Abstract
We study CoFe∕Al2O3 multilayers, varying from granular to continuous structure with CoFe nominal thickness (10Å⩽t⩽18Å). Structural percolation takes place at t=tc≈18Å, changing from activated to metallic conductance. A sharp anomaly in ac coercivity of magneto-optical Kerr effect and in magnetization M by superconducting quantum interferometer device is found at t=t*≈17Å, interpreted as transition from superferromagnetism (by dipolar coupled CoFe clusters) to exchange ferromagnetism. Onset at t=t* of anisotropic magnetoresistance (AMR) is attributed to skew scattering of bandlike electrons within giant fractal clusters. These structures may cause the observed Barkhausen noises in M and AMR at t≈t*, disappearing at t>t*.
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