Large Magnetic Circular Dichroism Research Articles

Induced circular dichroism from helicoidal nano substrates to porphyrins: the role of chiral self-assembly.

Because the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free base- and metallo-porphyrins with silica nano helices, using a variety of elaboration methods, and have studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks.

Magnetic optical rotary dispersion and magnetic circular dichroism in methylammonium lead halide perovskites.

Large magnetic optical rotary dispersion (Faraday rotation) has been demonstrated recently in methylammonium lead bromide. Here, we investigate the prospect of extending the active spectral range by altering the halogen. We also investigate the origins of large Faraday rotation in these diamagnetic materials using magnetic circular dichroism (MCD) spectroscopy and the Kramers-Kronig relations. We find that, while MAPbCl3 (MA = methylammonium) single crystals exhibit a large Verdet constant in the blue, no appreciable Faraday rotation is observed in the red/near infra-red for MAPbI3 single crystals. However, in all film samples, we find clear evidence of large MCD resulting from the Zeeman splitting of the highly resonant 1s exciton state. Our Kramers-Kronig calculations of Faraday rotation based on MCD data matches well with the dispersion of our experimental data for MAPbCl3 and MAPbBr3 , with some deviation in magnitude-demonstrating the excitonic nature of Faraday rotation in these materials. However, our calculations predict significant Faraday rotation in MAPbI3 , contrary to our experimental results, indicating a potential discrepancy between the properties of the thin film and single crystal.

1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism as a probe for the local and non-local orbitals in CrO2

We have determined the magnetic ground state of the half-metal CrO2 based on 1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism (RIXS-MCD) experiments. The two-dimensional RIXS-MCD map displays the 1s X-ray absorption spectrum combined with the 1s2p X-ray emission decay, where there is a large MCD contrast in the final state involving the 2p core hole.Our measurements show that the Cr K pre-edge structure is dominated by dipolar contributions and the quadrupole peak is invisible in direct K pre-edge absorption. Using RIXS-MCD, we reveal that the quadrupole 1s3d pre-edge has a large MCD contrast, which appears at lower energy with respect to the K pre-edge maximum.We use crystal field multiplet calculations to model the excitonic RIXS-MCD spectral shape in tetragonal (D4h) symmetry. The RIXS-MCD is strongly sensitive to the ground state distortion of the Cr4+ sites. The calculations of the RIXS-MCD maps suggest that the 3d spin–orbit interaction is fully quenched (ζ3d=0meV) and the ground state electron configuration must contain a 3B2g (D4h) contribution, which is required to explain the appearance of the Magnetic Circular Dichroism (MCD) in the Cr K pre-edge. This is in apparent contrast with the compressed tetragonal distortion.

Magnetic circular dichroism of non-local surface lattice resonances in magnetic nanoparticle arrays.

Subwavelength metallic particles support plasmon resonances that allow extreme confinement of light down to the nanoscale. Irradiation with left- and right hand circularly polarized light results in the excitation of circular plasmon modes with opposite helicity. The Lorenz force lifts the degeneracy of the two modes in magnetic nanoparticles. Consequently, the confinement and frequency of localized surface plasmon resonances can be tuned by an external magnetic field. In this paper, we experimentally demonstrate this effect for nickel nanoparticles using magnetic circular dichroism (MCD). Besides, we show that non-local surface lattice resonances in periodic arrays of the same nanoparticles significantly enhance the MCD signal. A numerical model based on the modified long wavelength approximation is used to reproduce the main features in the experimental spectra and provide design rules for large MCD effects in sensing applications.

Large magnetic circular dichroism of Co clusters in Co-doped ZnO

Large magnetic circular dichroism (MCD) in Co-doped ZnO films is reported. The Co-(∼20%) doped ZnO films with oxygen deficiencies show ferromagnetic behavior at room temperature. While paramagnetic films exhibit a small MCD signal, broad MCD spectra with large amplitude of ∼20kdeg∕cm are observed for the ferromagnetic films. In the ferromagnetic films, Co cluster formation occurs, especially near the interface between the substrate and the film. The amount of the Co clusters is consistent with the spontaneous magnetization and the amplitude of the MCD.

MAGNETIC CIRCULAR DICHROISM OF 4d–4f RESONANT X-RAY EMISSION FOR GADOLINIUM AND TERBIUM

We have measured the magnetic circular dichroism (MCD) of X-ray emission spectra (XES) for gadolinium and terbium in the 4d–4f resonant excitation region. Clear MCD signals in the XES were observed and they dramatically changed as the excitation energy changed. Especially, an extremely large MCD signal appeared with the 8D resonant excitation for gadolinium. Comparison between the MCD for XES and total photoelectron yield (TEY) shows that the XES are very insensitive to the surface state. As a result, the measurement of XES was proved to be a superior technique for estimating the genuine magnetic moment in the bulk.

Magnetic Circular Dichroism of X-Ray Emission for Gadolinium in 4d–4f Excitation Region

We have measured magnetic circular dichroism of x-ray emission spectra (XES) for gadolinium in the 4 d –4 f excitation region. At a pre-threshold excitation energy, a large magnetic circular dichroism (MCD) signal appeared in a Raman scattering and the dramatic dependence of the MCD spectra on the excitation energy was observed. Theoretical calculation shows that the magnetic moment estimated with total photoelectron yield (TEY) spectra was much smaller than that with the emission spectra. This indicates that the MCD for the TEY reflects the magnetic state on the surface whereas the MCD for the XES reflects that in the bulk. We also observed the MCD spectra for total photon yield (TPY) and found the great difference of the MCD spectra for the TEY and TPY.

Magnetic circular dichroism at transition metal L2,3 edges in D03-type (Fe(1-x)Vx)3Al alloys.

Magnetic circular dichroism (MCD) spectra have been measured at the Fe and V L2,3 edges of DO3-type (Fe(1-x)Vx)3Al in order to investigate their local magnetic moments and electronic structures. Large MCD is observed at the Fe L2,3 edges, while the V L2,3 MCD shows relatively small intensity with complicated features. Signs of these MCD spectra indicate an antiferromagnetic coupling between the magnetic moments on Fe and V. According to the analysis based on the magneto-optical sum rules, the magnetic moment decreases with x, but remains fairly large for Fe2VAl, which might arise from its marginally magnetic nature.

Magnetic circular dichroism in X-ray fluorescence of Heusler alloys at threshold excitation

The results of fluorescence measurements of magnetic circular dichroism (MCD) in Mn L 2,3 X-ray emission and absorption for Heusler alloys NiMnSb and Co 2MnSb are presented. Very intense resonance Mn L 3 emission is found at the Mn 2p 3/2 threshold and attributed to a peculiarity of the threshold excitation in materials with a half-metallic character of the electronic structure. The excitation energy dependence of Mn L 2,3 X-ray emission spectra (XES) was measured at beamline ID12B at the European Synchrotron Radiation Facility (ESRF, Grenoble) using 83% circularly polarized X-rays. A very large MCD effect found in XES is attributed to strong exchange splitting of spin-up and spin-down Mn 3d states. The anomalously high ratio of L 2 emission intensity to L 3 emission intensity is found.

Magnetic circular dichroism in 5pphotoemission from Gd and Tb metal

The isotropic and magnetic-circular-dichroism (MCD) 5$p$ core-level photoemission (PE) spectra of Gd(0001) and Tb(0001) are presented. Large MCD effects were observed at the low-binding-energy side, gradually reducing in amplitude at higher binding energies. Both the isotropic and the MCD-PE spectra can be well reproduced by a many-particle intermediate-coupling approach taking spin-orbit, Coulomb, and exchange interactions properly into account. We also present a simple model to understand the global line shape of the MCD spectra. The MCD permits an accurate determination of the values of the Coulomb and exchange interactions in the final state.

Application of MCD in 4f photoemission as an element-specific probe for near-surface magnetization in lanthanide materials

4 f photoemission (PE) spectra from magnetically ordered lanthanide materials exhibit large magnetic circular dichroism (MCD), i. e. the intensities of individual PE multiplet lines depend strongly on the relative orientation between sample magnetization and photon momentum. We apply MCD-PE as a probe for near-surface magnetization with atomic-layer resolution to the (0001) surfaces of Gd and Tb, where the magnetization of the topmost atomic layer - by help of the surface core-level shift - can be separated spectroscopically from the sub-surface magnetization. In multicomponent magnetic thin films containing different lanthanide elements, MCD-PE allows to monitor the magnetization in an element-specific way. This is demonstrated for the example of the heteromagnetic interface Eu/Gd(0001).

Application of MCD in 4 f photoemission as an element-specific probe for near-surface magnetization in lanthanide materials

4 f photoemission (PE) spectra from magnetically ordered lanthanide materials exhibit large magnetic circular dichroism (MCD), i. e. the intensities of individual PE multiplet lines depend strongly on the relative orientation between sample magnetization and photon momentum. We apply MCD-PE as a probe for near-surface magnetization with atomic-layer resolution to the (0001) surfaces of Gd and Tb, where the magnetization of the topmost atomic layer - by help of the surface core-level shift - can be separated spectroscopically from the sub-surface magnetization. In multicomponent magnetic thin films containing different lanthanide elements, MCD-PE allows to monitor the magnetization in an element-specific way. This is demonstrated for the example of the heteromagnetic interface Eu/Gd(0001).

Interfacial magnetism studied by MCD in 4 f photoemission: [formula omitted

We report on an element-specific study of the interfacial magnetic properties of 1 monolayer (ML) Eu on top of the Gd(0001) surface using magnetic circular dichroism (MCD) in 4 f photoemission (PE). Divalent Eu forms a thermodynamically stable abrupt interface with trivalent Gd(0001). 1 ML can be ordered as a commensurate two-dimensional hexagonal lattice (6×6 phase) on Gd(0001). 4 f 6 final-state PE multiplets, which are well separated for Eu and Gd, show large MCD at 15 K. It has the same sign for both elements. This demonstrates that 1 ML Eu is ferromagnetically aligned with respect to the Gd(0001) substrate.

Magnetic circular dichroism in core-level photoemission from Gd, Tb, and Dy in ferromagnetic materials.

Core-level photoemission (PE) spectra from magnetically ordered lanthanide materials display strong magnetic circular dichroism (MCD). Particularly large MCD effects were observed in 4f PE from Gd, Tb, and Dy, with the intensities of resolved 4f PE lines changing up to 62%. Substantial MCD effects were also found in 4d core-level PE. The perspectives of applying MCD in 4f PE as an element-specific tool for studies of surface and interface magnetism and as a sensor for the degree of circular polarization of soft x rays are discussed.

Circular dichroism in 4f photoemission from magnetically ordered rare-earth materials

4f Photoemission (PE) spectra from magnetically ordered rare-earth materials using circularly polarized X-rays exhibit strong Magnetic Circular Dichroism (MCD), i.e., the intensities of the individual multiplet lines depend on the relative orientation of sample magnetization and photon spin. On the example of the wellresolved Tb 4f PE multiplet, it is shown that in relevant cases 4f PE lines are essentially only observed for one magnetization direction, either parallel or antiparallel to the photon spin. These large MCD effects in 4f PE open new perspectives in the analysis of surface and thin-film magnetism and provide a sensor for the degree of circular polarization of soft X-rays over a wide photon-energy range. To demonstrate the potential of MCD in 4f PE as a magnetometer, we studied Gd(0001) and Tb(0001), where the magnetization of the topmost atomic (0001) layer can be easily separated from the bulk magnetization via the surface core-level shift. In multicomponent magnetic thin films containing different rare-earth elements 4f PE allows to monitor magnetization in an element-specilic way, e.g., in case of the hetero-magnetic interface 1 ML En/Gd(0001).

Magnetic circular dichroism in photoemission from rare earth materials: a new technique in surface and thin-film magnetism

Using circularly polarized X-rays to excite photoelectrons from magnetically ordered rare-earth materials gives rise to large magnetic circular dichroism in photoemission (MCD-PE), i.e. the intensity of 4f-photoemission lines strongly depends on the orientation of sample magnetization with respect to the photon spin. To explore its potential for application as near-surface magnetometer with atomic-layer resolution, we studied MCD-PE at the (0001)-surfaces of Gd and Tb where the magnetization of the topmost atomic layer-by help of the surface core-level shift-is spectroscopically separated from the bulk magnetization. In thin films containing different rare earths MCD-PE allows to monitor the magnetization in an element specific way. This is demonstrated using the ordered heteromagnetic interface Eu/Gd as an example.

Magnetic x-ray dichroism in 2p absorption spectra of Fe/Cu(001)

Large magnetic circular dichroism using circularly polarized synchrotron radiation has been observed at the L2,3 absorption edges of thin (1–12 monolayers) fcc Fe films grown on Cu(001). Dramatic changes in the 2p branching ratio are observed when the orientation of photon helicity and sample magnetization are varied from parallel to antiparallel. The temperature and film thickness dependence of the perpendicular anisotropy in these films could be monitored by variations in the 2p branching ratio. Finally, our results are described in a simple theoretical framework that allows a determination between the predicted low- and high-spin phases for fcc Fe. Our data suggest a high-spin phase with a moment of 2.0–2.5 μB/atom. Interestingly, thicker films with remanant magnetic moments in the film plane present smaller branching ratio variations consistent with either a reduced moment or with domain closure for these films. The surface sensitivity and elemental specificity of this technique make it particularly attractive for the study of surface and thin film magnetism.

Radiation-detected optical pumping in solids

Radiation-detected optical pumping in solids has been developed to investigate the structure of unstable nuclei. Appreciable nuclear polarization of implanted or doped unstable-nuclei in a host crystal is achieved with the optical pumping in solids. The nuclear polarization achieved is enhanced/reduced by applying a radio-frequency magnetic field together with the optical pumping, radiation-detected magnetic resonance being thus observed to get information on electromagnetic properties of unstable nuclei. Two schemes have been successfully developed for the optical pumping of unstable nuclei in solids. One is to directly pump the atoms by the excitation from the ground state to a broad absorption band in visible and UV regions which shows a large magnetic circular dichroism. This scheme is applicable to many rare-earth atoms in alkaline-earth fluoride host. The other scheme is to pump the electrons in the conduction band of direct-type semiconductor and thus indirectly polarize the nuclei in the host material via a hyperfine interaction between the nuclei and the polarized conduction electrons. This scheme can be especially applied to the III to VI families of atoms in direct-type semiconductors. Principle of the methods, on-line experimental system, and a few examples of the results obtained so far are presented and discussed.

Dynamics of the Optical-Pumping Cycle ofFCenters in Alkali Halides-Theory and Application to Detection of Electron-Spin and Electron-Nuclear-Double-Spin Resonance in the Relaxed-Excited State

The dynamics of the optical-pumping cycle of $F$ centers in alkali halides are governed by two prime factors: the large magnetic circular dichroism exhibited by the $F$ band and a large electron-spin memory over the entire cycle. A model of the pump cycle and associated rate equations are given. The most general solutions are given, and important special cases are treated in detail. A number of new experimental techniques are based on the special characteristics of the rate-equation solutions. Among these are purely optical techniques for measuring (i) the degree of spin memory in the entire pumping cycle and (ii) the spin-lattice relaxation rate in the relaxed-excited state (RES). But the most important new development based on the rate-equation solutions is the optical-trigger detection of both electron-spin resonance (ESR) and electron-nuclear-double-spin resonance of the RES. Results of these experiments, including ESR $g$ factors and hyperfine splittings for the RES are given and discussed.

Polarization ofTm169andF19in CaF2:Tm2+by Optical Pumping

Nuclear polarization of $^{169}\mathrm{Tm}(I=\frac{1}{2})$ is achieved in a crystal of Ca${\mathrm{F}}_{2}$ containing a small fraction of paramagnetic ${\mathrm{Tm}}^{2+}$ ions ($S=\frac{1}{2}$) by optically pumping with circularly polarized light in the region 5400-6000 \AA{}, where there is a large magnetic circular dichroism. By optical pumping alone at $T=1.65$ \ifmmode^\circ\else\textdegree\fi{} K and in a field $H=750$ G a nuclear polarization of 9% is observed. If the ${I}_{+}{S}_{\ensuremath{-}}$ rf transition is simultaneously saturated, the polarization increases to 18%. The polarization is reversed by changing from left- to right-circularly polarized light. These results are understood in terms of a model in which a large degree (\ensuremath{\sim}90%) of nuclear-spin memory exists in the optical-pumping cycle. The experiment proves the feasibility of significant nuclear polarizations in solids by optical pumping. A small polarization of the abundant $^{19}\mathrm{F}$ nuclei is produced by a three-spin cross-relaxation process with two optically pumped $^{169}\mathrm{Tm}^{2+}$ ions.