~ Beamline Practice ~30th September and 1st October, 2011

BL39XU (Magnetic Materials Beamline)

Beamline scientists:
Motohiro Suzuki (JASRI/SPring-8, Leader of MCD team)Naomi Kawamura (JASRI/SPring-8)
Masaichiro Mizumaki (JASRI/SPring-8)
Title:
Handling of X-ray polarization and application to X-ray magnetic circulardichroism spectroscopy
Abstract:
X-ray magnetic circular dichroism (XMCD) is a useful tool to investigate magnetic
properties because of an element specificity, electronic shell selectivity andangular-momentum sensitivity. In particular, unnecessary ultrahigh-vacuum in hard
X-ray region is effective to measure the XMCD under multiple extreme conditions
such as high-magnetic field, extreme low temperature, and high pressure. Tomeasure XMCD spectra with high accuracy, it is necessary to optimize the tuning of
the relation among the undulator, monochromator, and X-ray phase retarder (XPR)
precisely; that is, the undulator gap and the XPR condition must be adjusted
according to the X-ray energy. In addition, a helicity-modulation technique, which
provides a dichroic signal of less than 0.01% and with a good signal-to-noise ratio,requires a combination of fast switching of the helicity and a phase-sensitive (lock-in)
detection system. Therefore, extremely high-quality XMCD spectra are obtained in ashort acquisition time.
In this study, you can understand polarization control using XPR and the basis of
the helicity-modulation method.

-1-Time Table:
09:00 – 09:15 Introduction of BL39XU
09:15 – 11:30 Measurement of undulator spectrum and controlling beamline
optics11:30 – 13:00 Lunch
13:00 – 14:50 Controlling X-ray phase retarder and polarization measurement
using a simple polarimeter
14:50 – 15:10 Rest
15:10 – 16:45 X-ray magnetic circular dichroism measurement
16:45 – 17:00 Summary-2-Beamline:
BL39XU is a hard X-ray beamline covering X-ray energy of 5 – 37 keV.[1,2] Main
optical components consist of the undulator, double-crystal monochromator,
X-ray phase retarder, and higher-harmonics reduction mirror. Layout of thecomponents of BL39XU is shown in figure 1.

Optics hutch

Front end

30 m

Experimental hutch

Double-crystal Screen monitor
Downstream shutter
monochromator
XY slit 1
Phase plateγ-stopperXY slit 2Mirror

535 m

540 m

Be window

45 m

X-ray stopper

50 m

Figure 1. Layout of the optical components of BL39XU.

Undulator:
The light source of BL39XU is an in-vacuum undulator.[3] This undulator
provides with extremely high-brilliance X-rays linearly polarized in thehorizontal plane. The small X-ray source size and low divergence is perfectlycompatible with X-ray diffractive phase retarder [4] for handling the polarization
states. Table 1 summarizes the parameters of the undulator.
Table 1. BL39XU undulator source parameters.
Total length /Period /Number of period

4.5 m /32 mm /140

Minimum gap /Kmax

8 mm /2.46

Tunable energy range (1st, 3rd and 5th)

5 – 70 keVPeak brilliance

1.4 × 1020 photons/s/mrad2/mm2/0.1%b.w./100 mA

Total power /Power density

11 kW (K = 2.3) /470 kW/mrad2

Polarization

horizontal-3-

Let’s measure the undulator spectrum !
The undulator spectrum depends on the gap value (also, the K parameter).
The energy of the maximum intensity (peak energy) is changeable bychanging the gap value. Here, we investigate the relationship between thegap value G (mm) and peak energy E (keV).
Gap value: G (mm)Peak energy: E (keV)1015202530
The relation of the peak energy E (keV) and the gap value G (mm) can beapproximated by

β

(1)
G = α ln − 1 + γ ,E 
where, α, β and γ are parameters dependent on the undulator property. We
determine these parameters by a least-square fitting of our result.
α=β=γ=In SPring-8 users are allowed to change the undulator gap anytime,
without giving any influence of the stored electron beam (independenttuning of undulator). At BL39XU the undulator gap is tuned to follow theX-ray energy determined by the monochromator at each energy point whilea spectral scan. As a result, very smooth incident X-ray (I0) spectrum isobtainable as if using a white source. This feature is important to record anEXAFS or XMCD spectrum in a wide energy range.

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Put a graph of the undulator spectrum.

Put a graph of the relation between G (mm) and E (keV).

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X-ray phase retarder (XPR):The diamond X-ray phase retarder (XPR) provides us with a freedom of theX-ray polarization: the XPR is used to convert horizontal linear polarization of thesource into either circular, vertical, or elliptical polarization.[4] At BL39XU a highrate of circular polarization of PC > 0.9 over 5 – 16 keV is achieved with the use ofa synthetic diamond crystal of appropriate thickness.

Let’s change the X-ray polarization !
Using the XPR, polarization states is controllable by changing the offset
angle Δθ (crystal angle measured from the Bragg condition). The offset angleΔθ for circular (±π/2 retardation) or vertical polarization (±π retardation)
depends on the X-ray energy (wavelength), thickness of the XPR crystal, andreflection plane; the phase retardation δ generated by an XPR is given by [4]δ =−π

C hkl ⋅2

λ3 sin 2θ B 
 t,Δθ


(2)where, Chkl represents a constant related to a structure factor of hkl Braggreflection, θB and t represent the Bragg angle and the path length of X-rays in
the crystal, respectively. When δ = ±π/2 (±π) circularly (vertically) polarized
X-rays are generated.
X-ray polarization state can be determined using a simple polarimeter,
which consists of Kapton scatter and two scintillation counters as shown infigure 2. When non-polarization component is negligible, the degree of
linear polarization is determined by
I − IxPL = y,(3)
Iy + Ix
where, Iy and Ix represent X-ray intensities scattered vertically andhorizontally, respectively. In this case, PL = 1, 0, and -1 means horizontally
linear, circularly, and vertically linear polarization, respectively.
Here, we measure PL at several X-ray energies using the simple
polarimeter. Moreover, we determine the condition of generating circularly
and vertically polarized X-rays by analyzing our result.

-6-Thickness of a diamond XPR: t (mm) =ΔθC (arcsec)

E (keV)ΔθV (arcsec)

6.5
7.5
8.5
y
NaI scintilation counter 1
vertical scattering (Iy)

Kapton sheet
ion chamberx
diamond x-ray
phase plate
NaI scintilation counter 2
horizontal scattering (Ix)incoming x-rays(horiz. polarization)
45ºFigure 2. Configuration of the simple polarimeter system.

Put a graph of the result of XPR θ scan.

-7-X-ray magnetic circular dichroism (XMCD):
X-ray magnetic circular dichroism (XMCD) is defined by the difference
between the X-ray absorption of a sample for right- and left-circularly polarized
X-rays. XMCD is a powerful tool to study the electronic and magnetic states of theferromagnetic materials.[5,6] In particular, its element-specific feature provides us
with detailed information for the magnetic properties.[7]The XMCD spectrometer consists of a magnet and X-ray detectors. The
electromagnet and superconducting magnet are available at BL39XU. In XMCD
measurement in the transmission mode, two ionization chambers are used before
and after the sample to monitor the incident (I0) and transmitted (I) X-ray
intensities. In the fluorescence mode, an Si drift detector (SDD) is usually used tomonitor the fluorescence X-ray intensity from the sample, instead of the I
ionization chamber. XMCD spectrum is obtained by measuring and subtracting
the X-ray absorption intensities with switching the X-ray helicities at each energypoint.

Figure 3. The XMCD spectrometer using the electromagnet.

-8-Let’s measure the XMCD spectrum !
Here, we measure an XMCD spectrum in pure Fe foil using ahelicity-reversal (static) method. To avoid the demagnetizing effect,magnetic field is applied in-plane of the foil. XMCD spectrum is measured
by the transmission-mode using two ionization chambers.

Measurement condition
Applied magnetic field: H (T)=Sample:Sample thickness: t (μm) =Gain of current amplifier: I0 @ 10
Energy range: E (keV) =Flipping times / Counting time:/ I1 @ 10~/sec

Put a graph of the XMCD spectrum measured by helicity-reversal method.

-9-Helicity-modulation XMCD measurement:
To actualize XMCD measurement with high-accuracy, the helicity-modulation
method [8] was developed. This technique allows a measurement of dichroic
signals of the order of 0.01% with a good signal-to-noise ratio. Extremely
high-quality XMCD spectra are obtainable in a short acquisition time.
The helicity-modulation technique requires a combination of fast switching of
the X-ray helicity and a phase-sensitive (lock-in) detection system. Block diagramof the helicity-modulation technique is shown in figure 4.

Monochromatic x-rays
from planer undulator(linear polarization)

Diamond x-ray phaseretarderIonization(generating circularchamber (I0)polarization)

Electromagnet

Ionizationchamber (I)H

Piezo-driven
oscillation stageSample_μt (dc)

Current amplifier
Piezo driver

V(I0)Digital voltmeter

Logarithmic converter

V(μt)

AΔμt (ac)ln(A/B)

Current amplifier

V(I)Lock-in amplifier

B

Function
generator

40 Hz

Figure 4. Block diagram of the helicity-modulation technique.

Let’s experience the helicity-modulation technique !
Here, we record an XMCD spectrum of the same Fe foil using thehelicity-modulation method in the same condition of the helcity-reversalmethod. Then we compare the obtained spectra using the different methods,
with respect to the statistic accuracy, signal-to-noise ratio, amd theacquisition time.

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Measurement condition
Applied magnetic field: H (T)=Sample:Sample thickness: t (μm) =Gain of current amplifier: I0 @ 10
Energy range: E (keV) =Modulation frequency: f (Hz) =Accumulation times:/ I1 @ 10~

Put a graph of the XMCD spectrum measured by helicity-modulation method.

References:[1] Show in URL: http://www.spring8.or.jp/en/users/current_user/bl/beamline/BLtable/[2] H. Maruyama et al.: J. Synchrotron Rad. 6 (1999) 1133.
[3] T. Hara et al.: J. Synchrotron Rad. 5 (1998) 403.
[4] K. Hirano and H. Maruyama: Jpn. J. Appl. Phys. 36 (1997) L1272.
[5] N. Ishimatsu et al.: J. Phys. Soc. Jpn. 76 (2007) 064703.
[6] N. Kawamura et al.: J. Phys. Soc. Jpn. 76 (2007) 074716.
[7] A good example is given in : A. Koizumi et al.: Phys. Rev. B61 (2000) R14909.
[8] M. Suzuki et al.: Jpn. J. Appl. Phys. 37 (1998) L488.

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