The magneto-optical Kerr effect (MOKE) is usually the most suitable methods to characterize magnetic properties of ultrathin films since the rotation angle of the electric-field vector of linearly polarized lights is nearly proportional to the magnetization of the films. Although we are planning to use x-ray magnetic circular dichroism (XMCD) in UVSOR, in-laboratory experiments is also important for several reasons. First, detailed and time-consuming characterization is required especially for the preparation of new magnetic materials. Since the beamtime in UVSOR is limited, some in-laboratory equipment is essential. Second, the measurements in the applied magnetic field is easier in MOKE since MOKE detects polarization of the reflected lights while XMCD scales emitted electrons.

We have just constructed a new ultrahigh vacuum (UHV, <1×10-8 Pa) system in which in situ MOKE can be measured (Figs. 1-4). 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 90-100 K using liquid nitrogen and to 25-40 K using liquid He. A magnetic field can be applied to samples in UHV with a simple electromagnet up to 3000 Gauss (Fig. 4). The magnetic field is not so large but is usually sufficient for ultrathin films of 3d elements. Both the polar MOKE geometry for perpendicular magnetization and the longitudinal MOKE for in-plane magnetization are available (Fig. 5).

Figure 1: Schematic view of the UHV MOKE chamber.

Figure 2: Front view of the UHV MOKE chamber. The magnet part was now replaced　as in Fig. 4.

Figure 3: Rear-side view of the UHV MOKE chamber.

Figure 4: UHV compatible electromagnet (ICF253) made of Au-coated pure Fe.

Figure 5: Typical MOKE geometries.

Typical MOKE data are shown in Fig. 6. The samples were in-plane magnetized Co (2 and 5 monolayer thickness) films on Cu(001) before and after NO (nitric oxide) adsorption. The signal-to-noise ratio is already sufficient to investigate these types of magnetic films.
	Figure 6: Longitudinal MOKE data of 2 and 5 ML epitaxial Co films on Cu(001) before and after NO adsorption at 100 K. Changes in the Kerr intensity and the coercivity can be found.

Using the UHV MOKE 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.