K. Stojak

Enhanced magnetism in highly ordered magnetite nanoparticle-filled nanohole arrays.

A new approach to develop highly ordered magnetite (Fe3O4) nanoparticle-patterned nanohole arrays with desirable magnetic properties for a variety of technological applications is presented. In this work, the sub-100 nm nanohole arrays are successfully fabricated from a pre-ceramic polymer mold using spin-on nanoprinting (SNAP). These nanoholes a then filled with monodispersed, spherical Fe3O4 nanoparticles of about 10 nm diameter using a novel magnetic drag and drop procedure. The nanohole arrays filled with magnetic nanoparticles a imaged using magnetic force microscopy (MFM). Magnetometry and MFM measurements reveal room temperature ferromagnetism in the Fe3O4-filled nanohole arrays, while the as-synthesized Fe3O4 nanoparticles exhibit superparamagnetic behavior. As revealed by MFM measurements, the enhanced magnetism in the Fe3O4-filled nanohole arrays originates mainly from the enhanced magnetic dipole interactions of Fe3 O4 nanoparticles within the nanoholes and between adjacent nanoholes. Nanoparticle filled nanohole arrays can be highly beneficial in magnetic data storage and other applications such as microwave devices and biosensor arrays that require tunable and anisotropic magnetic properties.

Tunable Magneto-Dielectric Polymer Nanocomposites for Microwave Applications

Magneto-dielectric polymer nanocomposites are investigated as a new class of functional materials well suited for RF and microwave device applications. Magnetite (Fe3O4) nanoparticles are homogeneously dispersed in a polymer matrix, which exhibits low losses at microwave frequencies. The monodispersion of the magnetic nanoparticles, with sub-10-nm diameters and tight size distribution, enhances the microwave properties of the engineered composite material by increasing the relative permeability and relative permittivity. Moreover, complex permeability and permittivity of the nanocomposite material can be tuned by an externally applied dc magnetic field with a strength that is achievable with commercial permanent magnets. Multilayered microstrip test fixtures and preexisting measurement techniques were conjointly implemented to extract the microwave properties of the nanocomposite material in the frequency range between 1-6 GHz. The measured data revealed tunability of 5.5% in the permittivity, 37% in the permeability, and more than 100× reduction in the loss tangent at specific frequencies under externally applied magnetic fields.

Polymer nanocomposites exhibiting magnetically tunable microwave properties

Polymer nanocomposites (PNCs) have been synthesized using Rogers polymer and CoFe₂O₄ nanoparticles (CFO NPs). X-ray diffraction (XRD) confirms the inverse spinel crystal structure of CFO NPs and transmission electron microscopy (TEM) images show the uniform dispersion of nanoparticles (10 nm ± 1) into the polymer matrix. Magnetic measurements indicate superparamagnetic response near room temperature for all PNCs. A blocking temperature T(B)~298 K was observed and does not vary for different loading fractions of CFO NPs for the PNCs. The saturation magnetization (M(s)) was found to be 11 emu g⁻¹ for 30 wt% CFO, increasing to 32 emu g⁻¹ for the 80 wt% CFO loaded PNC. A large value of coercivity (H(c) = 19 kOe) is also observed at 10 K and is not affected by varying CFO loading. Microwave measurements show significant absorption in the 80 wt% CFO loading PNC and the quality factor shows a strong enhancement with applied magnetic field.

Enhanced magnetoimpedance effect in Co-based amorphous ribbons coated with carbon nanotubes

We report upon the enhancement of the giant magnetoimpedance (GMI) effect in Co-based amorphous ribbons coated with non-magnetic carbon nanotubes (CNTs). In our study, the CNTs were drop-casted onto the surface of a Metglas® 2714 A ribbon with three different concentrations (5, 10, and 15 μL of CNTs). Relative to the plain ribbon, a 15% enhancement of the GMI effect was observed in the ribbon coated with 10 μL of CNTs. The GMI effect first increased with the CNT concentration, to a maximum of 10 μL of CNTs, and then decreased at higher concentrations. Noticeably, at a measured frequency of 10 MHz, the magnetic field-induced ac resistance change was about 35% larger for the ribbon coated with 10 μL of CNTs than for the plain ribbon. These observations may reveal a new perspective for developing CNT-based gas sensors that operate using the principle of the GMI effect.

Magnetically tunable Nanocomposites for microwave applications

In this work, tunability of the microwave properties of Magneto-Dielectric Polymer Nanocomposites (MDPNCs) has been demonstrated for the first time. Particularly, permeability, permittivity and loss factor were tuned by an externally applied DC magnetic field with strength achievable by commercial permanent magnets. The considerable tunability, ease of manufacturing, compatibility with multilayer RF laminates and substrate implementation make this material well-suited for use in tunable microwave circuits. Two custom-built microstrip test fixtures using multi-layer substrates were employed to extract the permeability, permittivity and loss tangent of MDPNCs using S-parameter measurements.