Takeshi Nakagawa

Enhancements of Spin and Orbital Magnetic Moments of Submonolayer Co on Cu(001) Studied by X-ray Magnetic Circular Dichroism Using Superconducting Magnet and Liquid He Cryostat

Magnetic properties of 0.4 monolayer Co grown epitaxially on Cu(001) were investigated by X-ray magnetic circular dichroism (XMCD) using our newly constructed ultrahigh vacuum system equipped with a 7 T superconducting magnet and a liquid He cryostat. The angle-dependent XMCD spectra for the saturated magnetization recorded at 6.0 K allowed us to evaluate separately the spin and orbital magnetic moments along the surface parallel and normal directions. Enhancements of the magnetic moments compared with the corresponding bulk values were clearly elucidated: ~15% for the spin magnetic moment and ~96 and ~53% for the orbital magnetic moments along the surface parallel and normal directions, respectively.

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 for surface and thin film magnetism: Measurement techniques and surface chemical applications

The technical development of the characterization of magnetic thin films is an urgent subject especially for further improvement of high-density and high-speed recording media. This article focuses attention on the fundamental methodology and recent exploitations of various magnetic circular dichroism (MCD) techniques. First, basic theories and experimental methods of the magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) are described. MOKE is a conventional but usually the most useful method to characterize macroscopic magnetization of metal thin films using visible lasers. Moreover, recent development of MOKE allows one to perform optical microscopic and ultrafast time resolved investigations. XMCD has now become a mature technique by virtue of the developments of soft and hard X-ray synchrotron radiation sources. Since XMCD is based on core absorption spectroscopy, the technique provides information on element specific magnetization. Using the so-called sum rules, one can determine the microscopic spin and orbital magnetic moments. The experimental method and an example using a superconducting magnet system combined with a liquid helium sample cryostat are described. Moreover, by combining XMCD with photoelectron emission microscopy (PEEM), one can perform spatiotemporal measurements, whose spatial resolution reaches several tens of nanometres. Magnetization induced second harmonic generation (MSHG) is also described. This is a unique technique for its inherently high sensitivity to surfaces and interfaces since MSHG is inhibited in the bulk of centrosymmetric crystals. The drastic polarization dependence of MSHG based on the selection rules is also discussed. As a last method addressed in this article, the threshold photoemission MCD technique is reviewed. The technique has recently been proposed and has demonstrated the possibility of an ultrafast spatiotemporal method by combining PEEM. Applications of these various MCD families to surface physical chemistry are described. Here, drastic spin reorientation transitions (SRT) of some metal thin films induced by adsorption of atoms and molecules are discussed from the macroscopic and microscopic points of view. Consequently, future aspects in the MCD techniques and surface and thin film magnetism are addressed.

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.