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Resonant optical cavities are essential components in mid-infrared applications. However, typical film-type cavities require multilayer stacks with a micron-thick spacer due to mid-infrared wavelengths, and their performance is limited by narrow frequency tunability and angular sensitivity. We propose and experimentally demonstrate the subwavelength-scale (≈λ0/150) resonant nanocavity arrays that enhance the absorption spectrum of the device in the mid-infrared (10–12 microns) via excitation of coupled surface plasmon–phonon polaritons. The proposed metal–insulator–polar dielectric (gold–silicon–silicon carbide) structure supports a guided mode of the coupled surface polaritons in the lateral direction while vertically confining the mid-infrared wave within the 80 nm thick dielectric spacer. In particular, the subwavelength-scale (≈λ0/10) gratings are imposed to form Fabry–Pérot cavity arrays displaying angle-insensitive and frequency-tunable absorption of up to 80% of the optical power in the mid-infrared. Our work should benefit diverse mid-infrared applications and novel designs of polariton-based photonic devices.

A high-throughput top-down nanomanufacturing approach for making metal–polymer nanoparticles with tunable magnetic properties using nanoimprint lithography is reported. The nanoparticles comprise of a layered structure that includes a Co/Ru/Co synthetic antiferromagnet (SAF) and a polymethyl methacrylate (PMMA) or a hydrogen silsesquioxane (HSQ) layer. The Co/Ru/Co structure deposited by magnetron sputtering was tuned to achieve antiferromagnetic coupling between Co layers, desired magnetic susceptibility, and saturation magnetization. Ultrahigh density positive tone thermal nanoimprint lithography molds designed to maximize the depth and minimize the thickness of imprint trenches in nanoimprint resist (PMMA or HSQ) were fabricated. The molds were used to pattern large arrays of 200 nm × 200 nm, 400 nm × 400 nm, and 600 nm × 600 nm SAF nanostructures with superparamagneticlike magnetic characteristics. The nanostructures were released from the wafers into an aqueous suspension by dissolving the sacrificial underlayers, Cu for SAF/PMMA nanoparticles and PMMA for SAF/HSQ nanoparticles. [ABSTRACT FROM AUTHOR]

The modulated field sweep ferromagnetic resonance (FMR) spectroscopy was used to study the magnetization dynamics in large arrays of interacting cubic nanomagnets. A 60nm thick permalloy (Ni80Fe20) thin films were patterned using a lift-off process into several large arrays of 60 x 60 nm2 nanostructures where the spacing between the magnetic nanocubes was varied to control the strength of dipolar coupling. Electron beam lithography was used for device patterning using lift-off. The permalloy films were deposited using magnetron sputtering. DC magnetic properties were evaluated using alternating gradient force magnetometer (AGFM). In the FMR measurements, the orientation of DC bias magnetic field was varied from the in-plane to out-of-plane with respect to the 2D plane of the arrays. The FMR peak splitting and multiple FMR modes were observed in the evolution of the FMR spectra as the function of the bias field orientation, and were strongly influenced by the nanomagnet geometry and the spacing between the nanomagnets. Two resonance modes, shape and lattice, were particularly well pronounced in the observed FMR spectra. These modes are characterized by effective demagnetizing factors representing different symmetries of the system of interacting nanomagnets. Micromagnetic modeling suggest that micromagnetic texture within the nanomagnets modulated by the stray fields from the neighbors correlates with the FMR spectra. [ABSTRACT FROM AUTHOR]

Single or a-few-atoms-thick sheets of graphene and graphene oxide (GO) exhibit a range of unique mechanical, optical, thermal, and electrical properties. While these properties have been extensively studied, research into the magnetics of graphene and GO systems is still in its early stages. A number of theoretical studies, which have yet to be verified experimentally, predict that various types of magnetism can be induced in otherwise diamagnetic carbon systems through (1) the introduction of defects in conjugated carbon networks (e.g., vacancies and covalently bonded functional groups that are abundantly present in GO), (2) the substitution of heteroatoms, and (3) edge effects. Although pristine graphene shows in many respects some of the most interesting features, unlike GO, it is difficult to produce pristine graphene in sufficient quantities to characterize or harvest its magnetic properties for real-world applications. In this study, we examine derivatives of graphene and describe an approach to induce a defect-based frustrated magnetic (spin-glass) phase in GO powders using a rather simple chemical process of surface modification by sodium sulfate (SS). Magnetization as a function of applied field (M-H loops) and the temperature dependence of magnetization in zero and nonzero field-cooling experiments show behavior that is typical for superparamagnets and frustrated magnetic systems such as spin glasses. Characterization of the magnetic properties paired with XPS and Raman spectroscopy reveal the nature of the chemical bonds and suggest that the exposure of paramagnetic GO to SS leads to the development of magnetic sites across the GO surface that form spin-glass-like phases similar to other spin-glass phases in highly disordered magnetic materials. [ABSTRACT FROM AUTHOR]

Magnetic sensing utilizes the detection of biomolecule-conjugated magnetic nanoparticles (MNPs). Our new strategy offers a novel approach to magnetic sensing where in situ conversion produces a “loss of signal” in the sensing device. This report demonstrates the enzymatic conversion of Fe3O4 MNPs to a non-magnetic precipitate via reduction by l-ascorbic acid generated by the action of alkaline phosphatase. [ABSTRACT FROM AUTHOR]

We report the first in vitro enzymatic synthesis of paramagnetic and antiferromagnetic nanoparticles toward magnetic ELISA reporting. With our procedure, alkaline phosphatase catalyzes the dephosphorylation of L-ascorbic-2-phosphate, which then serves as a reducing agent for salts of iron, gadolinium, and holmium, forming magnetic precipitates of Fe45±14Gd5±2O50±15 and Fe42±4Ho6±4O52±5. The nanoparticles were found to be paramagnetic at 300 K and antiferromagnetic under 25 K. Although weakly magnetic at 300 K, the room-temperature magnetization of the nanoparticles found here is considerably greater than that of analogous chemically-synthesized LnxFeyOz (Ln = Gd, Ho) samples reported previously. At 5 K, the nanoparticles showed a significantly higher saturation magnetization of 45 and 30 emu/g for Fe45±14Gd5±2O50±15 and Fe42±4Ho6±4O52±5, respectively. Our approach of enzymatically synthesizing magnetic labels reduces the cost and avoids diffusional mass-transfer limitations associated with pre-synthesized magnetic reporter particles, while retaining the advantages of magnetic sensing. [ABSTRACT FROM AUTHOR]

By systematically varying the reactionparameters in a liquid-phasereduction reaction, large FeCo nanocubes with tunable body diagonallengths of 175, 350, and 450 nm were synthesized. The nanocubes wereinitially stabilized with poly(vinyl pyrrolidone) (PVP) and then coatedwith a relatively thin layer of silica (∼55 nm thick), whichallowed them to retain their cubic shape. The magnetization curvesshowed that the PVP-stabilized nanocubes exhibited a high saturationmagnetization of 167 ± 4 emu/g. The saturation magnetization,however, decreased upon coating with silica to 146 ± 13 emu/gfor the particles with 350 and 450 nm FeCo cores and 48 ± 1 emu/gfor the particles with 175 nm FeCo cores. The silica-coated FeCo nanocubeswere then functionalized with 3-(aminopropyl)trimethoxysilane (APTMS),and a layer of surface-bound nanoparticle was generated by exposingthe resultant amine-functionalized nanocubes to self-assembled monolayers(SAMs) on gold terminated with carboxylic-acid groups. [ABSTRACT FROM AUTHOR]

We describe a low-energy glow-discharge process using reactive ion etching system that enables non-circular device patterns, such as squares or hexagons, to be formed from a precursor array of uniform circular openings in polymethyl methacrylate, PMMA, defined by electron beam lithography. This technique is of a particular interest for bit-patterned magnetic recording medium fabrication, where close packed square magnetic bits may improve its recording performance. The process and results of generating close packed square patterns by self-limiting low-energy glow-discharge are investigated. Dense magnetic arrays formed by electrochemical deposition of nickel over self-limiting formed molds are demonstrated. [ABSTRACT FROM AUTHOR]

Sensitivity and specificity are among the most important parameters for viable sensor technologies based on magnetic nanoparticles. In this work, we describe synthetic routes and analytical approaches to improve both aspects. Magnetic iron oxide particles having diameters of 120, 440, and 700 nm were synthesized, and their surfaces were specifically functionalized. The larger particles showed significantly stronger magnetic signals and responses when compared to commercially available magnetic particles (Dynabeads). A force-based detection method was used to distinguish specifically bound particles (via protein interactions) and nonspecifically bound ones (e.g., via physisorption). In addition, an exchange platform, denoted as exchange-induced remnant magnetization (EXIRM), was developed and utilized to detect label-free proteins specifically. Using EXIRM, the 700 nm magnetic particles showed a 7-fold increase in detection sensitivity when compared to the markedly larger commercially available Dynabeads; furthermore, EXIRM exhibited high specificity, even in a 100-fold increase of nontargeted protein. More generally, particle size effects, reaction times, and dynamic ranges are evaluated and discussed herein.

This paper highlights the relation between the shape of iron oxide (Fe 3 O 4 ) particles and their magnetic sensing ability. We synthesized Fe 3 O 4 nanocubes and nanospheres having tunable sizes via solvothermal and thermal decomposition synthesis reactions, respectively, to obtain samples in which the volumes and body diagonals/diameters were equivalent. Vibrating sample magnetometry (VSM) data showed that the saturation magnetization ( M s ) and coercivity of 100-225 nm cubic magnetic nanoparticles (MNPs) were, respectively, 1.4-3.0 and 1.1-8.4 times those of spherical MNPs on a same-volume and same-body diagonal/diameter basis. The Curie temperature for the cubic Fe 3 O 4 MNPs for each size was also higher than that of the corresponding spherical MNPs; furthermore, the cubic Fe 3 O 4 MNPs were more crystalline than the corresponding spherical MNPs. For applications relying on both higher contact area and enhanced magnetic properties, higher- M s Fe 3 O 4 nanocubes offer distinct advantages over Fe 3 O 4 nanospheres of the same-volume or same-body diagonal/diameter. We evaluated the sensing potential of our synthesized MNPs using giant magnetoresistive (GMR) sensing and force-induced remnant magnetization spectroscopy (FIRMS). Preliminary data obtained by GMR sensing confirmed that the nanocubes exhibited a distinct sensitivity advantage over the nanospheres. Similarly, FIRMS data showed that when subjected to the same force at the same initial concentration, a greater number of nanocubes remained bound to the sensor surface because of higher surface contact area. Because greater binding and higher M s translate to stronger signal and better analytical sensitivity, nanocubes are an attractive alternative to nanospheres in sensing applications.