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Chapter 9 Photoemission Electron Microscopy (PEEM) 685

remanent loops were acquired by detecting the XMCD intensity in a

single domain as a function of the strength of the pulsed ﬁ eld. Selected

images of a full loop (about 30–40 images) are shown in Figure 9–24.

After applying a ﬁ eld pulse in the +y direction, dark regions that were

magnetized into the −y direction switched into the +y direction and

became bright. Gray domains were magnetized in the ±x direction and

did not switch, held by the uniaxial anisotropy induced by the anti-

ferromagnet. Below, remanent loops of two selected domains, marked in

the domain pictures, are plotted. The local bias appears as a horizontal

shift of the magnetization loop. The local bias reached values of up to

30 Oe. Also shown is a loop averaged over a large area that shows no

macroscopic bias. The intensity of the loop is only half the intensity of the

local loops because it averaged over ±y domains, which switched, and ±x

domains, which did not switch. A statistical analysis of the local switch-

ing ﬁ eld demonstrated that the average value of the local bias increased

linearly with decreasing inverse domain diameter (Scholl et al., 2004b).

5.4 Linear Dichroism Imaging of Ferromagnets

XMLD was also observed in experiments on ferromagnetic metals

(Schwickert et al., 1998). Since linear dichroism is a quadratic effect

with a 180° periodicity it allows us to distinguish between domains,

which have a 90° rotated magnetization, in contrast to XMCD, which

also distinguishes 180° domains. As an example PEEM images acquired

with X-PEEM of an Fe/NiO(001) sample are shown (Figure 9–25). The

Figure 9–25. X-Ray magnetic circular dichroism (left) and linear dichroism

(right) imaging of ferromagnetic Fe domains. Images were acquired at room

temperature (bottom) and at the Néel temperature of the NiO substrate (top).

686 J. Feng and A. Scholl

images in the left column were acquired using conventional XMCD at

the Fe L

3/2

edges; the right column shows the corresponding XMLD

images, acquired at the left and right side of the Fe L

edge using lin-

early polarized X-rays. In the XMCD image domains appear in three

colors, white, black, and gray, corresponding to domains with a mag-

netization pointing up, down, or left–right; the last are not distin-

guished. In the XMLD image only two colors appear and domains with

a magnetization pointing up or down are not distinguished. The

images in the bottom row were acquired at room temperature, and the

domain structure is a result of exchange coupling to the antiferromag-

netic NiO(001) substrate, which forces the magnetization in the Fe layer

along the four in-plane <110> directions. When the sample is heated

above the Néel temperature of NiO (523 K) the coupling vanishes and

only two Fe domains remain (up and down). Arrows indicate the local

magnetization direction. The XMLD image acquired at elevated tem-

perature shows contrast from Néel-type domain walls (dark lines at

the boundary between 180° domains).

6 Time-Resolved Microscopy

6.1 Introduction

Time-resolved X-ray microscopy of magnetization dynamics, although

still in its infancy, has made great progress over the past years. Storage

rings produce X-ray pulses, which are typically between 30 and 100 ps

long. Already with picosecond pulses interesting magnetic processes

can be studied: magnetostatic modes, magnetization precession, and

magnetic switching (Vogel et al., 2003; Stoll et al., 2004; Choe et al.,

2004; Schneider et al., 2004). On a fundamental level, magnetization

and spin dynamics have become increasingly i mportant in the ﬁ eld of

spin electronics, because the generation, transport, and decay of a spin

polarization are important steps in the function of a spin-electronic

device. The ﬂ ow of energy and angular momentum in a magnetic

material in response to an external excitation are therefore in the focus

of current research (Wolf et al., 2001).

X-Ray dichroism techniques have qualities that make them extremely

valuable for the study of magnetization dynamics. Besides elemental

and chemical speciﬁ city, the XMCD sum rules allow us to disentangle

and quantify spin and orbital momentum in each component of a

multielement system. The sensitivity to both ferro- and antiferromag-

netic order is particularly useful in magnetic oxides, which can be in

either magnetic state, depending on composition or temperature. High

spatial resolution is important for the investigation of nanoscale devices.

In a stroboscopic, or a pump-probe experiment using X-rays, the

response of the sample to an excitation (pump) pulse occurring at a

time t

is measured after a variable delay time ∆t (Figure 9–26). X-Ray

pulses of current synchrotron radiation sources do not contain a sufﬁ -

cient number of photons for a single shot experiment. Therefore, the

experiment has to be repeated at high frequency, the repetition fre-

quency of the X-ray and the excitation pulse, typically many kilohertz

Chapter 9 Photoemission Electron Microscopy (PEEM) 687

or megahertz. The signal is accumulated on the CCD detector over

millions of cycles and then read out. The time evolution of the event is

measured by varying the delay ∆t between pump and probe pulse.

Because of the signal averaging, only the repetitive or periodic compo-

nent of the dynamics of the system is measured. Stochastic events

cannot be measured by stroboscopic techniques. The limitation to

repeatable dynamics is a lesser constraint in mesoscopic magnetic

systems, in which the trajectory of the magnetization is simple and

often unique. In the following, experiments of the vortex dynamics in

small CoFe patterns will be summarized.

6.2 Experimental Setup

We will describe a technique developed at the Advanced Light Source

PEEM-2 (Choe et al., 2004a, 2004b). Similar techniques were developed

in other groups using electronic pulsers to produce fast electrical

pulses in a waveguide, a high bandwidth circuit (Voget et al., 2003;

Stoll et al., 2004; Schneider et al., 2004). At the Advanced Light Source,

optically triggered photoconductive switches were used for the pulse

generation. The photoconductive switches were triggered by a femto-

second laser, synchronized to the X-ray source at a frequency of

125 MHz, producing current pulses with very short rise time. An inter-

digited, ﬁ nger-like switch structure maximized the current. Lithogra-

phy techniques were used to integrate switch, waveguide, and magnetic

structure on the GaAs substrate. The magnetic structures were placed

on top or next to the waveguide in the magnetic ﬁ eld of the current

pulse. The dynamics of the structures were then monitored by X-

PEEM. A schematic drawing of sample and experiment is shown in

Figure 9–27 (Choe et al., 2004c).

6.3 In Situ Measurement of the Field Pulse

The electrical potential difference between the waveguide and the sur-

rounding ground plane during the ﬁ eld pulse caused a deﬂ ection of

the X-ray emitted electrons. The deﬂ ection of the electrons led to an

expansion or breathing of the PEEM image during the pulse. Figure

9–28 (left) shows narrow slices cut from PEEM images of the wave-

guide and assembled in a new image and ordered by delay time. The

measured expansion is proportional to the time-dependent potential

Figure 9–26. Pulse sequence in a laser-synchrotron pump-probe experiment.

A fast optical laser pulse excites the sample (pump) and an X-ray pulse probes

the dynamic after a variable delay ∆t.

688 J. Feng and A. Scholl

of the waveguide, and, assuming a resistive load, the current is pro-

portional to that potential. It can be determined quantitatively since

the average current drawn by the switch is known. To the right of

Figure 9–28, the thus estimated current and the strength of the ﬁ eld

pulse are plotted, showing a fast, rising edge, and a slower exponential

decay, modulated by a small-amplitude ringing, due to an imperfect

impedance match and reﬂ ections at the switch and the ground connec-

tion. The maximum current at 15 V bias was about 200 mA, which cor-

responded to a peak ﬁ eld of 13 mT (130 Oe).

6.4 Vortex Dynamics

Figure 9–29 shows a time sequence of PEEM XMCD images of a

2.5-µm-wide and 20-nm-thick triangular, soft-magnetic FeCo struc-

ture. The original images and images that were processed using a

gradient ﬁ lter are shown. The ﬁ lter enhances intensity gradients, e.g.,

from domain walls. It was observed that the center of the magnetic

vortex, a curling magnetization, which is the ground state of the trian-

gular pattern, rotated on a spiraling or gyrotropic trajectory around

Figure 9–27. Layout of the time-resolved PEEM-2 at the Advanced Light

Source experiment. The laser triggers a biased photoconductive GaAs switch

and generates a ﬁ eld pulse in a waveguide. The PEEM images the local dynam-

ics of the sample using an X-ray probe pulse. [Reprinted with permission from

Figure 9–28. Expansion of the waveguide image caused by the pulsed current

ﬂ owing through the circuit. A pulse with a fast rising edge and less than 500 ps

length was recorded. [Reprinted with permission from Choe et al. (2004c).

Chapter 9 Photoemission Electron Microscopy (PEEM) 689

the pattern center. This rotation of the vortex center occurred at a fre-

quency of 125 MHz, commensurate with the repetition rate of the pump

laser and the ﬁ eld pulse. The vortex dynamics were quantitatively

studied on a variety of patterns, using a particular advantage of full-

ﬁ eld imaging techniques: the ability to study the dynamics of a large

area in parallel. The vortex dynamics were analyzed in detail on four

rectangular structures, which contained a single vortex (Choe et al.,

2004a). Domain images and trajectories of the vortex center are shown

in Figure 9–30. In all patterns a gyrotropic rotation was observed

within the observation range of 8 ns, limited by the time gap between

consecutive ﬁ eld pulses. The trajectories showed an initial acceleration

parallel to the ﬁ eld pulse (blue arrow) and, subsequently, a gyrotropic

rotation of the vortex center around the center of the magnetic struc-

ture. Interestingly, when the trajectories of patterns II and III were

compared, different acceleration directions and rotation senses were

observed in two patterns, which had identical in-plane domain struc-

tures. This was the result of a hidden parameter in the system: the

magnetization in the center of the vortex, called the vortex core. Earlier

microscopic studies observed that the vortex core contained a nanome-

ter-sized region, in which the magnetization rotated out of the plane.

118

The time-resolved PEEM study found that the vortex core magnetiza-

tion is responsible for a handedness or chirality in the vortex pattern,

which governed the ﬁ eld response and picosecond dynamics of the

system. It was observed that a left-handed vortex accelerated parallel

to an applied ﬁ eld, whereas a right-handed vortex accelerated opposite

Figure 9–29. Vortex dynamics in a triangular FeCo structure after an in-plane

ﬁ eld pulse at t = 0. Gradient images are shown next to the XMCD images,

showing domain walls with enhanced contrast.

690 J. Feng and A. Scholl

to the ﬁ eld, explaining the observed, opposite rotation sense in two

patterns with identical in-plane domain structures but opposite core

magnetization. The availability of high-resolution microscopy was

essential for the real-space study of the vortex rotation, because of the

small amplitude of the vortex motion (∼100 nm).

7 Conclusion

X-PEEM as a hybrid technique stands between the worlds of electron

microscopy, from which it inherits its high spatial resolution, and of

X-ray spectroscopy with its sensitivity to the electronic, chemical, and

magnetic structure of solids. It excels at chemical and magnetic imaging

of thin ﬁ lms and surfaces, because of its surface sensitivity and its

sensitivity to the chemical environment and magnetic structure. Fast

dynamics on a picosecond time scale can be measured in pump-probe

style using the short and intense X-ray pulses of modern synchrotron

sources. New, faster X-ray sources and detection schemes are at the

brink of being realized and promise even better temporal resolution.

Figure 9–30. Top: Domain images of the in-plane magnetization of Pattern I (1 × 1 µm

), Patterns II

and III (1.5 × 1 µm

), and Pattern IV (2 × 1 µm

), taken at the speciﬁ ed delay times after the ﬁ eld pulse.

The images are part of a time series extending over 8 ns and were chosen so that the horizontal dis-

placement of the vortex has maximum amplitude. Hands illustrate the vortex handedness and the

out-of-plane core magnetization as determined from the vortex dynamics. Middle: Trajectories of the

vortex core. The dots represent sequential vortex positions (100 ps step). Bottom: Spin structure of a

left-handed (left side) and a right-handed (right side) square vortex. Red arrows represent the preces-

sional torque generated by the external magnetic ﬁ eld (purple arrow). [Reprinted with permission

Chapter 9 Photoemission Electron Microscopy (PEEM) 691

New schemes of aberration correction, employing electron mirrors,

will soon push down the resolution from a current 20–50 nm to only a

few nanometers, opening the door to many exciting applications in the

ﬁ eld of nanoscience.

Acknowledgments. The authors would like to thank H. Ohldag, S.-B.

Choe, and F. Nolting for their contributions to the experimental

work and a rewarding collaboration and in particular W. Wan, A.A.

MacDowell, N. Kelez, and R.D. Schlueter for their contributions to

the PEEM-3 design and development. This work was supported by

the Director, Ofﬁ ce of Science, Ofﬁ ce of Basic Energy Sciences,

Materials Sciences Division, of the U.S. Department of Energy under

Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National

Laboratory.

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