The x-ray magnetic circular dichroism (XMCD) is a unique technique to obtain information on microscopic magnetic properties. Especially, the orbital magnetic moment can be obtained by the so-called sum rule, which is hardly measureable by other conventional methods. Moreover, XMCD is element specific, implying that partial magnetization concerning one element can be separately derived. Such information is complementary to SQUID and other magentic techniques.

We can use BL4B in UVSOR, where the grazing-incidence varied-line spacing grating monochromator is installed. The beamline optics is given in Fig. 1. An aperture was placed in the upstream to obtain circularly polarized x rays as well as linearly polarized x rays. Figure 2 shows the calculated Stokes S3 parameter for BL4B, and the x rays with circularly polarization factor of 0.70 are avilable with a reasonable intensity. Since the electrons in the UVSOR storage ring rotaes counterclockwisely, the upper part of the synchrotron radiation gives positive helicity (negative S3) while the lower part yields negative helicity (positive S3).

Figure 1: Schematic plan and top views of UVSOR BL4B. Taken from Chem. Phys. 289 (2003) 15 by T. Gejo, Y. Takata, T. Hatsui, M. Nagasono, H. Oji, N. Kosugi and E. Shigemasa.		Figure 2: Stokes S3 parameters as a function of the beam position. Using the width of 1 mm, the x rays with the circularly polarization factor of 0.70 are available.

We have constructed a new ultrahigh vacuum (UHV, <1×10^-8 Pa) system in which in situ XMCD can be measured (Figs. 3-4). The system contains a standard UHV pumping unit, a surface cleaning system of sputtering and annealing of substrates, and a molecular beam epitaxy (MBE) system. The substrate can be cooled down to ~100 K using liquid nitrogen and to ~30 K using liquid He. A magnetic field can be applied to samples with a UHV compatible electromagnet (see Fig. 4) up to 3000 Gauss (see Fig. 4). The magnetic field is not so large but is usually sufficient for ultrathin films of 3d elements and the grazing incidence spectra can be measured for usual single crystals because of the pole gap is 30 mm.

	Figure 4: Inside view of the XMCD chamber.
Figure 3: XMCD chamber installed at UVSOR BL4B.

Typical XMCD data are shown in Fig. 5. The sample was an in-plane magnetized 3 monolayer (ML) Co epitaxial film on Cu(001) before and after 0.5 ML NO (nitric oxide) adsorption, taken at grazing x-ray incidence. The measurement conditions are given in Fig. 5. During the measurements, the magnetic field of 2000 Gauss was applied along the x-ray beam direction. Both the magnetic field and the x-ray helicity were changed to yield magnetic circular dichroism spectra (++, +-, -+ and -- spectra were taken). It took 30-60 minutes to get one complete data set, which is tolerable for these kinds of measurements. It can easily be found that NO adsorption suppress the magnetization significantly.

Figure 5: Co L-edge XMCD of 3ML Co epitaxial film on Cu(001) before and after 0.5 ML NO (nitric oxide) adsorption at 200 K, taken at grazing x-ray incidence.

Figures 6 and 7 show the N and O K-edge NEXAFS (near-edge x-ray absorption fine structure) spectra of 0.5 ML NO adsorbed on 3ML Co/Cu(001) using linear polarization x rays. The spectra were taken by measuring the sample current directly. Clear polarization dependence can be seen in the 2pi* and 6sigma* transitions, this implying that the NO molecule stands up on the surface with some small inclination angle. Although S/N ratio is sufficient, there can be found some second harmonics contributions in Figs. 6 and 7, where Co L2,3-edge and Cu L1-edge are visible respectively. At present, no mirrors for the elimination of higher-order harmonics are present, and this may be a future plan.

Figure 6: N K-edge NEXAFS spectra of 0.5 ML NO adsorbed on 3ML Co/Cu(001), taken at grazing incidence (30deg., red line) and normal incidence (90deg., blue line). The green line represents the I0 function. The photon energy was not calibrated.	Figure 7: O K-edge NEXAFS spectra of 0.5 ML NO adsorbed on 3ML Co/Cu(001). The photon energy was not calibrated.

Using the XMCD system, we will investigate magnetic properties of ultrathin metal films, nanowires and nanodots, especially control of magnetism using surface chemical techniques such as gas deposition.