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

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

Structural and magnetic properties of SrFe12O19 hexaferrite synthesized by a modified chemical co-precipitation method

Microwave absorption properties of La-substituted M-type strontium ferrites

We investigated La+Co-doped ${\mathrm{BaFe}}_{12}{\mathrm{O}}_{19}$ and ${\mathrm{SrFe}}_{12}{\mathrm{O}}_{19},$ a recently discovered improved variant of the most popular hard magnetic M-type hexaferrites, with nuclear magnetic resonance at low temperature, and in magnetic fields up to 5 T. Two satellite lines were observed between 70 and 76 MHz in the zero-field powder spectra of ${}^{57}\mathrm{Fe}.$ Satellites in Co+Ti-doped samples at the same frequencies have been assigned in the literature to k-Fe with a $f1$-Co neighbor, and a-Fe with a $f1$-Co neighbor. We report observation of a ${}^{59}\mathrm{Co}$-resonance in this structure. The signal is due to ${\mathrm{Co}}^{2+}$ in a low-spin state. Together with the field dependence of the hyperfine field this gives a very strong indication that ${\mathrm{Co}}^{2+}$ substitutes in the presence of La for $f2$-Fe, in contrast to the behavior assumed in Co+Ti-doped samples.

NMR characterization of Co sites in La+Co-doped Sr hexaferrites with enhanced magnetic anisotropy

We report on recent investigations on substituted hexaferrites of the type SrM, Sr 1-x La x Fe 12-x Co x O 19 (0 ≤ x ≤ 0.4). It was already proved that the simultaneous substitution of Sr and Fe by La and Co, respectively, significantly improves the permanent magnetic properties of the material. In order to get a deeper insight into the magetic order from an atomistic point of view, 57 Fe Mossbauer spectroscopy and NMR ( 57 Fe, 59 Co) studies were carried out. It is demonstrated that by applying local probe hyperfine techniques the knowledge about this complex material can be considerably improved. The main results concern the preferred Co 2+ occupation on the 4f 2 and the 2a sites and the occurrence of Fe 2+ ions in the substituted material.

Substituted ferrites studied by nuclear methods

NMR analysis of La+Co doped M-type ferrites

La-substituted Sr hexaferrite with different La concentrations $x$ were prepared and investigated in order to observe changes in magnetic and structural properties with the substitution. With diffraction methods a change in crystal structure below approximately $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from hexagonal with spacegroup $P{6}_{3}∕mmc$ to orthorhombic with spacegroup $Cmcm$ was observed for the fully substituted La hexaferrite and attributed to a lattice distortion. A large increase in magnetocrystalline anisotropy of ${\mathrm{LaFe}}_{12}{\mathrm{O}}_{19}$ at low temperatures was confirmed by measurements and associated with the formation of an orbital momentum of ${\mathrm{Fe}}^{2+}$ together with the lattice distortion. The localization of the ${\mathrm{Fe}}^{2+}$ ion at the $2a$ site was determined from the M\"ossbauer spectrum, showing also an indication for an orbital momentum in the hyperfine field. In contrast, neither a structural phase transition nor the formation of ${\mathrm{Fe}}^{2+}$ was observed at $xl1$ in x-ray diffraction or NMR spectroscopy.

Structural phase transition and magnetic anisotropy of La-substituted M -type Sr hexaferrite

The strongly exchange enhanced Pauli paramagnet LaCo$_9$Si$_4$ is found to exhibit an itinerant metamagnetic phase transition with indications for metamagnetic quantum criticality. Our investigation comprises magnetic, specific heat, and NMR measurements as well as ab-initio electronic structure calculations. The critical field is about 3.5 T for $H||c$ and 6 T for $H\bot c$, which is the lowest value ever found for rare earth intermetallic compounds. In the ferromagnetic state there appears a moment of about 0.2 $\mu_B$/Co at the $16k$ Co-sites, but sigificantly smaller moments at the 4d and $16l$ Co-sites.

M. W. Pieper

Papers

NMR characterization of Co sites in La+Co-doped Sr hexaferrites with enhanced magnetic anisotropy

Substituted ferrites studied by nuclear methods

NMR analysis of La+Co doped M-type ferrites

Structural phase transition and magnetic anisotropy of La-substituted M -type Sr hexaferrite

Itinerant electron metamagnetism in LaCo 9 Si 4