Ising-Glauber spin cluster model for temperature-dependent magnetization noise in SQUIDs

Clusters of interacting two-level-systems, likely due to Farbe+(F(+)) centers at the metal-insulator interface, are shown to self-consistently lead to 1/f(α) magnetization noise [with α(T)≲1] in SQUIDs. Model calculations, based on a new method of obtaining correlation functions, explains various puzzling experimental features. It is shown why the inductance noise is inherently temperature dependent while the flux noise is not, despite the same underlying microscopics. Magnetic ordering in these systems, established by three-point correlation functions, explains the observed flux-inductance-noise cross correlations. Since long-range ferromagnetic interactions are shown to lead to a more weakly temperature dependent flux noise when compared to short-range interactions, the time reversal symmetry of the clusters is also not likely broken by the same mechanism which mediates surface ferromagnetism in nanoparticles and thin films of the same insulator materials.