Zachary J. Huba

Plasmonics and Enhanced Magneto-Optics in Core−Shell Co−Ag Nanoparticles

We present theoretical and experimental studies that explain the observed strong enhancement of the magneto-optical (MO) Faraday rotation in all-metal core-shell Co-Ag nanoparticles (NPs) attributed to localized surface plasmon resonance (LSPR). We also explain why the optical absorption and MO spectra peaks appear blue-shifted with increased Co core size while keeping the NP size constant. Further, we demonstrate direct correlation between the strong LSPR induced electromagnetic fields and the enhanced MO activity of the NPs.

Magnetic properties of Co2C and Co3C nanoparticles and their assemblies

Nano-composite material consisting of Co2C and Co3C nanoparticles has recently been shown to exhibit unusually large coercivities and energy products. Experimental studies that can delineate the properties of individual phases have been undertaken and provide information on how the coercivities and the energy product change with the size and composition of the nanoparticles. The studies indicate that while both phases are magnetic, the Co3C has higher magnetization and coercivity compared to Co2C. Through first principles electronic structure studies using a GGA+U functional, we provide insight on the role of C intercalation on enhancing the magnetic anisotropy of the individual phases.

Synthesis of high magnetization FeCo alloys prepared by a modified polyol process

High magnetization, soft ferromagnetic FeCo alloy nanoparticles were synthesized at various Fe to Co ratios using a modified polyol process. Transmission electron microscopy images revealed that Fe-rich particles had a cubic shape with a mean particle size of 100 nm, while Co-rich particles had a spherical shape. A maximum saturation magnetization of 212 emu/g was recorded for both Fe60Co40 and Fe75Co25 particles. X-ray diffraction scans at room temperature of synthesized particles were characteristic of body-centered-cubic single-phase FeCo. Variable temperature x-ray diffraction scans under N2 gas revealed an order–disorder transition at 600 °C and a transition to a face-centered-cubic crystal structure at 1000 °C.

Enhanced magnetic anisotropy in cobalt-carbide nanoparticles

An outstanding problem in nano-magnetism is to stabilize the magnetic order in nanoparticles at room temperatures. For ordinary ferromagnetic materials, reduction in size leads to a decrease in the magnetic anisotropy resulting in superparamagnetic relaxations at nanoscopic sizes. In this work, we demonstrate that using wet chemical synthesis, it is possible to stabilize cobalt carbide nanoparticles which have blocking temperatures exceeding 570 K even for particles with magnetic domains of 8 nm. First principles theoretical investigations show that the observed behavior is rooted in the giant magnetocrystalline anisotropies due to controlled mixing between C p- and Co d-states.

Large-scale synthesis of high moment FeCo nanoparticles using modified polyol synthesis

Binary alloys of Fe and Co have among the highest magnetizations of any transition metal alloy systems, but their affinity to form oxides act to reduce the magnetization of nanoparticles as their size is reduced below ∼30 nm. Here, we demonstrate the synthesis of single phase, size-controlled FeCo nanoparticles having magnetization greater than 200 emu/g via a non-aqueous method in which ethylene glycol served as solvent and reducing agent as well as surfactant. Experiments indicated pure-phase FeCo nanoparticles, having saturation magnetization up to 221 emu/g for sizes of 20–30 nm, in single batch processes resulting in > 2 g/batch. Post-synthesis oxidation of nanoparticles was investigated until very stable nanoparticles were realized with constant magnetization over time.

Size and phase control of cobalt-carbide nanoparticles using OH- and Cl- anions in a polyol process

Exchange coupled cobalt–carbide nanocomposites and single-phase Co2C nanoparticles were synthesized using the polyol process. Hydroxide and chloride anions were used to control carbide phase and particle shape. Synthesized CoxC nanocomposites exhibited average diameters around 300 nm. CoxC nanocomposites synthesized at 0.25 M [OH−] and [Cl−] formed clusters of capped nanorods, whereas synthesis at 0.37 M [OH−] and [Cl−] produced clusters of long blade-like particles. For single-phase Co2C, an [OH−] and [Cl−] of 0.71 M was used and produced clusters of ellipsoidal grains. The CoxC nanocomposites comprised of capped nanorods possessed a BHmax of 1.65 MGOe with a magnetic saturation and coercivity values of 38 emu/g and 2.4 kOe, respectively. Co2C possessed a saturation magnetization of 16 emu/g and coercivity of 1.3 kOe.

A versatile synthetic approach for the synthesis of CoO, CoxC, and Co based nanocomposites: tuning kinetics and crystal phase with different polyhydric alcohols

The solution synthesis of magnetic nanoparticles allows for precise control of a particles shape, size, and crystal phase on the nanoscale, key parameters in tuning the intrinsic magnetic properties of nanoparticles. In this study, we investigate the role of polyhydric alcohols for the solution synthesis of cobalt carbide nanoparticles, a newly discovered hard ferromagnetic material. The oxidative stability of the polyhydric alcohol at reaction temperatures plays a significant role in the kinetics of carbide formation, as well as the phase produced (Co2C vs. Co3C). By tuning the oxidation rate of the polyhydric alcohol, composites of CoO, CoxC, and Co phases can be produced, allowing for magnetic composites comprised of anti ferromagnetic, hard ferromagnetic and soft ferromagnetic phases.

Ethanol assisted reduction and nucleation of ferromagnetic Co and Ni nanocrystalline particles

In this report, we demonstrate the ability of ethanol to act as a solvent and reducing agent to nucleate nanocrystalline Co and Ni particles. Under solvothermal conditions, Co and Ni particles can be produced at 200 °C. The Ni and Co particles crystallized into FCC and a mixture of FCC and HCP crystal phases, respectively. Ni particles possessed a spherical morphology with diameters in the range of 200 nm to 300 nm. Co particles took on an ellipsoidal morphology, with diameters greater than 500 nm. Magnetizations for the Ni and Co particles were commensurate with bulk values, showing their high crystallinity and the presence of little oxide impurity. By finding inexpensive solvents with a lowered environmental impact, steps towards a “greener” synthetic process for ferromagnetic nanoparticles can be established.

Continuous-Flow Synthesis of Cu-M (M=Ni, Co) Core-Shell Nanocomposites

AbstractMagnetic nanomaterials have many applications in the fields of catalysis, medicine, and environmental studies. An emerging synthetic method capable of large-scale production of nanomaterials is a continuous-flow microreactor. However, translating known conventional benchtop reactions to a continuous-flow system can be difficult; reaction parameters such as reaction time and viscosity of the solution are significant limitations in flow-based systems. In this study, nanocrystalline Cu-Ni and Cu-Co core-shell materials were successfully synthesized using a capillary microreactor in a one-step process. Ethanol was used as solvent, allowing for faster reaction times and reduced reaction solution viscosity, compared to similar bench top synthetic protocols. Both nanocomposites were tested for activity in Fischer-Tropsch and showed activity above 220 °C. This study shows that a continuous-flow capillary microreactor has the capabilities to make complex metallic nanomaterials at short reaction times with proper selection of reaction solvent systems.