C.C. Yao

Magnetic domain studies of permalloy wire-based structures with junctions

Permalloy (Ni/sub 80/Fe/sub 20/) wire-based structures (30 nm thick and 1/spl middot/w/spl middot/10 /spl mu/m wide) with junctions (crosses, networks, H-shapes, rectangular chains and ring chains) prepared on a GaAs(100) substrate were observed in both their demagnetized and remanent states by magnetic force microscopy (MFM) in order to investigate the role of junction geometry in domain formation. Except in ring chains, two classes of domain configuration are found at the junction: (i) a domain wall-like feature due to abrupt spin rotation and (ii) a triangle-shape domain consistent with a flux closure configuration. Ring chains, on the other hand, form vortex domains at every other junction. The MFM observations are compared with micromagnetic calculations which qualitatively support the magnetic domain configurations.

Magnetic domain evolution in permalloy mesoscopic dots

Permalloy (Ni/sub 80/Fe/sub 20/) squares (30 nm thick and w /spl mu/m wide; 1/spl les/w/spl les/200 /spl mu/m) and circular disks (30 nm thick and r /spl mu/m diameter; 1/spl les/r/spl les/200 /spl mu/m) prepared on a GaAs(100) substrate were observed in both their demagnetized and remanent states by magnetic force microscopy (MFM) associated with non-contact atomic force microscopy (NC-AFM). The squares (2/spl les/w /spl mu/m) exhibited conventional closure domains and the corner plays a very important role in creating new walls. The circular disks, on the other hand, formed either vortex domain (5/spl les/r/spl les/20 /spl mu/m) or multi-domain (50/spl les/r /spl mu/m) states. The magnetization rotation is observed by MFM to change according to the size and shape of the elements. The MFM observations are supported by micromagnetic calculations which confirm the effect of the corner on the domain wall formation.

MAGNETORESISTANCE IN MODULATED WIDTH NI80FE20 WIRES

The magnetization reversal processes and magnetoresistance behavior in micron-sized Ni80Fe20 wires with periodically modulated width have been studied. The wires were fabricated by electron beam lithography and a lift-off process. A combination of the magneto-optical Kerr effect and magnetoresistance measurements shows that the lateral shape of the wires greatly influences the magnetic and transport properties. For the field applied along the wire axis, the hysteresis loops are strongly influenced by the wire shapes. In contrast to the fixed width wires, the modulated width wires show an additional transverse magnetoresistance, which has been attributed to the shape-dependent demagnetizing fields and the inhomogeneous current density. The resistance of the modulated width wires is dominated by the contribution due to the narrow part of the wires; however, the inhomogeneous current density in the wide part of the wires contributes a significant transverse magnetoresistance.

Magnetization reversal and magnetic anisotropy in Co network nanostructures

The magnetization reversal and magnetic anisotropy in Co network structures have been studied using magneto-optic Kerr effect (MOKE). An enhancement of the coercivity is observed in the network structures and is attributed to the pinning of domain walls by the hole edges in the vicinity of which the demagnetizing field spatially varies. We find that the magnetization reversal process is dominated by the intrinsic uniaxial anisotropy (2K/sub u//M/sub s//spl ap/200 Oe) in spite of the shape anisotropy induced by the hole edges. The influence of the cross-junction on the competition between the intrinsic uniaxial anisotropy and the induced shape anisotropy is discussed using micromagnetic simulations.

Effect of junction geometry on switching field and reversal behavior in permalloy wires

We report on the magnetization reversal process and switching field behavior in permalloy (Ni/sub 80/Fe/sub 20/) wire structures investigated by magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements. A combination of electron beam lithography and a lift-off process has been utilized to fabricate three sets of wire structures from a 30 /spl Aring/ Au/300 /spl Aring/ Ni/sub 80/Fe/sub 20//GaAs(100) continuous film: straight wires, elbow wires, and cross wires with width w=0.5-50 /spl mu/m and length l=200 /spl mu/m. We found that the switching field and reversal behavior in the elbow and the cross wires depend upon the junction geometry, which nucleates or suppresses reverse domains according to the wire width. Consequently the junction determines the magnetization reversal process and switching field in the elbow and cross wires.

Pseudo-Hall effect and anisotropic magnetoresistance in a micron-scale Ni/sub 80/Fe/sub 20/ device

The pseudo-Hall effect (PHE) and anisotropic magnetoresistance (AMR) in a micron-scale Ni/sub 80/Fe/sub 20/ six-terminal device, fabricated by optical lithography and wet chemical etching from a high quality UHV grown 30 /spl Aring/ Au/300 A Ni/sub 80/Fe/sub 20/ film, have been studied. The magnetisation reversal in different parts of the device has been measured using magneto-optical Kerr effect (MOKE). The device gives a 50% change in PHE voltage with an ultrahigh sensitivity of 7.3%Oe/sup -1/ at room temperature. The correlation between the magnetisation, magneto-transport properties, lateral shape of the device and directions of the external applied field is discussed based on extensive MOKE, AMR and PHE results.

Influence of lateral geometry on magnetoresistance and magnetisation reversal in Ni/sub 80/Fe/sub 20/ wires

The magnetisation reversal processes and magnetoresistance behaviour in micron-sized Ni/sub 80/Fe/sub 20/ wires with triangular and rectangular modulated width have been studied. The wires were fabricated by electron beam lithography and a lift-off process. A combination of magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE) and magnetoresistance (MR) measurements shows that the lateral geometry of the wires greatly influences the magnetic and transport properties. The width modulations modify not only the shape-dependent demagnetising fields, but also the current density. The correlation between the lateral geometry the magnetic and the transport properties is discussed based on MFM, MOKE and MR results.

Domain wall trapping and magneto-transport in mesoscopic ferromagnets

Nanofabrication of mesoscopic ferromagnets provides an opportunity lo address key issues in nanomagnetism, such as the novel spin-electronics of domain wall (DW).scattering[l, 21, as the domain structure can be controlled to confine DW. In this paper, we have designed and fabricated a simple cmss shape wire structure using advanced e-beam lithography. We demonstrate the trapping of the DW in a junction of submicron size and the direct observation of the MR effeci associated with a single 180-degree domain wall in these mesoscopic ferromagnetic crosses.