D. C. Chen

Variation of magnetization reversal in pseudo-spin-valve elliptical rings

We studied nanoscale elliptical ring shaped NiFe/Cu/NiFe trilayer pseudo-spin-valve structures. The magnetization reversal processes showed simultaneous-reversal single-step transition or double-step transition involving flux closure states. For various aspect ratios (short axis to long axis) and linewidths, transition between single-step and double-step magnetization reversals was measured to form a phase diagram. When the linewidth was reduced, edge roughness became important. Simulations of the magnetization reversal behavior agreed qualitatively with our results.

Magnetic switching and reversal process in a tip ring structure

Patterned Permalloy submicron-size structures have been fabricated by e-beam lithography in the shape of a ring with a tip. A tip was intentionally added into the ring as a geometrical defect to interrupt the continuity of the magnetization, which aligns along the ring, in order to pin the domain wall. Magnetic switching and reversal process have been measured by the magnetoresistance measurement. The switching field about 260 Oe was obtained. At the remanent state, there was a 0.21% difference in the magnetoresistance between the angles of 90° and 0° that was equivalent to the domain wall magnetoresistance. By applying an external field, the domain wall moved along the ring under a lower field (100 Oe), which is smaller than the switching field. A drop (0.24%) in the resistance between the angle of 70°–120° has been observed that means the domain wall was moving into the voltage measuring region during the rotation.

Vortex domain wall depinning by polarized current in submicron half-ring wires

Domain wall pinning force in the junctions (corners), with different shapes of square, semicircle, or triangle, of half-ring in-series wires is considered to study the current injection induced wall movements. This geometry has less thermal activation at the region of domain wall nucleation in contrast to notch structures. The wires with square corners have the largest domain pinning force to resist polarized current-induced magnetization reversal, judging from the largest slope in the current-field dependence (ΔI∕ΔH=0.274).

Determining vortex chirality in ferromagnetic ring by lateral nonlocal spin valve

We demonstrate detecting chirality of vortex state in a magnetic ring by lateral nonlocal spin-valve (NLSV) measurement. A Permalloy (Py) ring, a Py narrow wire, and copper contacts were used as spin injector, detector, and the normal-metal diffusive channel, respectively. By comparing the anisotropic magnetoresistance loop of the individual ring with NLSV loop, the vortex chirality and the related switching field of the ring can be determined. Both onion-to-vortex transition field and vortex chirality were found to alternate in both the same and different current probe arrangements on the magnetic ring.

Observation of anomalous Hall effect in Cu-Py-crossed structure with in-plane magnetization

We investigate a series of Cu-Py-crossed stripe devices in which the contact magnetoresistance behavior exhibits hysteretic loops originating from anomalous Hall effect in Py at the Cu/Py contact area with in-plane magnetization. These highly reproducible hysteretic loops relate directly to the magnetization switching of Py wires at the crossed regions. The nonzero magnitude of resistance difference (ΔR) between the two remnant states at zero Oe depends on the width of Cu by a roughly reciprocal relation, but is independent of that of Py wire. The ΔR ranges between 0.1 ∼ 0.6 mΩ with Cu width of 100 nm ∼ 500 nm and Cu/Py thickness of (65 nm)/(31 nm) ∼ (50 nm)/(20 nm). The results provide understanding of how the Hall voltage is induced and sensed.

Domain walls detection in a single magnetic ring by MR measuring

We demonstrated the results of the MR measurement in a single magnetic ring. The measurement was performed at room temperature with applying magnetic fields of constant magnitudes in the direction of the ring plane, while rotating the magnetic ring in its plane. The anisotropic field of the narrow ring forces the local moment aligned to the direction of perimeter of the ring, hence the onion state forms. Besides, the field of 250 Gauss can still drag the 2 domain walls at the onion state along the perimeter of the ring without changing the direction of other local moment which is not in the region of the 2 domain walls. While the first domain wall dragged into the region between voltage contacts, the local moment in this region is no longer parallel to the perimeter of the ring, hence a drop appears (or the resistance decreases). With sharp drops, the rotating MR measurement can be used to detect domain walls.

Dynamic study of domain wall in a patterned permalloy submicron wires

In summary, we have successfully demonstrated the domain wall motion along the direction of perimeter of a ferromagnetic ring at onion state. As the results of the MR measurement with rotating the ring at different constant fields, the critical field to form onion state is near 200 Oe and the lowest field which can still drag the domain wall is between 100 and 50 Oe.

Domain wall device of Permalloy submicron half-ring in series wire

The submicron Permalloy half-ring in series wire were fabricated. This was used to indicate different magnetic states by domain. Different magnitude longitudinal external fields were applied to the wire. The numbers of jump in the loop of magnetoresistances (MR) were different and might be the function of the maximum external field. The pattern structures of magnetic wires were fabricated by electron-beam lithography. Permalloy (Ni/sub 80/Fe/sub 20/) film was deposited by DC magnetron sputtering. MR measurement was made by standard 4-probe technique on two gold pads at 77 K and 300 K. Magnetic field was applied in the substrate plane. Longitudinal magnetoresistance were measured with different maximum field. From the measurements, jumps in the MR loops were observed which were attributed to domain wall depinning at the intersection of two half-ring or the domain walls were driven out of the half-ring wire.

Domain wall magnetoresistance in permalloy half-ring wires

Submicron permalloy wires with half-ring and s-shape structure are designed. Magnetoresistance loops are measured between the sweeping field /spl plusmn/4000 Oe at varying angles between external field and longitudinal axis. The resistances in remanent state of s-shape wire are found to be almost same value for every angle. On the other hand, the resistances of half-ring wire are found to be decreasing with increasing angle. Magnetic force microscopy images of the wires confirm the varied resistances result from different domain structures on the corners with domain wall or without domain wall. The domain wall resistance (DWR) in half-ring wires is estimated to be about 0.31%. Theoretical calculations based on impurity scattering model and spin accumulation model are done to validate the experimental data. DWR from spin accumulation model is 0.012%/spl sim/0.031% and from impurity scattering model, it is 0.15%/spl sim/0.33%.