http://www2.physics.colostate.edu/groups/WuGroup/publications/MMM_2009_Recent%20advances%20in%20processing%20and%20applications%20of%20microwave%20ferrites.pdf

Recent advances in processing and applications of microwave ferrites

The development and current status of microwave ferrite technology is reviewed in this paper. An introduction to the physics and fundamentals of key ferrite devices is provided, followed by a historical account of the development of ferrimagnetic spinel and garnet (YIG) materials. Key ferrite components, i.e., circulators and isolators, phase shifters, tunable filters, and nonlinear devices are also discussed separately.

Ferrite devices and materials

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.

Modern Microwave Ferrites

Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.

/pdf/microwave-ferrites-part-1-fundamental-properties-124qghgxm4.pdf

Microwave ferrites, part 1: fundamental properties

Aspects of the subject disclosure may include, for example, receiving, by a feed point of a dielectric antenna, electromagnetic waves from a dielectric core coupled to the feed point without an electrical return path, where at least a portion of the dielectric antenna comprises a conductive surface, directing, by the feed point, the electromagnetic waves to a proximal portion of the dielectric antenna, and radiating, via an aperture of the dielectric antenna, a wireless signal responsive to the electromagnetic waves being received at the aperture. Other embodiments are disclosed.

Method and apparatus for coupling an antenna to a device

An array antenna includes a ferrite substrate having a two-dimensional planar array of antenna elements disposed over a first surface thereof and means for applying an external magnetic field having a magnitude and direction which can be varied at least in a plane in which the antenna elements lie. The amplitude and direction of the external magnetic biasing field is varied to control the relative phases between each of the plurality of antenna elements and to steer the direction of a main antenna beam in two dimensions.

Antenna array having two dimensional beam steering

The authors report on the fabrication and testing of millimeter-wave microstrip/stripline Y-junction circulator operating at K/sub a/ band. By exploiting the properties of high-anisotropy magnetic field, external magnetic field requirements are reduced. The design of microstrip/stripline circulators involves calculations of the radius of the ferrite disks and the port suspension angles in terms of the microstrip dielectric constant and saturation magnetization of the ferrite material. In addition, the operation frequency is determined largely by the uniaxial anisotropy field of the barium-strontium hexagonal ferrite. >

Self-biasing circulators operating at K/sub a/-band utilizing M-type hexagonal ferrites

Narrow bridge structures were precisely fabricated (using novel ion-beam-milling techniques) on pulsed-laser-deposited ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films. The bridges have a length scale down to L\ensuremath{\simeq}500 \AA{}. Supercurrents were observed to flow across the bridge structures with critical current densities up to ${\mathit{J}}_{\mathit{c}}$\ensuremath{\simeq}1.3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ A/${\mathrm{cm}}^{2}$, the highest critical current densities yet reported. The current-density magnitudes are consistent with the Onsager-Feynman vortex-ring-creation mechanism (for producing voltages across narrow constrictions) in the length scale region of \ensuremath{\xi}\ensuremath{\ll}L\ensuremath{\le}\ensuremath{\Lambda}, with \ensuremath{\xi} the coherence length and \ensuremath{\Lambda} the London penetration depth. The highest ${\mathit{J}}_{\mathit{c}}$ is in good agreement with the critical depairing mechanism.

https://repository.library.northeastern.edu/files/neu:330718/fulltext.pdf

Observation of ultrahigh critical current densities in high-Tc superconducting bridge constrictions.

We have calculated the S-parameters and losses in ferrite-film-junction circulators using a new effective-field theory assuming TEM-like propagation. Conductivity loss dominates the dielectric and magnetic losses in Y-junction circulators fabricated on ferrite films with thicknesses less than 200 /spl mu/m. It is plausible to fabricate Y-junction thin-film circulator at X-band with insertion loss less than 0.5 dB if the film thickness is larger than 100 /spl mu/m. The quality of the conductor plane is important in reducing the overall insertion loss of the thin-film-junction circulator.

Theory and experiment of thin-film junction circulator

A theoretical formulation has been developed to calculate the coupling coefficient, London penetration depth, and surface resistance of a coplanar waveguide resonator fabricated from films of superconducting YBCO material. Experimental data of the reflection coefficient as a function of temperature and frequency agree reasonably well with calculations. The formulation is of sufficient generality to be applicable to other guided structures. >

Microwave characteristics of high T/sub c/ superconducting coplanar waveguide resonator

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