S. D. Yoon

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 3.4 kOe and maximum energy products greater than 20 kJ m−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 510 K at temperatures approaching 700 K 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.

Ferromagnetism in pure wurtzite zinc oxide

The ferromagnetism induced by the intrinsic point defects in wurtzite zinc oxide is studied by using ab initio calculation based on density functional theory. The calculations show that both oxygen interstitial and zinc vacancy may induce ferromagnetism into this material. The calculations also show that zinc oxide with oxygen interstitial may be a ferromagnetic semiconductor. Based on the simplified electronic configuration of the defect molecules, we explain the total magnetic moment, electronic structure, and ferromagnetism.

Microwave and magnetic properties of double-sided hexaferrite films on (111) magnesium oxide substrates

We have deposited hexaferrite films of BaFe12O19 on both sides of a magnesium oxide, (111) MgO, substrate by the liquid phase epitaxy (LPE) deposition technique. The procedure entailed the deposition of seed layers of BaFe12O19 onto the substrate by laser ablation deposition and then dipping of the substrate into a molten flux. Whereas the seed layers were the order of 0.5 μm, the total thickness after LPE deposition was ∼45 μm. The ferrimagnetic resonance linewidth at 60 GHz, 27 Oe, is as low as it has ever been measured before in BaFe12O19 to the best of our knowledge. The static field measurements of the saturation magnetization and anisotropy field were in agreement with previous measured bulk values.

Multiferroic heterostructure fringe field tuning of meander line microstrip ferrite phase shifter

Magnetic fringe fields emanating from a multiferroic heterostructure composite of Terfenol-D and lead magnesium niobate-lead titanate were utilized to actively tune a meander line microstrip ferrite phase shifter operating above ferrimagnetic resonance at C-band. Differential phase shifts of 65° were measured when tuned with an applied voltage to the multiferroic heterostructure. This demonstration of magnetoelectric field generation provides an alternative approach to tuning broadband planar microwave magnetic devices where neither strain nor direct electromagnetic coupling is experienced between device and multiferroic transducer.

MMIC circulators using hexaferrites

MMIC circulators operating at K/sub A/-band have been fabricated from single crystal and polycrystalline hexaferrite platelets integrated into glass-microwave integrated circuit wafers. Measurements on circulators utilizing single crystal BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ showed two-band circulation, below-FMR (at 36 GHz) and above-FMR (at 24 GHz), while those using single crystal BaFe/sub 12/O/sub 19/ showed above-FMR circulation at 32-35 GHz. Self-biased circulators using SrFe/sub 12/O/sub 19/ ceramics showed above-FMR circulation at 32 GHz. All circulators showed reasonable insertion losses and isolation, with the best insertion loss of 1.2 dB at an isolation of 18 dB being found for a BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ circulator operating below-FMR.

Large induced magnetic anisotropy in manganese spinel ferrite films

The oxygen pressure dependence of magnetic anisotropy in pulse laser deposited manganese ferrite (MnFe2O4) films was investigated. Magnetic anisotropy fields (Ha) are shown to exceed 5kOe when films were processed at oxygen pressures below 5mTorr. Further, it is shown that the magnetically preferred direction of Ha can be aligned either along the film plane (pO2 8mTorr). The ability to induce large perpendicular magnetic anisotropy in spinel ferrites allows for new applications (i.e., phase shifters, filters, isolators, and circulators) near or above X-band frequencies to be considered.

Thick M-type barium hexaferrite films grown on garnet substrates

The deposition and characterization of thick M-type barium hexaferrite (BaFe12O19) films produced by a modified liquid phase epitaxy deposition technique are reported. The films are deposited on (111) oriented single crystal garnet (Gd3Ga5O12) substrates. The thickness of the films ranged between 45 and 80μm with growth rates of up to 40μm∕h. This growth rate is about five times greater than films grown on substrates of (111) magnesium oxide. Although the films are relatively thick, the ferrimagnetic resonance linewidth is remarkably narrow (∼50Oe) for external fields applied along the c axis.

Effect of growth temperature on the magnetic, microwave, and cation inversion properties on NiFe2O4 thin films deposited by pulsed laser ablation deposition

First principles band structure calculations suggest that the preferential occupation of Ni2+ ions on the tetrahedral sites in NiFe2O4 would lead to an enhancement of the exchange integral and subsequently the Neel temperature and magnetization. To this end, we have deposited NiFe2O4 films on MgO substrates by pulsed laser deposition. The substrate temperature was varied from 700to900°C at 5mTorr of O2 pressure. The films were annealed at 1000°C for different times prior to their characterization. X-ray diffraction spectra showed either (100) or (111) orientation with the spinel structure dependent on the substrate orientation. Magnetic studies showed a magnetization value of 2.7kG at 300K. The magnetic moment was increased to the bulk value as a result of postdeposition annealing at 1000°C. The as produced films show that the ferromagnetic resonance linewidth at 9.61GHz was 1.5kOe, and it was reduced to 0.34kOe after postannealing at 1000°C. This suggests that the annealing led to the redistribution of N...

Relaxation mechanism for ordered magnetic materials

We have formulated a relaxation mechanism for ferrites and ferromagnets (insulators and metals) whereby the coupling between the magnetic motion and lattice is based purely on continuum arguments concerning magnetostriction. This theoretical approach contrasts with previous mechanisms based on microscopic formulations of spin-phonon interactions employing a discrete lattice. Our model explains the scaling of the intrinsic ferromagnetic resonance linewidth with frequency, with temperature

Functionalization of FeCo alloy nanoparticles with highly dielectric amorphous oxide coatings

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