V. Metlushko

Nanocomputing by field-coupled nanomagnets

Demonstrates through simulations the feasibility of using magnetically coupled nanometer-scale ferromagnetic dots for digital information processing. Microelectronic circuits provide the input and output of the magnetic nanostructure, but the signal is processed via magnetic dot-dot interactions. Logic functions can be defined by the proper placements of dots. We introduce a SPICE macromodel of interacting nanomagnets and use this tool to design and simulate the proposed nanomagnet logic units. This SPICE model allows us to simulate such magnetic information processing devices within the same framework as conventional electronic circuits.

On-chip manipulation of protein-coated magnetic beads via domain-wall conduits.

For this reasonmanipulationatthenanoscaleofsurfacefunctionalizedmagneticbeads in suspension is of paramount importance in biotechnol-ogy, nanochemistry, and nanomedicine as it leads to a precisecontrol of the tagged biological entity.In the past few years many approaches have been developedboth for the manipulation and transport of a massive particlepopulation or of a single particle, e.g., microfabricated current-carrying wires,

Magnetic QCA systems

The field-coupled QCA architecture has emerged as a candidate for providing local interconnectivity for nanodevices, and offers the possibility to perform very dense, high speed, and low power computing in an altogether new paradigm. Magnetic interactions between nanomagnets are sufficiently strong to allow room-temperature operation. We are investigating the fabrication and testing of arrays of nanomagnets for this purpose, and have found that by tailoring their shapes, strong coupling can be observed. This paper will present recent work of the Notre Dame group on magnetically coupled QCA.

Domain wall displacement in Py square ring for single nanometric magnetic bead detection

physical properties of a domain wall DW localized at a geometric corner. The concept illustrated in the present paper relies on a previous experimental work made on square rings of Permalloy Py for application in magnetic storage of information. 11 In this design head-to-head and tail-to-tail DWs having a transverse structure Neel type 11 can be positioned at a given corner, and their position can be read electrically thanks to the AMR effect: when a DW is present between two sensing leads a reduction in the resistance is observed since some of the magnetization of the DW points perpendicularly to the current flow. Otherwise, when there is no DW present between the two sensing leads, the magnetization follows the direction of the perimeter of the ring and the resistance is higher. In this work we adapted this device to demonstrate a detection concept suitable for the detection of magnetic nanobeads. Panel a of Fig. 1 shows the scanning electron microscopy image of the structure used in the present experiment. The 30 nm thick Py square ring structures have been lithographically patterned on top of 20 nm thick and 100 nm wide Au contacts, previously fabricated on a SiO2 /Si substrate. The outside size of the rings is 1.0 1.0 m 2 , the width of each segment is about 180 nm, and the slit is about 80 nm wide. For the magnetoresistance measurements presented here, the voltage drop was measured

Shape effect on magnetization reversal in chains of interacting ferromagnetic elements

The magnetization reversal in the chains of submicron square- and disk-shaped Permalloy dots with lateral size of 800 nm, thickness of 50 nm and variable inter dot distance was investigated by using the magneto-optical Kerr effect technique, magnetic force microscopy and micromagnetic modeling. We have found that the particle shape strongly affects the characteristic switching fields of well-separated dots, and has almost no influence on strength of inter dot interaction in chains of magnetostatically coupled elements.

Nanosized corners for trapping and detecting magnetic nanoparticles

We present a device concept based on controlled micromagnetic configurations in a corner-shaped permalloy nanostructure terminated with two circular disks, specifically designed for the capture and detection of a small number of magnetic beads in suspension. A transverse head-to-head domain wall (TDW) placed at the corner of the structure plays the role of an attracting pole for magnetic beads. The TDW is annihilated in the terminating disks by applying an appropriate magnetic field, whose value is affected by the presence of beads chemically bound to the surface. In the case where the beads are not chemically bound to the surface, the annihilation of the TDW causes their release into the suspension. The variation of the voltage drop across the corner, due to the anisotropic magnetoresistance (AMR) while sweeping the magnetic field, is used to detect the presence of a chemically bound bead. The device response has been characterized by using both synthetic antiferromagnetic nanoparticles (disks of 70 nm diameter and 20 nm height) and magnetic nanobeads, for different thicknesses of the protective capping layer. We demonstrate the detection down to a single nanoparticle, therefore the device holds the potential for the localization and detection of small numbers of molecules immobilized on the particles functionalized surface.

Spin waves in circular soft magnetic dots at the crossover between vortex and single domain state

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

Magnetoresistance of single magnetic vortices

The magnetoresistance in a 1μm Permalloy disk, that develops a vortex state during reversal, has been experimentally measured and modeled. The agreement between measurements and numerical simulations shows that the conventional anisotropic magnetoresistance effect is the main source of magnetoresistance. The results demonstrate that magnetoresistance can be used to determine the chirality of the vortex thereby improving the chances that patterned dot arrays could be used in data storage technology.

Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by magnetic force microscopy imaging, the magnetic state of both systems can be adjusted to produce arrays of almost pointlike magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role in the transport properties of this hybrid system.

Magnetization reversal and configurational anisotropy of dense permalloy dot arrays

Electron beam patterned permalloy circular dots of 700 nm diameter with small separations were studied by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. Images in the demagnetized state show that the dot is in a vortex state with a vortex core (singularity) in the center. Local hysteresis loops, measured by cantilever frequency shift in an external field, indicate that the magnetization reversal of individual disks is a vortex nucleation and annihilation process. By carefully doing MFM, nucleation and annihilation fields without MFM tip stray field distortions are obtained. Configurational anisotropy originated from magnetostatic coupling is found through hysteresis loops.