Exchange hardening of nanostructured two-phase systems composed of an aligned hard phase and a soft phase with high magnetization is investigated using an approach which yields analytic nucleation fields from the micromagnetic vector equation, and accounts for interactions between the soft regions. In suitable structures the nucleation field is proportional to the volume-averaged anisotropy constant. For example, a multilayer composed of alternating 2.4 nm hard-magnetic ${\mathrm{Sm}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{N}}_{3}$ layers and 9 nm ${\mathrm{Fe}}_{65}$${\mathrm{Co}}_{35}$ layers can have an energy product as high as 1 MJ/${\mathrm{m}}^{3}$ (120 MG Oe), with a rare-earth content of only 5 wt %. Giant energy products may also be achieved in suitable cellular and disordered structures.

/pdf/giant-energy-product-in-nanostructured-two-phase-magnets-44b3bql0kz.pdf

Giant energy product in nanostructured two-phase magnets.

Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.

https://iopscience.iop.org/article/10.1088/0022-3727/38/23/R01/pdf

Developments in magnetocaloric refrigeration

Coercivity enhancement of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B powders by the diffusion of Nd–Cu eutectic alloys

Micromagnetic calculations are used to investigate coercivity and energy products of magnets consisting of an aligned hard‐magnetic skeleton phase and a soft‐magnetic phase with high saturation magnetization. Compared to the present‐day theoretical limit of 516 kJ/m3 for single‐phase Nd2Fe14B, the energy product in suitable nanostructured Sm2Fe17N3/Fe65Co35 composites is predicted to be as high as 1090 kJ/m3. The influence of the skeleton’s texture and shape is discussed, and aligned nanocrystalline two‐phase magnets are compared with remanence‐enhanced isotropic magnets. In particular, it is shown how the nucleation‐based analytical approach breaks down in the isotropic limit. Finally, we outline conceivable processing methods and discuss potential applications of ‘‘megajoule’’ magnets.

/pdf/aligned-two-phase-magnets-permanent-magnetism-of-the-future-3xx3tknb90.pdf

Aligned two‐phase magnets: Permanent magnetism of the future? (invited)

Nanostructured permanent magnetic materials, including exchange-coupled nanocomposite permanent magnets, are considered as the next generation of high-strength magnets for future applications in energy-saving and renewable energy technologies. However, fabrication of bulk nanostructured magnets remains very challenging because conventional compaction and sintering techniques cannot be used for nanostructured bulk material processing. In this paper we review recent efforts at producing bulk nanostructured single-phase and composite magnetic materials with emphasis on grain size control, anisotropy generation and interface modification.

Fabrication of bulk nanostructured permanent magnets with high energy density: challenges and approaches.

Measured magnetization and 57 Fe hyperfine fields at T∼0 K for Y 2 Fe 17 and Y 2 Fe 17 Z 3-δ , with Z=H, C, or N and δ∼0. 5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1 μg is very close to the fully saturated value of 39.4 μg

/pdf/magnetization-and-57fe-hyperfine-fields-in-y2fe17z3-delta-z-4xo9sl8f4o.pdf

Magnetization and 57Fe hyperfine fields in Y2Fe17Z3- delta (Z=H, C, or N) interstitial compounds.

For pt. I see ibid., vol.4, p.5573 (1992) The magnetization, Curie temperature and 57Fe hyperfine fields are measured for Y(Fe11Ti) and the interstitial nitride and carbide. Data are compared with the results of electronic structure calculations by the OLCAO method. All compounds are weak ferromagnets. The increase of magnetization from 19.0 mu B FU-1 in the parent compound to 21.6 mu B FU-1 in the carbide and nitride is essentially due to volume expansion. A corresponding increase in average 57Fe hyperfine field is observed in the nitride, but not in the carbide due to different 4s transferred hyperfine fields.

Gas-phase interstitially modified intermetallics R(Fe11Ti) Z1- delta . II. 3d magnetization of the compounds Y(Fe11Ti) Z1- delta (Z=N, C)

Maghemite in a tuffite deposit from Patos de Minas, Brazil, has been isolated by highgradient magnetic separation and analyzed by X-ray diffraction, electron microprobe analysis, Mossbauer spectroscopy, and magnetization measurements The maghemite constitutes 5 wt% of the tuffite, where it is present as separate grains in the 1-400-~m-size range, some with associated anatase inclusions or intergrowths The lattice parameter is ao = 08380(2) nm No Fe2+ is detected, but the composition of the grains shows Fe ~ Mg 2:: Ti The averaged composition and proposed cation distribution is [FeO88SioOlM~1l]{FeO96M~30 Tio32Aloo7Cro03Mnoo2Do30}04, where [ ] and { } denote A and B sites, respectively, of the spinel lattice, and D denotes cation vacancies Magnetization, O"s,is 18-31 J/(T kg) at room temperature, and the Curie temperature, Tc, is 320-360 °C The Mgrich maghemite is inherited by magnetic soils forming on the tuffite

/pdf/characterization-of-mg-rich-maghemite-from-tuffite-i3kp3b2261.pdf

Characterization of Mg-rich maghemite from tuffite

Nd(Fe12−xMox)Ny (x=1.25 and 1.50, y=0.9) 57Fe Mossbauer spectra of parent compounds and nitrides have been measured at 15 and 293 K. Hyperfine fields are little changed by the presence of nitrogen at 15 K, where the average value is about 25 T but at 293 K there is a change in average hyperfine field from 19 T for parent compounds to 24 T for nitrides due to the increase in Curie temperature. Spin reorientations occur in the parent compounds in the temperature range between 160–190 K but not in the nitrides, which show c‐axis anisotropy throughout this range.

57Fe Mössbauer studies of Nd(Fe12−xMox) and Nd(Fe12−xMox)N0.9 (x=1.25 and 1.50)

The iron nitrides Fe3N, Fe4N and Fe8N (impure) have been prepared by treating iron in NH3/H2 gas mixtures followed by appropriate heat treatment. Fe8N has also been prepared by mechanical alloying. Mossbauer spectra are analysed in terms of iron sites with different nitrogen environments, and results are compared with magnetization measurements and band-structure calculations. Moments and hyperfine fields on sites with nitrogen neighbours are reduced by p-d hybridization, whereas the sites farthest from nitrogen have the lowest charge and the highest moments. There is no evidence of an exceptionally high magnetization in any of these compounds.

Mössbauer spectra and magnetic properties of iron nitrides

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