In situ measurement of thickness dependence of magnetoresistance and magnetic hysteresis loops of ultrathin Co films on a SiO2/Si(1 1 1) substrate

Ultrathin Co films ranging from 1 to 19 monolayers (ML) thick were grown by thermal evaporations on native-oxide-covered Si(1 1 1) surfaces in ultrahigh vacuum (UHV). The thickness-dependent magnetoresistance (MR) and thickness-dependent magnetic hysteresis loops were measured in situ by a collinear four-point probe and surface magneto-optical Kerr effect (SMOKE) techniques, respectively. The magnetoresistance of the Co films, measured with the applied magnetic field parallel to the film plane and perpendicular to the current direction, was obtained only when the Co film thickness reached ∼7 ML. The MR increased monotonically from ∼0.01% at ∼7 ML to ∼0.11% at ∼19 ML. The onset of measurable magnetic hysteresis loops from the Co film occurred at ∼3 ML, earlier than the onset thickness ∼7 ML for measurable MR. The coercivity Hc of the Co film decreased with the film thickness d in the range of 3-19 ML and followed a power law Hc∝ d-n with n=0.33±0.05. The MR change was attributed to scattering from domain walls. The coercivity decrease as a function of thickness was attributed to the decrease of surface domain-wall pinning