X-Ray magnetic circular dichroism spectroscopy of Fe II Nb IV - - PDF document

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

1

X-Ray magnetic circular dichroism spectroscopy

• f FeII – NbIV molecular magnet

Dominik Czernia

• 1. Molecular magnets

The phenomenon of magnetism fascinate mankind for thousands of years. Although magnetic attraction has been observed in antiquity, process of understanding its origin has lasted for a very long time. A sudden progression in this field of both knowledge and technology came

• ut just in the last century. Nowadays, most common conventional magnets, made of transition

metals and rare-earths (like iron and neodymium) can be found everywhere: in cars, telephones, computers, scientific or medical instruments. Futhermore, since the 80s of XXth century a new, rapidly expanding magnetism branch - molecular magnetism - has introduced a vast variety of novel materials with unique promising for future applications properties. They offer multifunctionality: lightness, transparency, solubility, biocompatibility, magneto-optical and electrical properties. Nevertheless, the biggest advantage of molecular magnets over conventional magnets is that their properties can be tuned. By chosing appropriate molecules and by manipulation of synthesis parameters (e.g. temperature, pressure or electromagnetic radiation) one can remarkably modify the final

• utcome.

The vast majority of molecular magnetic materials in the literature contain first row transition metal ions (3d) as the main magnetic moment carriers. The reason of their popularity is that they are easier to handle than their 4d and 5d congener. Nonetheless, the use of the heavier transition metal ions offers significant benefits which are not present in lighter ions. They are characterized by [1]:  more radially extended valence orbitals (5d > 4d >> 3d) which translate into stronger exchange interactions,  large spin – orbit coupling parameters which often lead to highly anisotropic g factors,  variety of oxidation states with redox events which can be triggered photochemically or electronically.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

2

Molecular 4d and 5d magnets exhibit anisotropic long-range order, magnetic bistability and slow magnetic relaxation. Due to these inherently unusual quantum magnetic properties, they are of great interest in potential applications in spintronics and quantum computing [1]. The easy changes in oxidation states and the ability to promote these changes using external stimuli (e.g. light) allowed to produce photomagnetic compounds where magnetism of a system changes after absorption of a photon [2].

• 2. Studied sample

Examined compound {[FeII(H2O)2]2[NbIV(CN)8⸱4H2O}4, in the further part of this work indicated by an abbreviation FeII – NbIV, was first time synthesized and initially characterized by Dawid Pinkowicz et al. from Jagiellonian University in Cracow, Poland [3]. FeII – NbIV crystallizes in the tetragonal system with space group I4/m which was resolved using single crystal X-Ray diffraction (fig. 1a). Architecture comprises iron and niobium ions connected with each other through cyano-bridges (CN), caged in coordination spheres (fig. 1b). Magnetic properties of FeII – NbIV derive from eight high-spin iron (s = 2, g = 2.2) and four niobium (s =

1 2, g = 2) ions per mol. For temperatures above 100 K magnetic susceptibility

𝜓𝑒𝑑(𝑈) (fig. 2) measured in a bias magnetic field 𝐼𝑒𝑑 = 2 𝑙𝑃𝑓 obeys Curie – Weiss law (eqn. 1) with the positive Weiss temperature θ = 47 ± 1 𝐿 and the Curie constant 𝐷 = 7.7 ± 0.2 𝑑𝑛3𝐿

𝑛𝑝𝑚 [3].

𝜓𝑒𝑑(𝑈) =

𝐷 𝑈−θ (1)

Figure 1. Crystal structure of FeII – NbIV (a), connectivity and geometry of the coordination spheres (b). Water molecules are omitted for clarity [3].

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

3

The transition to the long-range ordered magnetic state occurs at critical temperature about 𝑈

𝑑 = 43 𝐿 what can be estimated from sharp increase of DC susceptibility 𝜓𝑒𝑑(𝑈) curve

and from the strong peak in the AC susceptibility 𝜓𝑏𝑑(𝑈) signal (fig. 2). The virgin magnetization and magnetization loop of FeII – NbIV are presented in figure 3. The compound is a soft magnet with the coercive field 𝐼𝑑 = 130 𝑃𝑓 and the remanence 𝑁𝑆 = 1.24 𝑂𝛾 [3]. Authors of [3] performed calculations to estimate the value of the exchange coupling constant 𝐾 between FeII and NbIV assuming that the exchange interaction mediated through CN bridge is ferromagnetic and obtain 𝐾 = +8.1 1

𝑑𝑛3. However, they also

emphasized that the shape of the magnetization curve suggest non-collinear ordering of magnetic moments and therefore FeII – NbIV might be not ferro- but ferrimagnetic.

Figure 2. Magnetic 𝜓𝑒𝑑𝑈(𝑈) and 𝜓𝑏𝑑𝑈(𝑈) (inset) dependances of FeII – NbIV [3]. Figure 3. Virgin magnetization and hysteresis loop (inset) of FeII – NbIV [3].

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

4

• 3. Synchrotron X-rays measurment techniques

Molecular magnet FeII – NbIV represent the foundation in the enginnering of FeNb cyano-bridged materials with diverse topologies and required multifunctionality [3]. Taking into account a high scientific and technological value of above group of materials to complement previously done measurments on this compound, X-ray Magnetic Circular Dichroism spectrocopy (XMCD) was performed at ID12 beamline at European Synchrotron Radiation Facility (ESRF) in Grenoble.

3.1. X-ray absorption spectroscopy (XAS)

Among many reasearch techniques which benefit from X-rays, an absorption spectroscopy was strongly developed due to arise of synchrotron radiation. Possibility to use monochromatic beam and fast change of photons wavelengths significantly eased application of that technique. XAS measure the ability of the sample to absorb photon of a particular wavelength. Probability of this process possess three main characteristic properties:  tendency to decrease with increasing energies of photons as a consequence of fotoelectric effect - cross section 𝜏 depends on energy 𝐹 and atomic numer Z [4]: 𝜏 ∝

𝑎5 𝐹7/2

(2).  occurrence of the sharp peaks for the specific photon energies - so called absorption

• edges. These discontinuities arise when the energy of an absorber photon is high

enough to excite electron from one energy level to a higher energy level above Fermi

• level. The nomenclature used in XAS is as follows: K, L, M, N edges correspond to

the principal quantum number of core electron 1, 2, 3 and 4 respectively. The subscripts 2, 3, 4 and 5 refer to 𝑞1/2 , 𝑞3/2, 𝑒3/2, 𝑒5/2 initial core states respictively [5]. Assuming dipole approximation: absorption on K and L1 edges always lead to electronic transition to unfilled p shells and absorption on L2,3 edges to s (2% cases) or d (98% cases) shells [4]. Analysis of shape and position of absorption edge is an entity

• f X-ray Absorption Near Edge Structure (XANES) which is the mostly performed for

photons energies from about 10 eV below edge to about 30 eV above it. This technique is element selective and allows to determine electronic density of unfilled states.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

5

 oscillations of absorption above absorption edges. From an amplitude (which is usually

• f an order of few percentage of jump on absorption edge) and from frequency of
• scillations one can obtain information about local atomic structure around absorbing
• atom. Analysis is conducted in the range starting from about 20 to 1600 eV above

absorption edge and is so called Extended X-ray Absorption Fine Structure (EXAFS). Figure 4. shows schematic example of absorption spectrum with marked area of applicability of XANES and EXAFS.

3.2. X-ray Magnetic Circular Dichroism (XMCD)

In most cases the influence of magnetic moments on XAS is negligible because theoretical cross section of that process is around 10-5-10-6 times smaller than other absorption processes [4]. However, near absorption edges interaction between magnetic moments and X-rays raises and can be observed by using sensitive instruments and high intensity radiation. Moreover, by using circularly polarized X-rays one can observe difference in absorption in magnetized material between right and left polarized photons. This property is used in X-ray Magnetic Circular Dichroism (XMCD). The basic principle of XMCD can be explained in two steps. Circularly polarized X-ray photon carries an angular momentum: +ℏ for a left polarized photon and −ℏ for a

Figure 4. XAS spectrum of a molecule in solution illustrating two regions: the low energy XANES and the high energy EXAFS [6].

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

6

right polarized photon (that correspond to a helicity vector being parallel or antiparallel to the direction of propagation). In the first step a core electron is excited by circularly polarized X-ray photon which transfers entire angular momentum to this photoelectron. Both the orbital and the spin angular momenta of the photoelectron can be affected depending on the initial core state and the nature of interactions. Let consider the photoelectrons excited from the initial core states 2𝑞1/2, 2𝑞3/2 (𝑀2 and 𝑀3 absorption edges) by absorption of the left circularly polarized photon (+ℏ). For dipole transition approximation into a d-like final state selection rules are as following: Δ𝑘 = 0, +1; Δ𝑡 = 0; Δ𝑚 = +1; Δ𝑛𝑚 = +1. The probability of particular dipole transitions can be calculated using Wigner-Eckart theory. Assuming that transitions into final s-states are negligible, for the 𝑀2 edge the spin polarization of the photoelectron amounts to 𝑄

𝑡𝑓(𝑀2) = −0.5 and the angular polarization amounts to 𝑄 𝑚𝑓(𝑀2) = 0.75. For the 𝑀3 edge

spin polarization amounts 𝑄

𝑡𝑓(𝑀3) = 0.25 and angular polarization 𝑄 𝑚𝑓(𝑀3) = 0.75. Right

circularly polarized photon (−ℏ) does the opposite. Above mentioned spin 𝑄

𝑡𝑓 and orbital

𝑄

𝑚𝑓 polarizations are defined as below [4]:

𝑄

𝑡𝑓 = 𝑜↑↑−𝑜↑↓ 𝑜↑↑+𝑜↑↓ 𝑄 𝑚𝑓 = ∑ 𝑛𝑚∙𝑜𝑛𝑚

𝑛𝑚

∑ 𝑚∙𝑜𝑛𝑚

𝑛𝑚

(3), where 𝑜↑↑ is a number of photoelectrons that have spin parallel to photon wavevector and 𝑜↑↓ is a number of antiparallel photoelectrons, 𝑜𝑛𝑚 is a number of photoelectrons excited to the state with magnetic quantum number 𝑛𝑚. The result of the second step depends on the magnetic properties of the compound. Final states determined by the spectroscopic selection rules act as a spin and/or orbital moment detector of the excited photoelectron. Because of the spin conservation in an

• ptical transition, the photoelectrons excited from 2𝑞1/2 by left circularly polarized photon

(𝑄

𝑡𝑓(𝑀2) = −0.5 ⇒ 75 % spin down photoelectrons) probe mostly the spin up d-states

above the Fermi level. And similarly, the photoelectrons excited from 2𝑞3/2 by left circularly polarized photon (𝑄

𝑡𝑓(𝑀3) = 0.25 ⇒ 62.5 % spin up electrons) probe mostly the

spin down d-states. In a non magnetic materials the total (spin up + spin down) intensities are the same for the left and right photon circular polarizations. As soon as there is an unbalance in the number of available empty spin up and spin down states (magnetic materials) the absorption of the two photon polarizations will be different.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

7

If the spin moment carried by the d states is aligned parallel to the X-ray wavevector, the absorption of the left circularly polarized photon will be enhanced at the 𝑀2 edge and reduced at the 𝑀3 edge with respect to unpolarized absorption. The opposite effect is

• bserved for right circularly polarized photons. The difference in the absorption is the

X-ray magnetic circular dichroism. The XMCD signal is proportional to the change in the scalar product between the magnetization vector 𝑁 ⃗ ⃗ and the helicity vector of X-rays. Therefore, XMCD spectra could be recorded either by reversing the direction of the sample magnetization or by changing the helicity of the X-rays. Absorption spectra of 𝑀2,3 edges are presented in figure 6a. of an atom with a partially

• ccupied d-shell. If one assumes that the d-states of the absorbing atom carry the spin

moment only, then a corresponding XMCD signal (fig. 6b.) is negative at 𝑀3 edge and positive at the 𝑀2 edge. The integrals are the same except from sign. In the hypothetical case of pure orbital magnetization of the d-states (no unpaired electrons, spin moment is zero), one would expect the XMCD spectrum as shown in fig. 6c.

Figure 5. Schematic sketch of the excitation. Due to magnetic field d shell has more electrons with spin up (more available states for spin down). Therefore, the 𝑀2 (𝑞1/2) absorption edge will be enhanced because majority of excited photoelectrons has spin down and the 𝑀3 (𝑞3/2) absorption edge will be reduced because majority of excited photoelectrons has spin up.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

8

Information about spin and orbital moment carried by an absorbing atom can be provided by applying so called magneto-optical sum rules. After many assumptions, sum rules for 𝑀2,3 edges can be given as below [5]: ∫ (𝐽− − 𝐽+)𝑒𝐹 = −

3 𝑚 Ω𝑚𝑛𝑚 𝑘++𝑘−

(4), ∫ (𝐽− − 𝐽+)𝑒𝐹 −

𝑑+1 𝑑 ∫ (𝐽− − 𝐽+)𝑒𝐹 𝑘−

= −Ω𝑚 [𝑛𝑡 +

2(2𝑚+3) 𝑚

〈𝑈

𝑨〉] 𝑘+

(5), where:

• 𝑑 is an orbital quantum number of the core state,
• 𝑚 is an orbital quantum number of the valence state,
• 𝑘± = 𝑑 ±

1 2 identifies core state,

• 𝐽0,± stand for intensities of left (+), right (-) circular and linear (0) polarized photons

absorption signal,

• Ω𝑚 =

∫ (𝐽−+𝐽++𝐽0)𝑒𝐹

𝑘++𝑘−

3(4𝑚+2−𝑜)

is a X-ray absorption cross section per hole of symmetry l and 𝑜 electrons in the valence state,

• 〈𝑈

𝑨〉 is the expectation value of the z-component of the magnetic dipole operator

𝑈 = ∑ 𝒕𝒋 − 3𝒔𝒋(𝒔𝒋 ∙ 𝒕𝑗)/𝒔𝒋

2 𝑗

, where the sum over i extends over all the electrons of the l shell. In the wide majority of practial cases, 〈𝑈

𝑨〉 is much smaller than the spin moment and its

contribution could be neglected.

Figure 6. (a) Schematic representation of X-ray absorption spectrum at the 𝑀2,3 edges of an atom with partially filled d-shell. (b) Hypothetical XMCD signal for an atom with pure spin polarization in the d-shell. (c) Hypothetical XMCD spectrum for an atom with magnetic moment in the d-shell of orbital origin only. [5]

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

9

From XMCD spectra one can obtain another valuable magnetic property of compound which is local magnetization of specific elements. Such measurment consist of collecting XMCD signal in a function of applied magnetic field using photons that energy equals to chosen element’s absorption edge energy. In this way it is possible to evaluate the contribution of each atomic species to the magnetic proporties of the material. After normalization of the XAS spectra and adequate scaling of the XMCD spectra with the same factor, the measured magnitude is the average moment of all the present atoms corresponding to the selected element edge [8].

3.3. ID12 beamline

Unique worldwide experimental set-up at ESRF ID12 beamline is dedicated to XMCD measurments in the temperature down to 2.2 K and under high magnetic field up to 17 T. The beamline is capable of offering fast switching circular and linear polarisation of X-rays in the energy range 2.05 to 15 keV [7]. Figure 7. present absorption edges of specific elements which can be studied on beamline ID12 with full control of polarization of photon beam.

Figure 7. Periodic table of elements and absorption edges possible to study at beamline ID12.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

10

As it can be seen in figure 7. a broad range of elements can be measured including metals from studied compound FeII – NbIV. During experiment 𝐿 edge of iron and 𝑀2,3 egdes of niobium were probed (fig. 8). It is worth emphasizing that beamline ID12 at ESRF is the only facility in the world that has posibility to examine niobium 𝑀2,3 edges with circularly polarized photon beam. This unique feature gives an opportunity to enrich knowledge and understanding

• f Nb compounds.

Experimental set-up is presented in figure 9. A major concern in circular dichroism experiments is the circular polarization rate of the monochromatic beam, which depends not

• nly on the polarization of the undulator radiation but also on the polarization transfer by all
• ptical components including monochromator [5]. Results of numerical simulations of the

figure of merit for X-ray magnetic circular dichroism experiments at the ESRF beamline ID12 equipped with a Si (111) double crystal monochromator for two helical undulators are shown in figure 10. XMCD signal should be then normalized by polarization rate correction acquired from figure of merit.

Figure 8. Absorption edges and their energies (in eV) for iron and niobium. Figure 9. Schematic of an experimental set-up.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

11

Second correction that should be taken into consideration is self-absorption

• correction. Self-absorption effects occur when emmited photons are re-absorbed before they

can exit the sample. To make a correction of such effect one can use Henke’s tables which can be extended by experimental or theoretical data [8]. Unfortunately, self – absorption correction was not conducted due to limited time of project. Therefore, we have obtained

• nly qualitative results.

The experimental station which is the largest static magnetic field presently available for XMCD spectrocopy is shown in figure 11 and figure 12 present a studied sample FeII – NbIV already inserted into sample holder.

Figure 10. Figure of merit of a circular dichroism experiment at the ESRF beamline ID12. Figure 11. High field XMCD experimental station at the ESRF beamline ID12. Figure 12. Studied compound FeII – NbIV inserted into sample holder.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

12

• 4. Data analysis

Two different measurments were performed on FeII – NbIV molecular magnet. Firstly, absorption spectroscopy was measured in the three photon ranges which comprised three absorption edges energies:  K – edge of iron, 7100 – 7185 eV,  L2 – edge of niobium, 2440 – 2500 eV,  L3 – edge of niobium, 2335 – 2410 eV. For most accurate results every single measurment was performed six times with left and right circularly polarized X-rays and in two directions of the magnetic field each (parallel and antiparallel to the X-rays wavevector). Average value of spectrum was taken into futher consideration. Figures 13. and 14. show obtained XANES and XMCD spectra after polarization rate correction. Absorption edge energies are in well agreement with tabular data (fig. 8). Three essential regions can be observed near iron K - edge XANES/XMCD spectra. First region (1) correspond to the quadrupole transition 1𝑡 → 3𝑒. Next region (2) stands for the dipole transition 1𝑡 → 4𝑞 (K – edge). The last interesing area (3) arised due to so called Multi-electron excitation (MEE): part of the energy of the highly energetic photon is absorbed by 1𝑡 electron (1𝑡 → 4𝑞) and then the excess energy of this photon is big enough to excite 3𝑞 electron (3𝑞 → 3𝑒).

Figure 13. XAS and XMCD spectra for iron K - edge. Marked regions (1-4) are described in text.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

13

Integral over measured range of energy is negative what together applied sum rules means that Fe 3d state has an orbital magnetic moment parallel to the magnetic field. Unfortunately, spin magnetic moment cannot be deduced using hard x-rays. Nonetheless,

• btained results are cinsistent with macroscopic observations (Fe g = 2.2).

One can notice on niobium L2,3 edges (2𝑞1/2 → 4𝑒 and 2𝑞3/2 → 4𝑒) XANES spectra that both absorption peaks (1) and (2) are divided into two others. This phenomenon is caused by the ligand field splitting of 4d states of niobium ion. By applying sum rules one can notice that for first equation (4) integral is negative and therefore orbital moment contribution has to be positive and parallel to the magnetic

• field. On the other hand, sum of integrals (5) has to be positive (c = 1 for L - edges) – spin

moment contribution has negative value and is aligned antiparallel to the magnetic field (as expected for less than half filled d-shell). Obtained results are not consistent with macroscopically deduced g = 2 for niobium [3] therefore this simple model of a spin-only Nb magnetism needs to be revised. Secondly, XMCD signal was measured in the function of applied magnetic field with below condition:  photon energy 7128 eV (Fe) and magnetic field from -6 T to 6 T,  photon energy 2369 eV (Nb) and magnetic field from -6 T to 6 T. Direction of applied magnetic field was parallel or antiparallel to the photon wavevector thus magnetization is proportional to the obtained XMCD signal. Comparision

Figure 14. XAS and XMCD spectra for niobium L2,3 - edges. Marked peaks (1-2) are described in text.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

14

between macroscopic and microscopic shapes of magnetization curve are shown in figure

• 15. in the magnetic field range from -6 T to 6 T. One can see that microscopic magnetization
• f Fe follows macroscopic magnetization. However, Nb behavior is completely different. It

seems to be essential to perform futher investigations at higher magnetic fields. As one can see in figure 15. both iron and niobium average magnetic moments are alligned parallel to the magnetization vector. It denotes that FeII – NbIV compound exhibit ferromagnetic long-range ordering.

• 5. Summary

Discovery of Magnetic Circular Dichroism in x-ray absorption ushered in a new era

• f magnetism research with objectives that previously would have been unattainable.

XMCD benefits of the unique advantage to be element/edge selective since it originates from electric dipole transitions of a core electron of an absorbing arom into the empty states above the Fermi level. Moreover, derivation of magneto-optical sum rules has greatly strengthened the XMCD, offering a capability to deduce from the experimental spectra the

• rbital and spin contributions to the total magnetic moment carried by the absorbing atom.

The other important advatage of XMCD is the possibility to measure the element- specific magnetic hysteresis. One can evaluate the contribution of each atomic species to the magnetic properties of the material.

Figure 15. Macroscopic magnetization of FeII – NbIV and microscopic XMCD magnetization of Fe and Nb in the magnetic field range from -6 T to 6 T.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

15

Magnetic properties of FeII – NbIV molecular magnet were examined obtaining valuable information. The study of compounds based on 4d and 5d transition metals is emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of samples. XMCD performed at the ESRF beamline ID12 provided valuable information of magnetic properties of FeII – NbIV which cannot be obtained anywhere else. As deduced from macroscopic measurments iron carries a spin and a weak orbital

• moment. Suprisingly, niobium magnetism is dominated by orbital moment with a weaker

spin being antiparallel. Niobium and iron mangetizations are coupled ferromagnetically but with very distinct field dependent behavior.

• 6. References

[1] X. Y. Wang, C. Avendaño, K. R. Dunbar, Molecular magnetic materials based on 4d and 5d transition metals, Chem. Soc. Rev., 2011, 40, 3213-3238, [2] A. Bleuzen, V. Marvaud, C. Mathoniere, B. Sieklucka, M. Verdaguer, Photomagnetism in Clusters and Extended Molecule-Based Magnets, Inorg. Chem., 2009, 48, 3453 – 3466, [3] D. Pinkowicz, R. Podgajny, R. Pełka, W. Nitek, M. Bałanda, M. Makarewicz, M. Czapla,

• J. Żukrowski, C. Kapusta, D. Zając, B. Sieklucka, Iron(II)-octacyanoniobate(IV)

ferromagnet with Tc 43 K, Dalton Trans., 2009, 7771-7777, [4] M. Sikora, Magnetyzm związków La2/3-yREyCa1/3MnO3 badany metodą magnetycznego dichroizmu promieniowania X, PhD thesis, AGH University of Science and Technology, Cracow, 2002, [5] A. Rogalev, F. Wilhelm, Magnetic Circular Dichroism in the Hard X-ray Range, The Physics of Metals and Metallography, 2015, 13, 1285-1336,

Figure 16. Qualitative schematic of deduced orbital and spin moment contributions.

X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble

16

[6] L. Nanai, S. Beke, K. Sugioka, Chapter 9: Ultrafast Time-resolved Spectroscopy, Femtosecond-Scale Optics, InTech, 2011, [7] ID12 – Circular Polarisation Beamline, www.esrf.eu/UsersAndScience /Experiments/EMD/ID12, access on 22.09.2017, [8] G. Laan, A. Figueroa, X-ray magnetic circular dichroism – A versatile tool to study magnetism, Coordination Chemistry Reviews, 2014, 277-278, 95-129.