Chunghee Nam

Interactions between 180° and 360° domain walls in magnetic multilayer stripes

Magnetostatic interactions between 360° and transverse 180° domain walls in the NiFe and Co layers of Co/Cu/NiFe multilayer stripes are investigated by micromagnetic simulations. In 200 nm wide Co (5 nm)/Cu (5 nm) /NiFe (5 nm) stripes, stray fields from 360° domain walls in the Co layer strongly influence the magnetic behavior of the NiFe layer, promoting reverse domain nucleation and providing a pinning potential of order 100 Oe which impedes domain wall propagation. 360° domain walls may be useful as programmable pinning sites in magnetoelectronic logic or memory devices.

Magnetostatic control of vortex chirality in Co thin film rings

The vortex chirality in an elliptical Co ring spaced 60 nm from a circular ring has been controlled by magnetostatic interaction. One of the two domain walls (DWs) in the elliptical ring interacts with a neighboring wall in the circular ring, while the other is unaffected by the stray field of the circular ring. The direction of motion of the DWs, and the chirality of the resulting vortex state in the elliptical ring, can be selected by the field direction and history.

Current-driven vortex formation in a magnetic multilayer ring

Current-driven domain wall (DW) motion has been studied in the NiFe layer of a Co/Cu/NiFe thin film ring using giant-magnetoresistance measurements in a four-point contact geometry. The NiFe layer is initially in an onion state configuration with two 180° DWs. An electric current drives the walls around the ring so that they annihilate and the NiFe layer forms a DW-free vortex state. The direction of motion of the two DWs is determined by the current polarity, enabling the vortex chirality to be selected.

Effect of magnetic field direction on the remanent resistance levels and vortex chirality of a multilayered magnetic ring

The effect of applied field direction on the magnetoresistance response of a 5 μm diameter Co/Cu/NiFe thin film ring has been examined. When the Co layer of the ring is placed in a vortex state, four possible remanent magnetization configurations exist, in which the NiFe layer is in one of four states: forward onion, reverse onion, clockwise vortex or counterclockwise vortex. The resistance levels of these four remanent states depend on the field angle with respect to the electrical contact leads, and measured values agree well with an electrical model. The chirality of the Co vortex can be determined from measurements of the minor loop, and the chirality of the NiFe vortex can be set using two-step field cycling at two different field angles.

360° domain wall mediated reversal in rhombic Co/Cu/NiFe magnetic rings

The reversal process of thin film micron-scale Co/Cu/NiFe rhombic rings in an in-plane magnetic field is investigated by micromagnetic simulation and magnetoresistance measurements. Simulations show that the impingement of reverse domains leads to the formation of multiple 360° domain walls in the ring during low-field cycling. Two types of reversal process can be identified experimentally which are attributed to the presence or absence of residual 360° domain walls in the ring. The reversal path depends on the field history, which affects the population of walls in the ring.

Effects of Interlayer Coupling in Elongated

We describe the magnetization reversal processes in multilayer elongated Ni80Fe20(10 nm)/Au(t = 0 to 20 nm)/ Co(20 nm)/Si(001) nanorings. For a field applied in-plane along the long axis, the hysteresis loops display a two-step switching for both t = 0 nm and 2 nm, and a three-step switching for t = 10 nm and 20 nm. Onion-to-vortex and vortex-to-reverse onion state transitions occur for a thin Au spacer due to strong exchange coupling between the layers. For a thick Au spacer layer, the Ni80Fe20 layer switches first, then a two-step reversal of the Co, correlated with domain motion in the Ni80Fe20, occurs without the formation of vortex states as a result of magnetostatic interactions. There is a good agreement between the experimental results and micromagnetic simulations.

\hbox{Ni}_{80}\hbox{Fe}_{20}/\hbox{Au/Co}

Anisotropic magnetoresistance (AMR) measurements have been used to probe the detailed reversal mechanism of 3??m diameter, 15?nm thick NiFe and Co rings. In the vortex state, small changes in the resistance are associated with distortion or buckling in the section of the ring magnetized antiparallel to the applied field, and the resistance changes can be similar in magnitude to the domain-wall resistance. Micromagnetic simulations showed that a distorted-vortex state forms just before the vortex?onion transition, and a reversible change between the distorted-vortex state and a fully symmetric vortex state is expected during minor loop magnetic cycling. The distorted-vortex state enables the vortex chirality in a single magnetic ring to be detected using AMR measurements.

Nanorings

In a nanowire consisting of a ferromagnet/insulator/superconductor multilayer structure, the superconductivity is shown to depend strongly on the configuration of the magnetic domain walls in the neighboring ferromagnetic layer, yielding a high magnetoresistance within a temperature range near the superconducting transition temperature TC. Micromagnetic simulations confirmed that out-of-plane stray magnetic fields from uncompensated magnetic poles play a dominant role in inducing magnetoresistance in this particular system.