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

Ferromagnetic resonance (FMR) has been studied in CoFe(t)/Al 2 O 3 (30 A) multilayers. With growing nominal thickness t of CoFe layers, transitions from superparamagnetic (SP) to ferromagnetic (FM) state at t 1 13 A and from granular to continuous structural state at t 2 18 A, previously revealed by magnetotransport studies on this system, are confirmed by the FMR data. The effective anisotropy field H eff (t) clearly displays three distinct regions: (i) SP at t < t 1 , (ii) mixed SP + FM at t 1 < t < t 2 , and (iii) FM at t 2 < t. The FMR linewidth AH(t) increases when passing from the continuous to discontinuous regime, mainly due to static fluctuations of internal field on granules. At room temperature AH(t) peaks near t 1 , the magnetic percolation point, while at 77 K it still grows below this point, probably due to getting below the AC blocking temperature.