Enhanced magnetoresistance of ultrathin(AuCo)nmultilayers with perpendicular anisotropy

We present magnetoresistance (MR) measurements performed on two Au/Co/Au sandwiches with ultrathin cobalt layers (0.32 and 0.76 nm) and on a bilayer Au/Co/Au/Co/Au (0.75 nm Co). The easy magnetization axis is shown to be perpendicular to the films, in agreement with previous magnetization and ferromagnetic resonance measurements. The hysteretic behavior is easily observed by measuring the MR, leading to a determination of the coercive fields of a few ${10}^2$G. The Co bilayer exhibits a drastic enhancement of the MR effect with respect to the corresponding monolayer ($\delta \frac{R}{R=1.3\%}$ at 300 K and 3% at 4.2 K) and a square hysteresis loop. Different mechanisms for the enhancement of the MR are proposed.