Next generation magnetic microwave devices require ferrite films to be thick ͑Ͼ300 m͒, self-biased ͑high remanent magnetization͒, and low loss in the microwave and millimeter wave bands. Here we examine recent advances in the processing of thick Ba-hexaferrite ͑M-type͒ films using pulsed laser deposition ͑PLD͒, 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.

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.

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<100Oe), it may be possible to reduce to 100Oe by eliminating pores, cracks, local grain boundary, and nonuniformity. In terms of material preparation, we believe that it is cost effective in the production of future microwave devices.

Next generation magnetic microwave devices require ferrite films to be thick ͑Ͼ300 m͒, self-biased ͑high remanent magnetization͒, and low loss in the microwave and millimeter wave bands. Here we examine recent advances in the processing of thick Ba-hexaferrite ͑M-type͒ films using pulsed laser deposition ͑PLD͒, 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. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2165145͔ INTRODUCTIONDriven by radar electronics and wireless technologies, the next generation of magnetic microwave devices ͑isola-tors, filters, phase shifters, and circulators and related components͒ will be planar, self-biased, and low loss, and operate well beyond the performance metrics of today's devices. Self-biasing is an important property that eliminates the need for a biasing field that is provided by a comparatively large permanent magnet. The elimination of this magnet is an essential step in making these devices smaller and planar. Integration with semiconductor devices continues to be a desirable property that requires ferrite fabrication techniques to be compatible with complementary metal-oxide semiconductor ͑CMOS͒ processing. This is a difficult task considering that most ferrite fabrication techniques require temperatures Ͼ900°C to produce high-quality films.In order to achieve these goals, magnetic materials must possess high saturation magnetization ͑4M s ͒, high remanent magnetization ͑M r ͒, adjustable magnetic anisotropy fields ͑H A ͒, low microwave losses ͓i.e., low ferromagnetic resonance ͑FMR͒ linewidths ⌬H FMR ͔, and for many applications, have the easy axis of magnetization perpendicular to the film plane ͑i.e., perpendicular magnetic anisotropy͒. In physical terms, the films should be thick ͑Ͼ300 m͒, dense ͑low levels of porosity that are responsible for added microwave loss͒, and pure phase. For many applications the microstructure should possess a strong crystallographic orientation, although true epitaxy is not required.In this paper, we focus on recent advances made in the processing of Ba hexaferrite films for applications in microwave and millimeter-wave devices, with special emphasis on circulator devices. We will compare and contrast different film processing technologies including pulsed laser deposition ͑PLD͒, liquid-phase epitaxy ͑LPE͒, and screen printing.Ba ͑M-type͒ hexaferrite ͑henceforth BaM͒ has the magnetoplumbite structure and a stoichiometry of BaFe 12 O 19 . This structure has 32 atoms/ f.u. and 64 atoms in a single unit cell ͑see Fig. 1͒. One property of this compound that is of particular value in microwave device design is the strong uniaxial anisotropy with the easy direction being along the c axis ͑H A ϳ 17 000 Oe͒.1,2 The high magnetic anisotropy field can b...

Eu 3+ -doped SiO 2 , Al 2 O 3 -SiO 2 , and P 2 O 5 -SiO 2 glasses were prepared by a sol-gel method from metal alkoxides, and their persistent spectral hole burning (PSHB) properties were investigated in relation to the local environment of the Eu 3+ ions in glass. Fluorescence line narrowing spectra indicated that Eu 3+ clustering occurred in SiO 2 glass. The addition of Al 2 O 3 or P 2 O 5 promoted better dispersion of Eu 3+ in the glass matrix, though two sites for Eu 3+ ions seemed to exist in Al 2 O 3 -SiO 2 glass. The holes were burned in the 7 F 0 f 5 D 0 line of the Eu 3+ ions using a Rhodamine 6G dye laser, the hole area of which increased proportionally with the content of hydroxyl groups. The holes that were burned by the photoinduced rearrangement of the OH bonds were thermally refilled and the barrier height of the burnt-state was determined as 0.14, 0.30, and 0.40 eV for P 2 O 5 -SiO 2 , SiO 2 , and Al 2 O 3 -SiO 2 glass, respectively.