Magnetization reversal dynamics in epitaxial spin-valve structures

We report the dynamic hysteresis behavior of epitaxial single ferromagnetic NiFe, Co layers, and NiFe/Cu/Co spin-valve structures investigated as a fu nction of field sweep rate H(dH/dt) in the range 0.01-270 kOe/sec using the magneto-optic Kerr effect. In situ reflection high-energy electron-diffrac tion images confirmed that the NiFe, Cu, and Co layers grew epitaxially in the (100) orientation where the fee NiFe, Co(110) in-plane directions corre spond to the Si(100) directions. For Cu/60 Angstrom NiFe/Cu/Si (H-c = 5 Oe) and Cu/40 Angstrom Co/Cu/Si (H-c = 104 Oe) single magnetic layer structure s, the hysteresis loop area A is found to follow the scaling relation A pro portional 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, re spectively. This result indicates that the NiFe and Co layers in the spin-v alve structures can be expected to show distinct scaling behavior at high s weep rate. We found that the "double-switching" behavior which occurs at lo w sweep rates transforms to "single switching" at similar to 154 kOe/sec an d similar to 192 kOe/sec, respectively, for the single and double spin valv es due to the different dynamic response of the NiFe and Co layers. Our res ults provide direct experimental evidence that the magnetic anisotropy stre ngth affects dynamic hysteresis scaling in ultrathin magnetic films, in con trast with the predictions of current theoretical models.