The Fe/Si multilayers are grown in the ion-beam sputtering (IBS) chamber whose layout is shown schematically in Figure 1. The system base pressure is typically about 2×10-8 torr. The ion gun is a 3 cm Kauffman source with focusing optics. The energy of the ions leaving the gun can be modulated by raising and lowering the voltage on the acceleration grids, creating in effect an electrical shutter. The Ar ions are incident on the sputter target at 1000V at an angle of about 45°. The Ar pressure is maintained in the 2-3×10-4 range by a flow-controller coupled to a capacitance manometer. Four 3" diameter sputter targets are mounted on a tray which can be rotated by a stepper motor. Layer thickness is monitored by a quartz-crystal oscillator which is placed in close proximity to the substrates. The substrates are about 25 cm above the targets, clamped to a copper tray. The temperature of the tray is monitored by a thermocouple and can be varied between -150°C and +200°C. Three films are grown per chamber pumpdown.
The thickness monitor, the controller for the stepper motor and the ion-beam power supply are all interfaced to a personal computer which has been programmed using the ASYST instrument control package. When the system is depositing a multilayer, the computer sends the material parameters to the thickness monitor, rotates the stepper motor to its new orientation, and turns the ion gun on. When the desired thickness is reached, the thickness monitor turns the ion-gun off and prompts the computer for the next layer. The basic design of the system is similar to one previously described by Kingon et al.
The substrates for multilayers growth include glass coverslips, oxidized silicon wafers, MgO (001) and Al2O3(0211). The first two substrates, which are used for growth of polycrystalline films, were rinsed in solvents before loading into the vacuum chamber. The second two, which are used for epitaxial growth, are cleaned according to a recipe reported by Farrow and coworkers. The typical deposition rate for Fe is 0.2Å/s while that for Si is about 0.3Å/s. All films are capped with a 200Å Ge oxidation barrier. The magnetic and structural properties of the films are stable for at least one year. Ge is used for capping instead of Si in order to prevent interference with element-specific soft x-ray fluorescence measurements, which will be reported elsewhere.
X-ray diffraction characterization has been performed using a 18kW rotating anode system outfitted with a graphite monochromator. All spectra are taken using the Cu Kalpha wavelength. Conventional high-resolution electron microscopy and electron diffraction have been performed in order to characterize the microstructure of the as-deposited films in cross-section. Magnetization curves are obtained using a vibrating sample magnetometer. All the data shown here were taken at room temperature.