Magnetic Circular Dichroism Spectra Research Articles

Growth, magnetic and magneto-optical properties of CaDyAlO4 crystals

Abstract CaDyAlO4 (CDA) crystals with dimensions of Φ 25 × 28 mm3 were grown by the Czochralski method. The presence of a small amount of oxygen in the growth atmosphere will cause the coloration of CDA crystal, and correspondingly reduce the transmittance in the range of 400–730 nm. Rietveld structural refinement of XRD patterns revealed a slight difference in the unit cell parameters of the brown and yellow-green CDA crystals. The angular and temperature dependencies of the magnetic susceptibility were measured, indicating that CDA crystal has remarkable magnetic anisotropy and abnormal magnetic behavior in the a and c direction below 150 K. Faraday rotation characteristics of CDA crystal were investigated at room temperature. Verdet constant of CDA at 500–700 nm band is about 1.1 times as large as that of TGG, but only 0.69 fold of that for isostructural CaTbAlO4 (CTA). Based on the Van Vleck-Hebb theory, the effective transition wavelength and probability of three crystals were calculated and compared. The absorption and magnetic circular dichroism spectra (MCD) of 4f-4f transitions in Dy3+ ions in CDA were studied.

Magnetic Circular Dichroism of Naphthalene Derivatives: A Coupled Cluster Singles and Approximate Doubles and Time-Dependent Density Functional Theory Study.

The UV-vis absorption and magnetic circular dichroism spectra of naphthalene and some of its derivatives have been simulated at the Coupled Cluster Singles and Approximate Doubles (CC2) level of theory, and at the Time-Dependent Density Functional Theory (TD-DFT) level using the B3LYP and CAM-B3LYP functionals. DFT and CC2 predict in general opposite energetic ordering of the Lb and La transitions (in gas phase), as previously observed in adenine. The CC2 simulations of UV and MCD spectra show the best agreement with the experimental data. Analysis of the Cartesian components of the electric dipole transition strengths and the magnetic dipole transition moment between the excited states have been considered in the interpretation of the electronic transitions and the Faraday term inversion among the naphthalene derivatives.

Characterization of “Free Base” and Metal Complex Thioalkyl Porphyrazines by Magnetic Circular Dichroism and TDDFT Calculations

UV–vis absorption and magnetic circular dichroism (MCD) spectra of octakis thioethyl “free base” porphyrazine H2OESPz and its metal complexes MOESPz (M = Mg, Zn, Ni, Pd, Cu), as well as of [MnOESPz(SH)] were recorded. In the last case, MCD proved to have quite good sensitivity to the coordination of this complex with 1-methylimidazole (1-mim) in benzene. Time-dependent density functional theory (TDDFT) calculations were carried out for the considered porphyrazine complexes and showed good performance on comparing with MCD and UV–vis experimental spectra, even in the open-shell Cu and Mn cases. Calculations accounted for the red shift observed in the thioalkyl compounds and allowed us to reveal the role of sulfur atoms in spectroscopically relevant molecular orbitals and to highlight the importance of the conformations of the thioethyl external groups. Calculated MCD spectra of [MnOESPz(SH)] confirm the Mn(III) → Mn(II) redox process, which leads to the [Mn(OESPz)(1-mim)2] species, and the relevance of the spin state for MCD is revealed.

Detailed Insight into the Interaction of Bicyclic Somatostatin Analogue with Cu(II) Ions.

Somatostatin analogues are useful pharmaceuticals in peptide receptor radionuclide therapy. In previous studies, we analyzed a new bicyclic somatostatin analogue (BCS) in connection with Cu(II) ions. Two characteristic sites were present in the peptide chain: the receptor- and the metal-binding site. We have already shown that this ligand can form very stable imidazole complexes with the metal ion. In this work, our aim was to characterize the intramolecular interaction that occurs in the peptide molecule. Therefore, we analyzed the coordination abilities of two cyclic ligands, i.e., P1 only with the metal binding site and P2 with both sites, but without the disulfide bond. Furthermore, we used magnetic circular dichroism (MCD) spectroscopy to better understand the coordination process. We applied this method to analyze spectra of P1, P2, and BCS, which we have described previously. Additionally, we analyzed the MCD spectra of P3 ligand, which has only the receptor binding site in its structure. We have unequivocally shown that the presence of the Phe-Trp-Lys-Thr motif and the disulfide bond significantly increases the metal binding efficiency.

In vitro antiproliferative effect of four ball-type phthalocyanines linked by t-butylcatechol and high singlet oxygen production

Four ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) containing 4-tert-butylcatechol substituents at alpha-positions were synthesized from 3,3′-(4-tert-butyl-1,2-phenylene)bis(oxy)diphthalonitrile. The structures were elucidated by elemental analysis and ultraviolet-visible, Fourier-transform infrared, proton nuclear magnetic resonance, and Maldi-Tof mass spectroscopies. The photophysicochemical properties of H2Pc, MgPc, and ZnPc were investigated in dimethylsulphoxide. The fluorescence quantum yields (ΦF), triplet state quantum yields (ΦT), and singlet oxygen quantum yields (ΦΔ) with respect to H2Pc, MgPc and ZnPc were experimentally obtained. Furthermore, the degradation quantum yield (Φd) was evaluated for all complexes. Finally, the magnetic circular dichroism spectra of the four Pcs were obtained. Among these ball-type phthalocyanines, MgPc had the highest triplet lifetime and ZnPc had the highest singlet oxygen quantum yield. Moreover, the in vitro antiproliferative effect of ball-type phthalocyanines (H2Pc, MgPc, CoPc, and ZnPc) was evaluated on MCF-7 cell line originated from luminal A breast cancer and MDA-MB-231 cell line originated from triple negative breast cancer. For this purpose, cell viability MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used. For all compounds, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2, 102.4 µM concentrations were applied to both cell lines. There was a significant decrease in cell viability for MCF-7 and MDA-MB-231 cells (p < 0.05).

Incipient antiferromagnetism in the Eu-doped topological insulatorBi2Te3

Rare earth ions typically exhibit larger magnetic moments than transition metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet, in a recent photoemission study of Eu-doped Bi$_2$Te$_3$ films, the spectra remained gapless down to $T = 20\;\text{K}$. Here, we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the $M_{4,5}$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu$^{2+}$ valence and confirms a large magnetic moment, consistent with a $4f^7 \; {^8}S_{7/2}$ ground state. At temperatures below $10\;\text{K}$, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth doped Bi$_2$Te$_3$ and call for spectroscopic studies in the kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.

Magnetooptics of Nanocomposites Based on Iron Chalcogenide Nanoparticles

Morphology, structure, magnetic properties and magnetic circular dichroism (MCD) were studied for the FexSy and FexSey nanoparticles (NPs) synthesized with the polyol mediated and the thermal decomposition processes, correspondingly, at different regimes. For all NPs samples, but Fe3S4, MCD was measured for the first time. It has been shown that in the case of sulfides, the character of the MCD spectrum changes as the NPs phase composition transforms from Fe3S4 to FeS with the synthesis temperature increase. At the same time, the shape of the MCD spectrum is almost independent of the NPs phase in the case of iron selenides

Electronic structure and magnetic properties of the CoFeMnZ (Z=As and Si) Heuslers by XAS, XMCD and MOKE: A DFT study

The electronic and magnetic and magneto-optic properties of the Heusler alloys of CoFeMnSi and CoFeMnAs were investigated by density functional theory. The density of states and magnetic moments were analyzed for both compounds and their magnetic moments obey the Slater-Pauling rule. The X-ray absorption and X-ray magnetic circular dichroism spectra of the constituent transition metals of these Heusler compounds were investigated and the origin of the XMCD spectra in these Heusler compounds was also studied through their density of states. The MOKE studies revealed the highest magnitude of Kerr rotation of 0.21° and 0.12° for CoFeMnAs and CoFeMnSi, respectively.

Molecular beam epitaxy of antiferromagnetic (MnBi2Te4)(Bi2Te3) thin films on BaF2 (111)

The layered van der Waals compounds (MnBi2Te4)(Bi2Te3) were recently established as the first intrinsic magnetic topological insulators. We report a study on the epitaxial growth of (MnBi2Te4)m(Bi2Te3)n films based on the co-deposition of MnTe and Bi2Te3 on BaF2 (111) substrates. X-ray diffraction and scanning transmission electron microscopy evidence the formation of multilayers of stacked MnBi2Te4 septuple layers and Bi2Te3 quintuple layers with a predominance of MnBi2Te4. The elemental composition and morphology of the films is further characterized by x-ray photoemission spectroscopy and atomic force microscopy. X-ray magnetic circular and linear dichroism spectra are comparable to those obtained for MnBi2Te4 single crystals and confirm antiferromagnetic order in the films.

Interfacial-hybridization-modified Ir ferromagnetism and electronic structure in LaMnO3/SrIrO3 superlattices

Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further reveal the mechanism of the SL-period dependence of the interfacial electronic structure and the local properties of the Ir moments, confirming that the formation of Ir-Mn molecular orbital is responsible for the interfacial coupling effects. The SL-period dependence of the ratio between spin and orbital components of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital. Our results clarify the nature of interfacial coupling in this prototypical 3$d$/5$d$ SL system and the conclusion will shed light on the study of other strongly correlated and spin-orbit coupled oxide hetero-interfaces.

Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins.

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups (n = 0–4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0–0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Magnetic circular dichroism spectra from resonant and damped coupled cluster response theory.

A computational expression for the Faraday A term of magnetic circular dichroism (MCD) is derived within coupled cluster response theory and alternative computational expressions for the B term are discussed. Moreover, an approach to compute the (temperature-independent) MCD ellipticity in the context of coupled cluster damped response is presented, and its equivalence with the stick-spectrum approach in the limit of infinite lifetimes is demonstrated. The damped response approach has advantages for molecular systems or spectral ranges with a high density of states. Illustrative results are reported at the coupled cluster singles and doubles level and compared to time-dependent density functional theory results.

Distinct spectral response in M -edge magnetic circular dichroism

Experimental investigations of ultrafast magnetization dynamics increasingly employ resonant magnetic spectroscopy in the ultraviolet spectral range. Besides allowing to disentangle the element-specific transient response of functional magnetic systems, these techniques also promise to access attosecond to few-femtosecond dynamics of spin excitations. Here, we report on a systematic study of transient magnetic circular dichroism (MCD) on the transition metals Fe, Co and Ni as well as on a FeNi and GdFe alloy and reveal a delayed onset between the electronic and magnetic response. Supported by \textit{ab-initio} calculations, we attribute our observation to a transient energy shift of the absorption and MCD spectra at the corresponding elemental resonances due to non-equilibrium changes of electron occupations.

On the electronic transitions of α-Fe2O3 hematite nanoparticles with different size and morphology: Analysis by simultaneous deconvolution of UV–vis absorption and MCD spectra

Nanostructures of the naturally-existing most stable iron oxide, hematite (α-Fe2O3), gain significant research interest in various recent applications. In this study, we put the main focus on magneto-optical (MO) properties of hematite nanoparticles synthesized by a solvothermal approach, and the effects of size, morphology, and particle aggregation on the magnetic circular dichroism (MCD) and conventional absorption spectra are investigated. Simultaneous deconvolution of UV–vis absorption and MCD spectra allows us to discuss the nature of electronic transitions of various hematite nanoarchitectures on the basis of possible transitions in the 3d5 system, giving enhanced spectral resolution and detailed assignments. Consequently, we find (i) characteristic size- and shape-dependent MCD responses at 450–470 nm; and (ii) the presence of both spin-allowed pair excitation and Laporte-forbidden d–d transition with opposite MCD signs in close energy at around 550 nm.

First principles calculations of intersite disorder influence on the electronic structure and X-ray magnetic circular dichroism in CoFeMnSi Heusler alloy

Electronic structure, X-ray absorption, and magnetic circular dichroism (XMCD) spectra in the CoFeMnSi Heusler alloy were studied from first principles. Fully relativistic Dirac linear muffin-tin orbital band structure method was implemented with various exchange–correlation functionals tested. The supercell approach was used to study the influence of intersite disorder, at the levels of 6.25%, 12.5%, and 25% within transition metal sites, on the XMCD spectra at [Formula: see text] edges and spin polarization (SP) at the Fermi level. It is found that most sensitive to Fe–Mn and Co–Fe disorder are XMCD spectra at [Formula: see text] edges of Fe, while the sensitivity decreases from Mn to Co. It is shown that magnetic moments estimated with the use of magneto-optical (MO) sum rules agree with the ab initio calculated ones to within [Formula: see text], [Formula: see text], and [Formula: see text], for Co, Fe, and Mn, respectively. The calculated SP decreases from 99% for ordered CoFeMnSi alloy, to 96% upon 25% Co–Fe disorder, to 83% for Fe–Mn disorder, and to 42% in the case of Co–Mn disorder. The calculated spectra agree well with the available experimental data. The rich XMCD spectral structures are predicted from first principles at Fe, Co, Mn and Si [Formula: see text] edges.

Effect of site disorder on the electronic, magnetic, and ferroelectric properties of gallium ferrite

The electronic and magnetic properties of the magnetoelectric solid ${\mathrm{GaFeO}}_{3}$ are investigated within the generalized gradient approximation including the Hubbard interaction ($U$) on the localized $d$ orbitals of iron. It was found that using an onsite $U=8$ eV describes consistently the experimental results. The origin of ferrimagnetism was attributed to the cationic site disorders. The density of states at the iron sites, in a octahedral geometry, show that the occupied ${e}_{g}$ states are below the ${t}_{2g}$ states in contradiction with the crystal-field splitting obtained by a point-charge model. However, for the unoccupied states the ab initio data agree qualitatively with the model, showing the complexity of the electron-electron interaction in ${\mathrm{GaFeO}}_{3}$. The computed electric polarization of the system as a function of the temperature in the linear regime shows a monotonic decreasing trend. To determine the nature of the magnetoelectric coupling, the computed electric polarization as a function of the rotation of the magnetization axis indicates that the magnetoelectric effect observed experimentally could not have been due to a direct coupling between the electric and magnetic order parameters. Finally, the calculated x-ray absorption and x-ray magnetic circular dichroism spectra for the disordered system are shown to be in good agreement with experiment.

Low Current Densities Toggle Optical Polarization Switching in Pt/Yttrium Iron Garnet Magnetic Heterostructures Using Energy Resolution

Spin current, which is converted from the charge current in heavy metals, is widely used to manipulate magnetization in heavy metal/magnetic insulator systems. However, there is scant research on how the spin‐polarized density of states (DOS) is affected by the absorption of spin current. This study investigates the spin‐current‐induced magnetic variation of an yttrium iron garnet (YIG) film driven by charge current in an overlayer of Pt that uses energy‐resolved magnetic circular dichroism (MCD) spectra as an example. MCD intensity can be manipulated reversibly using an applied current at near d–d transitions. Furthermore, the MCD sign, which is associated with an elliptically polarized direction, is reversed by a relatively low current density (≈108 A m−2) near YIG charge transfer transitions, even without the assistance of a magnetic field. The spin accumulation in Fe orbits and the corresponding change in spin‐polarized DOS explain the observed phenomenon. This paves the way to low‐dissipation spintronics and electrically controlled magneto‐optical devices based on magnetic insulators.

Characterization of the iron oxide phases formed during the synthesis of core–shell FexOy@C nanoparticles modified with Ag

Core–shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 · 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.

Magnetic circular dichroism and absorption spectra of f–f transitions 5I8 → 5F2 and 5F3 in the HoFe3(BO3)4 single crystal

Magnetic circular dichroism (MCD) and absorption spectra of multiferroic HoFe3(BO3)4 of 5I8 → 5F2 and 5F3 f–f transitions are measured at T = 90 K. The absorption spectra are expanded into Lorentzian components, and the transition intensities are obtained. The Zeeman splitting of some transitions is determined using the MCD and absorption spectra. The MCD spectra and Zeeman splitting are theoretically analyzed in the free-atom |J, ±MJ〉 wave function approximation by using the concept of a crystalline quantum number. An inconsistency is revealed when this concept is applied to ions with an integer moment, and a modification of the concept is proposed. Anomalously intense vibronic repetitions of electronic transitions from excited sublevels of the ground multiplet are found.

On Interpretation of Spectra of Photochemically Induced Circular Dichroism of Semiconductor Nanocrystals

We propose an approach to the analysis and interpretation of spectra of photoinduced circular dichroism (PICD) of semiconductor nanocrystals, in particular, quantum dots, based on comparison of these spectra with the spectra of magnetic circular dichroism (MCD). We show that MCD and PICD spectra have similar features. The values of parameters similar to the well-known parameters A and B of the MCD spectra, which characterize the splitting and mixing of states, are determined from the PICD spectra.