field pulse at the location of the sample to be

studied. After a variable delay time the sample is flashed with

an X-ray pulse (probe) which then sees the actual magnetiza-

tion state of the specimen. Since at third generation

synchrotron sources one single pulse does not provide enough

photons to generate a full X-ray image this pump-probe

sequence is repeated many times, however this requires that the

sample fully relaxes into its groundstate before the next pump-

probe sequence starts. The repetition frequency in the so-called

two bunch mode operation of the storage ring is at 3 MHz. In

that mode only two electron bunches are circulating in the ring

with a lifetime of about 2 h each carrying an initial current of

about 20 mA.

3. Results

In the following the specific features of magnetic full-field

soft X-ray transmission microscopy will be illustrated by

typical results.

The elemental specificity of X-MCD as the magnetic

contrast mechanism, which is inherent to any analytical

techniques which is based upon X-MCD, such as X-ray

scattering, X-ray absorption or X-ray reflectivity can be used to

either address the local magnetization of individual com-

ponents in multicomponent, amorphous ferromagnetic systems

or, in the case of multilayered thin film systems, to serve as a

layer sensitive magnetic probe.

Fig. 3 shows M-TXM images obtained both at the Fe L3

(706 eV) (a) and at the GdM5 (1189eV) (b) edges of an

amorphous Gd25Fe75 system prepared by magnetron sputtering

onto a 350 nm polyimid membrane (Fischer et al., 2003).

Limited by the available flux at the bending magnet the

illumination time needed for each image amounts to a few

seconds only. With an on-line contrast of more than 25% given

by the maximum dichroic absorption close to the absorption

edge dark and light areas can be clearly identified, which can be

attributed to magnetic domains, where the direction of the local

Fe magnetization points in and out of the paper plane,

respectively.

A morphologically identical domain pattern is found at the

corresponding M5 edge of Gd (Fig. 3b), which is to be expected

for such an amorphous system. However, although the spin-

orbit coupling of both edges is identical, i.e. parallel, the

observed reversal of magnetic contrast between the Fe and the

Gd results can be explained by taking into account the well-

known antiparallel coupling between the RE (Gd) and the TM

(Fe) atoms.

As a pure photon based technique, a major advantage of

M-TXM is the capability to record high resolution magnetic

images in any varying external magnetic fields, which gives

information on the magnetization dependent evolution of

magnetic domains within a complete hysteresis loop. This is of

utmost interest both for fundamental studies of micromagnet-

ism and in particularly for technologically relevant magnetic

systems in magnetic sensor and storage applications.

As a typical example Fig. 4 shows results from a study of the

reversal behavior in stray-field coupled magnetic microcon-

tacts (Meier et al., 2004). These two-layer systems were

prepared on silicon nitride (Si3N4) membranes consisting of

11 nm Al as seed layer, 30 nm Fe as lower and 30 nm Ni as top

ferromagnetic layer. A 6 nm thick Al cap serves as protection.

The contacts were defined by electron-beam lithography,

in situ electron-beam evaporation of the two ferromagnetic

Fig. 4. (a) Micromagnetic calculations of the magnetic domain structure in 2!

2 mm2 and 2!4 mm2 sized microcontacts deposited 0.4 mm apart at an external

field of K8 mT. (b)–(d) Corresponding M-TXM images at K8, 0 and 3.8 mT

(Meier, 2004).

P. Fischer et al. / Micron 37 (2006) 296–300 299

layers at a rate of 0.1 nm/s with a base pressure in the 10K8

mbar range, and lift-off processing.

Fig. 4 shows the M-TXM results obtained at the Fe L3 edge

of two 2!2 and 2!4 mm2 sized microcontacts deposited

0.4 mm apart and the corresponding micromagnetic simulation

for the external magnetic field of K8 mT (Meier et al., 2004).

The M-TXM images were recorded at various external

magnetic field between positive and negative saturation while

the field was applied along the short axis of the elements. As a

result of the patterning process the shapes show deviations

from perfect rectangles and the two ferromagnetic layers are

not perfectly aligned. The element specificity of MTXM

provides a signal from the Fe layer solely. Because of strong

parallel coupling of the two ferromagnetic layers within the

observed field range of G40 mT the domain structure in both

layers is identical and can thus serve to interpret the reversal of

the contact systems. As input parameters for the micromagnetic

simulations obtained with OOMMF (Donahue and Porter,

1999) using the fully three-dimensional code for the deposited

layer sequence we used a saturation magnetization of 1700 kA/

m (490 kA/m), an anisotropy constant of 48.0 kJ/m3 (K5.7 kJ/

m3), an exchange constant of 21!10K12 J/m (9!10K12 J/m),

and a cell size of 10 nm in each direction for the iron (nickel)

layer, respectively. The evaporation process yields

Fig. 5. Top: Series of images taken at various delay times showing the time varia

Cu/50 nm Co) squared element. Bottom: Corresponding micromagnetic simulation

polycrystalline films with virtually no texture thus justifying

the choice of a random distribution of anisotropy axes in the

simulations.

Although the M-TXM results shown in Fig. 4b–d clearly

show a complex magnetic domain structure due to the size of

the elements and the stray-field dominated behavior of the

magnetically weak ferromagnetic behavior of Fe, across the

400 nm gap a symmetric magnetic coupling in the closure

domains of the upper and lower contacts is clearly visible.

Micromagnetic simulations taking into account the aforemen-

tioned parameters allowed us to determine the stray field

strength in the distance of 400 nm away from the lower contact

to be 1.1 mT, obviously sufficient to cause the parallel

coupling.

Spin dynamics, i.e. the temporal development of the

magnetization is described by the Landau–Lifshitz–Gilbert

equation (LLG) of motion which takes into account a

precession of the magnetization in an external magnetic field

Heff quantified by the well known gyromagnetic ratio g and the

relaxation and damping of the system after excitation which is

generally characterized by a phenomenological constant a.

This damping constant depends strongly on the local geometry,

anisotropy and morphology and the mechanisms governing the

relaxation processes are only poorly understood so far.

Fig. 5 shows a typical results of time-resolved magnetic

images with M-TXM. The system that we have studied was a

4!4 mm2 [3 nm Al/50 nm Ni80Fe20 (PY)/2 nm Cu/50 nm Co]

element (Stoll et al., 2004). It was patterned by e-beam

lithography onto a 100 nm thin Si3N4 membrane. The images

were recorded at the Fe L3 absorption edge (706 eV). A micro-

coil with an inner diameter of 6 mm surrounding the element

was prepared onto the same membrane and created a magnetic

field pulse of about 100 kA/m pointing perpendicular to the

surface of the magnetic element.

The magnetic ground state configuration in squared

ferromagnetic PY elements is a four domain closure domain

pattern (Landau state) with its magnetization direction lying in

the element’s film plane. The magnetic field pulse created in

the microcoil points perpendicular to the static magnetization.

Therefore, the associated magnetic torque that launches the

precessional motion described in the LLG equation tips the

tion of the z-component in a 4!4 mm2 (3 nm Al/50 nm Ni80Fe20 (PY)/2 nm

s (OOMMF) (Stoll et al., 2004).

P. Fischer et al. / Micron 37 (2006) 296–300300

magnetization out of the plane. In order to eliminate the static

magnetization the elements were viewed with the photon

propagation direction (z-axis) perpendicular to the film plane,

so that only the time varying z-component of the magnetization

showed up in the images.

The top panel of Fig. 5 shows a series of M-TXM images

taken at various delay times between the pump pulse and the

X-ray probe. Each of the displayed image consists of an

accumulation of approx 500 samples illuminated for 4 s with a

pump-probe frequency of 3 MHz, which amounts to roughly

109 stroboscopic cycles. To enhance the magnetic contrast the

direction of the magnetic field pulse was inverted for every

second image. At a delay time of K400 ps, i.e. the time when

the X-ray flash hit the sample before the magnetic pulse was

delivered, no magnetic contrast shows up since the sample is

still in its relaxed Landau ground state. However, at positive

time delays pronounced features show up indicating the time

evolution of the magnetic domain structures. Interestingly two

different features can be observed that can be attributed to

different precessional modes, out of which one is located at the

domain wall regions. This is consistent with findings from

time-resolved Kerr microscopy.

The lower panel of Fig. 5 displays micromagnetic

simulations of the time evolution in these elements, which

clearly reproduce the experimental findings taking into account

the relevant parameters for the PY element (Stoll et al., 2004).

The pronounced vortex structure with a size of approximately

10 nm at the center of the squared elements cannot be resolved

with our current experimental resolution.

4. Conclusion and outlook

Full-field magnetic soft X-ray microscopy exhibits a

combination of features that are essential to study the

nanomagnetic systems both for fundamental and applied

reasons. The major advantage is the high lateral resolution

that will approach the 10 nm scale in the near future by

achievements with nanotechnology to fabricate Fresnel zone

plates with high efficiency and small outermost zone width

both for the condenser and the microzone plate optics. The time

resolution is currently limited by the performance of current

synchrotron radiation sources. This allows for intense studies

of domain wall motions e.g. in confined geometries and

spintronic logical elements. In-situ time resolved magnetiza-

tion reversal studies require a picosecond time resolution,

which will be accessible at improved synchrotron sources.

Finally, at X-ray free electron laser systems or similar fourth

generation synchrotron sources, the ultimate femtosecond

regime is feasible, that will allow fundamental insight into

the time scale of exchange interactions.

Acknowledgements

The continuous help of the technical staff of CXRO and

the ALS is highly appreciated. We would like to thank G.

Meier, M. Barthelmes, R. Eiselt, M. Bolte (U. Hamburg) for

preparing excellent samples. The time resolved studies were

performed in collaboration with H. Stoll, A. Puzic, B.v.

Waeyenberge (Dept Schutz MPI Stuttgart) and J. Raabe, M.

Buess, T. Haug, R. Hollinger, C.H. Back, D. Weiss

(U Regensburg).

B.S. Kang would like to thank the Korean Institute of

Science & Technology Evaluation and Planning (KISTEP) and

Ministry of Science & Technology (MOST) of Korean

government through national nuclear fellowship of Korean

nuclear R&D program for financial support.

This work was supported by the Director, Office of Science,

Office of Basic Energy Sciences, of the US Department of

Energy under Contract No. DE-AC03-76SF00098.

References

Baibich, M.N., Broto, J.M., Fert, A., Nguyen Van Dau, F., Petroff, F., Eitenne,

P., Creuzet, G., Friederich, a., Chazelas, J., 1988. Giant Magnetoresistance

of (001)Fe/(001)Cr Magnetic Superlattices. Phys. Rev. Lett. 61,

2472–2475.

Chao, W., Harteneck, B.H., Liddle, J.A., Anderson, E.H., Attwood, D.T., 2005.

Soft X-ray microscopy at a spatial resolution better than 15 nm. Nature 435,

1210–1213.

Donahue, M.J., Porter, D.G., 1999. OOMMF User’s Guide, Version 1.0

Interagency Report NISTIR 6376, National Institute of Standards and

Technology, Gaithersburg, MD.

Fischer, P., Schutz, G., Schmahl, G., Guttmann, P., Raasch, D., 1996. Imaging

of magnetic domains with the X-ray microscope at BESSY using X-ray

magnetic circular dichroism. Z.f. Physik B 101, 313–316.

Fischer, P., Eimuller, T., Schutz, G., Denbeaux, G., Lucero, A., Johnson, L.,

Attwood, D., Tsunashima, S., Kumazawa, M., Takagi, N., Kohler, M.,

Bayreuther, G., 2001. Element-specific imaging of magnetic domains at 25

nm spatial resolution using soft X-ray microscopy. Rev. Sci. Instr. 72 (5),

2322–2324.

Fischer, P., Eimuller, T., Schutz, G., Denbeaux, G., 2003. Imaging magnetic

domain structures with Soft X-ray microscopy. Struct. Chem. 14 (1), 39–47.

Grunberg, P., Schreiber;, R., Pang, Y., Brodsky, M.B., Sowers, H., 1986.

Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of

Fe Layers across Cr Interlayers. Phys. Rev. Lett. 57, 2442–2445.

Kang, B.S., Kim, D.H., Anderson, E., Fischer, P., Cho, G., 2005. Polarization

modulated magnetic soft X-ray transmission microscopy, Appl. Phys. Lett.

Meier, G., Eiselt, R., Bolte, M., Barthelmes, M., Eimuller, T., Fischer, P., 2004.

Comparative study of magnetization reversal in isolated and strayfield

coupled microcontacts. Appl. Phys. Lett. 85, 1193–1195.

Prinz, G.A., 1998. Magnetoelectronics. Science 282, 1660–1663.

Stoll, H., Puzic, A., v.Waeyenberge, B., Fischer, P., Raabe, J., Buess, M.,

Haug, T., Hollinger, R., Back, C.H., Weiss, D., Denbeaux, G., 2004. High

resolution imaging of fast magnetization dynamics in magnetic nanos-

tructures. Appl. Phys. Lett. 84, 3328–3330.