Single-particle blocking and collective magnetic states in discontinuous CoFe/Al2O3 multilayers

Discontinuous metal-insulator multilayers (DMIMs) of [CoFe(tn)/Al2O3]m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness tn = 0.5 nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5 nm < tn < 1.1 nm, the particles encounter strong interactions which give rise to a superspin glass (SSG) phase. The SSG phase has been characterized by memory effect, ageing, dynamic scaling, etc. With further increase in particle concentration (1.1 nm < tn < 1.4 nm) and, hence, smaller interparticle distances, strong interactions lead to a FM-like state which is called superferromagnetic (SFM). The SFM state has been characterized by several techniques, e.g. dynamic hysteresis, Cole-Cole plots extracted from ac susceptibility, polarized neutron reflectometry, etc. Moreover, the SFM domains could be imaged by x-ray photoemission electron microscopy and magneto-optic Kerr effect microscopy. At tn > 1.4 nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.