  Superconducting quantum interference devices (SQUIDs) and related circuits
made of aluminum are known to display $1/\omega$ flux noise, where $\omega$ is
frequency. A recent experiment showed that the application of an external
magnetic field in the $10-100$~G range changed the noise to a single Lorentzian
peaked at $\omega=0$. Here it is shown that a model based on independent
impurity spin flips with coexisting cross and direct mechanisms of spin
relaxation may explain these experiments. The model shows that application of
an external magnetic field can be used to reduce the impact of flux noise in
qubits.
