Absorption Magnetic Circular Dichroism Research Articles

Electronic structure and magnetic properties of the half-metallic ferrimagnetMn2VAlprobed by soft x-ray spectroscopies

We have studied the electronic structure of ferrimagnetic Mn2VAl single crystal by means of soft X-ray absorption spectroscopy (XAS), X-ray absorption magnetic circular dichroism (XMCD) and resonant soft X-ray inelastic scattering (RIXS). We have successfully observed the XMCD signals for all constitute elements, supporting the spin polarized states at the Fermi level. The Mn $L_{2,3}$ XAS and XMCD spectra are reproduced by the spectral simulation based on density-functional theory (DFT), indicating itinerant character of the Mn 3d states. On the other hand, V $3d$ electrons are rather localized since the ionic model can qualitatively explain the V $L_{2,3}$ XAS and XMCD spectra as well as the local dd excitation revealed by V $L_3$ RIXS.

Interface electronic structure at the topological insulator–ferrimagnetic insulator junction

An interface electron state at the junction between a three-dimensional topological insulator film, Bi2Se3, and a ferrimagnetic insulator film, Y3Fe5O12 (YIG), was investigated by measurements of angle-resolved photoelectron spectroscopy and x-ray absorption magnetic circular dichroism. The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe3+ in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film.

Strong magnetic enhancement in self-assembled multiferroic-ferrimagnetic nanostructures

In the past decade, self-assembled vertical nano-heterostructures have drawn considerable attention because a high interface-to-volume ratio can be used to tailor or create functionalities. We have systematically investigated the magnetic properties of oxide heterostructures consisting of the CoFe2O4 nanopillars embedded in the BiFeO3 matrix using macroscopic magnetization measurements and element-selective soft X-ray absorption magnetic circular dichroism (XMCD) at the Co- and Fe-L2,3 edge. The magnetization and XMCD data show that the total ordered magnetic moment of Co(2+) in CoFe2O4-BiFeO3 nano-heterostructures is strongly enhanced. This study clearly indicates that the high interface-to-volume ratio vertical nanostructure creates a strong ferromagnetic and antiferromagnetic magnetic coupling via an interface. Furthermore, the magnetic coupling can be tuned in the multiferroic-ferrimagnetic self-assembled heterostructures by controlling the spacing between nanopillars.

X-ray Absorption Magnetic Circular Dichroism (XMCD)

X‐ray absorption magnetic circular dichroism (XMCD), which utilizes synchrotron radiation, has become one of the most powerful tools to investigate microscopic information on the magnetic properties of ferromagnetic materials. In a ferromagnetic system, the X‐ray absorption rate becomes different for different photon helicity vectors, right and left circular polarized light, and the difference in the absorption rate provides information on the spin and the orbital magnetic moments separately. The orbital magnetic moment is mostly quenched in solids and is often neglected. However, because the magnetic anisotropy is determined by this small orbital magnetic moment through the spin‐orbit coupling, it plays a critical role in the magnetic behavior. Further, the core level absorption energy is characterized by the element; thus, XMCD is a unique way to determine the element-specific spin and orbital magnetic moments. This article presents a brief introduction to X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism experimental techniques.

Magnetic field dependence of the circular dichroism in EuTe

The absorption magnetic circular dichroism in EuTe was investigated as a function of the magnetic fieldat T = 2 K. In order to describe the optical spectra theoretically, we developed a model thattakes into account the magnetic order of the lattice as a function of the magneticfield. The model allows us to calculate the absorption spectrum as a continuousfunction of the magnetic field intensity. The theory accounts for the stronglydichroic absorption seen at high fields, and for its continuous evolution towards theB = 0 spectrum, which is described by a much weaker and much broader absorptionline. The drastic changes observed in the experimental spectra as afunction of magnetic field are fully explained by the dependence of theEu2+ spin arrangement on the external magnetic field.

Soft X-ray Absorption Magnetic Circular Dichroism Study of Ferromagnetic Superconductor UGe2

X-ray absorption magnetic circular dichroism (XMCD) at the U N 4,5 edges was measured for the ferromagnetic superconductor UGe 2 in the normal state. The orbital and spin magneic moments deduced from the sum rule analysis of the XMCD data indicate that the U atom in UGe 2 is considered to be closer to the trivalent state rather than the tetravalent state. The temperature dependence of the XMCD signal does not show any significant anomaly at T * ∼30 K, where the resistivity anomaly was observed at ambient pressure. The XMCD measurement at the U N 2,3 edge indicates that the U 6 d electrons have negligibly small magnetic contributions.

Calculation of spin and orbital moments of 3d transition metals using X-ray magnetic circular dichroism in absorption

X-ray magnetic circular dichroism(XMCD) in absorption has been extensively used to determine spin and orbital moments by applying the sum rules to the absorption spectra of specified atoms. Due to quench of orbital moments in 3d transition metals, it is necessary to reduce errors in experiments and data analysis. In this article we focas on the data analysis in 4 aspects. 1) The influence of applied magnetic field H on signal intensity. Experiments reveal that, for H-α; for H>200Gs, it changes little with the increase of H. 2) There is a separation between absorption spectra resulting from the iron-core remanence being in the parallel or antiparallel directions. This separation is independent of incident X-ray polarization and can be eliminated by multiplying by a constant. 3) An analytic form of absorption spectrum was obtained by fitting experimental data using XPSPEAK 4.1. It can be used as a criterion to determine which numerical integration method is suitable under certain experimental conditions. And, 4) by choosing an error function as a background to be subtracted from the X-ray absorption spectra, a method of calculation of spin and orbital moments from XMCD absorption spectrum is also developed Finally, the spin moment, of 1.314μB and orbital moment of 0.141μB of cobalt atom in a 20nm thick cobalt film are figured out based on Bode rule numerical integration.

Research on the effects of thickness of cobalt layers on the orbital and spin moments of Co atoms using x-ray magnetic circular dichroism in absorption

Orbital and spin moments of Co atoms in films of different thickness deposited by magnetron-sputtering on silicon wafers have been investigated by use of soft x -ray magnetic circular dichroism in absorption. Layers with thickness 2nm, 10nm and 30nm are covered by 0.8—1nm of protection overlayer. Orbital and spin momen ts of 0.249—0.195μB and 1.230—1.734μB are found from X MCD sum rules. Increase in orbital moments and decrease in spin moments are obse rved with reducing cobalt film thickness. There is a ratio variation from 0.101 to 0.168 of orbital moment to total moment, that is, contribution of orbital mom ent of cobalt atoms to total moment in 2nm film is 83% greater than that in the 30nm film.

About the blue and green colours in natural fluorapatite

Different natural blue and green fluorapatite samples from Brazil were investigated by electron paramagnetic resonance (EPR), optical absorption, photoluminescence and the magnetic circular dichroism of absorption (MCDA). The main objective of our work is to confirm the unusual valence state of Mn (V), with high spin configuration S = 1, in tetrahedral coordination. The EPR spectra measured at low temperatures (10 K – 30 K) in X-band (10 GHz) show a complicated structure that can be explained through static Jahn-Teller effect acting on the excited states of Mn (V). Photoluminescence measurements showing a multi line spectrum in the near infrared region caused by transitions between the 1E - 3A2with phonon replica confirm the presence of Mn (V). However, strong evidence for high spin configuration of Mn (V) with S = 1 was found from measurements of the MCDA as a function of temperature and magnetic fields. Using the Brillouin function for the magnetization, a spin S = 1 for the ground state for all optical absorption bands in the visible spectral range was determined, confirming that Mn (V) is responsible for the blue colour in natural fluorapatite. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray magnetic circular dichroism: Orbital and spin moments of iron single-crystal thin film deposited on MgO substrate

X-ray magnetic circular dichroism in absorption of the single-crystal iron layer deposited epitaxially on MgO substrate is studied. Spin and orbital moment, 0.069 and 2.33 μB respectively, are calculated in terms of the XMCD sum rules. Our results are accordant to those published. Experiments show that the orbital moment would be decreased to that in bulk materials as iron film is thinned down, but spin moment changes little.

Linear Dichroism in X-Ray Absorption and Orbital Orderings

Linear Dichroism in X-Ray Absorption and Orbital Orderings

RESONANT SCATTERING OF POLARIZED X-RAYS AT THE 4d EDGE OF GADOLINIUM

The impact of resonant scattering of polarized X-rays is illustrated taking the gadolinium 4d edge energy range as an example. It is shown that: (i) the study of the reflection of linearly polarized light by a magnetized Gd sample gives information on the electronic states of the system, similar to that given by magnetic circular dichroism in absorption; (ii) the energy-loss peaks corresponding to spin-flip transitions in the 4d → 4f excitation exhibit a strong circular dichroism.

Magnetic circular dichroism in resonant x-ray emission from impurities: Results at theL2,3edges of Mn in Ni

We present a study of Mn magnetic impurities (about 2%) in Ni with resonant x-ray emission spectroscopy excited at the ${L}_{2,3}$ edges of Mn using circularly polarized x rays incident almost parallel/antiparallel to the magnetization. The results are discussed jointly with absorption magnetic circular dichroism and complemented with elastic reflectivity measurements. Moreover, we show that emission spectroscopy is particularly suitable to the impurity problem, since the self-absorption/saturation corrections to the measurements are rather limited. The problem is treated also theoretically with a full multiplet splitting calculation of a single ${\mathrm{Mn}}^{2+}$ ${(3d}^{5})$ ion with cubic crystal field. The theory reproduces satisfactorily all experimental trends of the features dispersing with the incident photon energy in the whole ${L}_{3}$ region, whereas it is not adequate just below the ${L}_{2}$ excitation. The joint use of the theory and of the measurements allows us to extract the nondispersive component from the measured spectra. It is shown that this component does not correlate directly with the $3d$-Mn local density of states. The general implications of the results are discussed.

Magneto-optical and ODEPR investigations of native defects in substrate-free LT-MBE grown GaAs

As-grown substrate-free LT-GaAs (200°C) and annealed samples (up to 660°C) have been investigated with magnetic circular dichroism of absorption (MCDA) and optically detected electron paramagnetic resonance (MCDA-EPR) in K- and W-bands. MCDA-EPR spectra of all samples reveal several As Ga-related defects with different excitation spectra, that indicate their different structures in as-grown and annealed material. A strong change occurs in samples annealed above 400°C. The ODEPR spectra show another yet unknown defect which we attribute to a Ga vacancy-related defect. Samples annealed at 500°C exhibit EL2-like MCDA spectra. From the above band gap MCDA spectra of the EL2-like defects evidence is found for their low symmetry. The bleaching properties and spin-lattice relaxation times of the different As Ga-related defects were investigated and compared with the properties of the EL2 defect in bulk semi-insulating GaAs.

Native point defects in non-stoichiometric GaAs doped with beryllium

Native point defects in non-stoichiometric GaAs layers doped with Be were observed. Ga vacancies were identified by positron annihilation. As Ga antisite defects were investigated by near infrared absorption and magnetic circular dichroism of absorption, respectively. We found the properties of the As Ga antisites very similar to that of As Ga in bulk GaAs, i.e. they exhibit the same MCDA spectra and quenching behavior. The concentration of native point defects was not influenced by the Be doping. Therefore, the thermal stability of non-stoichiometric GaAs : Be can not be explained with a different incorporation of native defects.

Magnetic circular dichroism in soft X-ray resonant inelastic scattering

We discuss the application of resonant inelastic X-ray scattering to the study of magnetic systems in the soft X-ray range. To this end we distinguish two broad areas. In the first the layout of the experiment is such that the absorption magnetic circular dichroism (MCD) is not zero. In the second the magnetisation is perpendicular to the incident helical beam so that the absorption MCD is zero. In the first area we summarise published results on Fe-Co alloys and we present new data on Mn impurities in Ni together with calculations. In the second area we summarise published results on Ni in Ni-ferrite with final 3s shell excitation and we present new results on Co-metal and Co in Co-ferrite measured with a new approach. This is based on the incident energy dependence of the integral of the Raman spectrum in inner shell excitation (integrated resonant Raman scattering). The potentialities and the limitations of the above methods are critically presented.

Magnetic resonance investigation of F centers in Lif caused by ionizing radiation

Damage caused in lithium fluoride crystals by γ-rays, electrons, and heavy ions includes, besides other point defects, F centers and their agglomerates. We have studied the paramagnetic F centers by measurements of the nuclear spin relaxation (NSR) of 19F and 7Li, the return of the magnetic circular dichroism of absorption (MCDA) after switching off the resonant microwave field and by optical absorption. The magnetic resonance experiments were performed over a range of defect concentrations to measure the longitudinal spin lattice relaxation times T 1e of the F centers. Over several orders of magnitude, the NSR rate T −1 ln is proportional to the concentration of F centers. Measurements at different frequencies and temperatures suggest a spin diffusion limited NSR process. Raising the temperature of the crystal initiates an annealing process that can be observed as a decrease of the NSR rate.

Magnetic resonance investigation of the dynamics of F centers in LiF

Damage caused in lithium fluoride crystals by γ-rays and electrons includes, besides other point defects, F centers and their agglomerates. We have studied the paramagnetic F centers created by radiation doses that vary by several orders of magnitude. We measured the electronic spin relaxation time T 1e of the F centers at low temperatures by the recovery of the magnetic circular dichroism of absorption (MCDA) as well as the temperature dependence of the 19F and 7Li nuclear spin relaxation (NSR) times, T 1n. Our results indicate that the nuclear spin relaxation is dominated by spin-diffusion limited paramagnetic relaxation. The electron spin correlation function is determined by the electronic spin–lattice relaxation. In the studied temperature range from 4 to 300 K, the electron spin–lattice relaxation time T 1e is long compared to the nuclear Larmor period, ω L T 1e≫1 ( ω L: nuclear Larmor frequency). The temperature variation of T 1n indicates that the electron spin–lattice relaxation is dominated by phonon-F center interactions. At temperatures T greater than 360 K, an annealing process takes place that eliminates the F centers and their agglomerates.

Electronic and magnetic states of RCo 5− xB x investigated by photoemission and soft X-ray MCD

Electronic and magnetic states of CeCo 5− x B x are studied element-specifically by means of high-resolution bulk-sensitive resonant photoemission spectroscopy (RPES) and soft X-ray absorption magnetic circular dichroism (MCD). Judging from MCD of the ferromagnetic CeCo 5, the spins of Ce 4f and Co 3d electrons couple antiferromagnetically with each other. RPES spectra show that the Ce 4f states are itinerant and form strongly mixed bands with the Co 3d states.

Use of optically detected magnetic resonance to correlate germaniumelectron centres with UV absorption bands in x-ray irradiated germanosilicateglasses

The relationship between paramagnetic defect centres and UV absorption bandssimultaneously generated by ionizing radiation in Ge-doped SiO2 glass isinvestigated using magneto-optical techniques. A sample of 7.0 mol% Ge-dopedSiO2 was exposed to x-ray radiation, which resulted in the formation ofabsorption bands centred at 4.4 eV and 5.7 eV as well as electron spinresonance (ESR) signals attributed to the Ge(1) and Ge(2) defects. To isolateparamagnetic contributions to the induced optical absorption, the magneticcircular dichroism absorption (MCDA) spectrum was measured at severaldifferent temperatures over the range 2.00-6.21 eV. The optically detectedmagnetic resonance (ODMR) spectrum was then obtained at 4.42 eV viamicrowave-induced changes in the MCDA signals in order to unambiguouslycorrelate this optical transition with a particular ESR signal. The ODMR dataindicate that the Ge(1) centre is responsible for this absorption band,providing the first unequivocal correlation of ESR and optical properties forthis defect. In addition, evidence of a weakly absorbing paramagnetic defect,that appears to be related to the Ge(2) centre, was found at 5.65 eV.