This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substra...

Recent progress in some composite materials and structures for specific electromagnetic applications

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.

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Microwave ferrites, part 1: fundamental properties

Neutron diffraction studies of some hexagonal ferrites: BaFe12O19, BaMg2W and BaCo2W

The static and high frequency magnetic properties have been studied for BaW ferrites BaZn2−xCoxFe16O27 with x=0, 0.5, 0.7, 1.0, 1.5, and 2.0. The results showed that Co ions are able to modify the anisotropy from c axis to c plane at x=0.6–0.7. For the BaW composites with c-plane anisotropy, as the Co concentration x increases, the resonance frequencies fR are shifted to higher frequency, from 2.5 GHz at x=0.7 to 12.0 GHz at x=1.5. The permeabilities μ0′ and μmax″ are 2.4–2.2 and ∼0.8, respectively, for composites with 35% (by volume) BaW ferrite powders at x=0.7 and 1.0. The parameters indicate that BaZn2−xCoxFe16O27 composites have suitable high frequency properties for use as electromagnetic materials with low reflectivity at microwave frequencies. In addition, for BaW composites with c-axis anisotropy, there exist both the domain wall and natural resonances. However, only natural resonance is observed for composites with c-plane anisotropy.

High-frequency magnetic properties of W-type barium–ferrite BaZn2−xCoxFe16O27 composites

The complex permeability and permittivity, the relationship between dynamic and static properties, and attenuation properties have been studied for BaZn2−xCoxFe16O27 composites with x=0.7–2.0. The results show that the resonance frequency fR is controlled by the out-of-plane anisotropy field Hθ, and the static permeability μ0′ is related to the in-plane anisotropy field Hφ. For appropriate resonance frequency and high complex permeability, the percentage bandwidths of ΔW=Δf/f0>90% for attenuation of 10 dB or more can be achieved by composites with x=0.7 and 1.0. Therefore, W-type barium ferrites with easy c-plane anisotropy are good candidates for electromagnetic materials with low reflectivity over broad frequency range at microwaves.

Microwave attenuation properties of W-type barium ferrite BaZn2−xCoxFe16O27 composites

Macroscopic magnetic measurements, Mossbauer, and neutron diffraction studies have been carried out on BaCoxZn2−xFe16O27 [(Zn−Co)2‐W] single‐crystal hexaferrites. The extrapolated saturation magnetization values σs(0) are slightly lower than those found in the literature. Such differences have been justified as due to the different Co and Zn distributions among the tetrahedral sites.The Co ions induce a strong planar contribution to the magnetic anisotropy. At low temperature, the spin order, found from the macroscopic magnetic measurements and preliminary neutron diffraction studies, is planar, while the Mossbauer data indicate a conical order with an angle of 70° with respect to the c axis. By increasing the temperature, the cobalt anisotropy decreases and a spin reorientation to easy axis is observed.

Magnetic, Mössbauer, and neutron diffraction investigations of W‐type hexaferrite BaZn2−xCoxFe16O27 single crystals

Polycrystauine samples of composition BaZn2−xCox
16O27 (O<x<2) have been examined by thermomagnetic analysis and Mossbauer spectroscopy. Cobalt ions enter preferentially the octahedral sites of the spinel block and give rise to strong perturbations of the magnetic order. The 12k sublattice splits into two magnetically inequivalent sublattices. A spin reorientation transition is observed at a temperature that increases with increasing cobalt content. Co2-W undergoes a transition from planar to conical order at 515±5 K.

Mössbauer investigation of W-type hexaferrite of composition BaZn2−xCoxFe16O27

Etruscan trade amphoras are a kind of pottery widely diffused in the archaic period (VII–VI century B.C.) in many archaeological sites of the Mediterranean area. It is only recently that the problem of their origin with regard to their similarity with Phoenician amphoras has been taken into consideration. Indeed, from the morphologic point of view, the earlier Etruscan and Phoenician trade amphoras are rather similar in shape. It is therefore difficult to distinguish, on archaeological grounds alone, the different production centers. We have therefore performed a detailed Mossbauer investigation on amphoras found in different sites of Central Italy and Sicily. The obtained data allowed us to assess with certainty the Etruscan origin of a peculiar type of amphoras for which a Phoenician origin had previously been proposed.

Mössbauer investigation on archaic trade amphoras of Phoenician and Etruscan origin

Fine powder samples of substituted Barium M-type haxaferrites with composition BaFe12−2xCoxTixO19 (0.72<x<0.84) have been studied by macroscopic magnetic measurements (Curie temperature, magnetization, anisotropy field, coercive field) and Mossbauer spectroscopy. The analysis of the magnetization data and of the distribution of Co−Ti among the iron sublattices suggest that the Gorter spin arrangement is no longer valid. The Mossbauer data allow to rule out the possible presence of superparamagnetic fine particles as the origin of the observed scattering in the magnetization values.

E. Calabrese

Papers

Magnetic, Mössbauer, and neutron diffraction investigations of W‐type hexaferrite BaZn2−xCoxFe16O27 single crystals

Mössbauer investigation of W-type hexaferrite of composition BaZn2−xCoxFe16O27

Mössbauer investigation on archaic trade amphoras of Phoenician and Etruscan origin

Superparamagnetic effects and cation distribution in industrially produced hexaferrite fine powders