Soack Dae Yoon

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.

Oriented barium hexaferrite thick films with narrow ferromagnetic resonance linewidth

Hexagonal BaFe12O19 ferrite films, having thicknesses ranging from 200–500μm, were prepared by a screen printing process followed by sintering heat treatments. Structural, magnetic, and microwave measurements confirmed that the polycrystalline films were suitable for applications in self-biasing microwave devices in that they exhibited a large remanence (4πMr=3800G), high hysteresis loop squareness (Mr∕Ms=0.96) and low microwave loss. A derivative linewidth ΔH of 310 Oe was measured at 55.6 GHz. This represents the lowest ΔH measured in polycrystalline hexaferrite materials. ΔH can be further improved by reducing porosity and improving the c-axis orientation of grains in polycrystalline ferrite.

Bias Field Effects on Microwave Frequency Behavior of PZT/YIG Magnetoelectric Bilayer

Magnetoelectric behavior of a yttrium iron garnet (YIG)/zirconate titanate (PZT) magnetoelectric bilayer composite was studied over 1-7 GHz under different bias magnetic fields and electric fields by using a broadband air-gap microstrip with the PZT/YIG loaded in the air gap. Electrostatically induced ferrimagnetic resonance (FMR) frequency shifts of the YIG/PZT bilayer composite were studied. The FMR frequency shift was negligible at bias fields when the YIG was not saturated. After saturation, the FMR frequency increased nearly linearly from 15 MHz at a bias field of 100 Oe to 30 MHz at 1200 Oe and dropped suddenly at a field of 1300 Oe to about 20 MHz. This nonlinear bias magnetic held dependence was due to magnetic domain activities when the YIG was not saturated and due to the interference between the uniform mode and other magnetostatic spin waves after saturation. An electrostatically tunable band-reject filter device was demonstrated which has a peak attenuation of greater than 50 dB, 40 dB rejection band of 10 MHz, and pass band insertion loss of <5 dB at ~4.6 GHz.

Screen printed thick self-biased, low-loss, barium hexaferrite films by hot-press sintering

BaFe12O19 hexaferrite thick films having high hysteresis loop squareness (Mr∕Ms) and low ferromagnetic resonance (FMR) linewidths were processed through the use of a screen printing technique coupled with hot-press sintering. Scanning electron microscopy and x-ray diffraction measurements exhibit strong crystallographic c-axis alignment of crystals perpendicular to the film plane. Static magnetic resonance and FMR measurements were performed to determine the effect of the preparation technique on magnetic hysteresis and microwave properties. Hot pressing during sintering produced dense thick films having high squareness (>0.95) and reduced coercivity (∼1900Oe). Of greater importance was the measurement of a minimum peak-to-peak FMR linewidth, 320Oe at the U band, for films ranging in thickness from 100to500μm. Theoretic estimates suggest that such narrow linewidths can be attributed to the reduction in porosity and the improvement in c-axis orientation of crystallites in polycrystalline barium ferrite fil...

Enhanced Néel temperature in Mn ferrite nanoparticles linked to growth-rate-induced cation inversion

Mn ferrite (MnFe(2)O(4)) nanoparticles, having diameters from 4 to 50 nm, were synthesized using a modified co-precipitation technique in which mixed metal chloride solutions were added to different concentrations of boiling NaOH solutions to control particle growth rate. Thermomagnetization measurements indicated an increase in Néel temperature corresponding to increased particle growth rate and particle size. The Néel temperature is also found to increase inversely proportionally to the cation inversion parameter, delta, appearing in the formula (Mn(1-delta)Fe(delta))(tet)[Mn(delta)Fe(2-delta)](oct)O(4). These results contradict previously published reports of trends between Néel temperature and particle size, and demonstrate the dominance of cation inversion in determining the strength of superexchange interactions and subsequently Néel temperature in ferrite systems. The particle surface chemistry, structure, and magnetic spin configuration play secondary roles.

Microwave and magnetic properties of self-biased barium hexaferrite screen printed thick films

The interest in barium hexaferrite thick films, particularly with high remanent magnetization, is driven by the development of small planar ferrite microwave devices. We report here processing and microwave characterization of BaFe12O19 ferrite thick films (100–400μm). The films were deposited on silicon and alumina substrates by screen printing, oriented under a magnetic field of 8kOe, then annealed at 250°C and sintered at temperatures ranging from 850to1300°C. Scanning electron microscopy and x-ray diffraction exhibited strong crystallographic alignment of c-axis crystals perpendicular to the film plane. The magnetization measurement indicated that a typical dense film with 270μm thickness yielded a high squareness (Mr∕Ms) of 0.93. Ferrimagnetic resonance (FMR) measurements were performed in the frequency range of 40–55GHz. From the linear dependence of FMR frequency on the external field, a g factor of 2.03±0.08 was deduced, while the smallest linewidth was obtained to be 1.2kOe at 40GHz. The broadeni...

Large tunability of Néel temperature by growth-rate-induced cation inversion in Mn-ferrite nanoparticles

The tuning of Neel temperature by greater than 100 K in nanoparticle Mn-ferrite was demonstrated by a growth-rate-induced cation inversion. Mn-ferrite nanoparticles, having diameters from 4 to 50 nm, were synthesized via coprecipitation synthesis. The Neel temperature (TN) increased inversely to the cation inversion parameter, δ (i.e., defined as (Mn1−δFeδ)tet[MnδFe2−δ]octO4). Concomitantly, TN increased with increased particle growth rate and particle size. These results unambiguously establish cation inversion as the dominant mechanism in modifying the superexchange leading to enhanced TN. The ability to tailor TN enables greater flexibility in applying nanoparticle ferrites in emerging technologies.

Structure and Magnetism of Ba-Hexaferrite Films Grown on Single Crystal 6-H SiC With Graduated Interfacial MgO Buffer Layers

M-type barium hexaferrite films were processed by pulsed laser deposition on single-crystal 6-H silicon carbide substrates. MgO buffer and barrier layers were introduced to improve the film quality. Samples were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and ferromagnetic resonance (FMR). X-ray thetas-2thetas diffraction measurements indicated a strong (0, 0, 2n) crystallographic alignment. The magnetization of the BaM film is comparable to bulk values (4piMs~4320 G). A derivative power FMR linewidth of 500 Oe was measured at 55 GHz for the as-deposited films. This paper explores a potential next generation of microwave and millimeter-wave monolithic integrated circuit technology based upon a wide band-gap semiconducting material

Low Bias Field Hexagonal Y-Type Ferrite Phase Shifters at

A microstrip line fabricated on a single crystal hexagonal Y-type ferrite substrate was utilized to achieve phase shifts of ~65deg/cm at KU-band with an external bias field of only 100 Oe. This approach utilizes the rapid variation of the Y-type ferrite tensor permeability near the ferromagnetic antiresonance frequency. The magnetic and microwave properties of the substrate material were characterized by vibrating sample magnetometry and ferromagnetic resonance measurements. Numerical methods based on Galerkins approach adapted to spectral domain were utilized to model the performance of the device while taking into account the correct form of the tensor magnetic permeability of the anisotropic and gyromagnetic substrate material. Good agreement between the numerical results and experimental data was obtained. A miniature electromagnet, based on a Metglastrade laminated yoke to minimize demagnetizing fields, was fabricated. Phase shifts of 50deg/cm were achieved with continuous drive currents of only 100 mA.