H.T. Leung

Micromagnetism in mesoscopic epitaxial Fe dot arrays

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.

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.

Domain nucleation processes in mesoscopic Ni80Fe20 wire junctions

The magnetization reversal process in permalloy (Ni80Fe20) wire junction structures has been investigated using magnetoresistance (MR) measurements and scanning Kerr microscopy. A combination of electron beam lithography and a lift-off process has been utilized to fabricate wires consisting of two 200 μm length regions with distinct widths w1 and w2 in the range 1–5 μm. Longitudinal MR measurements and magneto-optic Kerr effect hysteresis loops demonstrate that the magnetization reversal of the complete structure is predominantly determined by the wider region for fields applied parallel to the wire axis. Magnetic force microscopy and micromagnetic calculations show that several domain walls nucleate in the wider part and are trapped in the junction area. This implies that domain nucleation at the junction of the wire initiates magnetization reversal in the narrow half. As a consequence, the switching fields are found to be identical in both halves in this case. These results suggest the possibility of de...

Magnetic anisotropies and magnetic switching in Co films

Abstract We have used the magneto-optical Kerr effect to investigate the role of the substrate and growth conditions in determining the magnetic switching behaviour of Co films in the thickness range 100–200 A supported by GaAs(001) and Si(111) substrates. We discuss the anisotropic magnetic hysteresis behaviour observed for Co/GaAs and Co/Si films in terms of coherent rotation of the magnetisation vector during magnetic switching. Equivalent films supported by glass substrates are found to be almost isotropic in-plane. The in-plane coercive and saturation fields are observed to lie in the range 20–80 Oe but perpendicular saturation fields of 25 and 19 kOe are found for the Co/Si and Co/GaAs systems respectively which substantially exceed the demagnetising field in each case. The measured perpendicular anisotropy fields differ strongly from the values for hcp and bcc Co and are attributed to the details of the interface and film structure. We also report strongly frequency dependent magnetic switching behaviour in these Co films.

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.

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.

Domain wall trapping in controlled submicron ferromagnetic elements

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.

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.

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

There have been several recent efforts to control the magnetization reversal behaviour in ferromagnetic wires. Kirk et a l [2] reported that the switching field and magnetization reversal process depend strongly on the end shape as well as the width of Permalloy and CO wires. This behaviour is attributed to the formation of end domains which are crucial in the reversal mechanism [l-41. In this paper, we have investigated the effect of mesoscopic junction on the switching field associated with magnetization reversal and the domain configuration in Permalloy (NisaFezo) wire structures using scanning Kerr microscopy and magnetoresistance (MR) measurements.

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.