FMR in CoFe/Al2O3 multilayers: from continuous to discontinuous regime

Ferromagnetic resonance (FMR) has been studied in CoFe(t)/Al2O3(30 Angstrom ) multilayers. With growing nominal thickness t of CoFe layers, transitions from superparamagnetic (SP) to Ferromagnetic (FM) state at t(1) approximat e to 13 Angstrom and from granular to continuous structural state at t(2) a pproximate to 18 Angstrom, previously revealed by magnetotransport studies on this system, are confirmed by the FMR data. The effective anisotropy fie ld H-eff(t) clearly displays three distinct regions: (i) SP at t < t(1), (i i) mixed SP + FM at t(1) < t < t(2), and (iii) FM at t(2) < t. The FMR line width DeltaH(t) increases when passing from the continuous to discontinuous regime, mainly due to static fluctuations of internal field on granules. A t room temperature DeltaH(t) peaks near t(1), the magnetic percolation poin t, while at 77 K it still grows below this point, probably due to getting b elow the AC blocking temperature, (C) 2001 Elsevier Science B.V. All rights reserved.