A. K. Gangopadhyay

Self-organized bimetallic Ag-Co nanoparticles with tunable localized surface plasmons showing high environmental stability and sensitivity.

We demonstrate a promising synthesis route based on pulsed laser dewetting of bilayer films (Ag and Co) to make bimetallic nanoparticle arrays. By combining experiment and theory we establish a parameter space for the independent control of composition and diameter for the bimetallic nanoparticles. As a result, physical properties, such as the localized surface plasmon resonance (LSPR), that depend on particle size and composition can be readily tuned over a wavelength range one order of magnitude greater than for pure Ag nanoparticles. The LSPR detection sensitivity of the bimetallic nanoparticles with narrow size distribution was found to be high-comparable with pure Ag (∼60 nm/RIU). Moreover, they showed significantly higher long-term environmental stability over pure Ag.

Ferroplasmons: Intense Localized Surface Plasmons in Metal-Ferromagnetic Nanoparticles

Interaction of photons with matter at length scales far below their wavelengths has given rise to many novel phenomena, including localized surface plasmon resonance (LSPR). However, LSPR with narrow bandwidth (BW) is observed only in a select few noble metals, and ferromagnets are not among them. Here, we report the discovery of LSPR in ferromagnetic Co and CoFe alloy (8% Fe) in contact with Ag in the form of bimetallic nanoparticles prepared by pulsed laser dewetting. These plasmons in metal-ferromagnetic nanostructures, or ferroplasmons (FP) for short, are in the visible spectrum with comparable intensity and BW to those of the LSPRs from the Ag regions. This finding was enabled by electron energy-loss mapping across individual nanoparticles in a monochromated scanning transmission electron microscope. The appearance of the FP is likely due to plasmonic interaction between the contacting Ag and Co nanoparticles. Since there is no previous evidence for materials that simultaneously show ferromagnetism and such intense LSPRs, this discovery may lead to the design of improved plasmonic materials and applications. It also demonstrates that materials with interesting plasmonic properties can be synthesized using bimetallic nanostructures in contact with each other.

Unusual size-dependent magnetization in near hemispherical Co nanomagnets on SiO2 from fast pulsed laser processing

Nanosecond pulsed laser melting of ultrathin metal films can lead to self-organized arrays of spherical nanoparticles. We have applied this technique to assemble arrays of nanoparticles of the soft elemental ferromagnet Co on SiO2. Surface morphology studies by using scanning electron microscopy and atomic force microscopy established that the nanoparticles were nearly hemispherical with an average contact angle of ∼104±22°. Magnetic properties of these nanoparticles in the size range of 30–250nm diameter were investigated by magnetic force microscopy under zero applied field in conjunction with simulations of the magnetic tip-particle interaction. Particles up to 180nm diameter were found to be single domain with the magnetization direction oriented predominantly in-plane for the smaller particles (⩽75nm) and out-of-plane for the larger particles (⩽180nm). Multidomain behavior was observed for particles larger than 180nm. Magnetic hysteresis measurements at room temperature confirmed that the arrays cons...

Heterogeneous nucleation of amorphous alloys on catalytic nanoparticles to produce 2D patterned nanocrystal arrays

Templates are widely used to produce artificial nanostructures. Here, laser-assisted self-organization has been used to form one-xa0and two-dimensional (D) nanoarrays of Cu nanocrystals. Using these nanoarrays as a template, a 2D patterned ferromagnetic nanostructure of FeCrSi nanocrystals has been produced by heterogeneous nucleation and growth of nanocrystals by partial devitrification from an amorphous Fe64.5Cr10Si13.5B9Nb3 alloy with the Cu nanoparticles acting as catalytic nucleation sites. The interaction among the ferromagnetic nanocrystals via the residual amorphous matrix can be controlled by suitable choice of the amorphous alloy composition. Although demonstrated for a ferromagnetic system, the processing method may have much wider applicability for producing artificial nanostructures of a wide variety of materials when materials-specific catalysts and amorphous alloy compositions are judiciously chosen.

Demagnetization-borne microscale skyrmions

Magnetic systems are an exciting realm of study that is being explored on smaller and smaller scales. One extremely interesting magnetic state that has gained momentum in recent years is the skyrmionic state. It is characterized by a vortex where the edge magnetic moments point opposite to the core. Although skyrmions have many possible realizations, in practice, creating them in a laboratory is a difficult task to accomplish. In this work, different methods for skyrmion generation and customization are suggested. Skyrmionic behavior was numerically observed in minimally customized simulations of spheres, hemisphere, ellipsoids, and hemiellipsoids, for typical Cobalt parameters, in a range approximately

Fe nanomagnets with unusual size-dependent magnetization directions produced by fast laser-induced self-organization

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Magnetic measurement of pulsed laser-induced nanomagnetic arrays using Surface Magneto-Optic Kerr Effect

in diameter simply by applying a field.

Link between liquid structure and the nucleation barrier for icosahedral quasicrystal, polytetrahedral, and simple crystalline phases in Ti-Zr-Ni alloys : Verification of Frank's hypothesis

Laser-induced melting of ultrathin films can lead to self-organized arrays of hemispherical particles. We have applied this procedure to assemble arrays of Fe nanomagnets on SiO2 substrates. Morphological studies showed presence of spatial short range order (SRO) in the array. Magnetic properties were studied at room temperature using zero-field magnetic force microscopy (MFM). The particles upto 55 nm in diameter showed in-plane (≤ 45°), compared to out-of-plane magnetization directions (≥ 45°) for the larger particles. The size-dependent orientation of magnetization for these hemispherical particles, was attributed to the dominating magnetostrictive energy and a size-dependent residual strain.

Nanoparticle ordering by dewetting of Co on SiO2

Efficient and dependable characterization methods of magnetic-plasmonic nanostructures are essential towards the implementation of new nanoscale materials in magneto-optical applications. Surface magneto-optic Kerr effect (SMOKE) is a powerful characterization technique, because of its simplicity and high sensitive to even monolayer thick magnetic materials. It relies on the measurement of polarization and absorption changes of reflected light in the presence of a magnetic field. While SMOKE has been applied in the past to investigate the magnetic information of continuous films, there is little work on applying it to characterize arrays of nanoparticles with variable magnetic and optical properties. Here, we have used it to investigate the magnetic behavior of nanoparticle arrays made by nanosecond pulsed laser self-organization. This technique produces an array of single-domain magnetic nanoparticles with size-dependent magnetic orientation. Nanoparticle arrays of Co and Ni were prepared on SiO2 substrates. SMOKE measurements were performed for a variety of different particle sizes and material. Systematic differences in saturation and coercivity were observed for the different samples. These results demonstrated that SMOKE is a reliable technique to rapidly characterize the magnetic behavior of nanoparticle arrays.