Antidot density-dependent reversal dynamics in ultrathin epitaxial Fe/GaAs(001)

Easy axis dynamic magneto-optic Kerr effect loops have been obtained from ultrathin (20 Å) epitaxial Fe/GaAs(001) patterned with antidot arrays of different densities (antidot spacings s=10 and 50 ?m). The external field was driven sinusoidally in time with frequency in the range 0.01 Hz-2.3 kHz. In the low-frequency regime (f<20 Hz) coercivity Hc increases with 1/s, in agreement with existing phenomenological laws of geometric coercivity scaling in quasistatic fields, e.g., Hc=?t/x, where t is the film thickness, x is a length parameter of the magnetic structure, and ? is a constant of proportionality. However, in the transitional region between the low- and high-frequency regimes (20 Hz<f<1000 Hz), we find that scaling parameter ? is no longer constant and increases with frequency: at low antidot density (s=50 ?m) the coercivity in the transitional region falls to a minimum, while at high antidot density (s=10?m) it remains almost a constant. The dip in coercivity for s=50 ?m is attributed to a resonance of the sweeping applied field with the domain-wall propagation in the film. Meanwhile, the suppression of the dip for s=10 ?m is ascribed to a change in the relative importance of wall propagation and domain nucleation mechanisms in dynamic magnetization reversal.