Adsorbed Oxygen Molecules as a Possible Source of Flux Noise in SQUIDs

One of the dominant source of flux noise in SQUIDs is flux noise which has been attributed to mysterious fluctuating magnetic spins on the surface. We propose that the spins producing flux noise could be adsorbed O$_2$ molecules that have a magnetic moment of about 2 $\mu_B$. Using density functional calculations, we studied O$_2$ molecules adsorbed on a sapphire surface. We find that the barrier for spin rotation is small enough to allow almost free spin reorientation due to thermal excitations at low temperatures. Monte Carlo simulations of a 2D XY spin model yields $1/f$ noise where $f$ is frequency.