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

A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy effi ciency in the total energy lifecycle, has accelerated research into energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the effi ciency and performance of devices in electric power generation, conditioning, conversion, transportation, and other energy-use sectors of the economy. This review focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials, with an emphasis on their optimization for energy applications. Specifi cally, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure‐property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy effi ciency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

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Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient

Self-ordering electrochemistry: a review on growth and functionality of TiO2nanotubes and other self-aligned MOx structures

The current status of research and development in Fe-based bulk metallic glasses (BMGs) is reviewed. Bulk metallic glasses are relatively new materials possessing a glassy structure and large section thickness. These materials have an exciting combination of properties such as high mechanical strength, good thermal stability, large supercooled liquid region and potential for easy forming. Ever since the first synthesis of an Fe-based BMG in an Fe–Al–Ga–P–C–B system in 1995, there has been intense activity on the synthesis and characterisation of Fe-based BMGs. These BMGs exhibit some unique characteristics which have not been obtained in conventional Fe-based crystalline alloys. This uniqueness has led to practical uses of these bulk glassy alloys as soft magnetic and structural materials. This review presents the recent results on the glass-forming ability, structure, thermal stability, mechanical properties, corrosion behaviour, soft magnetic properties and applications of Fe-based bulk glassy a...

https://www.tandfonline.com/doi/pdf/10.1179/1743280412Y.0000000007

Iron-based bulk metallic glasses

We discuss the anomalous Hall effect in a two-dimensional electron gas subject to a spatially varying magnetization. This topological Hall effect does not require any spin-orbit coupling and arises solely from Berry phase acquired by an electron moving in a smoothly varying magnetization. We propose an experiment with a structure containing 2D electrons or holes of diluted magnetic semiconductor subject to the stray field of a lattice of magnetic nanocylinders. The striking behavior predicted for such a system (of which all relevant parameters are well known) allows one to observe unambiguously the topological Hall effect and to distinguish it from other mechanisms.

/pdf/topological-hall-effect-and-berry-phase-in-magnetic-4jtnbfna58.pdf

Topological Hall effect and Berry phase in magnetic nanostructures.

In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/65/5/202/pdf

Magneto-optical studies of current distributions in high-Tc superconductors

A comprehensive and systematic study on Sm(Co/sub bal/Fe/sub v/Cu/sub y/Zr/sub x/)/sub z/ magnets is made to completely understand the effects of composition and processing on their magnetic properties. These studies include the compositions required for the formation of cellular/lamellar microstructure, the effects of compositions (x, y, v, z) and processing (aging temperature and time) on microstructure and magnetic properties, the evolution of microstructure and magnetic properties and the magnetic hardening. All of these effects and results may be summed up as changing microstructure and microchemistry, which leads to a variation of the distribution and amount of Cu at the 1:5 cell boundaries. This becomes the most predominant factor controlling the pinning strength and therefore the coercivity and its temperature dependence.

High temperature 2:17 magnets: relationship of magnetic properties to microstructure and processing

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/3885_02.pdf

High density hexagonal nickel nanowire array

Sm2(Co,Cu,Fe,Zr)17 permanent magnets with their three-phase precipitation structure (cells, cell walls, and lamellae) show two characteristic features which so far are difficult to interpret but which are the prerequisites for high-temperature applications: (1) The hard magnetic properties only develop during the final step of the three-step annealing procedure consisting of homogenization, isothermal aging, and cooling. (2) Depending on the composition and on the annealing parameters, the temperature dependence of the coercivity can be easily changed from the conventional monotonic to the recent nonmonotonic behavior showing coercivities up to 1T even at 500K. The magnetic hardening during cooling is due to the fact that the cell walls order chemically and structurally during the cooling process. From an analysis of electron diffraction patterns of the superimposed structures existing before and after cooling it could be proven that a phase transition from a phase mixture of defective phases 2:17, 2:7, a...

Micromagnetism and the microstructure of high-temperature permanent magnets

Planar phase boundaries in hard-magnetic materials are found to act as strong repulsive or trapping barriers for the displacement of domain walls if the magnetocrystalline anisotropies of the two phases are different from each other. On the basis of the continuum theory of micromagnetism the coercive field of the phase boundaries is determined under the general conditions of different material parameters of the two phases (spontaneous magnetization J s , magnetocrystalline anisotropy K 1 , exchange constant A ). The results are applied to analyze the coercive field of the three-phase nanocrystalline Sm 2 Co 17 -based magnets.

H. Kronmüller

Papers

Magneto-optical studies of current distributions in high-Tc superconductors

High temperature 2:17 magnets: relationship of magnetic properties to microstructure and processing

High density hexagonal nickel nanowire array

Micromagnetism and the microstructure of high-temperature permanent magnets

Micromagnetic theory of the pinning of domain walls at phase boundaries