Magnetic-circular-dichroism study of the valence states of perpendicularly magnetized Ni(001) films

Direct observation of magnetic contrast obtained by photoemission electron microscopy with deep ultra-violet laser excitation

Magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) have been investigated in a photoemission electron microscopy (PEEM) system excited by a deep ultra-violet (DUV) laser (with λ = 177.3 nm and hυ = 7.0 eV) for the first time. High resolution PEEM magnetic images (down to 43.2 nm) were directly obtained on a (001)-oriented magnetic FePt film surface with a circularly-polarized light under normal incidence. Furthermore, a stepped Cr seeding layer was applied to induce the formation of large-area epitaxial FePt films with (001) and (111) two orientations, where MLD with large asymmetry was observed in the transition area of two phases. It demonstrates that DUV laser can be a powerful source for high resolution magnetic imaging in the laboratory in absence of synchrotron facilities.

Spin texture and circular dichroism in photoelectron spectroscopy from the topological insulator Bi2Te3: first-principles photoemission calculations

By relativistic first-principles photoemission calculations for the topological insulator Bi2Te3, we study how the spin texture of the Dirac state manifests itself in circular dichroism. On one hand, there are significant modifications of the initial state's spin texture, which are explained by final-state effects and the symmetry of the photoemission setup. On the other hand, a highly symmetric setup allows us to draw conclusions about the detailed Dirac state's spin texture. Our study supports that circular dichroism in angular distribution successfully complements spin- and angle-resolved photoelectron spectroscopy from topological insulators.

Magnetic circular dichroism photoemission electron microscopy using laser and threshold photoemission

We demonstrate that laser induced valence band photoemission can be used in the observation of magnetic domain structures with a magnetic circular dichroism photoemission electron microscope (MCD-PEEM). It has been widely considered that valence band photoemission MCD asymmetry is rather small compared to that obtained with x-rays because of its weak spin-orbit coupling. However, we show that the MCD asymmetry is high near the photoemission threshold, permitting us to perform MCD-PEEM experiments. The use of intense and pulsed lasers as excitation sources enables PEEM studies of two-photon photoemission MCD and all optical time resolved MCD.

Measurements of threshold photoemission magnetic dichroism using ultraviolet lasers and a photoelastic modulator

A magneto-optical method based on valence band photoemission by laser excitation is described. Total photoexcited electron dichroism is detected using wave plates or a photoelastic modulator. Compared to the direct current method using wave plates, a modulation technique assisted by a photoelastic modulator has an advantage to improve the signal-to-noise ratio of magnetic dichroism by a factor of 10. The magnetic circular and linear dichroism can be investigated with this technique. An application to magnetic domain imaging using photoemission electron microscope is also demonstrated.

Magnetic circular dichroism near the Fermi level

We report the observation of enhanced magnetic circular dichroism (MCD) near the Fermi level using visible and ultraviolet lasers. More than 10% MCD asymmetry is achieved for a perpendicularly magnetized 12 ML (monolayer) Ni film on Cu(001). By changing the work function with the aid of cesium adsorption, the MCD asymmetry of is found to be enhanced only near the photoemission threshold and to drop down to 0.1% at the photon energy larger than the work function by 0.6 eV. A theoretical calculation also shows enhanced MCD near the photoemission threshold, qualitatively in agreement with the experimental results. Other ultrathin films of 6 ML Ni, 15 ML Co, and 3 and 15 ML Fe on Cu(001) are also investigated. It is found that the perpendicularly magnetized films show much larger MCD asymmetries than the in-plane magnetized films as in the Kerr effect.

Magneto-optical linear dichroism in threshold photoemission electron microscopy of polycrystalline Fe films

Magnetic linear dichroism in threshold photoemission has been exploited to obtain magnetic contrast in a photoemission electron microscope using a mercury arc lamp. The dichroism at threshold can be described similar to the magneto-optical Kerr effect in the region of visible light. The asymmetry of electron intensity observed for a 100 nm polycrystalline Fe film on silicon is A=(0.37+/-0.05)%. The asymmetry occurs for the geometry of the transverse Kerr effect. For unpolarized light the asymmetry was about half the value observed for linearly polarized light. Threshold photoemission microscopy has a large potential for high resolution magnetic domain imaging with fast data acquisition.