Permanent magnet development has historically been driven by the need to supply larger magnetic energy in ever smaller volumes for incorporation in an enormous variety of applications that include consumer products, transportation components, military hardware, and clean energy technologies such as wind turbine generators and hybrid vehicle regenerative motors. Since the 1960s, the so-called rare-earth “supermagnets,” composed of iron, cobalt, and rare-earth elements such as Nd, Pr, and Sm, have accounted for the majority of global sales of high-energy–product permanent magnets for advanced applications. In rare-earth magnets, the transition-metal components provide high magnetization, and the rare-earth components contribute a very large magnetocrystalline anisotropy that donates high resistance to demagnetization. However, at the end of 2009, geopolitical influences created a worldwide strategic shortage of rare-earth elements that may be addressed, among other actions, through the development of rare-earth-free magnetic materials harnessing sources of magnetic anisotropy other than that provided by the rare-earth components. Materials engineering at the micron scale, nanoscale, and Angstrom scales, accompanied by improvements in the understanding and characterization of nanoscale magnetic phenomena, is anticipated to result in new types of permanent magnetic materials with superior performance.

Perspectives on Permanent Magnetic Materials for Energy Conversion and Power Generation

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.

In this paper a computational high-throughput screening (HTS) approach to the search for alternative permanent magnetic materials is presented. Systems considered for a start are binary intermetallic compounds composed of rare-earth (RE) and transition metal (TM) elements. With the tight-binding-linear muffin-tin-orbital-atomic-sphere-approximation (TB-LMTO-ASA) method of density functional theory (DFT) a variety of RE?TM intermetallic phases is investigated and their magnetic properties are obtained at rather low computational costs. Next, interstitial elements such as boron, carbon and nitrogen in these phases are considered. For promising candidate phases with high and stable spontaneous ferromagnetic polarization, the calculated local magnetic moments and exchange coupling parameters, as obtained from TB-LMTO-ASA calculations, are then used for Monte Carlo simulations to identify candidates with sufficiently high Curie temperatures (Tc). Finally, magnetocrystalline anisotropy constants (K1) of the most promising candidate phases are calculated with accurate, potential-shape-unrestricted DFT calculations using the Vienna ab initio simulation package. The computational HTS procedure is illustrated by results for a selection of hard-magnetic RE?TM phases like RETM5, RE2TM17 and RE2TM14B.

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Ab initio screening methodology applied to the search for new permanent magnetic materials

Abstract The magnetic properties of the rhombohedral Ce 2 Fe 17 − x Ga x solid solutions, with x equal to 0.3, 0.5, 0.7, 1.0, and 2.0, have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. Magnetization studies indicate that, as the unit cell volume expands with Ga addition, the Curie temperature increases from 238 K for Ce 2 Fe 17 to 406 K for Ce 2 Fe 15 Ga 2 . Powder neutron diffraction measurements at 295 and 14 K indicate that the Fe moments are both oriented in the basal plane and increase with increasing Ga content. The Mossbauer spectra, which have been fit with a binomial distribution of the near-neighbor environments, indicate that the maximum hyperfine field, H max , increases with increasing Ga content and is largest for the 6c site. The decremental field, ΔH , per added Ga atom is virtually independent of temperature and Ga content, indicating that the next-nearest-neighbor interactions are very small. The compositional dependence of the isomer shifts for R 2 Fe 17 − x M x , where R is Ce or Tb and M is Al, Si, or Ga, is analyzed in terms of volume and chemical terms, the second of which reveals the importance of the electronic configuration of M in understanding its influence on the magnetic properties of these solid solutions. Finally, the similarities and differences in the changes in crystallographic, magnetic, and Mossbauer spectral properties of R 2 Fe 17 − x M x , at low x values, indicate that electronic rather than structural factors are more important in modifying the magnetic properties and indicate the need for extensive band structure calculations on these solid solutions.

A Magnetic, Neutron Diffraction, and Mössbauer Spectral Study of the Ce_2Fe_17-xGa x Solid Solutions.

R 2 Fe 17 (R = rare earth) intermetallic compounds constitute one of the most important classes of materials identified as high-energy permanent magnet materials. They crystallize either in the rhombohedral Th 2 Zn 17 structure (for light R) or in the hexagonal Th 2 Ni 17 structure (for heavy R). In this article, we discuss the variations in the lattice parameters (unit cell volume), site occupancies and Curie temperature when non-transition and transition metals are substituted for Fe in R 2 Fe 17 compounds.

On the Structural and Magnetic Properties of R2Fe17—x(A,T)x (R = Rare Earth; A = Al, Si, Ga; T = Transition Metal) Compounds

The structural and magnetic properties of 2 : 17 compounds, their nitrides and carbides and the recent development of the novel 3 : 29 compounds are discussed.

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Recent advances in 2 : 17 and 3 : 29 permanent magnet materials

Extended x-ray absorption fine structure measurements of the Fe K, and Pr and Er LIII absorption edges, were carried out to elucidate the relationship between the local structure and magnetism in Al substituted, partially nitrogenated (Er0.5Pr0.5)2Fe17 permanent magnets. We find that the nitrogenation acts to dilate both the Fe–Fe and the (Pr,Er)–Fe bonds, thus raising the TC via a magnetovolume effect. However, nitrogenation of Al-substituted samples acts to decrease the average Fe–Fe bond distance, thus reducing the exchange on the Fe sublattice and lowering TC relative to the nitrogenated parent compound. This is opposite to the trend measured in systems when N is absent.

K. G. Suresh

Papers

On the Structural and Magnetic Properties of R2Fe17—x(A,T)x (R = Rare Earth; A = Al, Si, Ga; T = Transition Metal) Compounds

Recent advances in 2 : 17 and 3 : 29 permanent magnet materials

Effect of Al substitution on the local environments and magnetic properties of partially nitrogenated (Er0.5Pr0.5)2Fe17 permanent magnets

On the Structural and Magnetic Properties of R2Fe17‐x(A,T)x (R: Rare Earth; A: Al, Si, Ga; T: Transition Metal) Compounds.

The effect of Al substitution on the local structure and magnetism in (Er/sub 0.5/Pr/sub 0.5/ /sub 2/ (Fe/sub 1-x/Al/sub x/)/sub 17/ N/sub y/