Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

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

It was shown by X-ray diffraction that rare earth compounds of the type R2Fe17 can dissolve appreciable amounts of carbon up to x = 0.6 in R2Fe17Cx at 900 °C. The increase in the lattice constants is accompanied by a strong increase in Curie temperature. From a structure determination (R = 7%) it was found that the carbon atoms dissolve interstitially and occupy the 9e position in the Th2Zn17 structure type (R3m). This ternary carbide is present as the main phase when compounds of the type R2Fe14C are heated above their decomposition temperatures. The decomposition proceeds by means of a solid state transformation and the corresponding transformation temperatures Tt vary strongly with the R component. In several of the light rare earth systems Tt becomes rather low and makes an annealing treatment ineffective with regard to the formation of R2Fe14C phases. The magnetic properties of the R2Fe14C phases are compared with those of R2Fe14B. It is shown that the former follow the magnetic behaviour of the latter almost completely.

Formation and magnetic properties of the compounds R2Fe14C

New permanent magnet materials

RETM5 and RE2TM17 (RE=rare earth, TM=transition metal) permanent magnets are discussed in this review. The scientific and technological considerations which have guided their development are described. Along with a discussion of the various research efforts reported in the literature, they help establish the framework within which most of the work can be integrated and understood. In regard to the RETM5 alloys, the review limits itself to the technologically significant SmCo5 materials; others are discussed minimally, and only to the extent that they contributed to overall magnet development efforts. Likewise, from among the fabrication technologies, the review is limited to those processes which have been demonstrated as capable of producing magnets in bulk useful form. Areas covered in this review include phase diagrams, crystal structures, magnetocrystalline anisotropy, coercivity mechanisms, fabrication technologies, processing‐microstructure‐property interrelationships, and thermal effects related to...

RETM5 and RE2TM17 permanent magnets development

A magnetic characterization of the CO 1-x Pt x system with 0\leqx\leq0.8 has been made. The anisotropy field (H A ) of both the hard magnetic phases, i.e. hexagonal at low Pt content and tetragonal around the equiatomic composition, has been measured by the singular point detection technique (SPD). The dependence of H A on composition and temperature is given. The solid state transformation diagram at low Pt content has been completed by using the thermomagnetic analysis (TMA).

Magnetocrystalline anisotropy and phase transformation in Co-Pt alloy

Magnetic properties and anisotropy of Co-Cr alloy

A systematic study of the effects of Co substitution on the crystal structure and magnetic anisotropy of Y2Fe14−xCoxB and Nd2Fe14−xCoxB has been carried out in order to clarify the different roles and contributions to the anisotropy of both rare earth and transition metals. The substitution of Co leads to an enhancement of the Curie temperature in both series of compounds. The Co has been found to provide a planar anisotropy which is larger than that of the axial anisotropy of Fe. A marked site preference has been observed. Two distinct contributions of the Co to the anisotropy have been observed.

3d and 4f magnetism in Nd2Fe14−xCoxB and Y2Fe14−xCoxB compounds

High field first order transitions in PrCo5±x: The role of the K3 anisotropy constant

The effects of substitution of Ba2+ ions for some trivalent rare-earth ions on the magnetic properties and phase formation of M-, W- and X-type hexaferrites are studied. The compounds considered were: Ba1-REx/2Nax/2Fe12O19 with RE = gadolinium, lanthanum, lutetium, samarium and 0.0 ⩽x ⩽ 0.3; Ba0.9La0.05Na0.05Zn2Fe16O27 and Ba1.9La0.05Na0.05Zn2Fe28O46. The samples were prepared by standard ceramic procedures and have been investigated by thermomagnetic analysis, X-ray diffraction, Mossbauer spectroscopy, scanning electron microscope and magnetic measurements. Monophasic BaREx/2-M compounds were found for 0.0⩽x<0.2 giving an indication of the solubility range of the rare-earth (RE) atoms in this phase. The intrinsic magnetic properties and the coercive field are lowered with the exception of the lanthanum-substituted hexaferrite. No significant differences in the phase formation and magnetic properties were observed for the W- and X-type hexaferrites.

F. Leccabue

Papers

Magnetocrystalline anisotropy and phase transformation in Co-Pt alloy

Magnetic properties and anisotropy of Co-Cr alloy

3d and 4f magnetism in Nd2Fe14−xCoxB and Y2Fe14−xCoxB compounds

High field first order transitions in PrCo5±x: The role of the K3 anisotropy constant

Magnetic and mössbauer study of rare-earth-substituted M-, W- and X-type hexagonal ferrites