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.

The domain structures of epitaxial Fe (20 nm)/GaAs(100) circular dot arrays (diameters from 50 to 1 μm) were studied with magnetic force microscopy. A transition from a single domain to a multidomain remanent state was observed upon reducing the dot diameter beneath 10 μm in dot arrays with the separation twice the dot diameter. When the separation is reduced to half the dot diameter, the single domain states were found to “collapse” into stripe-like multidomain states due to local dipole coupling between dots. Micromagnetic simulations further suggest that for ultrathin Fe dots of less than about 2 nm thickness the diameter does not have a significant influence on the domain structures due to a dramatic reduction of the dipole energy.

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.

The magnetoresistance (MR) and domain structure of submicron NiFe wires and crosses fabricated using advanced electron beam lithography techniques have been studied in order to investigate the dependence of MR on the detailed domain configurations. While the 0.5 μm wire shows almost no longitudinal MR, the cross sample clearly shows a variation of the resistance upon sweeping the magnetic field, indicating an MR effect associated with the domain structures which form at the junction. By correlating the MR curves with the domain configurations obtained from magnetic force microscopy, we found that a 180° domain wall trapped in the junction of this 0.5 μm cross contributes a negative MR effect.

The domain configuration in permalloy wires (30 nm thick, 10 μm wide, and 205 μm long) with a wide size range of a narrow central bridge (5 μm long and w μm wide; 0.5⩽w⩽10 μm) were investigated in both their demagnetized and remanent states using magnetic force microscopy and the results were confirmed by micromagnetic calculations. At the bridge region, domain walls were found to be shifted by a small external field. Scanning magneto-optical Kerr effect revealed that the coercivity in these structures are the same as that in a straight wire, suggesting that domain wall movement is the dominant process in the magnetization reversal of these structures.