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Sabiá

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Sabiá (Mimosa caesalpinaefolia) is a tree found in the Northeast and North Regions of Brazil. It is also the name of a wide range of species of the Turdus genus, found all over the world. (Photo: Claudio Oliveira Lima )

OVERVIEW

Sabia (Soft X-Ray ABsorption Spectroscopy and ImAging) is a beamline for soft X-rays using undulators  with polarization control and a Planar Grating Monochromator. The main analysis possible in this beamline will be X-rays Photoemission and Absorption Spectroscopy. In particular, Angle Resolved Photoemission Spectroscopy (ARPES) which is one of the most powerful experimental techniques to research the electronic structure of materials.

In addition, several variations of dichroism in x-ray absorption enable the investigation of structural and magnetic properties with chemical selectivity. This aspect will benefit from the photon energy range, which corresponds to L-edge of 3d transition metals, such as manganese, iron, cobalt, the K-edge of light elements such as carbon, nitrogen and oxygen, and to the M4 and M5-edges of rare earths. Finally, the Sabiá beamline will also feature a photoelectron emission microscope, which provides a measure of absorption, including its dichroism variants, with sub micrometric spatial resolution.

In Sabia line will be privileged a higher photon flux at the expense of an energy resolution not as great as that of Ipe line. For this reason, the optical design will target a small number of plane mirrors and flat-elliptical and more gratings optimized for high efficiency.

A special feature of Sabiá beamline will be the availability of a very versatile system for preparation and pre-characterization of in-situ samples, allowing its users to grow thin films by various techniques, and transfer under conditions of ultra-high-vacuum for analysis chambers with X-ray.

LAYOUT & OPTICAL ELEMENTS


 

ElementTypePosition [m]Description
SOURCEInsetion Device0.0Ondulador Helicoidal (EPU) com possibilidade de controle da polarização do feixe de raios X (linear ou circular).
M1Plane Mirror27.5Absorvedor da carga térmica principal proveniente do anel.
M2Plane Mirror28.0Parte da óptica do Monocromador.
VLSPGDiffraction Grating28.5Elemento principal do monocromador. Grade plana com densidade de linha variável.
M3, M4, M5Toroidal MirrorsVariableCom função de refocalização do feixe nos diversos instrumentos.

PARAMETERS

ParameterValueCondition
Energy Range [eV]100 - 2500
Energy Resolution [$\Delta$E/E]$ 10^{-4}$
Energy ScanYes
Beam Size [$ \mu \rm m^{2}$]15 x 15ARPES
Beam Size [$ \mu \rm m^{2}$]20 x 20Magnet
Beam Size [$\mu \rm m^{2}$]Variable, up to 1 x 1PEEM

EXPERIMENTAL TECHNIQUES


Linear dichroism Natural or Magnetic (XLD, XMLD)

Variations in the crystal structure of the materials makes the absorption of X-rays different depending on the orientation of the electric field of the beam and the crystal axes of the specimen. This effect is known as X-ray Linear Dichroism (XLD) and is a powerful source of information on changes in the structure of interfaces and surfaces of thin films and multilayers. In addition, the absorption of linearly polarized radiation may vary with the magnetization of the sample. In this case, we are dealing with Linear X-ray Magnetic Dichroism (XLMD) and  we have magnetic information with chemical sensitivity.

Magnetic Circular Dichroism (XMCD)

Materials which have a nonzero magnetic moment absorb differently in the two possible helicities of circularly polarized X-rays (left and right circular polarization). This difference is called circular dichroism, and when applied to the absorption edge of the components of the sample, it allows for the magnetic contribution of each chemical element independently. Furthermore, in many cases it is possible to determine the spin and orbital components of the magnetism. The dichroism signal is overall maximum in the peak of the absorption edge of each element. Keeping the fixed energy at this point of the spectrum and varying the applied magnetic field, it is possible to obtain curves of magnetic hysteresis for each element in the composition of the sample. This is particularly important in the characterization of new permanent magnets.

Photoemission Spectroscopy with Angular Resolution (ARPES)

The electron analyzer that equips the SABIÁ beamline does not only determine the kinetic energy of the photoelectrons, but also its escape angle from the sample. Armed with this angle and kinetic energy, it is possible to determine the momentum of the emitted photoelectron. Applying the principle of conservation of momentum, it is possible to then infer the momentum of the electron in the crystal (before being emmited) and thus build the energy dispersion curve as a function of the angular momentum. This is one of the most important information about the electronic structure materials.

Photoelectron Emission Microscopy

Upon interaction with the X-ray beam, the materials emit electrons. This effect is particularly important in the region of soft X-rays. Using column similar to an electronic transmission microscope, it is possible to obtain images based on the electrons emitted in the absorption process. This is the basic principle of PEEM (photoelectron emission microscopy), which allows obtaining spectroscopic information with a spatial resolution of up to a few tens of nanometers. Furthermore, the XMCD and XMLD methods are still valid and by the proper use of the polarization of the X-ray beam, magnetic information can be obtained with such a spatial resolution. This possibility is particularly interesting in the study of domain walls and its dynamics. Moreover, given the inherent chemical sensitivity to absorption of X-rays, the technique is of great potential to geosciences, environmental studies, among others, where it is important to locate the spatial concentrations of various chemical elements in the sample.

IN-SITU GROWING OF THIN FILMS


In the SABIÁ beamline, the X-ray beam can be used in both arms. At first, there will be a superconducting magnet for X-ray absorption measurements, followed by the photoelectron spectrometer for ARPES measurements. These two instruments are connected to an ultra-high vacuum system (vacuum tunnel) that allow the manufacture and transfer of samples without exposure to air. The second arm of the beamline will be dedicated to the photoelectron emission microscope.

For preparing in-situ samples, the community of users of the SABIÁ beamline will rely on a molecular beam epitaxy (MBE) system, pulsed laser deposition chamber (PLD) and a deposition chamber for organic molecules. All these systems are integrated in an ultra-high-vacuum tunnel, where users can also find a high end scanning microscope (which works in both the AFM and STM modes), to check the morphological quality of the films grown there.