| 1. |
- Jansing, C., et al.
(författare)
-
X-ray natural birefringence in reflection from graphene
- 2016
-
Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 94:4
-
Tidskriftsartikel (refereegranskat)abstract
- The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon 1s absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to 30 degrees, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to 17 degrees. Both graphene systems show resonantly enhanced effects at the pi* and sigma* energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of 140 degrees has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the pi* resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of pi* and sigma* bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced sigma* excitation energy and a strong tilt of 50% of the p(z) orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the p(z) orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting lambda/4 or lambda/2 plates in the design of future high-intensity light sources.
|
|
| 2. |
- Mertins, H. -Ch., et al.
(författare)
-
Giant magneto-optical Faraday effect of graphene on Co in the soft x-ray range
- 2018
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:6
-
Tidskriftsartikel (refereegranskat)abstract
- Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon 1s edge of single-layer graphene on Co, reaching Faraday rotation angles of 2.9 x 10(5)deg/mm. This value is of the order of those observed at the Co 3p and 2p edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned n orbitals of carbon atoms. It is induced by hybridization with the Co 3d(z)2 orbitals while carbon a bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of 0.14 mu(B) is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon p(z) and metal 3d orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co 3d(z)( )(2)orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with 3d(xy) and 3d(yz) orbitals of neighboring Co atoms forming tilted p(z) bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.
|
|
| 3. |
- Mertins, H-Ch., et al.
(författare)
-
Magneto-optical reflection spectroscopy on graphene/Co in the soft x-ray range
- 2017
-
Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 903
-
Tidskriftsartikel (refereegranskat)abstract
- The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.
|
|
| 4. |
- Legut, D., et al.
(författare)
-
Interference Effects in T-MOKE Spectra of Fe Thin Films at the 3p Edges - Theory and Experiment
- 2015
-
Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 127:2, s. 466-468
-
Tidskriftsartikel (refereegranskat)abstract
- We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.
|
|
| 5. |
- Legut, D., et al.
(författare)
-
Influence of the crystal structure of thin Co films on X-ray magnetic linear dichroism-Comparison of ab initio theory and reflectometry experiments
- 2014
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:17, s. 17E132-
-
Tidskriftsartikel (refereegranskat)abstract
- We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure. (C) 2014 AIP Publishing LLC.
|
|
| 6. |
- Tesch, M. F., et al.
(författare)
-
Magnetocrystalline anisotropy in x-ray magnetic linear dichroism at the 3 p edges of crystalline Fe thin films
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:14, s. 140404-
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3 p absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the 3 p semicore states on an equal footing.
|
|
| 7. |
- Valencia, S., et al.
(författare)
-
Faraday rotation spectra at shallow core levels : 3p edges of Fe, Co, and Ni
- 2006
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 8, s. 254-
-
Tidskriftsartikel (refereegranskat)abstract
- We present magneto-optical ( MO) Faraday spectra measured at the 3(p) edges of Fe, Co, and Ni. A polarization analysis of the final state of the transmitted radiation is employed to determine the Faraday rotation at these edges. The MO effect becomes resonantly enhanced at the 3p edges and accordingly large values for the intrinsic rotation constant k of 2.2 x 10(5), 1.5 x 10(5) and 0.8 x 10(5) deg/mm for the three ferromagnetic elements Fe, Co, and Ni, respectively, have been measured. These values are large and comparable to those observed at the 2p edges. This remarkable feature emphasizes the importance of the exchange ( EX) splitting over the spin-orbit ( SO) interaction of the core levels in determining MO effects in the extreme ultra-violet ( EUV) region. From these measurements the complete set of MO constants at the 3p edges has been derived.
|
|
