Alexandra Imre

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.

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.

Enhanced Raman scattering from focused surface plasmons

Surface plasmon polaritons launched at concentric arcs can be focused into a subwavelength wide focal spot of high near-field light intensity. The focused plasmons give rise to enhanced Raman scattering from R6G molecules placed in the focal area. By exploiting the polarization dependence of the focusing the authors establish an enhancement of the Raman signal by a factor of ∼6. The results show that focusing of propagating surface plasmons on flat metal surfaces may be an alternative to localized plasmons on metal nanostructures for achieving enhanced Raman scattering. In particular, a flat metal substrate enables better control over the local electric fields and the placement of analyte molecules, and, therefore, ultimately better fidelity of Raman spectra.

Nanoscale piezoresponse studies of ferroelectric domains in epitaxial BiFeO3 nanostructures

We report the dependence of the ferroelectric domain configuration and switching behavior on the shape (square versus round) of epitaxial BiFeO3 (BFO) nanostructures. We fabricated (001) oriented BFO(120 nm)/SrRuO3(SRO,125 nm) film layers on (001) SrTiO3 single crystals by rf magnetron sputter deposition, and patterned them to square (500×500 nm2) and round (502 nm in diameter) shaped nanostructures by focused ion-beam lithography. The surface morphology and the crystalline structure of the nanostructures were characterized by scanning electron microscopy and x-ray diffraction, respectively, while the domain configuration was investigated using piezoelectric force microscopy. We found that the square-shaped nanostructures exhibit a single variant domain configuration aligned along the [1¯11¯] direction, whereas the round-shaped nanostructures exhibit seven variants of domain configuration along the [1¯11¯], [11¯1¯], [111¯], [111], [1¯11], [11¯1], and [1¯1¯1] directions. Moreover, local d33 piezoelectric c...

Effects of cantilever buckling on vector piezoresponse force microscopy imaging of ferroelectric domains in BiFeO3 nanostructures

Systematic studies are presented on the effects of cantilever buckling in vector piezoresponse force microscopy (V-PFM) imaging of polarization domains in thin-film based (001)-oriented BiFeO3 nanostructures, as observed through the coupling of out-of-plane and in-plane PFM images. This effect is a strong function of the laser spot position on the cantilever, being strongest at the free end, and insignificant at 60% of the cantilever length from the pivot point. This finding provides a unique approach to V-PFM imaging of ferroelectric polarization domains, yielding three dimensional PFM images without sample rotation in the plane.

SERS enhancements via periodic arrays of gold nanoparticles on silver film structures.

We discuss surface enhanced Raman spectroscopy (SERS) structures aimed at providing robust and reproducible enhancements. The structures involve periodic arrays of gold nanospheres near silver film structures that may also be patterned. They enable one to excite Bloch wave surface plasmon polaritons (SPPs) that can also couple to local surface plasmons (LSPs) of the nanospheres, leading to the possibility of multiplicative enhancements. If the magnitude of the average electric field, /E/, between the particles is enhanced by g such that /E/ = g/E(0)/, /E(0)/ being the incident field, realistic finite-difference time-domain simulations show that under favorable circumstances g approximately equal 0.6 g(SPP) g(LSP), where g(SPP) and g(LSP) are enhancement factors associated with the individual components. SERS enhancements for the structures can be as high as O(g(4)) = 10(8).

Fabrication of high density, high aspect-ratio polyimide nanofilters.

A novel fabrication process produces high porosity polymer nanofilters with smooth, uniform, and straight pores with high aspect ratios. The process utilizes electron beam lithography and energetic neutral atom beam lithography and epitaxy techniques. The method has the potential to produce a new generation of high-precision, very-high-porosity, biocompatible filters with pore sizes down to 100nm.

Matching effect and dynamic phases of vortex matter in Bi2Sr2CaCu2O8 nanoribbon with a periodic array of holes

We report investigations on the dynamics of vortex matter with periodic pinning in crystalline Bi2Sr2CaCu2O8 nanoribbons containing an array of nanoscale holes. We found that the matching effect is enhanced near the melting field and persists to higher fields beyond the melting line. We attribute this enhancement to the existence of a soft-solid phase and a mixture of solid-liquid phases near the melting line, enabling the vortices to pin more effectively. We observed distinct regions in the voltage-current curves attributed to transitions of various dynamic phases which also account for the driving current dependent appearance of the matching effect.

Magnetic Logic Devices Based on Field-Coupled Nanomagnets

Nanomagnets that exhibit two distinct stable states of magnetization can be used to store digital bits. This phenomenon is already applied in today’s magnetic random access memories (MRAM). In addition, interacting networks of such nanomagnets, with physical spacing on the order of 10 nm between them, have been proposed to propagate and process binary information by means of magnetic coupling. The application of field-coupled nanomagnets for digital logic circuits was described in a concept called magnetic quantumdot cellular automata (MQCA) [1][2]. MQCA offer very low power dissipation and high integration density of functional devices. In addition, it can operate over a wide temperature range from near absolute zero to the Curie temperature of the employed ferromagnetic material. We introduce a logic gate similar to that proposed by Parish and Forshaw [3], which performs majority-logic operation. The nanomagnets are arranged in a cross-geometry as shown in Fig. 1, where the dipole coupling between the nanomagnets produces ferromagnetic and antiferromagnetic ordering of the magnetic states. Consider that the magnetic state of nanomagnets A, B, and C can be set by some inputs, and that a horizontal external magnetic field, called the clock-field, can allow the system to relax to its ground state. Then, majority logic operation can be performed by the central nanomagnet M, and the result can be transferred to another nanomagnet labeled as “out”. The cross-geometry can be extended, and inputs can be provided by adding nanomagnets that are oriented along the clock-field. Varying the position of these horizontally elongated nanomagnets, all eight input combinations in the majority-logic truth table can be tested. Figure 1. (a) An elongated polycrystalline NiFe alloy nanomagnet exhibits two stable magnetic states in the direction of its longest axis. (b) Majority gate geometry built up from nanomagnets. We demonstrate room temperature operation of majority gates made of NiFe alloy and fabricated by electron-beam lithography on silicon. Dipolar ordering in the nanomagnetnetworks is imaged by magnetic force microscopy (MFM), and the operation is explained by means of micromagnetic simulations. Figure 2 introduces a particular majority gate, in which the magnetic state of A is set to be the opposite to that of B and C. In this gate, A, B, and C all exert torque on the central dot’s magnetic moment in the same direction. Simulations show that, as the clock-field is reduced, the switching of the nanomagnets inside the gate begins at the input dots. The central dot switches after A, B, and C, and the switching propagates along the antiferromagnetically-coupled chain to the right. The figure demonstrates the final states after two independent experiments, in which the clock-field was applied in opposite directions. The MFM data shows the correct alignment of the magnetic moments in both cases.

Multiplexing surface plasmon polaritons on nanowires

The authors demonstrate a plasmonic device that generates and steers tightly focused plasmon beams between neighboring subwavelength metal-strip waveguides. By introducing a controlled phase shift into the plasmon condenser, they shifted the focused plasmon spot by microns with nanometer accuracy and realized the multiplexer functionality.