Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers $\left[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}\mathrm{}\right(t)/{\mathrm{Al}}_2{\mathrm{O}}_3(3\mathrm{}\mathrm{}\mathrm{n}\mathrm{m}){]}_{10}$ with nominal thickness $1.1<~t<~1.3\mathrm{nm}$ by use of ac susceptometry and dc magnetometry. At $t=1.0\mathrm{}\mathrm{nm},$ superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.

• Received 5 December 2000

DOI:https://doi.org/10.1103/PhysRevB.63.134423