  Discontinuous magnetic multilayers [CoFe/Al2O3] are studied by use of
magnetometry, susceptometry and numeric simulations. Soft ferromagnetic
Co80Fe20 nanoparticles are embedded in a diamagnetic insulating a-Al2O3 matrix
and can be considered as homogeneously magnetized superspins exhibiting
randomness of size (viz. moment), position and anisotropy. Lacking
intra-particle core-surface ordering, generic freezing processes into
collective states rather than individual particle blocking are encountered.
With increasing particle density one observes first superspin glass and then
superferromagnetic domain state behavior. The phase diagram resembles that of a
dilute disordered ferromagnet. Criteria for the identification of the
individual phases are given.
