  The dominant source of decoherence in contemporary frequency-tunable
superconducting qubits is 1/$f$ flux noise. To understand its origin and find
ways to minimize its impact, we systematically study flux noise amplitudes in
more than 50 flux qubits with varied SQUID geometry parameters and compare our
results to a microscopic model of magnetic spin defects located at the
interfaces surrounding the SQUID loops. Our data are in agreement with an
extension of the previously proposed model, based on numerical simulations of
the current distribution in the investigated SQUIDs. Our results and detailed
model provide a guide for minimizing the flux noise susceptibility in future
circuits.
