Magnetic biosensors: Modelling and simulation.

The current-induced domain wall motion along a thin cobalt ferromagnetic strip sandwiched in a multilayer (Pt/Co/AlO) is theoretically studied with emphasis on the roles of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction. The results point out that these ingredients, originated from the spin-orbit coupling, are consistent with recent experimental observations in three different scenarios. With the aim of clarifying which is the most plausible the influence of in-plane longitudinal and transversal fields is evaluated.

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Current-driven domain wall motion along high perpendicular anisotropy multilayers: The role of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics.

/pdf/the-non-random-walk-of-chiral-magnetic-charge-carriers-in-2rys14qxl0.pdf

The non-random walk of chiral magnetic charge carriers in artificial spin ice

Magnetic monopoles have eluded experimental detection since their prediction nearly a century ago by Dirac. Recently it has been shown that classical analogues of these enigmatic particles occur as excitations out of the topological ground state of a model magnetic system, dipolar spin ice. These quasi-particle excitations do not lead to a modification of Maxwell's equations, but they do interact via Coulombs law and they are of magnetic origin. In this paper we present an experimentally measurable signature of monopole dynamics and show that magnetic relaxation measurements in spin ice materials can be interpreted entirely in terms of their diffusive motion on a diamond lattice in the grand canonical ensemble. The monopole trajectories are constrained to lie on a network of Dirac strings filling the quasi-particle vacuum. We find quantitative agreement between the time scales for relaxation in the Dirac string network and the magnetic relaxation data for the spin ice material $Dy_{2}Ti_{2}O_{7}$ . In the presence of a magnetic field the topology of the network prevents charge flow in the steady state, but transient monopole currents do occur, as well as monopole density gradients near the surface of an open system.

/pdf/monopole-and-dirac-string-dynamics-in-spin-ice-55i3vga575.pdf

Monopole and Dirac string Dynamics in Spin Ice

The domain wall-related change in the anisotropic magnetoresistance in L-shaped permalloy nanowires is measured as a function of the magnitude and orientation of the applied magnetic field. The magnetoresistance curves, compiled into so-called domain wall magnetoresistance state space maps, are used to identify highly reproducible transitions between domain states. Magnetic force microscopy and micromagnetic modelling are correlated with the transport measurements of the devices in order to identify different magnetization states. Analysis allows to determine the optimal working parameters for specific devices, such as the minimal field required to switch magnetization or the most appropriate angle for maximal separation of the pinning/depinning fields. Moreover, the complete state space maps can be used to predict evolution of nanodevices in magnetic field without a need of additional electrical measurements and for repayable initialization of magnetic sensors into a well-specified state.

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Anisotropic Magnetoresistance State Space of Permalloy Nanowires with Domain Wall Pinning Geometry

The nucleation field of perpendicularly magnetized nanowires can be controlled by changing their width, so that below a critical width the nucleation field decreases as the width decreases. Placing pads at the ends of the nanowires prevents any reduction in coercivity with width, demonstrating that at small widths domain walls nucleate from the ends of the wires. Using this technique, we are able to create asymmetric nanowires with controlled nucleation at a defined point. We also show how dipole fields from a neighboring wire in close proximity can be used to shift the hysteresis loop of the asymmetric nanowire, creating a simple NOT gate. These results show how control of the in-plane shape of perpendicularly magnetized nanoscale elements can directly lead to device functionality.

Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape

Simultaneous anisotropic magnetoresistance and magneto-optical Kerr effect measurements have been performed on L-shaped Permalloy nanowires. It is shown that through magnetoresistance measurements at the corner of the device, the switching information of both arms in a single loop can be accessed. This is a very efficient method that allows for the characterization of the pinning properties in such a device as well as the understanding of the fundamental physics behind the nucleation and propagation processes in such a system. Experimental results are in good agreement with micromagnetic simulations.

Simultaneous magnetoresistance and magneto-optical measurements of domain wall properties in nanodevices

Supporting data for the paper 'Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape' published in APL in 2015 (http://dx.doi.org/10.1063/1.4930152). Each file is labelled by figure number.

Research data supporting "Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape"

Using out-of-plane magnetized layers, a lateral shift register made from discrete elements is demonstrated. By carefully designing the in-plane shape of the elements which make up the shift register, both the position of nucleation of new domains and the coercivity of the element can be controlled. The dipole field from a neighboring element, placed tens of nanometers away, creates a bias field on the nucleation site, which can be used to create a NOT gate. By chaining these NOT gates together, a shift register can be created where data bits consisting of neighboring layers with aligned magnetization are propagated synchronously under a symmetric applied magnetic field. The operation of a 16 element shift register is shown, including field coupled data injection.

https://iopscience.iop.org/article/10.1088/1361-6528/aa7f63/pdf

A magnetic shift register with out-of-plane magnetized layers

Considerable difficulties exist in generating appreciable magnetic fields, localized on nanometer length scales for future experiments and technologies. Here we experimentally demonstrate selective reversal of a ferromagnetic nanowire by the stray field from a domain wall. The use of a domain wall as a persistent, mobile source of magnetic field is an alternative to localized Oersted fields and current induced switching, with possible use in future domain wall based data storage schemes and magnetic random access memory applications.

/pdf/magnetic-domain-wall-induced-localized-nanowire-reversal-1zgcgditbs.pdf

A. Beguivin

Papers

Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape

Simultaneous magnetoresistance and magneto-optical measurements of domain wall properties in nanodevices

Research data supporting "Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape"

A magnetic shift register with out-of-plane magnetized layers

Magnetic domain wall induced, localized nanowire reversal