electronic reprint
Journal of
Synchrotron
Radiation
ISSN 0909-0495
Editor: G. Ice
X-Treme beamline at SLS: X-ray magnetic circular and linear
dichroism at h igh field and low temperature
Cinthia Piamonteze, Uwe Flechsig, Stefano Rusponi, Jan Dreiser, Jakoba
Heidler, Marcus Schmidt, Reto Wetter, Marco Calvi, Thomas Schmidt,
Helena Pruchova, Juraj Krempask y, Christoph Quitmann, Harald Brune
and Frithjof Nolting
J. Synchrotron Rad.
(2012). 19, 661–674
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J. Synchrotron Rad.
(2012). 19, 661–674 Cinthia Piamonteze
et al
. · X-Treme beamline at SLS
research papers
J. Synchrotron Rad. (2012). 19, 661–674 doi:10.1107/S0909049512027847 661
Journal of
Synchrotron
Radiation
ISSN 0909-0495
Received 6 February 2012
Accepted 19 June 2012
# 2012 International Union of Crystallography
Printed in Singapore – all rights reserved
X-Treme beamline at SLS: X-ray magnetic circular
and linear dichroism at high field and low
temperature
Cinthia Piamonteze,
a
* Uwe Flechsig,
a
Stefano Rusponi,
b
Jan Dreiser,
a
Jakoba Heidler,
a
Marcus Schmidt,
a
Reto Wetter,
a
Marco Calvi,
a
Thomas Schmidt,
a
Helena Pruchova,
a
Juraj Krempasky,
a
Christoph Quitmann,
a
Harald Brune
b
and
Frithjof Nolting
a
a
Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland, and
b
Institute of
Condensed Matter Physics (ICMP), E
´
cole Polytechnique Fe
´
de
´
rale de Lausanne (EPFL), CH-1015
Lausanne, Switzerland. E-mail: cinthia.piamonteze@psi.ch
X-Treme is a soft X-ray beamline recently built in the Swiss Light Source at
the Paul Scherrer Institut in collaboration with E
´
cole Polytechnique Fe
´
de
´
rale
de Lausanne. The beamline is dedicated to polarization-dependent X-ray
absorption spectroscopy at high magnetic fields and low temperature. The
source is an elliptically polarizing undulator. The end-station has a super-
conducting 7 T–2 T vector magnet, with sample temperature down to 2 K and
is equipped with an in situ sample preparation system for surface science. The
beamline commissioning measurements, which show a resolving power of 8000
and a maximum flux at the sample of 4.7 10
12
photons s
1
, are presented.
Scientific examples showing X-ray magnetic circular and X-ray magnetic linear
dichroism measurements are also presented.
Keywords: X-ray absorption spectroscopy; X-ray magnetic circular dichroism;
X-ray linear dichroism; soft X-rays; instrumentation.
1. Introduction
Polarization-dependent X-ray absorption spectroscopy (XAS)
includes a number of techniques, the most common being:
X-ray magneti c circular dichroism (XMCD), X-ray magneti c
linear dichroism (XMLD) and X-ray natural linear dichroism
(XNLD). A large magneto-optical effect was predicted in the
pioneering work of Erskine & Stern (1975) for Ni M
2,3
-edges
based on calculations taking into account the large spin–orbit
coupling of the 3p states and the spin polarization of the 3d
states. Ten years later, Thole et al. (1985) predicted a strong
dichroism in the M
4,5
-edges of rare-earth ions. Shortly after,
the first XMLD measurements were performed on lanthanides
M-edges by the same group (van der Laan et al., 1986). The
first XMCD spectrum was measured in the hard X-ray energy
range (Schu
¨
tz et al., 1987), followed a few years later by the
first measurement in the soft X-ray range by Chen et al. (1990)
on Ni bulk sample. Since its early times, XMCD has been
widely used for the study of a variety of questions related to
magnetism such as: magnetic anisotropy (Sto
¨
hr & Konig, 1995;
Sto
¨
hr, 1999) and exchange bias (Ohldag et al., 2003) at inter-
faces; element-specific magnetic properties of bimetallic
paramagnetic molecules (Arrio et al., 1999); magnetic aniso-
tropies, spin and orbital moments of single adatoms
(Gambardella et al., 2003), of a submonolayer of single-
molecule magnets (Mannini et al., 2009) or organometallic
molecules on magnetic surfaces (Wa
¨
ckerlin et al., 2010).
XMCD has also been used in areas as wide as mineralogy
(Pattrick et al., 2001), environmental sciences (Coker et al.,
2006) and in biological systems such as metalloproteins (Funk
et al., 2004, 2005). A few characteristics that make this tech-
nique unique are its element specificity, the direct access to the
states responsible for the chemical bonding and magnetic
properties as well as the possibility to extract quantitative
information on the spin and orbital moment separately
through sum rules (Thole et al., 1992; Carra et al., 1993). In
addition, its high sensitiv ity is unreached by other techniques
and enables the study of ultra-thin films and of magnetic
impurities.
The X-Treme beamline operates in the soft X-ray energy
range, from 400 to 1800 eV. This range includes, for example,
the L
2,3
-edges of 3d transition metals (2p ! 3d), the M
2,3
-
edges from 4d transition metals (3p ! 4d), the M
4,5
-edges
from lanthanides (3d ! 4f) and the K-edges of some common
ligands such as nitrogen, oxygen and fluorine (1s ! 2p). It is
therefore the most interesting energy range for the study of
the relevant states related to magnetism. The end-station
contains a superconducting vector-magnet allowing an applied
magnetic field of 7 T along the beam direction and of 2 T
perpendicular to it in the horizontal plane. The superposition
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of both fields can be used to create a
vector field up to 2 T. The sample
temperature can be adjusted from
370 K down to 2 K. The beamline is
installed in the Swiss Light Source
(SLS) at the Paul Scherrer Institut and
operated in collaboration with E
´
cole
Polytechnique Fe
´
de
´
rale de Lausanne
(EPFL). The SLS is a third-generation
source with an electron storage ring
energy of 2.4 GeV and a current of
400 mA, operated in top-up mode.
2. Beamline specifications
The X-Treme beamline is installed in
the straight section 7M of SLS. The
source is an Apple II undulator (Sasaki
et al., 1993) with 54 mm period and total
length of 1.7 m. The polarization can be
varied between circular right/left and
linear from 0
to 90
. The undulator
operates in the energy range from
150 eV up to 8000 eV, which corre-
sponds to operation up to the 31st
harmonic. The source is shared alter-
natingly between two beamlines:
X-Treme (400–1800 eV) and Phoenix
(800–8000 eV). The beam switch is
made by inserting or removing the first
mirror of each beamline. Fig. 1 shows a
top view of the two beamlines, with a zoom of the lead hutch
where the mirrors used for the switching are marked.
1
Downstream from the splitting chamber the vacuum system of
the two beamlines is separated.
The beamline design is based on a plane-grating mono-
chromator (PGM ) operated with collimated light.
2
In the
PGM design the fix-focus constant (c
ff
) is kept independent of
the energy (Petersen, 1982). c
ff
is given by
c
ff
¼ cos =cos ; ð1Þ
where and are indicated in the schematic representation
given in Fig. 2 and are defined in the grating equation,
m=d ¼ sin þ sin ; ð2Þ
where m is the diffraction order, is the photon wavelength
and d is the line spacing.
The deflection angle by the plane mirror (, see Fig. 2) is
given by
2 ¼ : ð3Þ
Therefore, the operation of the monochromator can be fully
described by and and the grating equation is rewritten as
m=d ¼ 2 cos sinð þ Þ: ð4Þ
The X-Treme monochromator is operated in inside diffraction
order (m >0,c
ff
> 1). The light accepted by the mono-
chromator is previously collimated in the vertical direction by
the first mirror. The PGM operation with collimated light
allows the value of c
ff
to freely vary (Follath & Senf, 1997).
This design gives an additional flexibility, since by changing
the c
ff
value the monochromator can be optimized for flux,
energy resolution or harmonic rejection as described by
Follath (2001).
research papers
662 Cinthia Piamonteze et al.
X-Treme beamline at SLS J. Synchrotron Rad. (2012). 19, 661–674
Figure 1
Floor layouts of the X-Treme and Phoenix beamlines. Top: overview of the two beamlines up to the
experimental station. Bottom: zoom into the lead hutch. The first mirror from the Phoenix beamline
and the collimating mirror from the X-Treme beamline are used for switching between the two
branches.
Figure 2
Scheme of the plane-grating monochromator indicating the relevant
angles.
1
The mirror chambers were manufactured by FMB Feinwerk & Messtechnik,
Berlin, Germany (http://www.fmb-berlin.de/).
2
The monochromator chamber was manufactured by Jenoptik, Jena,
Germany (http://www.jenoptik.com/).
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The beamline’s optical layout is schematically shown in
Fig. 3. The optical and geometrical specification for each
optical element are given in Table 1. The beamline front-end is
equipped with moving blades, which can be used to change the
accepted radiation cone coming from the undulator. The
collimating mirror has a toroidal shape used to collimate the
light vertically and focus horizontally at the exit slit position
(see Fig. 3). This same mirror is used to switch between
X-Treme and Phoenix as pointed out in Fig. 1. The vertical
focus at the exit slit is made by a cylindrical mirro r (focusing
mirror in Fig. 3) located downstream from the mono-
chromator. The dispersion length (distance between exit slit
and focusing mirror) is 6 m. The toroidal mirror after the exit
slit refocuses the beam both vertically and horizontally at the
end-station (refocusing mirror in Fig. 3). The focused spot at
the end-station is a 1:1 image of the exit slit. The calculated
focused spot size at the sample is 220 mm FWHM horizontally
while vertical ly it is the image of the adjustable exit slit
opening. The beamline also offers the possibility of a de-
focused beam at the sample. This feature is mostly used for
samples sensitive to radiation damage, since by not focusing
the beam the flux density at the sample is reduced. The change
between focused and defocused beam is made by retracting
the refocusing mirror, as pointed out in Fig. 3. For defocused
beam the spot size is 490 mm FWHM horizontally and verti-
cally it varies depending on c
ff
.Forc
ff
= 2.25 the defocused
vertical spot is 635 mm FWHM and 1.3 mm FWHM for c
ff
=5.
Fig. 4 shows the measured focused spot size at the sample
position for an exit slit opening of 40 mm. This image corre-
sponds to 230 mm FWHM horizontally and 35 mmFWHM
vertically.
We point out that another option for the optical layout
would have been to perform the horizontal focus by the
research papers
J. Synchrotron Rad. (2012). 19, 661–674 Cinthia Piamonteze et al.
X-Treme beamline at SLS 663
Figure 4
Focused beam spot measured with a CCD camera at the sample position
with the vertical exit slit set to 40 mm. The horizontal bar corresponds to
100 mm and the scale is the same for horizontal and vertical directions.
Table 1
Beamline optics specifications, measured radii and slope errors.
Collimating mirror†‡ Plane mirror† Plane grating§ Focusing mirror} Refocusing mirror}
Shape Toroidal Flat Flat Cylinder Toroidal
Position (m) 16.3 21.3 21.3 22.3 29.8
Source distance h/v (m) 16.3/16.3 – – –/1 1.5/1.5
Image distance h/v (m) 12.02/1 – – –/6.0 1.5/1.5
Total deflection angle (
) 176 155–180 155–180 178 178
Geometrical surface size (mm) 190 40 310 50 100 20 180 40 180 40
Optical surface size (mm) 140 20 275 20 90 15 150 15 100 5
Footprint (4) at 400 eV (mm) 100 2 100 425 1
Bulk material Si Si Si Si Fused silica
Water cooling Internal Internal Side None None
Roughness (r.m.s.) (nm) 0.1 0.15 0.16 0.161 0.374
Coating (30 nm) Pt Pt Au Pt Pt
Tangential radius (mm) 407000 – – – 89000
Sagittal radius (mm) 1138 – – 209.92 26.32
Tangential slope error (r.m.s.) (mrad) 2.4 0.67 0.37 1.9 2.2
Sagittal slope error (r.m.s.) (mrad) 6.78 0.66 0.9 2.61 6.8
Line density (1/mm) – – 1200 – –
Groove depth (nm) – – 10 – –
Groove width to period ratio – – 0.65 – –
† Manufacturer substrate: InSync Inc., Albuquerque NM, USA (http://www.insyncoptics.com/). ‡ Polishing: Carl Zeiss Laser Optics GmbH, Oberkochen, Germany (http://
www.zeiss.de/lo). § Manufac turer: Carl Zeiss Optronics GmbH, Oberkochen, Germany. } Manufacturer: WinlightX, Pertuis, France (http://www.winlightx.com/).
Figure 3
Optical layout of the X-Treme beamline. The optical elements are given
in the bottom legend. The red rectangles are adjustable apertures and the
orange marks are diagnostics as described in the text.
electronic reprint
focusing mirror. This is the design, for example, used in the
SIM beamline at SLS (Flechsig et al., 2010) and we will refer to
it as FM-focus for simplicity. In our case the collimating mirror
is responsible for the horizontal focusing (CM-focus). The
primary reason for choosing the CM-focus was that with the
FM-focus design it would not have been possible to accept the
full beam at the focusing mirror. This difference in acceptance
comes from the fact that our monochromator and focusing
mirror are further away from the source than in the SIM
beamline. This increased distance was necessary to allow
enough space for both X-Treme and Phoenix beamlines.
In the FM-design the beam is horizontally divergent until the
focusing mirror, which together with the increased distance
from the source would cause the beam horizontal footprint
to be larger than the mirror acceptance. Another difference
between both designs is that the FM-focus provides a smaller
horizontal spot size than the CM-focus. This increased spot
size is not so relevant for X-Treme since it is a beamline
dedicated to spectroscopy.
Fig. 3 points out the position of three diagnostics mounted
at the beamline: before and after the exit slit and after the
refocusing mirror. These diagnostics consist of horizontal
linear translation stages where two copper blades with sharp
ends in the vertical direction are mounted facing each other
forming a fixed vertical aperture. The copper blades are
connected to an electrical feedthrough allowing the
measurement of the total-electron-yield current between the
copper blade and ground, when the blade is positioned in front
of the beam. The translation stage is supported by a frame to
give stability and is driven by a five-phase motor allowing
a minimum step size of 300 nm. The distance between the
copper blades and alignment marks positioned outside the
linear stage is carefully measured before mounting. This
allows the calibration of the blade position according to the
theoretical beam path. Therefore, by measuring the electron
yield current to the copper blades while scanning their hori-
zontal position, one can measure the position and width of the
beam. If a beam imaging chamber is mounted in the end-
station position, the copper blades can also be used to analyze
the horizontal focus position of the mirrors. Using the blades
to cut the beam partially, the image at the end-station will
show a cut from the same or opposite side, depending on
whether the blade is before or after the focus. Performing this
imaging procedure for different angles of the mirrors one can
analyze how the focus position changes.
The translation stage length allows mounting more diag-
nostics besides the copper blades. The two stages after the exit
slit are equipped with a photodiode to measure the flux and
a gold mesh to measure the beam intensity arriving on the
sample, at the same time as the sample measurement. The
stage after the refocusing mirror has in addition a test sample
containing La, Fe, Co and O, which allows simple character-
ization of the beamline without the end-station. The diag-
nostics before the exit sl it has a 3 mm-thick Al filter used to
reduce the flux in case there is a problem with radiation
damage of the sample.
A further diagnostic tool for the horizontal beam position
and width is the analysis of the dispersion line image at the exit
slit position. The dispersion line image is formed by a camera
looking through a viewport pointing to a fluorescent screen
mounted directly above the exit slit.
The beamline is also eq uipped with three sets of adjustable
apertures to reduce the stray light, as shown in Fig. 3. The first
double-aperture set is positioned before the monochromator.
The second one consists of a vertical aperture and is posi-
tioned directly upstream from the focusing mirror. The third
one is positioned upstream from the refocusing mirror and
consists of a double-aperture set. They are all motorized and
controlled remotely. In addition, the blades are coated with
phosphor to allow a qualitative diagnostic of the beam image
by cameras mounted at view ports directed to the blades.
After the focusing mirror there is an additional screen coated
with phosphor to allow visualization of the dispersion line to
be as close as possible to the monochromator.
Fig. 5 shows the beamline efficiency calculated with the
REFLEC program (Schaefers et al., 2002) . The blue curve
shows the reflectivity for the three mirrors in the beamline, not
including the monochromator. The red curve shows the
reflectivity coming from the monochromator alone (plane
grating and plane mirror) for c
ff
= 2.25 and a laminar grating
with 1/d = 1200 lines mm
1
. The monochromator design
allows for three gratings to be mounted in parallel. Currently,
a 1200 lines mm
1
grating with laminar profile is installed.
Gratings with lower line density are foreseen to be installed in
the future. These gratings will be optimized for flux rather
than resolving power.
The working range of the 1200 lines mm
1
grating in terms
of c
ff
and photon energy is determ ined by a combination of
different mechanical constraints of the monochromator. The
limits imposed by each of these constraints are shown in
Figs. 6(a)–6(d) for a wide energy range. In this figure the
hatched areas represent the allowed working region. The first
two constraints are imposed by the angular range covered by
the grating (,Fig.6a) and mirro r (,Fig.6b). The minimum
angle reached by the grating (87.45
) limits the highest
research papers
664 Cinthia Piamonteze et al.
X-Treme beamline at SLS J. Synchrotron Rad. (2012). 19, 661–674
Figure 5
Reflectivity of the three mirrors in the beamline alone (blue curve) and
the plane-grating monochromator alone (red curve) for c
ff
= 2.25 and
laminar grating with 1200 lines mm
1
.
electronic reprint
