Magnetic circular dichroism and the nature of ferromagnetism in colloidal gold nanoparticles

The spectra of optical absorption and of magnetic circular dichroism (MCD) have been measured in the 350–1150 nm wavelength range for a set of colloidal solutions containing Au nanoparticles with the average size of 6 nm having thiolate coatings with different degrees of chirality. The form of absorption and MCD spectra suggests the dipole character of interband transitions involving the 5d-6(sp) orbitals. The absence (within the experimental error) of the dependence of the MCD spectra on the coating type rules out the hypothesis on the orbital nature of the observed magnetism. We argue that the spin polarization plays the dominant role in the magnetism of gold nanoparticles with a thiolate coating.     

Numerical simulation of the spectrum of X-ray natural circular dichroism in the CsCuCl3 crystal

The possibility of observing x-ray natural circular dichroism (XNCD) in enantiomorphous CsCuCl₃ crystals has been considered. The spectra of XNCD in CsCuCl₃ have been numerically simulated for the K, L1, L2, and L3 x-ray absorption edges of copper, cesium, and chlorine using FDMNES and LMTO programs. It has been shown that the largest value of dichroism signal can be expected near the copper and chlorine L1 edges.     

Stress-induced optical dichroism of V−-centers in MgO

The stress-induced optical dichroism and optical absorption spectra of the V^?-centers in electron irradiated MgO are reported. The dichroism spectrum can be largely attributed to a paraelastic alignment of inherently-dichroic V^?-centers. A curve-fit analysis of the lower-energy portions of the dichroism and absorption spectra yields the ratio of the transition dipole strengths, $\text{D}{}_{\mathrm{\parallel }}\text{D}{}_{\mathrm{\perp }}\text{= 1.55 ± 0.2}$, and the energy splitting, E_∥ ? E_⊥ = 0.22 ± 0.07 eV, for light polarized parallel (∥) and perpendicular (⊥) to the defect symmetry axis. The spectra also have considerable intensity in their high-energy tails, and it is speculated that a portion of this intensity is due to the V-center. The implications of the results with regard to the electronic structure of the center are discussed.     

Electron magnetic chiral dichroism in CrO2 thin films using monochromatic probe illumination in a transmission electron microscope

Electron magnetic chiral dichroism (EMCD) has been studied in CrO₂ thin films (with (100) and (110) growth orientations on TiO₂ substrates) using a gun monochromator in an aberration corrected transmission electron microscope operating at 300 kV. Excellent signal-to-noise ratio is obtained at spatial resolution ∼10 nm using a monochromatic probe as compared to conventional parallel illumination, large area convergent beam electron diffraction and scanning transmission electron microscopy techniques of EMCD. Relatively rapid exposure using mono probe illumination enables collection of EMCD spectra in total of 8–9 min in energy filtered imaging mode for a given Cr L_2,3 energy scan (energy range ∼35 eV). We compared the EMCD signal obtained by extracting the Cr L_2,3 spectra under three beam diffraction geometry of two different reciprocal vectors (namely g=110 and 200) and found that the g=200 vector enables acquisition of excellent EMCD signal from relatively thicker specimen area due to the associated larger extinction distance. Orbital to spin moment ratio has been calculated using EMCD sum rules for 3d elements and dichroic spectral features associated with CrO₂ are compared and discussed with XMCD theoretical spectra.     

Excited states of dinaphth[de-1, 2, 3; d′e′-5, 6, 7]azulene: Polarized absorption in stretched polymers and magnetic circular dichroism

UV-visible polarized absorption spectra of dinaphth[de-1,2,3; d′e′-5,6,7]azulene I have been determined from measurements of linear dichroism in stretched polymer sheets. Magnetic circular dichroic (MCD) spectra have been measured in solution and help to distinguish individual electronic transitions. π-Electron calculations of transition energies, oscillator strengths, and polarization directions are in good agreement with the experimental data and permit a straightforward correlation of the low-energy part of the spectrum of I with that of the parent pleiadiene II.     

Magnetic circular dichroism of transition metal ions in solids-I. MgO:Cr3+

Absorption and magnetic circular dichroism (MCD) spectra of MgO: Cr³⁺ have been studied over the temperature range 5–300 °K and with magnetic fields up to 77 kG. A theory for intensity calculation using a weak field coupling scheme is discussed. Spin-forbidden transitions to ²E and ²T₁, are observed in MCD and are interpreted as magnetic dipole zero-phonon lines. The spin allowed bands ⁴T₂ and ⁴T₁ are found to be vibration-induced electric dipole transitions in which the t_1u lattice modes are the dominant contributors to the intensity.     

X-ray natural circular dichroism in copper metaborate

The local electronic structure of copper ions in a copper metaborate CuB₂O₄ crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB₂O₄ to the absorption and X-ray circular dichroism spectra of the crystal.     

The absorption,magnetic circular dichroism and emission of Eu2+ in cubic elpasolites

The absorption, magnetic circular dichroism (MCD) and emission/excitation spectra of some $\text{4?}{}^7\text{→4?}{}^6\text{5d}$ transitions in Eu²⁺ in several hexachloro and hexabromoelpasolites have been measured. Data were collected between 5 and 300 K and up to energies of 42,000 cm^?1. The absorption and MCD were satisfactorily interpreted using a coupling scheme that treats the d-electron crystal field as being much greater than the f-electron spin orbit coupling and essentially ignores other interactions.     

Magnetic circular dichroism and Faraday rotation of YCrO3 in the R-lines region

The rich structure in magnetooptical spectra of ⁴A₂ → ²E transition in YCrO₃ was observed at liquid helium temperature. The magnetooptical lines are correlated with adsorption structure and attributed to magnon and phonon-assisted exciton transitions.     

Circular dichroism spectroscopy of chlorin <Emphasis Type="Italic">e</Emphasis>6 and its complexes with quantum dots in different media

The circular dichroism (CD) spectra of chlorin e6 and its complexes with ZnS:Mn/ZnS and CdSe/ZnS quantum dots (QDs) in aqueous solutions with different pH, in methanol, and in dimethyl sulfoxide (DMSO) have been experimentally investigated. The changes in the CD spectra of free chlorin e6 caused by its complexing with semiconductor QDs are analyzed. The application of CD spectroscopy made it possible to record for the first time the CD spectrum of luminescent dimer of chlorin e6 and reveal a nonluminescent aggregate of chlorin e6 (interpreted preliminary as a “tetramer”), the anisotropy factor of which exceeds that of its monomer by a factor of 40. An analysis of the experimental data shows that chlorin e6 in a complex with QDs can be either in the monomeric form or in the form of a nonluminescent “tetramer.” The interaction with a relatively low-stable luminescent dimer of chlorin e6 with QDs leads to its partial monomerization and formation of complexes where chlorin e6 is in the monomeric form.     

Relativistic multiple scattering theory of K-edge X-ray magnetic circular dichroism

We discuss the angular dependent K-edge X-ray magnetic circular dichroism (XMCD) spectra based on the non-relativistic Green’s function expansion of the relativistic 4×4 Green’s function developed by Gesztesy et al. [Ann. Inst. Henri Poincaré 40 (1984) 159]. For the core functions we use the Dirac equation solutions whereas the relativistic effects for photoelectrons are automatically taken into account in the Gesztesy expansion. Analyses of the angular anisotropy provides us useful information on local symmetry violation around X-ray absorbing atoms even though we include relativistic effects. We explicitly show three different types of the symmetry which give rise to the suppression of the sin β-dependence in XMCD spectra. We also present explicit formulas of XMCD for randomly oriented and spatially fixed systems. Discussion on Debye–Waller factors is given in the present theoretical framework. Some illustrative calculations are also shown to understand the relativistic effects on the XMCD. The results are given for the Gd L₁-edge and Fe K-edge XMCD.     

Magnetic circular dichroism of [Co/Pd] and [CoB/Pd] multilayered films

Magnetic circular dichroism (MCD) and X-ray absorption spectra (XAS) at the Co L_2,3-edge of [Co/Pd]₂₀ and [CoB/Pd]₂₀ multilayered films, which were fabricated at 260 °C with different magnetic layer thicknesses (δ), have been measured. The lineshapes of XAS–MCD show that the electronic state of Co 3d of the films hardly changes even when sputtered at higher temperatures. The expectation values of orbital and spin angular momentum (〈L_z〉 and 〈S_z〉) are estimated using the sum rule, and it is found that 〈L_z〉/〈S_z〉 in δ<0.5 nm is larger than that in δ>0.5 nm.     

Mössbauer dichroism

In analogy to optics, polarized γ-rays can be produced by absorbing one linearly or circularly polarized component. The transmitted γ-rays are polarized in the opposite direction. In samples with large effective thickness, such dichroism effects might cause an appearance in the spectra which is—at least at first glance—unexpected.     

Magnetic circular dichroism of the U center in KCl

The magnetic circular dichroism (MCD) and absorption spectrum of the U center in KCl have been measured at liquid helium temperature. The results are consistent with the usual ¹S(l s²) → ¹P(ls2p) assignment with the temperature independent MCD yielding an excited state magnetic moment only about 40 per cent that calculated for a hydride ion in an octahedral crystal field.     

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO₃ compare favorably with the experiments. Received: 20 October 1997 / Revised: 18 February 1998 / Accepted: 31 March 1998     

Magnetic linear dichroism in photoemission from an ultrathin iron silicide film

The effect of magnetic linear dichroism in photoemission of Fe 3p electrons was used to investigate the magnetic properties of the Si(100)2 × 1 surface on which iron films up to 10 monolayers thick were deposited at room temperature under ultrahigh vacuum. The experiments were performed with linearly polarized light (at a photon energy of 135 eV) incident at an angle of 30° to the surface. The photoelectron spectra were measured in a narrow solid angle oriented along the normal to the sample surface for two opposite magnetization directions which were parallel to the surface plane and perpendicular to the polarization vector of the light wave. An analysis of the data obtained showed that the effect has a threshold character and appears after deposition of eight Fe monolayers, when the ferromagnetic silicide Fe₃Si is formed on the surface.     

Revelation of Ni magnetic moment in GdNi single crystal by soft X-ray magnetic circular dichroism

Magnetic moment of Ni in GdNi single crystal was studied through the soft X-ray absorption spectra (XAS) and the magnetic circular dichroism (MCD) at the Ni L_2,3-edges and the Gd M_4,5-edges. Our experiment revealed for the first time that the Ni 3d band is not filled completely even at the content of 50 at.% of Gd and the Ni does retain a total magnetic moment coupling antiparallel to that of Gd. This result implied that the Ni in GdNi holds an intrinsic magnetic moment even at 50 at.% of Gd and contradicts the well-known charge transfer model. Further by employing the magneto-optical sum rule, the spin and orbital angular magnetic moment were evaluated and discussed.     

Torsion induced circular dichroism in hexagonal CdS

The influence of the torsion on the absorption properties of hexagonal CdS has been studied in our experiments. The undeformed CdS does not possess circular dichroism (CD) but it manifests linear dichroism (LD). An appearance of CD and a change of LD can be observed in the torsion deformed CdS crystals. The mentioned absorption phenomena occur in the vicinity of the isotropic point (i.p.), the crossing point of the curves of the refractive indicesn ₀() andn _e (). The sign of CD is connected with the direction of the torsional momentM. The magnitude ofM influences the shape of CD spectra as well as the values of LD. The appearance of CD and the change of LD are explained by the torsion induced gyrotropy in the CdS crystals, i.e. by the created chirality in the hexagonal CdS.     

Soft X-ray magnetic circular dichroism of Heusler-type alloy Co2MnGe

Co and Mn 2p core absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectra have been measured for the ferromagnetic ternary alloy Co₂MnGe. The observed Co 2p XAS spectrum can be understood on the basis of the unoccupied Co 3d partial density of states, whereas the overall features of the Mn 2p XAS and XMCD spectra have been partly reproduced by the Mn 2p⁵3d⁶ final state multiplets. We have found that the orbital polarization of the Co 3d and even the Mn 3d states are recognizable, which suggests that a spin-orbit coupling should be taken into account in the energy band structure in order to reproduce the half metallic nature of this alloy.     

First-principles calculations of circular dichroism of ligand-protected gold nanoparticles

Density functional theory calculations of the circular dichroism spectra of ligand-protected gold nanoclusters are performed to gain insight into the physical origin of their optical activity. The case of two different atomic models of the thiolated Au₃₈(SCH₃)₂₄ nanocluster is studied in detail, where the contribution to the circular dichroism spectra from the building parts of the nanoparticle, named core atoms, shell atoms, and ligand molecules, are analyzed separately. The results support the proposal of an intrinsically or ligand-induced chiral metallic core as the main responsible of their optical activity.