Mesofrequency switching dynamics in epitaxial CoFe and Fe thin films on GaAs(001)

Thin epitaxial Co27Fe73 and Fe films have been grown on GaAs(001), with thicknesses of 8.5 and 10 nm, respectively. The mesofrequency switching dynamics have been investigated by ac magneto-optic Kerr effect magnetometry. Dynamic hysteresis loops have been measured along the four major in-plane crystallographic axes [100], [110], [010], and [1(1) over bar 0] in each film in the sinusoidal applied field frequency range of 0.2-2 kHz. The coercive field H-c for the CoFe film is 1.4-2.9 times larger than that for the Fe film, depending on the measurement axis, indicating a larger effective energy barrier to reversal. Examining the coercive field as a function of frequency H-c(f) it is found that the time scales for the domain nucleation and wall propagation are 3-10 times longer in the CoFe than in the Fe film. As previously shown in epitaxial Fe films, a local minimum in H-c(f) can occur at the crossover from the wall propagation-dominated to the nucleation-dominated dynamic regime, and here we find that it is more pronounced for the CoFe than for the Fe film, indicating a reduction in domain-wall pinning. We conclude that the magnitude of the effective energy barrier to reversal sets the time scale for the domain nucleation and wall propagation processes and determines the characteristics of H-c(f) for the two films. (C) 2005 American Institute of Physics.