Multi-Color Imaging of Magnetic Co/Pt Multilayers [original]

IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017 6500905

Multi-Color Imaging of Magnetic Co/Pt Multilayers

D. Weder1, C. Von Korff Schmising1, F. Willems1, C. M. Günther2, M. Schneider1,B.Pfau

1,A.Merhe

3,4,

E. Jal3,4, B. Vodungbo3,4, J. Lüning3,4,B.Mahieu

5, F. Capotondi6,E.Pedersoli

6, and S. Eisebitt1,2

1Max-Born-Institute Berlin, 12489 Berlin, Germany

2Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany

3Sorbonne Universités, UPMC Université Paris 06, UMR 7614, LCPMR, 75005 Paris, France

4CNRS, UMR 7614, LCPMR, 75005 Paris, France

5Laboratoire d’Optique Appliquée, ENSTA ParisTech, CNRS, Ecole Polytechnique,

Université Paris-Saclay, 91762 Palaiseau Cedex, France

6Elettra-Sincrotrone Trieste, 34149 Trieste, Italy

We demonstrate for the first time the realization of a spatial resolved two color, element-specific imaging experiment at the

free-electron laser facility FERMI. Coherent imaging using Fourier transform holography was used to achieve direct real space

access to the nanometer length scale of magnetic domains of Co/Pt heterostructures via the element-specific magnetic dichroism in

the extreme ultraviolet spectral range. As a first step to implement this technique for studies of ultrafast phenomena we present

the spatially resolved response of magnetic domains upon femtosecond laser excitation.

Index Terms—Co/Pt, demagnetization, free-electron lasers (FEL), holography, magnetic domains, magnetic multilayers, perpen-

dicular magnetic anisotropy, X-ray magnetic circular dichroism (XMCD).

I. INTRODUCTION

ULTRATHIN perpendicularly magnetized heterostruc-

tures, such as the two-compound multilayer system

Co/Pt, show fascinating physical properties due to strong

spin-orbit coupling [1]. Co/Pt multilayers have a significant

interface contribution to the perpendicular magnetic anisotropy

which can be used for the control of domain wall motion

arising from an interplay of Dzyaloshinskii–Moriya interac-

tion [2], [3] at the Co/Pt interfaces and spin currents [4].

Also based on these mechanisms room-temperature skyrmions

can be created in thin Co/Pt films in racetrack memories [5].

Optical induced ultrafast spin currents in ferromagnetic thin-

film structures pave the way for future ultrafast spintronic

devices [6] and broadband terahertz emitters [7]. Moreover,

Co/Pt thin-film magnetism has a huge potential in magnetic

storage devices. Not only because of the fulfilled requirements

of long-term stability due to the strong magnetic anisotropy

but also because of recently discovered helicity-dependent

all-optical control of ferromagnetic multilayers [8], [9]. How-

ever, many underlying microscopic mechanisms on ultrafast

time and nanometer length scale are not fully understood and

currently under debate. In particular, the origin of all-optical

helicity-dependent switching or the fundamental microscopic

processes leading to an optically induced ultrafast demagne-

tization on a femtosecond time the scale of ferromagnetic

materials [10] raise many intriguing questions. To gain more

insight into the interaction between non-local spin currents

and interacting multi-compound perpendicularly magnetized

Manuscript received March 10, 2017; revised April 13, 2017; accepted

April 19, 2017. Date of publication May 18, 2017; date of current

version October 24, 2017. Corresponding author: D. Weder (e-mail:

weder@mbi-berlin.de).

Color versions of one or more of the figures in this paper are available

online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2017.2699560

heterostructures requires novel techniques that give a direct

and simultaneous view of the element-specific magnetization

on a femtosecond time and nanometer length scale.

Here, we present the first experimental demonstration of

two-color imaging with Fourier transform holography (FTH)

in the extreme ultraviolet (XUV) spectral range measured

at the free-electron laser (FEL) facility FERMI in Trieste,

Italy. The reconstructed magnetic domain pattern gives simul-

taneously element specific and real space access to the

magnetization of a two-compound Co/Pt multilayer system.

Furthermore, a time-resolved single-color approach reveals

spatially resolved frames of the optical induced ultrafast

demagnetization dynamics of Co/Pt. The feasibility and the

prospect of combining both, multi-color spectroscopy with

nanometer resolution and femtosecond spatial resolution are

discussed.

II. EXPERIMENTAL SETUP AND RESULTS

The coherent imaging experiment was performed at the FEL

facility FERMI at the end-station DiProI [11], [12] combining

femtosecond temporal and nanometer spatial resolution in an

optical pump—XUV probe setup. To image the magnetic

domains of a Co/Pt multilayer an FTH technique in trans-

mission configuration [13]–[15] were used. In this imaging

technique a hologram is formed by interference of an object

with a known reference wave. In the XUV to soft X-ray

spectral range this readily yields spatial resolutions on the

order of approximately 30 to 80 nm. For optimal stability

the sample and the reference holes are manufactured in a

monolithic mask design. The 250 nm thick gold mask with

no transmission in the XUV spectral range is evaporated on a

30 nm thin silicon nitride membrane supported by a 200 μm

thick silicon substrate. A circular hole with a diameter of dr=

2μm is drilled in the mask via a focused ion beam and defines

This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/

6500905 IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017

Fig. 1. Sketch of the FTH setup carried out at the FEL at FERMI. FEL

radiation in the XUV spectral range is created in the undulators Rad 1 and 2,

illuminating the Co/Pt multilayer that is structured with a holographic gold

mask. The diffracted radiation centered at the Co M-edge at 60 eV originating

from the reference wave and object wave interferes on a CCD detector placed

53 mm behind the sample. A beam stop cuts out the intense direct beam. The

difference hologram between left and right circular polarization contains only

magnetic information.

the object hole (field of view). Afterwards, the magnetic film

Al(10)/Pt(2)/[Co(0.6)/Pt(0.8)]20/Al(3) nm is deposited on the

opposite side of the membrane via magnetron sputtering.

Due to prior sample characterizations via magnetic force

microscopy and small angle X-ray scattering the domain size

is determined to be on the order of 80 nm. In the last step five

reference holes with diameters ranging from 60 to 80 nm are

drilled through the entire sample. Each reference will generate

a twin-image of the object in the reconstruction leading to ten

reference-object cross correlations. To increase the signal to

noise ratio of the reconstruction, images stemming from the

different reference holes can be added. To avoid an overlap of

the reconstructed objects with the object–object or reference–

reference auto correlation in the center of the reconstruction,

all references were arranged in a circle of |r|=5μm diameter

around the object hole, equidistantly placed with respect to

each other.

The time-resolved single-color and static two-color coherent

imaging experiment uses probe pulses in the XUV spectral

range. The radiation is tuned to the Co M-edge and Pt N-edge

for resonant small angle scattering due to the magnetic circular

dichroism (MCD) effect. Magnetic sensitivity to Pt is con-

fined to the Co/Pt interface where band hybridization induces

magnetism in Pt. The scattered intensity of reference and

object holes form the interference pattern (hologram), which

is recorded on a charge-coupled device (CCD) detector placed

53 mm behind the sample (Fig. 1). The difference image

between holograms recorded with left/right circularly polar-

ized radiation yields the magnetic information. To increase

the dynamic range of the recordable intensity and to avoid

damage to the CCD detector a beam stop was used to block

the intense direct beam.

The achievable spatial resolution is determined by the size

of the reference hole and the maximum-recorded scattering

vector qmax which is determined by

qmax =4π

λsin θmax (1)

with the wavelength λand the maximum scattering angle

θmax that is given by the size of the CCD chip and the

sample-CCD distance. The experimental setup enables a max-

imum recordable wave vector of qmax =77 μm−1at 20.8 nm

(Co M-edge) and qmax =92 μm−1at 17.3 nm (Pt N-edge)

leading to resolvable spatial frequencies of d=2π/qmax =

82 and 68 nm, respectively. Also note that in the XUV spectral

range wave guiding effects of the reference hole may further

influence the spatial resolution.

As a significant number of photons with large scattering

angles are required, we average several hundreds to thousands

of FEL pulses to form the hologram. At the same time,

the XUV intensity of the FEL pulses in the imaging experi-

ments has to be reduced to prevent non-reversible changes and

damage of the magnetic domain network [16], [17].

A. Time-Resolved Single-Color Imaging of Co/Pt

For the time-resolved single-color imaging of Co/Pt we

optically excite with laser radiation centered at 800 nm.

The laser-triggered ultrafast dynamics are probed by coherent

XUV radiation at the M-edge of Co at 60 eV. The laser fluenc

was set to approximately 3 mJ/cm2. The pulselength of the

optical pump beam and the XUV probe beam is on the order

of 100 fs. For each helicity we integrated 10 min with a 10 Hz

repetition rate, i.e., 6000 pulses.

In Fig. 2(a), we show the spatially resolved reconstruction

of the magnetic domains in an unpumped state 1 ps before

time zero. The clearly resolved magnetic domains with a

known periodicity of 160 nm allow a conservative estimation

of the spatial resolution below 80 nm. Black and white areas

represent out-of-plane worm-like domains pointing in and out

of the surface while the surrounding gray area represents

the border of the field of view. The cross section of the

domain contrast (line) is displayed in more detail in Fig. 3(b).

In Fig. 2(b), one can see the reconstructed domains within

the very same field of view but in an excited state 1 ps

after time zero. As the contrast settings of both images

are scaled in the same way a direct comparison between

the pumped and unpumped domain pattern is possible. The

excited Co/Pt film shows a strongly reduced magnetic domain

contrast caused by ultrafast demagnetization of the up and

down domains. This is further emphasized by the direct

comparison of the cross sections of the unpumped and pumped

contrast, shown in solid and dashed-dotted lines, respectively,

in Fig. 3(b). To investigate whether the magnetic domain

pattern shows any non-reversible changes either due to the

FEL probe or the optical excitation pulses, we subtract the

scaled pumped reconstruction from the unpumped reconstruc-

tion. The scaling factor is determined with a spatially resolved

minimization algorithm and yields 2.4, corresponding to a

global demagnetization rate of ∼42%. The result is depicted

in Fig. 3(a) and shows an almost perfect difference image and

no indication of large magnetic domain reconfiguration or non-

uniform demagnetization. Due to the high spatial resolution

and the high sensitivity for the magnetization, it is possible to

identify and localize small changes of the domain network

and, if required, take them into consideration in the data

evaluation. Careful inspection of Fig. 3(a) suggests that we

can indeed identify small areas of non-reversible changes, see

black and white dots in Fig. 3(a). This analysis is expected

WEDER et al.: MULTI-COLOR IMAGING OF MAGNETIC Co/Pt MULTILAYERS 6500905

Fig. 2. (a) Difference holograms of an unpumped and (b) pumped magnetic

domain pattern can be reconstructed with the help of a Fourier transformation.

The result is visible in the two top panels showing a worm-like domain

network within a field of view of 2 μm diameter.

to be particularly powerful in nano-structured samples where

multiple reflections and interference of the incident pump

light with the subwavelength dimension of the sample leads

to an inhomogeneous intensity distribution. Controlling such

complex light-matter interaction in nanostructures may allow

to induce tailored absorption for nanoscale confinement of

optically induced magnetization dynamics [12], [18].

Time resolved single-color imaging of Co/Pt using the

MCD contrast mechanism is a sensitive and powerful tool for

imaging magnetic heterostructures on a nanometer length

and ultrafast time scale without inducing permanent

changes or damaging their magnetic properties. That makes

it the ideal tool for repetitive investigations of domain wall

motion, skyrmion dynamics, and helicity-dependent all-optical

switching in real space.

B. Static Multi-Color Imaging of Co/Pt

For simultaneous probing of the element-specific response

of the magnetization of the Co film and the Co/Pt interface,

we performed a static two-color imaging. This is possible due

Fig. 3. Scaled difference of both reconstructions in (a) yields spatially

resolved information about optical induced permanent changes of the domain

structure. To compare the magnetic domain contrast of the Co/Pt multilayer

between the excited (dashed-dotted lines) and not excited (solid line) state

(b), two lineouts are plotted over the spatial coordinates. They not only show

a stable domain position but also indicate that the amplitude of magnetization

is quenched by approximately 40%.

to the recent achievement at FERMI to produce two-color FEL

radiation at integer multiples of the UV seed laser [19].

An experiment at the synchrotron facility BESSY II

revealed pronounced dichroic signals in the XUV spectral

range stemming from Co and the Co/Pt interface [20]. MCD of

comparable amplitude was found at 61.4 eV (20.2 nm) slightly

off resonant at the Co M2,3edge and at 71.6 eV (17.3 nm)

at the Pt N7edge. These FEL energies can be achieved by

seeding the FEL with UV light at 242.2 nm and tuning the

first part of the undulator, Rad1, to the 12th harmonic to

probe Co and the second part of the undulator, Rad 2, to the

14th harmonic to probe Pt.

For the physical dimensions of the holography mask con-

taining an object hole with a diameter of dr=2μmand

an object-reference distance of |r|=5μm, one expects

an overlap of the two reconstructions for the two respective

wavelengths of 17.3 and 20.2 nm. To ensure that both recon-

structions pertaining to Co and Pt do not spatially overlap,

one can either reduce the size of the field of view along the

dispersion direction ror increase the distance between object

6500905 IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017

Fig. 4. Reconstruction of the static multi-color hologram of Co/Pt. All auto correlations are overlapped in the middle of the image. They are surrounded by

a ring of 20 reconstructions of the object originating from the cross correlations between object and the five references for the two distinct energies.Thetwo

additionally references with |r|=13 μm lead to four clearly separated reconstructions of the magnetic domains coming from Co and the Co/Pt interface. For

a more detailed depiction see the magnified inset. The small dots that are distributed over the reconstruction (see dashed white line for example) stemming

from the cross correlations between the references sized between 60 and 80 nm.

and reference holes. More concretely, we have to fulfill the

condition |r|>drλ/λ. Since the FEL beam has a sufficient

transverse coherence length, we add two additional reference

holes at |r|=13 μm.

The energy of the two FEL pulses was set to 2 ±0.3μJat

61.4 eV and 3 ±0.3μJ at 71.6 eV. With a spot size of 180

μm×190 μm (full width at half maximum) this corresponds

to 8 ×106photons/μm2/pulse and 107photons/μm2/pulse

at the Co and Pt resonance, respectively. By adding up five

images in 10 Hz mode with an integration time of 600 s for

left and right helicity we obtain a hologram containing charge

and magnetic contribution. A Fourier transformation of the

difference hologram yields the direct visualization of a real

space, element-specific and simultaneously recorded image of

the magnetic domain network of Co and Pt with a high spatial

resolution on the order of 80 nm.

The very first reconstruction of such a simultaneously

probed two-compound system is shown in Fig. 4. It shows

the Fourier transformation of a single difference hologram

and reconstructs the magnetic domains from Co and from the

Co/Pt interface. In the center of the reconstruction, all auto cor-

relations (object–object and reference–reference) are located.

They are surrounded by the cross correlations between object

and reference holes. Note that we can also readily identify the

cross correlations of the different reference holes showing up

as white/black spots in the reconstruction. The fact that we can

easily resolve them corroborates our conservative estimate of

our spatial resolution on the order of 80 nm. As the position of

the reconstructed object is energy dependent [21] we observe

two displaced fields of view stemming from the two distinct

XUV wavelengths. The well-separated cross correlations at the

four edges originate from the two additional reference holes

and the object. In the lower left corner, we show a magnified

depiction of the element-specific reconstruction. Here we have

increased the signal to noise ratio by summing up the two

independent reconstructions from the two new reference holes.

A careful comparison of the reconstructions from Co and

from Pt yields an identical but inverted domain contrast.

This is due to an opposite sign of the MCD effect of the

Co M-edge (61.4 eV) and Pt N-edge (71.6 eV), i.e., the

magnetization of the Co film and the Co/Pt interface point

in the same direction [22].

Using this method in a dynamic manner can lead to a deeper

understanding of the influence of localized Co/Pt interfaces on

ultrafast demagnetization dynamics.

III. CONCLUSION

We demonstrated that single-color coherent imaging of

Co/Pt heterostructures via FTH in the XUV spectral range is a

powerful tool to gain detailed information about magnetization

dynamics on the nanometer length and ultrafast time scale.

Furthermore, we presented the very first implementation of

a two-color imaging at the FEL facility FERMI by taking

advantage of the MCD effect at the Co M-edge and the strong

dichroic signal stemming from the Pt N-edge. It allows simul-

taneous and element-specific access to thin-film magnetism.

By going from the M-edge to the L-edge of Co and Pt one

could push the limit of spatial resolution below 20 nm which

for example allows direct imaging of magnetic skyrmions.

We point out the feasibility of combining both approaches

for setting up a time resolved, simultaneous and multi-element

probing to unravel ultrafast processes in thin magnetic multi-

compound heterostructures on a nanometer scale.

WEDER et al.: MULTI-COLOR IMAGING OF MAGNETIC Co/Pt MULTILAYERS 6500905

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