BL practice at BL25SU
08/27/2013, T. Nakamura and T. Muro

Program of BL practice
We, at the BL25SU, plan MCD spectra measurements and angle-resolved photoemission
spectroscopy (ARPES) experiment, on 1st. and 2nd. of Oct, respectively. So, menus of the practice aredifference between the first and the second days.

Outline of BL25SU
BL25SU is designed for research on electronic structures, magnetic states and surface
structures of solids with high energy-resolution circularly-polarized soft X-rays.

Left- andright-handed circularly polarized radiation is obtained along the same optical axis by twin helicalundulators. The helicity of the circularly polarized radiation can be periodically switched at 0.1, 1 or
10 Hz by using kicker magnets distributed around the two undulators [1]. The beamline
monochromator is a constant deviation type with varied line-spacing plane gratings covering anenergy region of 0.12 ∼ 2 keV. The resolving power of the monochromator is more than 10,000 in thewhole energy region [2].
Four kinds of spectroscopic techniques are available for public use: high energy-resolution
photoemission spectroscopy, magnetic circular dichroism of core absorption, two-dimensional
angular distributions of photoelectrons, photoelectron emission microscope. Measurements areperformed in ultra high vacuum conditions down to 10-8 Pa.

Fig.1 Layouts of beamline optics at BL25SU. MCD apparatus is installed as astation-4 (ST4) downstream of the ST3.

Time schedule of 1st, Oct.
9:30-

Introduction of beamline design and major scientific activities.

10:30-

Samples preparation and introduction of the samples to a load lock chamber.

11:00-

Introduction of the helicity switching and MCD data acquisition techniques.

12:00-

Measurement of beam properties under helicity switching in the 1 Hz mode.

13:00-------- Lunch -----------

14:00-MCD measurements (Spectra and hysteresis loops). I propose having a time for exchangesof scientific interests and discussion on some technical subjects during the measurements.

16:30-Up to participant’s interests and requests.

-17:30Close

X-ray magnetic circular dichroism
Magnetic circular dichroism (MCD) of soft x-ray absorption is a powerful tool to study magneticand electronic states of ferromagnetic and ferrimagnetic materials [3-5]. At BL25SU, helicity
switching of circularly polarized radiation performed by the twin helical undulators are used for
MCD measurements. Two absorption spectra corresponding to an MCD are measured by one energy
scan switching the helicity at each energy point. The helicity-switching method is effective for
precise measurements. The switching frequency is currently 1 Hz [6].
Samples are magnetized by a water-cooled type electromagnet equipped with double yorks. The
electromagnet generates variable magnetic field up to 1.9 T at the sample position [7]. Sampletemperature can be controlled from 10 K to 300 K and from 300 K to 550 K. Absorption intensity ismeasured by means of the total electron yield (TEY). A combination of the helicity switching
technique and the apparatus provides element specific magnetic hysteresis (ESMH) measurements.
For more detailed information of XMCD at BL25SU, please find a review paper of ref. [8], which canbe downloaded free of charge.

Samples for the practiceIn the XMCD practice, we will use FeCo alloy thin films grown on Cu metal buffer layer. Using
these samples having different thickness of the FeCo layers, we will estimate probing depth of TEY
detection by recording Cu absorption in addition to standard XMCD measurement at the Co
L2,3-edges. The spin and orbital magnetic moments will be deduced using magneto-optical sum rules[9, 10].

Time schedule of 2nd, Oct.
9:30-

Introduction of beamline design and major scientific activities.

10:30-

Samples preparation and introduction of the samples to a load lock chamber.

11:00-

Introduction of ARPES data acquisition techniques.

12:00-

Sample surface preparation by cleaving.

13:00-------- Lunch -----------

14:00-

ARPES measurements.
It will take a couple of hours to finely tune the sample orientation against the ARPESanalyzer. If we have some extra time, we will also try to observe photoelectron diffraction
patterns using a two dimensional analyzer.

-17:30

Close

Angle-resolved photoemission spectroscopy
Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for observing electronic
band structures of solids. ARPES measurements are usually performed using vacuum ultraviolet(VUV) light with energies from a few tens to ~100 eV. In these cases, photoelectrons mainly comefrom surface layers because of their low penetration depths [11]. Thus, VUV-ARPES is quite usefulwhen, for example, studying the top layers of two dimensional crystals such as graphite or otherlayered compounds. However, when the materials of interest have three dimensional structures andtheir bulk electronic states are needed to be explored, we have to gain the bulk sensitivity of ARPES.
At BL25SU, such bulk-sensitive ARPES is available by virtue of using the high resolution soft x-rays
mentioned above [12].
ARPES spectra are measured with a hemispherical electron analyzer (VG Scienta AB, SES200).
When a sample emits photoelectrons from a soft-x-ray spot of ~100 μm on the surface, this analyzer
can simultaneously detect angle-dispersed photoelectrons emitted in a plane including the electron
lens axis of the analyzer. In our system, this plane is set vertical. Thus, vertically angle-dispersed
photoemission spectra can be measured in parallel with an acceptance angle of about ±5°. In mostcases, the acceptance angle is enough to cover the full first Brillouin zone when using soft x-rays.
Samples can be cooled down to about 9 K using a closed-cycle He cryostat. The details of the system
are described in Ref. [13].

Samples for the practiceIn this practice, we will measure a Si wafer as a typical material which has a three dimensional
crystal structure. A clean surface of the sample will be obtained by cleaving it in situ. The
cross-sectional surface of the cleaved 0.5-mm-thick Si wafer is large enough for the beam spot of ~100

μm [14]. The sample position will be tuned using an optical microscope mounted on themeasurement chamber [14]. We will measure band dispersions along the Γ-X direction in k-space
using a photon energy of about 0.9 keV. Both the sample angle and excitation energy will have to becarefully tuned to observe clear dispersions.

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