Ana-Vanessa Jausovec

Forgery: ‘Fingerprinting’ documents and packaging

We have found that almost all paper documents, plastic cards and product packaging contain a unique physical identity code formed from microscopic imperfections in the surface. This covert ‘fingerprint’ is intrinsic and virtually impossible to modify controllably. It can be rapidly read using a low-cost portable laser scanner. Most forms of document and branded-product fraud could be rendered obsolete by use of this code.

Domain wall pinning and potential landscapes created by constrictions and protrusions in ferromagnetic nanowires

The potential experienced by transverse domain walls (TDWs) in the vicinity of asymmetric constrictions or protrusions in thin Permalloy nanowires is probed using spatially resolved magneto-optical Kerr effect measurements. Both types of traps are found to act as pinning centers for DWs. The strength of pinning is found to depend on the trap type as well as on the chirality of the incoming DW; both types of traps are seen to act either as potential wells or potential barriers, also depending on the chirality of the DW. Micromagnetic simulations have been performed that are in good qualitative agreement with the experimental results.

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.

Magnetic domain wall pinning by a curved conduit

The pinning of a magnetic domain wall in a curved Permalloy (NiFe) nanostrip is experimentally studied. We examine the dependence of the pinning on both the radius of curvature of the bend and the chirality of the transverse domain wall. We find that bends act as potential wells or potential barriers depending on the chirality of the domain wall; the pinning field in both cases increases with decreasing radius of curvature. Micromagnetic simulations are consistent with the experimental results and show that both exchange and demagnetizing energies play an important role.

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.

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.

Cycle-by-cycle observation of single-domain-to-vortex transitions in magnetic nanodisks

The switching behavior of permalloy (Ni81Fe19) disks with dimensions close to the experimentally determined phase boundary separating the single domain and vortex ground states was investigated. The disks fabricated with electron beam lithography and thermal evaporation were 97 nm wide and 26 nm thick. The remanence curve, measured with a magneto-optical Kerr magnetometer, shows the presence of three different remanence states in different magnetic field regimes; vortex state at fields below 110 Oe, metastable state at fields 110Oe<H<520Oe, and single domain state at fields above 520 Oe. High sensitivity magneto-optical measurements combined with an advanced applied field sequence allowed a cycle-by-cycle observation of the collapse of the single domain state into the vortex ground state via a partially remanent metastable state.

Combined electrical and magneto-optical measurements of the magnetization reversal process at a domain wall trap.

We have performed combined electrical and magneto-optical Kerr effect measurements on Permalloy nanowires containing artificial symmetric protrusions. This has enabled us to construct a detailed picture of the energy landscape of such a trap, in excellent agreement with predictions based on recent results. In addition with the aid of micromagnetic simulations, we demonstrate how variations in the observed resistance with respect to the applied field can give us insight into the entire depinning and nucleation processes at domain wall traps.

Magnetisation reversal in permalloy nanowires controlled by near-field charge interactions

This study investigates the interaction between transverse head-to-head domain walls (THHDWs) in permalloy nanowires and magnetostatic charges located within a tripod structure in close proximity. This interaction enables remote pinning of the THHDW. Eight different interactions are measurable, depending on the initialising field sequence. The pinning strengths of an attractive potential well and a repulsive barrier are compared in this paper. Based on experimental measurements, micromagnetic simulations, and a monopolar analytical model, we demonstrate that pinning strength depends on the inhomogeneous charge distribution within the tripod and the relative orientation of the THHDW which faces the tripod.