ATOM OPTICS WITH MAGNETIC-SURFACES .2. MICROSCOPIC ANALYSIS OF THE FLOPPY DISK MIRROR

In a recent experiment we studied cold rubidium atoms bouncing on a ma gnetic mirror made from a flexible computer disk with sinusoidal magne tization. The motion was well described by a model in which the mirror was a perfect specular reflector, but complete agreement with the dat a required the reflecting surface to be slightly corrugated. Here we e xplore the physical origins of the corrugation both theoretically and experimentally. First, we develop a theory relating the reflecting for ce on the atoms to the magnetization of the mirror, taking into accoun t the finite thickness of the magnetic film. We find that if the signa l on the floppy disk is not harmonic the atoms appear to have been ref lected from a corrugated surface, as observed in our recent experiment . Next, we describe magnetic force microscope measurements which allow us to determine the distortion on the disk and hence to quantify its effect on the reflected atoms. We show that recording nonlinearity is indeed a major cause of the mirror roughness. We also consider other s ources of roughness and identify an important effect associated with t he boundaries between recorded tracks. Agreement between our experimen t and theory suggests that we have identified the limiting factors in a real atom-optical element made from a floppy disk. At present the an gular resolution of the mirror is approximately 35 mrad for atoms drop ped from a height of 4 cm. We discuss how this can be improved to reac h a level of 5 mrad or better.