A. Yang

High coercivity cobalt carbide nanoparticles processed via polyol reaction: a new permanent magnet material

Cobalt carbide nanoparticles were processed using polyol reduction chemistry that offers high product yields in a cost effective single-step process. Particles are shown to be acicular in morphology and typically assembled as clusters with room temperature coercivities greater than 3.4u2009kOe and maximum energy products greater than 20u2009kJu2009m−3. Consisting of Co3C and Co2C phases, the ratio of phase volume, particle size and particle morphology all play important roles in determining permanent magnet properties. Further, the acicular particle shape provides an enhancement to the coercivity via dipolar anisotropy energy as well as offering potential for particle alignment in nanocomposite cores. While Curie temperatures are near 510u2009K at temperatures approaching 700u2009K the carbide powders experience an irreversible dissociation to metallic cobalt and carbon thus limiting operational temperatures to near room temperature. These findings warrant more extensive investigation of this and other magnetic carbide systems in which particle size, chemistry and morphology are optimized.

Structure, magnetic, and microwave properties of thick Ba-hexaferrite films epitaxially grown on GaN/Al2O3 substrates

Thick barium hexaferrite [BaO⋅(Fe2O3)6] films, having the magnetoplumbite structure (i.e., Bau2009M), were epitaxially grown on c-axis oriented GaN/Al2O3 substrates by pulsed laser deposition followed by liquid phase epitaxy. X-ray diffraction showed (0,0,2n) crystallographic alignment with pole figure analyses confirming epitaxial growth. High resolution transmission electron microscopy images revealed magnetoplumbite unit cells stacked with limited interfacial mixing. Saturation magnetization, 4πMs, was measured for as-grown films to be 4.1±0.3u2002kG with a perpendicular magnetic anisotropy field of 16±0.3u2002kOe. Ferromagnetic resonance linewidth, the peak-to-peak power absorption derivative at 53 GHz, was 86 Oe. These properties will prove enabling for the integration of low loss Bau2009M ferrite microwave passive devices with active semiconductor circuit elements in systems-on-a-wafer architecture.

Epitaxial growth of PbFe12O19 thin films by alternating target laser ablation deposition of Fe2O3 and PbO

Oriented M-type hexaferrite thin films are deposited using the alternating target laser ablation deposition (ATLAD) technique utilizing PbO and Fe2O3 targets. Crystallographic, dc magnetic, and microwave characterization results confirming the presence of a hexagonal PbFe12O19 phase are presented. We conclude that the ATLAD technique holds great promise for layer by layer deposition of various hexaferrite materials, the properties of which can be adjusted by varying the composition of the targets as well as the number of laser shots from each target during the deposition process. This would provide control over the uniaxial anisotropy fields and saturation magnetization values that was not possible in the conventional single target LAD technique.

Microwave, magnetic, and structural properties of nanocrystalline exchange-coupled Ni11Co11Fe66Zr7B4Cu1 films for high frequency applications

Nanocrystalline structured films of the alloy Ni11Co11Fe66Zr7B4Cu1 were deposited by pulsed laser ablation deposition onto fused quartz substrates. A substrate temperature of 300°C was found to produce films consisting of body-centered-cubic (bcc) metallic grains suspended in an amorphous matrix. The bcc grain size ranged from 5–8nm for substrate temperatures up to 300°C. Measured values of coercivity were consistently below 4Oe for films having a saturation magnetization, as 4πMS, of ∼16kG with in-plane uniaxial anisotropy fields of 25to30Oe. The ferromagnetic resonance peak-to-peak derivative linewidth was measured to be 34Oe at a frequency of 9.61GHz. The zero magnetic field ferromagnetic resonance occurred at ∼2GHz. These films exhibit soft magnetic properties, high saturation induction, low microwave loss, and the structural stability desirable for thin film inductor applications.

Magnetism, Structure, and Cation Distribution in MnFe

A series of manganese ferrite thin film samples were prepared by alternating target laser ablation deposition and conventional pulsed laser deposition techniques on (111) MgO substrates. By extended X-ray absorption fine structure (EXAFS) analysis, we have discovered that the cation distribution was sensitive to the processing oxygen pressure and the preparation technique. Correspondingly, the saturation magnetization and uniaxial anisotropy fields change. Saturation magnetization was found to decrease while the percentage of Mn ions on the octahedral site increased, as a function of oxygen processing pressure. The highest magnetization (~4.5 kG) and anisotropy field (~0.5 kOe) corresponded to the sample grown at the lowest oxygen pressure

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An optimized alternating target laser ablation deposition (ATLAD) technique has been developed to grow high quality barium hexaferrite (BaFe12O19) thin films on basal plane oriented sapphire (Al2O3) substrates from barium monoferrite (BaFe2O4) and hematite (α-Fe2O3) targets. Crystallographic and structural characterization results show that the films possess low c-axis dispersion of Δω=0.259° and hexagonal terraced surface morphology. Saturation magnetization and uniaxial magnetic anisotropy field were determined to be consistent with reference data on high quality barium hexaferrite films and bulk single crystals grown by other techniques. Ferromagnetic resonance linewidth of 42Oe was measured at 52GHz by the shorted waveguide technique. We conclude that the ATLAD technique is capable of growing high quality barium hexaferrite films while providing unique opportunities to control the ionic distribution in the hexagonal unit cell by allowing different species of ions to be introduced from the respective t...

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In this paper, the growth of barium hexaferrite thin films on single crystal 6H silicon carbide was demonstrated. The preparation was done using pulsed laser deposition with a KrF excimer laser operating at 248 nm wavelength with 200 mJ energy per shot. The structure of the thin films were characterized using X-ray diffraction, AFM and SEM. The coercivities and the FMR linewidth of the films were determined using VSM.

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Exchange constants and band gap is predicted numerically for artificial nickel ferrite since they directly affect the magnetization and conductivity. These fundamental issues are addressed using density functional calculations to study the magnetic properties and the density of states of the material. The magnetization is deduced from the local magnetic moments calculated from the Mulliken population analysis.