FMR in CoFe/Al 2O 3 multilayers: from continuous to discontinuous regime

Ferromagnetic resonance (FMR) has been studied in CoFe( t)/Al ₂O ₃(30 Å) multilayers. With growing nominal thickness t of CoFe layers, transitions from superparamagnetic (SP) to ferromagnetic (FM) state at t ₁≈13 Å and from granular to continuous structural state at t ₂≈18 Å, 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< t₁, (ii) mixed SP+FM at t₁< t< t₂, and (iii) FM at t₂< t. The FMR linewidth Δ H( t) increases when passing from the continuous to discontinuous regime, mainly due to static fluctuations of internal field on granules. At room temperature Δ H( t) peaks near t₁, the magnetic percolation point, while at 77 K it still grows below this point, probably due to getting below the AC blocking temperature.