Simple window-compensation method for improving the signal-to-noise ratio in measurements of the magneto-optic Kerr effect in ultrathin films

Measurements of the magneto-optical Kerr effect in magnetic ultrathin films in ultrahigh vacuum (UHV), have a signal-to-noise ratio (S/N) which is limited by the extinction ratio of the apparatus. Although polarizing crystals with excellent extinction characteristics are usually employed for in situ measurements of films grown in UHV, the net extinction ratio is degraded by optical windows, resulting in poor signal contrast and poor S/N. A simple procedure for cancelling the effects of the windows is to slightly, but precisely, misalign the initial linear light polarization state from S or P (with respect to the sample). This technique does not require compensators or any additional optical components and is capable of an extinction ratio of 3×10−7, allowing the rapid acquisition of high quality Kerr-effect hysteresis loops with a very simple apparatus. The dependence of the extinction ratio on the initial light polarization is investigated and explained in terms of a Jones matrix analysis. Correcting for the windows is interpreted as a phase shift from the film substrate balancing phase shifts from the optical windows. The technique is applied to the acquisition of hysteresis loops for 4 ML Fe/W(001) films and a substantial improvement in S/N is demonstrated.