Circularly polarized soft X-ray generation by Co/Pt thin film polarizer with perpendicular magnetic anisotropy

We have experimentally demonstrated circular polarization generation from linear polarized soft X-ray at synchrotron by adopting a thin magnetic film polarizer. Polarizer is composed of Co/Pt multilayer with a perpendicular magnetic anisotropy, which allows us to easily accommodate without needing any tilting angle into the measurement setup since the circular polarization is generated for the X-ray with normal incidence and transmission. Generated circular polarization is examined by observing magnetic domain features based on the X-ray magnetic circular dichroism, where ∼11% of circular component is estimated compared to the case of full circular polarization.     

High spin state of Mn in an ideal monolayer on Ag(001)

We studied the magnetic properties of ultra-thin Mn films deposited on Ag (001) held at 80 K with soft X-ray absorption and magnetic circular dichroism. The observed shape and branching ratio of the Mn 2p absorption edge as a function of Mn coverage demonstrate that, up to , the Mn adopts a stable high spin state similar to the Mn atom Hund's rule ⁶ S _5/2 ground state. Above this coverage a rapid transition from localized high spin to itinerant low spin behavior of the Mn 3d electrons is evidenced. Magnetic circular dichroism shows no sign of long range ferromagnetic order in these films at 80 K. The data, first confirm the large atomic-like local magnetic moment, and second are in line with the in-plane antiferromagnetic order, reported recently (Phys. Rev. B 57, 1141 (1998)), for Mn in the nearly ideal on-top Mn monolayer formed by 0.9 ML deposited at 80 K. Received: 4 May 1998     

Soft X-ray resonant magnetic scattering of magnetic nanostructures

Soft X-ray resonant magnetic scattering offers a unique element-, site- and valence-specific probe to study magnetic structures on the nanoscopic length scale. This new technique, which combines X-ray scattering with X-ray magnetic circular and linear dichroism, is ideally suited to investigate magnetic superlattices and magnetic domain structures. The theoretical analysis of the polarization dependence to determine the vector magnetization profile is presented. This is illustrated with examples studying the closure domains in self-organising magnetic domain structures, the magnetic order in patterned samples, and the local configuration of magnetic nano-objects using coherent X-rays. To cite this article: G. van der Laan, C. R. Physique 9 (2008).     

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.     

Plasma-assisted molecular beam epitaxy of wurtzite GaMnN displaying ferromagnetism assessed by means of X-ray magnetic circular dichroism

We report on the optimization of the growth conditions of wurtzite GaMnN grown by plasma-assisted molecular beam epitaxy in order to obtain intrinsic ferromagnetic behavior. By growing with a Ga/N ratio lower than unity and by introduction of periodic growth interruptions, we succeed in synthesizing single-phase GaMnN epilayers containing up to 6.3 at.% of Mn. The structural quality of the GaMnN epilayer and the absence of secondary phases are demonstrated by means of X-ray diffraction experiments and X-ray linear dichroism measurements performed at the Mn and Ga K-edges. The intrinsic ferromagnetism for 6.3 at.% of Mn in our GaMnN epilayer is demonstrated by means of magnetization measurements in a 5 T Quantum Design superconducting quantum interference device (SQUID) and X-ray magnetic circular dichroism investigations performed at the K-edge of Mn. The Curie temperature thus determined is equal to 8 K and a spontaneous magnetic moment of 2.4μ_B per Mn atom is found at 2 K.     

Giant Asymmetric Transmission and Circular Dichroism with Angular Tunability in Chiral Terahertz Metamaterials

Inspired by the chiral structure from mode interaction, a chiral terahertz metamaterial and its complementary structure with the coexistence of asymmetric transmission and circular dichroism response are designed. The asymmetric transmission happens in the presence of electric and magnetic modes perpendicular to each other, resulting from intrinsic chirality under normal incidence. Circular dichroism exists in the presence of electric and magnetic modes parallel to each other under oblique incidence because of extrinsic chirality. Both asymmetric transmission and circular dichroism are enhanced by complementary structures, benefitting from the coupling between adjacent units. The maximum of chiral parameter κ can achieve 450, which is one hundred times higher than previously reported. The chiral response can be tuned by the incident angle and is sensitive to the environmental refractive index. The results highlight the potential applications of these metamaterials in chiral sensing and polarization transformation devices.     

Magnetic circular dichroism in Co 2p photoemission of Co/Cu(1 1 13): Separation of the fundamental spectra

We measured high-quality Co 2p magnetic circular dichroism (MCD) spectra in photoemission for > 5 ML Co films grown on Cu(1 1 13) using a “complete” experiment, where the sample magnetization and the light helicity vector were reversed separately. We show how the four measured spectra, M^±P^±, can be used to make new linear combinations, which correspond to the circular dichroism in the angular dependence (CDAD), magnetic linear dichroism in the angular dependence (MLDAD) and MCD spectra. The integrated signals of the MLDAD and CDAD can be used to estimate the error caused by the difference in the degrees of magnetization and light polarization, respectively, in the opposite alignments. The MCD signal integrated over the entire 2p region does not average to zero, as one would have expected from the sum rule for photoemission to a non-interacting continuum state. There is a strong MCD signal in the entire region between the 2p _3/2 and 2p _1/2 main lines with pronounced satellite structure. The differences between the measured and calculated results for an independent-particle and an atomic model indicate the presence of interatomic electron correlation effects and configurational mixing. Received 26 September 2000     

Chromium doped manganites: evidence for electronically phase-separated systems

X-ray absorption spectroscopy (XAS) and soft-X-ray magnetic circular dichroism (SXMCD) at the Mn L_2,3-, Cr L_2,3- and O-K edges of Sm_0.5Ca_0.5Mn_1-xCr_xO₃ () bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of a magnetic sublattice on each of the probed cations. Considering an electronically phase-separated system, results are compared with magnetization and resistivity measurements and a tentative correlation with magnetoresistance properties on such doped compounds is discussed. Received 7 January 2000     

First observation of nonreciprocal X-Ray gyrotropy

We report the first observation of a nonreciprocal x-ray linear dichroism caused by the time-reversal odd, real part zeta of the complex gyrotropy tensor zeta(*) which is dominated by electric dipole-electric quadrupole E1E2 interference terms. A nonreciprocal transverse anisotropy was observed in the low temperature insulating phase of a Cr doped V2O3 Mott crystal when a single antiferromagnetic domain was grown by magnetoelectric annealing along the hexagonal c axis. This new element (edge) specific spectroscopy could nicely complement x-ray magnetic circular dichroism which is silent for antiferromagnetic materials.     

Magnetooptical properties of YIG measured on a continuously working spectrometer

A spectrometer is described which allows continuous recording of four transmission magnetooptical effects: Magnetic linear and circular birefringence and magnetic linear and circular dichroism. The set-up has been used to measure magnetooptical spectra of Yttrium Iron Garnet (YIG) in the visible spectra range. Two of the spectra are reported here, namely, the magnetic linear dichroism and the magnetic linear birefringence (Cotton Mouton effect).     

Vibronic scheme of the franck-condon state of F centers that consistently elucidates MCD and resonance raman scattering

Abstract

The resonance Raman scattering (RRS) of F centers, particularly the photon-energy dependence of linear polarization of the RRS, has been studied on the basis of a vibronic viewpoint. With combinination of previous results that have explained magnetic circular dichroism (MCD) spectra, a vibronic scheme of the Franck-Condon state of F centers for KCl, KBr and KI crystals is established.     

The polarization-angular structure and elliptical dichroism of the cross sections for three-photon bound-bound transitions in atoms

Using the electric dipole approximation, we present, in invariant form, the cross section of an arbitrary three-photon transition between the discrete states of an atom with total angular momenta J _i and J _f. The cross section contains scalar and mixed products of the photon polarization vectors, and invariant atomic parameters dependent only on the photon frequencies. We determine the number of independent atomic parameters at fixed values of J _i and J _f and obtain their explicit expressions in terms of the reduced composite dipole matrix elements. The polarization dependence of the cross sections is expressed in terms of the degrees l and ξ of linear and circular photon polarizations. We analyze the phenomenon of dissipation-induced circular dichroism in three-photon processes, i.e., the difference Δ of the cross sections for opposite signs of the degree of circular polarization of all the photons. We study in detail the case of two identical photons and the phenomenon of elliptical dichroism, when Δ∼lξ holds and dichroism occurs only when the photons are elliptically polarized, with 0<|ξ|<1. Finally, we discuss the dissipation-induced effects of atom polarization in three-photon processes involving linearly polarized or unpolarized photons. Zh. éksp. Teor. Fiz. 111, 1984–2000 (June 1997)     

Band structure and atomic sum rules for X-ray dichroism

Corrections to the atomic orbital sum rule for circular magnetic X-ray dichroism in solids are derived using orthonormal LMTOs as a single-particle basis for electron band states. Received 4 September 2000     

X-ray linear dichroism dependence on ferroelectric polarization

X-ray absorption spectroscopy and photoemission electron microscopy are techniques commonly used to determine the magnetic properties of thin films, crystals, and heterostructures. Recently, these methods have been used in the study of magnetoelectrics and multiferroics. The analysis of such materials has been compromised by the presence of multiple order parameters and the lack of information on how to separate these coupled properties. In this work, we shed light on the manifestation of dichroism from ferroelectric polarization and atomic structure using photoemission electron microscopy and x-ray absorption spectroscopy. Linear dichroism arising from the ferroelectric order in the PbZr0:2Ti0:8O3 thin films was studied as a function of incident x-ray polarization and geometry to unambiguously determine the angular dependence of the ferroelectric contribution to the dichroism. These measurements allow us to examine the contribution of surface charges and ferroelectric polarization as potential mechanisms for linear dichroism. The x-ray linear dichroism from ferroelectric order revealed an angular dependence based on the angle between the ferroelectric polarization direction and the x-ray polarization axis, allowing a formula for linear dichroism in ferroelectric samples to be defined.     

Theoretical studies on magnetic atom imaging by use of photoelectron diffraction

We theoretically study possible methods how to obtain imaging of magnetic atoms by use of photoelectron diffraction (PD). We propose a novel method to apply Daimon effect where PD peaks are rotated around forward focusing peaks. In usual circular dichroism, we simply use the difference of the PD intensities for different X-ray circular polarization. In contrast to this dichroism, we rather use the difference of the PD intensities for different circular polarizations but - is used for the – circular polarization only in the data handling. This technique allows us to obtain clear atomic image only of spin polarized atoms, and to distinguish magnetic atoms with up-spins from those with down spins. Some illustrative calculations demonstrate the potential use and also the limitation of this technique.     

Orientation of Nd and Mn magnetic moments in a CMR Nd0.72Ba0.28MnO3 by X-ray magnetic circular dichroism

X-ray absorption and X-ray magnetic circular dichroism at Mn L_2,3- and Nd M_4,5-edges of a twinned Nd_0.72Ba_0.28MnO₃ single crystal have been performed versus temperature (between 10 and 300 K) and applied magnetic field (between 0 and 6.5 T). The results show that neodymium cations order below 70 K with the magnetic moments antiparallel to the manganese subnetwork in weak applied magnetic field (<1.5 T). For higher magnetic field, the neodymium moments turn parallel to the manganese ones showing that the antiferromagnetic exchange coupling between both subnetworks is weak. The presence of a metal–insulator transition below 70 K in the studied single crystal correlated to the magnetic ordering of neodymium cations can be understood on the basis of the cation mismatch introduced by barium substitution.     

The coupling of Cr to Fe studied by circular magnetic X-ray dichroism

The coupling of Cr to Fe in stacked Cr/Fe/Cr trilayers is studied by circular magnetic X-ray dichroism using fluorescence yield to monitor the absorption. From the dichroic spectra, we determine the thickness dependent average magnetic coupling of Cr to Fe for Cr layer thicknesses of 1, 2 and 3 ML. We compare our results to new calculated spectra. Additionally, all Cr spectra show significant contributions due to photon scattering processes at theL _{2, 3} absorption edges depending on the degree of circular polarization.     

Soft X-ray magnetic circular dichroism in Fe and Fe 0.50 Co 0.48 V 0.02 films: quantitative analysis of transmission

Magnetic circular dichroism (MCD) in X-ray absorption has been measured at the L _{2, 3} edges of Fe in ex-situ grown Fe and Fe _0.50 Co _0.48 V _0.02 films by means of the transmission method. A new approach is developed for fitting the observed transmittance, which describes the resonance lineshapes as (generalized) Fano profiles. Analytical integration of each single resonance allows a more reliable determination of the orbital and spin magnetic moments based on the MCD sum rules. The results are consistent with an increase of the Fe spin and orbital magnetic moment in Fe-Co alloys as obtained by other experiments and band structure calculations. Received 15 August 2000 and Received in final form 11 June 2001     

Magnetic and magneto-optical properties of Co/Nb-codoped TiO2 films deposited by gas flow sputtering

Titanium dioxide thin films codoped with Co and Nb (Co/Nb-codoped TiO₂ films) were fabricated by a low-energy deposition process, gas flow sputtering. No metallic Co or Nb phase was detected by X-ray diffraction and X-ray photoelectron spectroscopy, suggesting that the Co and Nb ions have the oxidation states 2+ and 5+, respectively. The films show a relatively large Faraday rotation and magnetic circular dichroism on the order of 10³ deg/cm.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.