Linear Dichroism In Photoemission Research Articles

Ultrathin epitaxial cobalt films formed under graphene

The intercalation of cobalt under a graphene monolayer grown on a Ni(111) single crystal film is studied. The experiments are conducted in ultrahigh vacuum. Samples are characterized in situ by low energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and magnetic linear dichroism in photoemission of Co 3p electrons. New data are obtained on the evolution of the atomic and electronic structure and magnetic properties of the system with increasing thickness of the intercalated cobalt layer in the range up to 2 nm. It is shown that a pseudomorphic epitaxial film of Co(111) having magnetization perpendicular to the surface is formed under the grapheme layer during intercalation in an anomalously wide range of thicknesses.

Intercalation synthesis of cobalt silicide under a graphene layer

The silicon intercalation under single-layer graphene formed on the surface of an epitaxial Co(0001) film was investigated. The experiments were performed under conditions of ultra-high vacuum. The thickness of silicon films was varied within the range of up to 1 nm, and the temperature of their annealing was 500°C. The characterization of the samples was carried out in situ by the methods of low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and magnetic linear dichroism in photoemission of Co 3p electrons. New data were obtained on the evolution of the atomic and electronic structure, as well as on the magnetic properties of the system with an increase in the amount of intercalated silicon. It was shown that the intercalation under a graphene layer is accompanied by the synthesis of surface silicide Co2Si and a solid solution of silicon in cobalt.

Intercalation synthesis of graphene-capped iron silicide atop Ni(111): Evolution of electronic structure and ferromagnetic ordering

A new method for synthesis of graphene-protected iron silicides has been tested, which consists in formation of graphene on Ni(111) followed by two-step intercalation of the system with Fe and Si. Characterization of the samples was performed in situ by low-energy electron diffraction, angular-resolved photoelectron spectroscopy, core-level photoelectron spectroscopy with synchrotron radiation and magnetic linear dichroism in photoemission of Fe 3p electrons. It is shown, that at 400°C the intercalation of graphene/Ni(111) with iron occurs in a range up to 14ML. The graphene layer strongly interacts with the topmost Fe atoms and stabilizes the fcc structure of the film. The in-plane ferromagnetic ordering of the film has a threshold nature and arises after the intercalation of 5ML Fe due to the thickness-driven spin reorientation transition. Subsequent intercalation of graphene/Fe/Ni(111) with Si leads to the formation of the inhomogeneous system consisted of intercalated and nonintercalated areas. The intercalated islands coalesce at 2ML Si when a Fe-Si solid solution covered with the Fe3Si surface silicide is formed. The Fe3Si silicide is ferromagnetic and has an ordered (√3×√3)R30° structure. The graphene layer is weakly electronically coupled to the silicide phase keeping its remarkable properties ready for use.

Formation and investigation of ultrathin layers of Co2FeSi ferromagnetic alloy synthesized on silicon covered with a CaF2 barrier layer

Ultrathin (∼2nm) films of Co2FeSi ferromagnetic alloy were formed on silicon by solid-phase epitaxy and studied in situ. Experiments were carried out in an ultrahigh vacuum (UHV) using substrates of Si(111) single crystals covered with a 5nm thick CaF2 barrier layer. The elemental and phase composition as well as the magnetic properties of the synthesized films were analyzed by photoelectron spectroscopy using synchrotron radiation and by magnetic linear dichroism in photoemission of Fe 3p and Co 3p electrons. The study shows that the synthesis of the Co2FeSi ferromagnetic alloy occurs in the temperature range of 200–400°C. At higher temperatures, the films become island-like and lose their ferromagnetic properties, as the CaF2 barrier layer is unable to prevent a mass transfer between the film and the Si substrate, which violates the stoichiometry of the alloy.

Formation and magnetic properties of the silicon-cobalt interface

Formation of the Si/Co interface and its magnetic properties have been studied by high-resolution photoelectron spectroscopy with synchrotron radiation. The experiments have been performed in situ in superhigh vacuum (5 × 10−10 Torr) with coating thicknesses up to 2 nm. It has been found that, in the initial stage of silicon deposition on the surface of polycrystalline cobalt maintained at room temperature, ultrathin layers of the Co3Si, Co2Si, CoSi, and CoSi2 silicides are formed. The three last phases are nonmagnetic, and their formation gives rise to fast decay of magnetic linear dichroism in photoemission of Co 3p electrons. At deposition doses in excess of ∼0.4 nm Si, a film of amorphous silicon grows on the sample surface. It has been established that the Si/Co interphase boundary is stable at temperatures up to ∼250°C and that further heating of the sample brings about escape of amorphous silicon from the sample surface and initiates processes involving silicide formation.

Electronic structure and magnetic properties of epitaxial FeRh(001) ultrathin films on W(100)

Epitaxial FeRh(100) films (CsCl structure, $\sim 10\ ML\ $ thick), prepared {\it in-situ} on a W(100) single crystal substrate, have been investigated via valence band and core level photoemission. The presence of the temperature-induced, first-order, antiferromagnetic to ferromagnetic (AF$\rightarrow$ FM) transition in these films has been verified via linear dichroism in photoemission from the Fe 3$p$ levels. Core level spectra indicate a large moment on the Fe atom, practically unchanged in the FM and AF phases. Judging from the valence band spectra, the metamagnetic transition takes place without substantial modification of the electronic structure. In the FM phase, the spin-resolved spectra compare satisfactorily to the calculated spin-polarized bulk band structure.

Ferromagnetic alignment of iron nanostructures on the silicon surface

The phase composition, electronic structure, and magnetic properties of nanostructures formed upon deposition of iron on the surface of the Si(556) vicinal face coated by a submonolayer silver film with a √3×√3-Ag structure have been studied using high-energy-resolution photoelectron spectroscopy and analysis of magnetic linear dichroism in photoemission of Fe 3p electrons. The effective thickness of the deposited iron layer is varied from 1 to 25 A. It has been shown that a 1- to 2-A Fe coverage leads to the formation of a metastable iron silicide thin layer with a CsCl-type structure on the surface of the sample. A further deposition of Fe (up to ≈7 A) brings about the formation of chains consisting of nonmagnetic islands of the Fe—Si solid solution on this layer, which are oriented along the steps of the substrate. A ferromagnetic alignment of the system along the surface of the sample appears only at coverages of approximately 10 A, when larger (≈100 nm) iron islands start to grow on the solid solution layer.

10.1007/s11451-008-3026-4

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.

Rashba effect at the surfaces of rare-earth metals and their monoxides

We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.

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.

Magnetic properties of Fe-based alloys

The magnetic properties of fcc FexNi1−x and FexCo1−x alloys grown on Cu(100) were investigated in an element-specific fashion. The technique employed was linear dichroism in photoemission, which by varying the chirality can also determine the magnetization axis. We observed a different behavior for the two alloys at Fe concentrations above 60%. At this concentration the FexNi1−x alloy shows a strong reduction of the Fe dichroism associated with the invar instability. This is in contrast to the FexCo1−x alloy, where the Fe dichroism stays essentially constant across the concentration. Despite these differences both systems show a change of the easy axis at roughly the same electron count. For small Fe concentrations the easy axis is in-plane along the [011] direction. This changes into the [001] direction at Fe63Ni37, which is at 0.7 excess electrons per atom when compared with Fe. This is different to the bulk, where a change occurs at Fe25Ni75. We find the easy axis change for FexCo1−x to occur at 49% Fe. This would be equivalent to 0.5 excess electrons when compared with Fe.

Relationship between magnetic circular and linear dichroism in photoemission from Fe3pcore level: An experimental and theoretical investigation

Relationship between magnetic circular and linear dichroism in photoemission from Fe<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>3</mml:mn><mml:mi>p</mml:mi></mml:math>core level: An experimental and theoretical investigation

Magnetic linear dichroism in photoemission from Fe on Cu 84Al 16(1 0 0) and Cu 3Au(1 0 0)

We have investigated the magnetic properties of monolayers of FCC Fe grown epitaxialy on Cu 84Al 16(1 0 0) and on ordered Cu 3Au(1 0 0), substrates that present lattice parameters 1% and 3.7% bigger than pure Cu. The magnetic measurements were carried out by linear magnetic dichroism in the angular distribution of photoelectrons (LMDAD), using linearly polarized synchrotron radiation. The LMDAD measurements of the Fe 3p core levels show that starting from 2.5 monolayers (ML) for Fe/Cu 84Al 16(1 0 0) and from 4 ML for Fe/Cu 3Au(1 0 0) these FCC Fe films present in-plane ferromagnetism. The 9% increase of the magnetic splitting of the Fe 3p LMDAD peak-to-peak asymmetry measured for Fe/Cu 3Au(1 0 0) suggests an increase of the Fe magnetic moment with increasing volume of the high-spin FCC Fe films.

Origin of spin polarization and magnetic dichroism in core-level photoemission.

Spin polarization and magnetic circular and linear dichroism in photoemission are interpreted in a many-electron approach. The six different ways to orient the three polarizations in the experiment (magnetization, electric vector of the light, and spin of the photoelectron) allow measurement of six different kinds of correlation between the corresponding atomic properties (valance spin, core-hole orbital momentum, and core-hole spin, respectively). This allows us to analyze exchange and hybridization effects and is illustrated for the 2p and 3p photoemission of ferromagnetic Ni metal.