Vincent G. Harris

Chemically prepared magnetic nanoparticles

Abstract Nanotechnology has spurred efforts to design and produce nanoscale components for incorporation into devices. Magnetic nanoparticles are an important class of functional materials, possessing unique magnetic properties due to their reduced size (below 100 nm) with potential for use in devices with reduced dimensions. Recent advances in processing by chemical synthesis and the characterisation of magnetic nanoparticles are the focus of this review. Emphasis has been placed on the various solution chemistry techniques used to synthesise particles, including: precipitation, borohydride reduction, hydrothermal, reverse micelles, polyol, sol–gel, thermolysis, photolysis, sonolysis, multisynthesis processing and electrochemical techniques. The challenges and methods for examining the structural, morphological, and magnetic properties of these materials are described.

Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys

The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the ...

Modern Microwave Ferrites

Microwave ferrites are ubiquitous in systems that send, receive, and manipulate electromagnetic signals across very high frequency to quasi-optical frequency bands. In this paper, modern microwave ferrites are reviewed including spinel, garnet, and hexaferrite systems as thin and thick films, powders and compacts, and metamaterials. Their fundamental properties and utility are examined in the context of high frequency applications ranging from the VHF to millimeter-wave bands. Perspective and outlook of advances in theory, processing, and devices occurring in the science and engineering communities since the year 2000 are presented and discussed.

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.

Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency thermal plasma torch

Nanocrystalline (NC) ferrite powders have been synthesized using a 50 kW–3 MHz rf thermal plasma torch for high-frequency soft magnet applications. A mixed powder of Ni and Fe (Ni:Fe=1:2), a NiFe permalloy powder with additional Fe powder (Ni:Fe=1:2), and a NiFe permalloy powder (Ni:Fe=1:1) were used as precursors for synthesis. Airflow into the reactor chamber was the source of oxygen for oxide formation. XRD patterns clearly show that the precursor powders were transformed into NC ferrite particles with an average particle size of 20–30 nm. SEM and TEM studies indicated that NC ferrite particles had well-defined polygonal growth forms with some exhibiting (111) faceting and many with truncated octahedral and truncated cubic shapes. The Ni content in the ferrite particles was observed to increase in going from mixed Ni and Fe to mixed permalloy and iron and finally to only permalloy starting precursor. The plasma-torch synthesized ferrite materials using exclusively the NiFe permalloy precursor had 40%–4...

MAGNETIC AND STRUCTURAL PROPERTIES OF NICKEL ZINC FERRITE NANOPARTICLES SYNTHESIZED AT ROOM TEMPERATURE

Nickel zinc ferrite nanoparticles (Ni0.20Zn0.44Fe2.36O4) have been produced at room temperature, without calcination, using a reverse micelle process. Particle size is approximately 7 nm as determined by x-ray powder diffraction and transmission electron microscopy. Saturation magnetization values are lower than anticipated, but are explained by elemental analysis, particle size, and cation occupancy within the spinel lattice. Extended x-ray absorption fine structure analysis suggests that a significant amount of Zn2+, which normally occupies tetrahedral sites, actually resides in octahedral coordination in a zinc-enriched outer layer of the particles. This “excess” of diamagnetic Zn can thus contribute to the overall decrease in magnetism. Further, this model can also be used to suggest a formation mechanism in which Zn2+ is incorporated at a later stage in the particle growth process.

Small Ultra-Wideband (UWB) Bandpass Filter With Notched Band

A compact ultra-wideband (UWB) bandpass filter (BPF) with notched band has been proposed and implemented in this letter. H-shaped slot is studied and adopted to tighten the coupling of inter-digital capacitor in order to improve the BPFs performance. Three pairs of tapered defected ground structures (DGS) are formed to assign their transmission zeros towards the out of band signal, thereby suppressing the spurious passband. Combining these two structures we obtain a small sized UWB BPF. Meander line slot is developed to reject the undesired wireless local-area network (WLAN) radio signals. An experimental UWB filter with notched band was fabricated with 35% less length as compared to an embedded open-circuited stub. The measured BPF insertion loss is less than 1.0 dB throughout the pass band of 2.8 to 10.8 GHz, the variation of group delay less than 0.20 ns in this band except for the notched band, and a wide stopband bandwidth with 20 dB attenuation up to at least 20.0 GHz.

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.

Photomagnetism and structure in cobalt ferrite nanoparticles

Light-induced changes in the coercivity ΔHc are reported for cobalt ferrite nanoparticles. These changes arise from charge transfer initiated by optical absorption near 2 eV. An insulator-to-metal transition at 170 K is an upper limit for significant ΔHc. The larger ΔHc for smaller (17 nm) particles was correlated with a distortion about the Co2+ ions at B sites, which increases the absorption probability.

Origin of high transport spin polarization in La 0.7 Sr 0.3 MnO 3 : Direct evidence for minority spin states

Using the point-contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite,