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

Effects of the grain boundary on the coercivity of barium ferrite BaFe 12 O 19

Recent Developments in Perpendicular Magnetic Anisotropy Thin Films for Data Storage Applications

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

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

We report the existence and characterization of the stable ternary phase Tb3(Fe1-xTix)29. The structural characterization by X-ray powder diffraction is evidence for a monoclinic structure (P21/c space group) with refined lattice parameters a=10.583(1) AA, b=8.5116(7) AA, c=9.6736(9) AA and beta =97.018(5) degrees . A large magnetovolume effect has been observed in the volume thermal expansion at the order temperature Tc=455 K. The anisotropy has been measured by using the singular point detection technique. Two distinct anisotropy fields of relatively high intensity have been detected. The HA values measured at 6.4 T and 1.9 T at 293 K and both increase markedly with decreasing temperature. Below 200 K, the approach to saturation corresponding to the highest HA develops into a first-order magnetization process.

https://iopscience.iop.org/article/10.1088/0953-8984/6/46/002/pdf

Structural and magnetic characterization of the new ternary phase Tb3(Fe1−xTix)29

Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 A)/Au(27 A)]27 multilayers grown at different sputtering pressures (0.8–9 Pa). All the as-prepared samples display perpendicular magnetic anisotropy that has its maximum value at intermediate pressures, while a monotonic decrease of the saturation magnetization was found with increasing pressure. As found by MFM, a relevant characteristic of these systems is the magnetic stripe domain structure. The application of the stripe domain model of Draaisma and de Jonge [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)] allows us to satisfactorily reproduce the magnetization processes and to determine the anisotropy constants (Keff) starting from the domain width. The obtained Keff values display the same behavior and order of magnitude as those deduced from the area enclosed between parallel and perpendicular magnetization curves. The measurement of the stripe domain size with MFM thus results in an easy...

G. Turilli

Papers

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

Structural and magnetic characterization of the new ternary phase Tb3(Fe1−xTix)29

Perpendicular magnetic anisotropy and stripe domains in ultrathin Co/Au sputtered multilayers

Influence of the particle size and intrinsic magnetic characteristics on the coercivity of sintered magnets

Investigation of magnetic and magnetotransport properties of Co-based multilayered granular films