Magnetization Reversal in Ferromagnetic Films Patterned with Antiferromagnetic Gratings of Various Sizes

The magnetic switching behavior in continuous NiFe films patterned with IrMn gratings is investigated experimentally and with micromagnetic simulations. The samples made by a two-step deposition process consist of a 10-nm-thick NiFe layer on which is placed 10-nm-thick IrMn stripes with width from 100 to 500 nm and period from 240 nm to 1  μm. Exchange bias is introduced by field cooling in directions parallel or perpendicular to the IrMn stripes. The samples display a two-step hysteresis loop for higher stripe width and period, as the pinned and unpinned regions of the NiFe reverse independently but a one-step loop for lower stripe periods. The transition between these regimes is reproduced by micromagnetic modeling.National Science Foundation (U.S.)Semiconductor Research Corporation. Interconnect Focus Cente

Plexcitons: Dirac points and topological modes

Plexcitons are polaritonic modes that result from the strong coupling between excitons and plasmons. We consider plexcitons emerging from the interaction of excitons in an organic molecular layer with surface plasmons in a metallic film. We predict the emergence of Dirac cones in the two-dimensional bandstructure of plexcitons due to the inherent alignment of the excitonic transitions in the organic layer. These Dirac cones may open up in energy by simultaneously interfacing the metal with a magneto-optical layer and subjecting the whole system to a perpendicular magnetic field. The resulting energy gap becomes populated with topologically protected one-way modes which travel at the interface of this plexcitonic system. Our theoretical proposal suggests that plexcitons are a convenient and simple platform for the exploration of exotic phases of matter as well as of novel ways to direct energy flow at the nanoscale

Perpendicular Block Copolymer Microdomains in High Aspect Ratio Templates

Perpendicular orientation of lamellar microdomains in a high interaction parameter block copolymer was obtained within high aspect ratio gratings functionalized with a preferential sidewall brush. The experiments used polystyrene-block-polydimethylsiloxane (PS-b-PDMS) with molecular weight 43 kg/mol within trenches made using interference lithography. The perpendicular alignment was obtained for both thermal and solvent annealing, using three different solvent vapors, for a range of film thicknesses and trench widths. A platinum (Pt) layer at the base of the trenches avoided the formation of a wetting layer, giving perpendicular orientation at the substrate surface. The results are interpreted using self-consistent field theory simulation and a Ginzburg–Landau analytic model to map the energies of lamellae of different orientations as a function of the grating aspect ratio and the surface energies of the sidewalls and top and bottom surfaces. The model results agree with the experiment and provide a set of guidelines for obtaining perpendicular microdomains within topographic features

Monolithic on-chip nonreciprocal photonics based on magneto-optical thin films

Monolithic integration of nonreciprocal optical devices on semiconductor substrates has been a long-sought goal of the photonics community. One promising route to achieve this goal is to deposit high quality magneto-optical (MO) oxide thin films directly on a semiconductor substrate. In this article, we will review our ongoing progress in material development and device engineering towards enabling a monolithically integrated, high-performance magneto-optical nonreciprocal photonics platform. In particular, we will discuss our recent work which has led to a new pulsed laser deposition (PLD) technique of Ce or Bi substituted yttrium iron garnet (YIG) thin films with reduced thermal budget, simplified growth protocols and improved magneto-optical characteristics. These materials were incorporated in monolithic resonator and interferometer based isolator devices to demonstrate on-chip optical isolation with improved device figure of merit. Challenges and opportunities for monolithic magneto-optical devices will be discussed in the context of our latest material and device performance metrics

Current-induced domain wall motion in compensated ferrimagnet

Due to the difficulty in detecting and manipulating magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. In this work, by employing heavy metal/rare earth-transition metal alloy bilayers, we experimentally studied current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum close to the angular momentum compensation. With experiment and modelling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy metal/ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.Comment: 13 pages, 3 figure

Large-area zinc oxide nanorod arrays templated by nanoimprint lithography: control of morphologies and optical properties

Vertically aligned, highly ordered, large area arrays of nanostructures are important building blocks for multifunctional devices. Here, ZnO nanorod arrays are selectively synthesized on Si substrates by a solution method within patterns created by nanoimprint lithography. The growth modes of two dimensional nucleation-driven wedding cakes and screw dislocation-driven spirals are inferred to determine the top end morphologies of the nanorods. Sub-bandgap photoluminescence of the nanorods is greatly enhanced by the manipulation of the hydrogen donors via a post-growth thermal treatment. Lasing behavior is facilitated in the nanorods with faceted top ends formed from wedding cakes growth mode. This work demonstrates the control of morphologies of oxide nanostructures in a large scale and the optimization of the optical performance

Oersted Field and Spin Current Effects on Magnetic Domains in [Co/Pd]

An out-of-plane Oersted field produced from a current-carrying Au wire is used to induce local domain formation in wires made from [Co/Pd][subscript 15] multilayers with perpendicular anisotropy. A 100 ns pulsed current of 56-110 mA injected into the Au wire created a reverse domain size of 120-290 nm in a Co/Pd nanowire on one side of the Au wire. A Biot-Savart model was used to estimate the position dependence of the Oersted field around the Au wire. The shape, size, and location of the reversed region of Co/Pd were consistent with the magnitude of the Oersted field and the switching field distribution of the unpatterned film. A current density of 6.2 × 10[superscript 11] Am[superscript -2] in the Co/Pd nanowire did not translate the domain walls due to low spin transfer efficiency, but the Joule heating promoted domain growth in a field below the coercive field.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR1419807

Monolithic integration of broadband optical isolators for polarization-diverse silicon photonics

Integrated optical isolators have been a longstanding challenge for photonic integrated circuits (PIC). An ideal integrated optical isolator for PIC should be made by a monolithic process, have a small footprint, exhibit broadband and polarization-diverse operation, and be compatible with multiple materials platforms. Despite significant progress, the optical isolators reported so far do not meet all these requirements. In this article we present monolithically integrated broadband magneto-optical isolators on silicon and silicon nitride (SiN) platforms operating for both TE and TM modes with record high performances, fulfilling all the essential characteristics for PIC applications. In particular, we demonstrate fully-TE broadband isolators by depositing high quality magneto-optical garnet thin films on the sidewalls of Si and SiN waveguides, a critical result for applications in TE-polarized on-chip lasers and amplifiers. This work demonstrates monolithic integration of high performance optical isolators on chip for polarization-diverse silicon photonic systems, enabling new pathways to impart nonreciprocal photonic functionality to a variety of integrated photonic devices