The dynamic magnetization reversal behavior of polycrystalline Ni 80 Fe 20 films ͑thickness 60 and 200 Å͒ was studied in the temperature range 90-300 K by applying a magnetic field along the easy magnetization axis of the samples. The loop area A is found to follow the scaling relation AϰH 0 ␣ ⍀  T Ϫ␥ with ␣Ϸ0.9, Ϸ0.8, and ␥ϭ0.38 for both thicknesses. This behavior is only qualitatively consistent with theoretical models if the magnetization reversal mechanism is identical for both films, independently of the applied field and sample temperature. The observed scaling exponent values indicate that domain nucleation and domain-wall motion process dominate the magnetization reversal process, which is not included in current theoretical models based on coherent rotation. Furthermore, the exponents ␣ and  are found to be independent of the temperature, indicating that the dynamic reversal mechanism is unchanged in this temperature range.
We report the dynamic hysteresis behavior of epitaxial single ferromagnetic NiFe, Co layers, and NiFe/Cu/Co spin-valve structures investigated as a function of field sweep rate Ḣ (dH/dt) in the range 0.01-270 kOe/sec using the magneto-optic Kerr effect. In situ reflection high-energy electron-diffraction images confirmed that the NiFe, Cu, and Co layers grew epitaxially in the ͑100͒ orientation where the fcc NiFe, Co͗110͘ in-plane directions correspond to the Si͗100͘ directions. For Cu/60 Å NiFe/Cu/Si (H c ϭ5 Oe) and Cu/40 Å Co/Cu/Si (H c ϭ104 Oe) single magnetic layer structures, the hysteresis loop area A is found to follow the scaling relation AϰḢ ␣ with ␣ϳ0.13 and ϳ0.02 at low sweep rates and ϳ0.70 and ϳ0.30 at high sweep rates, respectively. This result indicates that the NiFe and Co layers in the spin-valve structures can be expected to show distinct scaling behavior at high sweep rate. We found that the ''double-switching'' behavior which occurs at low sweep rates transforms to ''single switching'' at ϳ154 kOe/sec and ϳ192 kOe/sec, respectively, for the single and double spin valves due to the different dynamic response of the NiFe and Co layers. Our results provide direct experimental evidence that the magnetic anisotropy strength affects dynamic hysteresis scaling in ultrathin magnetic films, in contrast with the predictions of current theoretical models.
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 structures 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 is found to follow the scaling relation A∝Ḣα with α∼0.13 and ∼0.02 at low sweep rates and ∼0.70 and ∼0.30 at high sweep rates for 60 Å NiFe and 40 Å Co single magnetic layer structures, respectively. For the single and double spin valves, the “double-switching” behavior which occurs at low sweep rates transforms to “single switching” at ∼154 and ∼192 kOe/s, respectively. Our results provide direct experimental evidence that the magnetic anisotropy strength affects dynamic hysteresis scaling in ultrathin magnetic films.
Cu(50 Å)/NiFe(60 Å)/Cu(60 Å)/Co(20 Å) epitaxial spin-valve structures were grown on GaAs(001) substrates by molecular-beam epitaxy at room temperature. In situ reflection high-energy electron diffraction measurements indicate the stabilization of the bcc-Co(001) phase on 1×1 unreconstructed GaAs(001) for thicknesses up to 20 Å and the epitaxial growth of the fcc-Cu(001) spacer layer and fcc-FeNi(001) top magnetic layer. Magneto-optical Kerr effect and Brillouin light-scattering measurements of the composite structure showed that a fourfold cubic anisotropy is present but a twofold anisotropy also occurs directed along the 〈110〉 axes. The easy cubic axes are directed along the 〈100〉 axes, which implies that the cubic anisotropy constant Kl for bcc-Co is positive. The magnetic anisotropy of the bcc-Co layer has a striking influence on the magnetoresistance characteristics which were found to be angular dependent. A simulation of this mixed anisotropy behavior yields quantitative agreement with the experimental results.
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