Mark D. Mascaro

Low Energy Magnetic Domain Wall Logic in Short, Narrow, Ferromagnetic Wires

We present circuit simulation results of an implementation of universal logic that operates at low switching energy. Information is stored in the position of a single domain wall in a thin, short ferromagnetic wire. The gate is switched by current-driven domain wall motion, and information is read out using a magnetic tunnel junction. The inputs and outputs of the device are currents controlled by voltage clocks, making it compatible with CMOS. Using devices that operate at 100-1 mV, we simulate a shift register circuit and a full-adder circuit. The simulations show that the magnetic logic gates can operate at lower switching energy than CMOS electronics.

Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes

360°, 540°, and other complex transverse domain walls have been created in narrow cobalt wires connected to injection pads by cycling a magnetic field perpendicular to the wire length. The composite walls, formed by impingement of 180° transverse walls of alternating chirality, are stable over a wide field range. The structure of the walls observed at remanence by scanning electron microscopy with polarization analysis and by magnetic force microscopy is in good quantitative agreement with the prediction of micromagnetic simulations.

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.

Current-driven domain wall motion in heterostructured ferromagnetic nanowires

Micromagnetic modeling shows that the placement of non-magnetic conductive pads on a ferromagnetic wire affects the current-induced velocity of a domain wall (DW) in the wire and can act as a DW chirality filter. The pads shunt the current, causing a non-uniform spin current distribution inside the ferromagnetic wire and an Oersted field transverse to the wire. This suppresses Walker breakdown allowing higher current densities to be imposed before breakdown occurs. The transverse Oersted field pins the DW under some regimes of current density and pad geometry, selectively allowing transmission of DWs of only one chirality.

Magnetic remanent states and quasistatic switching behavior of Fe split-rings for spin field-effect-transistor applications

The magnetic remanent states and switching behavior of Fe thin-film split-rings are investigated using magnetic force microscopy, magnetoresistance measurements, and micromagnetic simulations in order to assess their suitability as spin-filter contacts for spin field-effect-transistors. The gaps between the two halves of each ring are found to absorb then emit domain walls and act as pinning sites for “virtual” domain walls so that the observed switching mechanisms are similar to those of continuous rings. It is shown that these rings offer advantages over rectangular spin-filter contacts owing to their reduced stray fields and easy accessibility of the necessary magnetic states.

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.

Electrical observation of asymmetric magnetization configurations in the vortex state of NiFe and Co rings

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.

Lateral Interaction of transverse magnetic domain walls

Magnetic domain walls (DWs) that are spatially close are formed with 360° (360DW) or 540° (540DW) configurations in Co magnetic rings. The 360DW is formed in a continuous magnetic structure whereas the 540DW configuration comprises the 360DW and 180DW with a narrow nonmagnetic gap. Individual transverse DWs play a key role in forming 360DWs or 540DWs because their magnetic polarity and chirality reduce the magnetostatic energy depending on the DW configurations. The magnetostatic interaction between individual DWs results in the variation of the local stray field, which is consistent with micromagnetic simulations.