In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

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

Aim This paper presents a tool for long-term global change studies; it is an update of the History Database of the Global Environment (HYDE) with estimates of some of the underlying demographic and agricultural driving factors.

Methods  Historical population, cropland and pasture statistics are combined with satellite information and specific allocation algorithms (which change over time) to create spatially explicit maps, which are fully consistent on a 5′ longitude/latitude grid resolution, and cover the period 10,000 bc to ad 2000.

Results  Cropland occupied roughly less than 1% of the global ice-free land area for a long time until ad 1000, similar to the area used for pasture. In the centuries that followed, the share of global cropland increased to 2% in ad 1700 (c. 3 million km2) and 11% in ad 2000 (15 million km2), while the share of pasture area grew from 2% in ad 1700 to 24% in ad 2000 (34 million km2) These profound land-use changes have had, and will continue to have, quite considerable consequences for global biogeochemical cycles, and subsequently global climate change.

Main conclusions  Some researchers suggest that humans have shifted from living in the Holocene (emergence of agriculture) into the Anthropocene (humans capable of changing the Earth's atmosphere) since the start of the Industrial Revolution. But in the light of the sheer size and magnitude of some historical land-use changes (e.g. as result of the depopulation of Europe due to the Black Death in the 14th century and the aftermath of the colonization of the Americas in the 16th century) we believe that this point might have occurred earlier in time. While there are still many uncertainties and gaps in our knowledge about the importance of land use (change) in the global biogeochemical cycle, we hope that this database can help global (climate) change modellers to close parts of this gap.

The HYDE 3.1 spatially explicit database of human‐induced global land‐use change over the past 12,000 years

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

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.

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

Journal De Physique. IV : JP

Influence du carbone sur les proprietes magnetiques du systeme Nd 2 Fe 14 B 1−x C x . Le champ d'anisotropie augmente avec x; l'aimantation a saturation et le point de Curie diminuent. A basse temperature, on observe des transitions magnetiques du premier ordre

J.L. Sanchez

Papers

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

Journal De Physique. IV : JP

Magnetic anisotropy of carbon doped Nd2Fe14B

Magnetic and Mössbauer study of Y2-xNdxFe14B

Magnetic properties of polycrystalline BaFe12O19 thin films prepared from metallorganic decomposition on ZrO2-coated Si substrates