Anton L. Geiler

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.

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.

Low-loss barium ferrite quasi-single-crystals for microwave application

Barium hexaferrites (BaFe12O19) are especially useful for microwave/millimeter devices. Due to large ferromagnetic resonance (FMR) loss (linewidths >2kOe), traditional compacts of polycrystalline Ba ferrites indeed hinder the utilization of the materials for practical devices. The present experiment demonstrates that the quasi-single-crystal Ba ferrite disks can be fabricated by a single solid-state reaction technique without liquid phase participation, combining with a processing of alignment for the ferrite seed crystals. The ferrite bulks show a pure hexagonal Ba ferrite phase, an expected 4πMs of 4.48kG, and coercivity of 10∼20Oe along the c axis, similar to the results of a typical single crystal. The FMR measurement indicates that the sample yields an anisotropy field of 16.0kOe and a linewidth of about 300Oe at U-band frequencies. Although the linewidth is broader than ideal Ba ferrite single crystals (ΔH 2kOe), traditional compacts of polycrystalline Ba ferrites indeed hinder the utilization of the materials for practical devices. The present experiment demonstrates that the quasi-single-crystal Ba ferrite disks can be fabricated by a single solid-state reaction technique without liquid phase participation, combining with a processing of alignment for the ferrite seed crystals. The ferrite bulks show a pure hexagonal Ba ferrite phase, an expected 4πMs of 4.48kG, and coercivity of 10∼20Oe along the c axis, similar to the results of a typical single crystal. The FMR measurement indicates that the sample yields an anisotropy field of 16.0kOe and a linewidth of about 300Oe at U-band frequencies. Although the linewidth is broader than ideal Ba ferrite single crystals (ΔH<100Oe), it may be possible to reduce to 100Oe by eliminating pores, cracks, local grain bounda...

Quasi-one-dimensional miniature multiferroic magnetic field sensor with high sensitivity at zero bias field

A miniature, quasi one dimensional, magnetic field sensor based on magnetoelectric coupling is presented. The magnetoelectric sensor makes use of the d31 coupling mode between a piezoelectric lead zirconate titanate tube and FeNi magnetostrictive wire. The sensors demonstrate high sensitivity, high signal-to-noise ratio, and low noise floor at zero DC magnetic bias field and at low frequency resulting in smaller, lower power consumption, and volumetric efficiency. Experiments indicate a zero bias field sensitivity of 16.5 mV/Oe at 100 Hz stemming from a magnetoelectric coefficient of 1.65 V/cm-Oe. The results are quantitatively described by a theoretical model of laminate composites.

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.

Improved Sensitivity and Noise in Magneto-Electric Magnetic Field Sensors by Use of Modulated AC Magnetostriction

A magnetic field sensor based on the nonlinear nature of the magnetostrictive response of a magneto-electric (ME) heterostructure has two orders of magnitude improvement in sensitivity and signal-to-noise ratio compared with a conventional dc-biased configuration. The sensor consists of a longitudinally magnetized and transversely poled lamination of iron-cobalt-boron (Metglas) and lead zirconate titanate (PZT). The ac-modulated sensor has enhanced environmental noise immunity and does not require a dc magnetic bias field. Combined, these advantages hold promise for the development of miniature ME sensor elements for applications with size and weight limitations.

Electronic tuning of magnetic permeability in Co2Z hexaferrite toward high frequency electromagnetic device miniaturization

The magnetic and magnetostriction properties of Z-type cobalt-doped barium hexaferrite with perpendicular c-axis crystallographic texture are presented. The hexaferrite was utilized as a component in Co2Z/lead magnesium niobate-lead titanate multiferroic heterostructures whose tunability of permeability with electric field in terms of ferromagnetic resonance shift was supported by experiments and theoretical calculation. A permeability change of 16% was measured by an induced magnetic field of 38 Oe under the application of 6 kV/cm of electric field. These findings lay the foundation for the application of Z-type hexaferrites in tunable rf and microwave devices valued for sending, receiving, and manipulating electromagnetic signals.

Self Biased Y-Junction Circulator at

A self-biased microstrip Y-junction circulator was designed, fabricated and tested at Ku band utilizing strontium M-type barium ferrite. The junction circuit consisted of a dielectric slab resting upon a polished thin composite plate of bulk strontium M-type hexaferrite. This approach proved to be mechanically rigid and compatible with the fabrication of integrated circuits yielding practical electrical characteristics of a circulator circuit. The measured isolation was 21 dB, and the insertion loss was 1.52 dB at 13.6 GHz. The measurements matched well with the HFSS simulation of the composite circulator design.

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The dynamic time domain response of the converse magnetoelectric effect in a multiferroic Metglas®/Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) heterostructure, under the application of a square waveform electric field excitation of 8 kV/cm at a frequency of 0.4 Hz, is reported. The relaxation behavior followed a stretched power-law function allowing the calculation of an intrinsic time constant. Aging behavior of magnetoelectric coupling was observed after polarization switching of 1000 cycles. These phenomena are predominantly attributed to the temporal response of polarization within the PMN-PT crystal. Results elucidate the dynamic properties of relaxor-based multiferroic heterostructures and importantly define operational constraints for low frequency device operation.

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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...