H. T. Zeng

Fast domain wall motion in magnetic comb structures

Modern fabrication technology has enabled the study of submicron ferromagnetic strips with a particularly simple domain structure, allowing single, well-defined domain walls to be isolated and characterized. However, these domain walls have complex field-driven dynamics. The wall velocity initially increases with field, but above a certain threshold the domain wall abruptly slows down, accompanied by periodic transformations of the domain wall structure. This behaviour is potentially detrimental to the speed and proper functioning of proposed domain-wall-based devices, and although methods for suppression of the breakdown have been demonstrated in simulations, a convincing experimental demonstration is lacking. Here, we show experimentally that a series of cross-shaped traps acts to prevent transformations of the domain wall structure and increase the domain wall velocity by a factor of four compared to the maximum velocity on a plain strip. Our results suggest a route to faster and more reliable domain wall devices for memory, logic and sensing.

Domain wall conduit behavior in cobalt nanowires grown by focused electron beam induced deposition

The domain wall nucleation and propagation fields in cobalt nanowires grown by focused electron beam induced deposition are measured using spatially resolved magneto-optical Kerr effect. The study was systematically done for wire widths from 600 to 150 nm, finding significant differences in the value of both fields for the wires, indicating high quality domain wall conduit behavior. The extreme simplicity and flexibility of this technique with respect to the multistep lithographic processes used nowadays opens a different route to create magnetic nanostructures with a good control of the domain wall motion.

Near-field interaction between domain walls in adjacent Permalloy nanowires.

The magnetostatic interaction between two oppositely charged transverse domain walls (TDWs) in adjacent Permalloy nanowires is experimentally demonstrated. The dependence of the pinning strength on wire separation is investigated for distances between 13 and 125 nm. The results can be described fully by considering the distribution of magnetic charge within rigid, isolated TDWs. Alternative DW internal structure cannot reproduce this observed dependence. Modeling suggests the TDW internal structure is not appreciably disturbed, and remains rigid although the pinning strength is significant.

Magnetization reversal in individual cobalt micro- and nanowires grown by focused-electron-beam-induced-deposition.

We systematically study individual micro- and nanometric polycrystalline cobalt wires grown by focused-electron-beam-induced-deposition. The deposits were grown in a range of aspect ratios varying from 1 up to 26. The minimum lateral dimension of the nanowires was 150 nm, for a thickness of 40 nm. Atomic force microscopy images show beam-current-dependent profiles, associated with different regimes of deposition. The magnetization reversal of individual nanowires is studied by means of the spatially resolved magneto-optical Kerr effect. Abrupt switching is observed, with a systematic dependence on the wires dimensions. This dependence of the coercive field is understood in magnetostatic terms, and agrees well with previous results on cobalt wires grown with different techniques. The influence of compositional gradients along the structural profile on the magnetic reversal is studied by using micromagnetic simulations. This work demonstrates the feasibility of using this technique to fabricate highly pure magnetic nanostructures, and highlights the advantages and disadvantages of the technique with respect to more conventional ones.

Six-fold configurational anisotropy and magnetic reversal in nanoscale Permalloy triangles

Six-fold configurational anisotropy was studied in Permalloy triangles, in which the shape symmetry order yields two energetically non-degenerate micromagnetic configurations of the spins, the so-called Y and “buckle” states. A 12-pointed switching astroid was measured using magneto-optical experiments and well reproduced numerically. Switching fields in different polar sectors were shown to reflect the different possible energy paths dictated by the configurational anisotropy during a 180° reversal of the magnetization, thereby giving a comprehensive view of the switching mechanism in these structures. A detailed analysis moreover highlighted the necessity to include the physical rounding of the structures in the simulations to account for the instability of the Y state.

Bidirectional magnetic nanowire shift register

We experimentally demonstrate a shift register based on an open-ended chain of ferromagnetic NOT gates which can support bidirectional data flow. Up to eight data bits are electrically input to the device, stored for extended periods without power, and then output either in a first in first out or last in first out scheme. Comparing to traditional transistor-based logic, this bidirectionality offers a range of devices that are reversible and not limited to only one mode of operation.

High efficiency domain wall gate in ferromagnetic nanowires

A transverse domain wall (DW) switchable gate with a very high efficiency is experimentally demonstrated in Permalloy nanowires using a transverse T-shaped structure. DWs are found to either travel undisturbed through the open gate or to be strongly trapped in front of the closed gate only able to travel backwards. The opening and closing of the gate depends on the magnetic configuration of the gate and is controlled using externally applied magnetic fields. Micromagnetic simulations confirm the experimental results.

Asymmetric magnetic NOT gate and shift registers for high density data storage

We have developed an asymmetric ferromagnetic NOT gate and shift register optimized on a square grid. This gives rise to a two-dimensional storage scheme built up by tessellating an elementary data unit, which is scalable down to very narrow wire widths. The areal footprint of each storage unit is 15F2, where F is the minimum feature size. We experimentally demonstrate NOT operations across a chain of three gates made from Permalloy with F=60 nm, and present a functional 15-gate, multichannel shift register with electrical injection, and optical readout.

Kinetic depinning of a magnetic domain wall above the Walker field

The dynamical interaction between a transverse domain wall and a T-shaped trap is investigated, for domain wall motion in the oscillatory regime above the Walker field. We demonstrate experimentally the existence of distinct static and kinetic depinning fields in this regime, and show that the oscillatory motion of the domain wall leads to a distribution of kinetic depinning fields. Micromagnetic simulations are in good qualitative agreement with our experimental results.

Magnetic imaging of the pinning mechanism of asymmetric transverse domain walls in ferromagnetic nanowires

The pinning of asymmetric transverse magnetic domain walls by constrictions and protrusions in thin permalloy nanowires is directly observed using the Fresnel mode of magnetic imaging. Different domain wall (DW)/trap configurations are initialized using in situ applied magnetic fields, and the resulting configurations are imaged both at remanence and under applied fields. The nature of the chirality dependent pinning potentials created by the traps is clearly observed. The effect of the asymmetry of the DW is discussed. Micromagnetic simulations are also presented, which are in excellent agreement with the experiments.