T. Liew

Subgroup decomposition of plasmonic resonances in hybrid oligomers: modeling the resonance lineshape.

Plasmonic resonances with a Fano lineshape observed in metallic nanoclusters often arise from the destructive interference between a dark, subradiant mode and a bright, super-radiant one. A flexible control over the Fano profile characterized by its linewidth and spectral contrast is crucial for many potential applications such as slowing light and biosensing. In this work, we show how one can easily but significantly tailor the overall spectral profile in plasmonic nanocluster systems, for example, quadrumers and pentamers, by selectively altering the particle shape without a need to change the particle size, interparticle distance, or the number of elements of the oligomers. This is achieved through decomposing the whole spectrum into two separate contributions from subgroups, which are efficiently excited at their spectral peak positions. We further show that different strengths of interference between the two subgroups must be considered for a full understanding of the resulting spectral lineshape. In some cases, each subgroup is separately active in distinct frequency windows with only small overlap, leading to a simple convolution of the subspectra. Variation in particle shape of either subgroup results in the tuning of the overall spectral lineshape, which opens a novel pathway for shaping the plasmonic response in small nanoclusters.

Spin transfer switching enhancement in perpendicular anisotropy magnetic tunnel junctions with a canted in-plane spin polarizer

We have studied spin transfer switching (STS) in a magnetic tunnel junction with perpendicular magnetic anisotropy for the reference and free layers using the Landau–Lifshitz–Gilbert formalism. We propose a multilayer structure in which the insertion of an additional spin polarizer with in-plane anisotropy can enhance the STS efficiency and switching speed of the device. It is revealed that a canted spin polarizer with an angle between 40° and 80° out of the film plane in the correct direction enhances the STS efficiency more than a fixed in-plane or perpendicular polarizer. Furthermore, we show that the spin transfer torque exerted on the in-plane polarizer layer by the free layer automatically tilts the in-plane polarizer in the direction that enhances STS for both magnetization states of the free layer.

Reduction in critical current for spin transfer switching in perpendicular anisotropy spin valves using an in-plane spin polarizer

We describe a strategy to reduce spin transfer switching (STS) currents in CoFe/Pd-based perpendicular anisotropy single spin valves (SSVs) by the insertion of an in-plane spin polarizer, thus creating a modified-dual spin valve (m-DSV). For SSV devices, concurrent STS of both magnetic layers was observed for positive currents, making the parallel-to-antiparallel (P→AP) transition impossible. In m-DSV devices, we observed a 60% reduction in the energy barrier for AP→P transitions and a 40% reduction in JcAP→P with 10 ns STS current pulses compared to SSV devices. Furthermore, the m-DSV structure enabled the soft layer to switch independently from the hard layer via STS.

Room temperature synthesis of c-AlN thin films by nitrogen-ion-assisted pulsed laser deposition

Cubic aluminum nitride (c-AlN) thin films have been deposited at room temperature on silicon substrates by nitrogen-ion-assisted pulsed laser ablation of a hexagonal AlN target. The deposited thin films exhibit good crystal properties with sharp x-ray diffraction peaks. The influences of the nitrogen ion energy on the morphological, compositional, and electronic properties of the AlN thin films have been studied. The nitrogen ions can effectively promote the formation of Al–N bonds and improve the crystal properties of the deposited thin films. A nitrogen ion energy of 400 eV is proposed to deposit high quality c-AlN thin films.

Frequency-tunable perpendicular spin torque oscillator

We present an experimental study on the frequency tunability of a perpendicular spin transfer torque oscillator with a perpendicularly magnetized (Co/Pd)5 reference electrode and a Co/(Pd/Co)4 composite oscillating layer of a giant magnetoresistive stack. A current-perpendicular-to-plane magnetoresistance ratio of 2.55% is obtained for the stack. Spin transfer torque induced steady oscillation with a narrow line-width has been obtained with direct current injection. As the injected dc current level is varied from 4.5 to 8.5 mA under a small constant applied field, the oscillation frequency increases from 5 to 6.2 GHz and back to 5.8 GHz. The oscillation with the narrowest linewidth occurs at the injection current magnitude at which the highest oscillation amplitude is also obtained. Micromagnetic modeling suggests that a nanomagnet above its critical single domain size is subjected to higher order spin wave generation at large current density. The spatial inhomogenity created across the nanomagnet can acc...

Multistate storage in dual spin valves with perpendicular magnetic anisotropy

We report the tunable separation of four distinct resistance levels in dual spin valves (DSV) with perpendicular magnetic anisotropy based on Co/Pd and CoFe/Pd multilayers. An optimal giant magnetoresistance (GMR) of 15.2% in the current in-plane geometry was obtained. By varying the spin filter layer thicknesses at the interfaces of one Cu spacer layer from 2 to 6 A, a linear dependence of GMR across the selected spacer layer was demonstrated without affecting the GMR contribution across the second Cu spacer layer in the DSV. Using this strategy, the intermediate resistance levels in a four-state perpendicular DSV can be adjusted independently, thus creating flexible platform for multistate storage.

Current driven oscillation and switching in Co/Pd perpendicular giant magnetoresistance multilayer

Spin torque transfer (STT) induced magnetization oscillation and switching in metallic spin valves with Co/Pd electrodes of perpendicular magnetic anisotropy are demonstrated. The bottom Co/Pd multilayer, acting as a perpendicular spin-polarizing/reference layer, is relatively thick with a strong perpendicular anisotropy and a perpendicular switching field of 8 kOe. An in-plane spin valve is placed on the top for reading back magnetization oscillation of the middle Co layer, whose thickness is varied from 6 to 30 A. When the middle Co layer is thin, current driven magnetization switching is observed. When the middle Co layer is relatively thick, perpendicular spin torque oscillation is clearly observed with oscillating frequency at 4 GHz. STT-included micromagnetic modeling has been performed which predicts the exact observed behavior and illustrates the significance of magnetization configuration of the Co layer on determining STT-induced dynamics.

Formation and Tribology Behavior of NiP Surface Textured by Argon Ion Laser

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