Dynamic hysteresis behavior in epitaxial spin-valve structures

We report the dynamic hysteresis behavior of epitaxial single ferromagnetic fcc NiFe(001), fcc Co(001) layers, and fcc NiFe/Cu/Co(001) spin-valve stru ctures investigated as a function of field sweep rate in the range of 0.01- 270 kOe/s using the magneto-optic Kerr effect. The hysteresis loop area A i s found to follow the scaling relation A proportional to H-alpha with alpha similar to 0.13 and similar to 0.02 at low sweep rates and similar to 0.70 and similar to 0.30 at high sweep rates for 60 Angstrom NiFe and 40 Angstr om Co single magnetic layer structures, respectively. For the single and do uble spin valves, the "double-switching" behavior which occurs at low sweep rates transforms to "single switching" at similar to 154 and similar to 19 2 kOe/s, respectively. Our results provide direct experimental evidence tha t the magnetic anisotropy strength affects dynamic hysteresis scaling in ul trathin magnetic films. (C) 2000 American Institute of Physics. [S0021-8979 (00)86708-0].