Katherine S. Ziemer

Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2−δ films

We demonstrate a semiconducting material, TiO2??, with ferromagnetism up to 880?K, without the introduction of magnetic ions. The magnetism in these films stems from the controlled introduction of anion defects from both the film?substrate interface as well as processing under an oxygen-deficient atmosphere. The room-temperature carriers are n-type with n~3 ? 1017?cm?3. The density of spins is ~1021?cm?3. Magnetism scales with conductivity, suggesting that a double exchange interaction is active. This represents a new approach in the design and refinement of magnetic semiconductor materials for spintronics device applications.

Ba-hexaferrite films for next generation microwave devices (invited)

Next generation magnetic microwave devices require ferrite films to be thick (>300μm), self-biased (high remanent magnetization), and low loss in the microwave and millimeter wave bands. Here we examine recent advances in the processing of thick Ba-hexaferrite (M-type) films using pulsed laser deposition (PLD), liquid-phase epitaxy, and screen printing. These techniques are compared and contrasted as to their suitability for microwave materials processing and industrial production. Recent advances include the PLD growth of BaM on wide-band-gap semiconductor substrates and the development of thick, self-biased, low-loss BaM films by screen printing.

Synthesis of ordered arrays of multiferroic NiFe2O4-Pb(Zr0.52Ti0.48)O3 core-shell nanowires

A synthesis method was developed for producing core-shell nanowire arrays, which involved a combination of a modified sol-gel process, electrochemical deposition, and subsequent oxidization in anodized nanoporous alumina membranes. This method was applied to generate ordered arrays of one dimensional multiferroic NiFe2O4 core and Pb(Zr0.52Ti0.48)O3 (PZT) shell nanostructures. Extensive microstructural, magnetic, and ferroelectric characterizations confirmed that the regular arrays of core-shell multiferroic nanostructures were composed of a spinel NiFe2O4 core and perovskite PZT shell. This synthesis method can be readily extended to prepare different core-shell nanowire arrays and is expected to pave the way for one dimensional core-shell nanowire arrays.

Soft magnetism, magnetostriction and microwave properties of FeGaB thin films

A series of (Fe100−yGay)1−xBx (x=0–21 and y=9–17) films were deposited; their microstructure, soft magnetism, magnetostrictive behavior, and microwave properties were investigated. The addition of B changes the FeGaB films from polycrystalline to amorphous phase and leads to excellent magnetic softness with coercivity <1Oe, high 4πMs, self-biased ferromagnetic resonance (FMR) frequency of 1.85GHz, narrow FMR linewidth (X band) of 16–20Oe, and a high saturation magnetostriction constant of 70ppm. The combination of these properties makes the FeGaB films potential candidates for tunable magnetoelectric microwave devices and other rf/microwave magnetic device applications.

Electrical tuning of magnetism in Fe3O4/PZN–PT multiferroic heterostructures derived by reactive magnetron sputtering

Strong magnetoelectric (ME) coupling was demonstrated in Fe3O4/PZN–PT (lead zinc niobate–lead titanate) multiferroic heterostructures obtained through a sputter deposition process. The dependence of the magnetic anisotropy on the electric field (E-field) is theoretically predicted and experimentally observed by ferromagnetic resonance spectroscopy. A large tunable in-plane magnetic anisotropy of up to 600 Oe, and tunable out-of-plane anisotropy of up to 400 Oe were observed in the Fe3O4/PZN–PT multiferroic heterostructures, corresponding to a large ME coefficient of 100 Oe cm/kV in plane and 68 Oe cm/kV out of plane, which match well with predicted results. In addition, the electric field manipulation of magnetic anisotropy is also demonstrated by the electric fields dependence of magnetic hysteresis loops, showing a large squareness ratio change of 44%. These Fe3O4/PZN–PT multiferroic heterostructures exhibiting large E-field tunable magnetic properties provide great opportunities for novel electrostatic...

Chemical and physical modifications to poly(dimethylsiloxane) surfaces affect adhesion of Caco‐2 cells

Polydimethylsiloxane (PDMS) silicone elastomer is extensively used in soft lithography processes to fabricate microscale or nano scale systems for microfluidic or cell culture applications. Though PDMS is biocompatible, it is not an ideal material for cell culture due to its poor cell adhesion properties. In this study, PDMS surfaces were modified to promote intestinal cell adhesion, in the interest of testing feasibility of using microfabricated PDMS systems for high throughput drug screening. Modification techniques included changing chemical composition of PDMS (i.e., varying curing to mixing agent ratio, and oxidization of PDMS surface by oxygen plasma), surface treatment of PDMS by coating with charged molecules (i.e., poly-D-lysine, L-alpha-phosphatidylcholine, and a layer bylayer coating), and deposition of extracellular matrix (ECM) proteins (i.e., laminin, fibronectin, and collagen). The influence of these modifications on PDMS properties, including elastic modulus and surface properties (wettability, chemical composition, topography, and protein adsorption) were characterized. Modification techniques were all found to change PDMS properties and influence the attachment and proliferation of Caco-2 cells over three days of culture to varying degrees. Generally, Caco-2 cells preferred to attach on collagen-coated, fibronectin-coated, and fibronectin-coated oxygen-plasma treated PDMS. The results highlight the importance of considering multiple physical and chemical factors that may be influenced by biomaterial modification and result in altered cell attachment to microfabricated systems, including surface hydrophobicity, chemical composition, stiffness, and topography. This study provides a foundation for further miniaturization, utilizing soft lithography techniques, of Caco-2 cell-based system for high-throughput screening of drug intestinal absorption during lead optimization in drug discovery. The understanding of different surface modifications on adjusting cell adhesion on PDMS allows systemic design of Biomicroelectromechanical Systems (BioMEMS) with tunable cell adhesion properties.

Epitaxial growth of M-type Ba-hexaferrite films on MgO (111)‖SiC (0001) with low ferromagnetic resonance linewidths

Barium hexaferrite (BaM) films were deposited on 10nm MgO (111) films on 6H silicon carbide (0001) substrates by pulsed laser deposition from a homogeneous BaFe12O19 target. The MgO layer, deposited by molecular beam epitaxy, alleviated lattice mismatch and interdiffusion between film and substrate. X-ray diffraction showed strong crystallographic alignment while pole figures exhibited reflections consistent with epitaxial growth. After optimized annealing, these BaM films have a perpendicular magnetic anisotropy field of 16900Oe, a magnetization (as 4πMs) of 4.4kG, and a ferromagnetic resonance peak-to-peak derivative linewidth at 53GHz of 96Oe, thus demonstrating sufficient properties for microwave device applications.

Spin-spray deposited multiferroic composite Ni0.23Fe2.77O4∕Pb(Zr,Ti)O3 with strong interface adhesion

Ni0.23Fe2.77O4 (NFO)/Pb(Zr,Ti)O3 (PZT) multiferroic composites were synthesized by spin-spray deposition of NFO film onto PZT at 90°C. Strong interface adhesion between NFO and PZT was observed, which was verified by high resolution transmission electron microscopy indicating excellent wetting between the NFO and PZT, and by the strong magnetoelectric coupling in the NFO/PZT multiferroic composite showing an electric field induced remnant magnetization change of 10%. This strong interface adhesion and low-temperature spin-spray synthesis of multiferroic materials provide an alternative route for novel integrated multiferroic materials and devices.

Cross-Linking and Degradation Properties of Plasma Enhanced Chemical Vapor Deposited Poly(2-hydroxyethyl methacrylate)

Plasma Enhanced Chemical Vapor Deposition (PECVD) of poly-2-hydroxyethyl methacrylate (pHEMA) biocompatible, biodegradable polymer films were produced alone and cross-linked with ethylene glycol diacrylate (EGDA). Degree of cross-linking was controlled via manipulation of the EGDA flow rate, which influenced the amount of swelling and the extent of degradation of the films in an aqueous solution over time. Noncross-linked pHEMA films swelled 10% more than cross-linked films after 24 h of incubation in an aqueous environment. Increasing degree of film cross-linking decreased degradation over time. Thus, PECVD pHEMA films with variable cross-linking properties enable tuning of gel formation and degradation properties, making these films useful in a variety of biologically significant applications.

Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy

Magnesium oxide (111) was grown epitaxially on hexagonal silicon carbide (6H-SiC) (0001) substrates at low temperatures by molecular beam epitaxy and a remote oxygen plasma source. The films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. Crystal structure, morphology, and growth rate of the magnesium oxide (MgO) films were found to be dependent on the magnesium flux, indicating a magnesium adsorption controlled growth mechanism. The single crystalline MgO thin films had an epitaxial relationship where MgO (111)‖6H-SiC (0001) and were stable in both air and 10−9Torr up to 1023K.