J. F. Herbst

Pr‐Fe and Nd‐Fe‐based materials: A new class of high‐performance permanent magnets (invited)

We report the properties of a new class of high‐performance permanent magnets prepared from Nd‐Fe‐B and Pr‐Fe‐B alloys. Magnetic hardening is achieved by rapid solidification. Energy products of these isotropic materials can exceed 14 MGOe with intrinsic coercivities of ∼15 kOe. X‐ray and microstructural analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites, strongly suggesting that the coercivity mechanism is of the single‐domain particle type. The crystallites are composed of an equilibrium R‐Fe‐B intermetallic phase having tetragonal symmetry, and the stability of this phase with respect to other rare earths and other metalloids has been investigated.

High‐energy product Nd‐Fe‐B permanent magnets

We have explored the hard magnetic properties of melt‐spun Nd‐Fe‐B alloys. A maximum energy product of 14.1 MG Oe has been observed, the highest value ever reported for a light rare earth‐iron material. X‐ray analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites of an equilibrium Nd‐Fe‐B intermetallic phase. The observed grain sizes are in or near the estimated single‐domain range, suggesting that the coercivity arises principally from the formation of single‐domain particles.

Preferential site occupation and magnetic structure of Nd2(CoxFe1−x)14B systems

Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(Co/x/Fe/1-x/)14B alloys (x = 0,0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. Throughout the entire composition range the Nd2Fe14B-type tetragonal crystal structure is maintained, with the lattice constants decreasing significantly as the Co concentration x increases. It is found that the J2-type transition-metal sites are preferentially occupied by Fe ions in the pseudoternary systems, a result which is analogous to the preferential Fe occupation of c sites previously observed in hexagonal Nd2(Co/x/Fe/1-x/)17 alloys.

Structural and magnetic properties of Nd2Fe14B (invited)

We describe detailed analyses of neutron powder diffraction data on Nd2Fe14B at several temperatures and discuss relationships between the crystal structure and the magnetic properties via comparison with other rare earth‐transition metal systems, Nd2Fe17 in particular. Diffraction studies have also been performed on optimum energy product melt‐spun Nd‐Fe‐B ribbons. Those results demonstrate that the ribbons are comprised of Nd2Fe14B particles with diameters of a few hundred angstroms.

Electronic structure calculations for LaNi5 and LaNi5H7: energetics and elastic properties

Abstract Density functional calculations of the electronic structure and enthalpy of formation Δ H of LaNi 5 and LaNi 5 H 7 are reported. Single-crystal elastic constants and Voigt–Reuss–Hill polycrystalline moduli were calculated for both materials using a stress-based least-squares fitting methodology. We obtain Δ H (0 K)=−40 kJ/mol H 2 for the hydride at zero temperature. Incorporating a Debye estimate of the phonon contribution we find Δ H (298 K)∼−39 kJ/mol/H 2 , a value that compares favorably with experimental determinations of −32 to −35 kJ/mol/H 2 . Our results indicate that the H–Ni and H–La interactions in the hydride are primarily metallic with a small ionic component. The calculated elastic moduli are in excellent accord with single-crystal measurements on LaNi 5 and with available data for polycrystalline samples of the parent and hydride.

Magnetic hardening of Ce2Fe14B

We report an effort to optimize the room-temperature permanent magnet properties of Ce-Fe-B materials rapidly solidified by melt spinning. Starting alloy compositions in the ternary phase diagram were selected systematically. Ribbons were melt spun from them at a quench wheel velocity of 35 m/s, corresponding to a solidification rate high enough to yield mostly amorphous or nanocrystalline material. Heat treatment above 450 °C crystallizes Ce2Fe14B, as x-ray diffraction clearly indicates, with the concomitant development of hard magnetic properties. The anneal temperature yielding optimum remanence Br, intrinsic coercivity Hci, and energy product (BH)max was determined in each case. For the ingot composition Ce17Fe78B6, we obtain Br = 4.9 kG, Hci = 6.2 kOe, and energy product (BH)max = 4.1 MGOe in ribbons comprised principally of Ce2Fe14B. This composition differs substantially from the optimum Nd13Fe82B5 stoichiometry for melt-spun magnets based on Nd2Fe14B and can be understood from a comparison of the ...

Crystal and magnetic structure of Pr2Fe14B and Dy2Fe14B

Rietveld analyses of neutron powder diffraction data on Pr2Fe14B and Dy2Fe14B are reported. Both phases form the Nd2Fe14B tetragonal crystal structure (P42/mnm). At 77 and 293 K our analyses indicate that all magnetic moments in each compound are collinear with the c axis. The rare‐earth moments in Pr2Fe14B are parallel to the Fe moments (ferromagnetic) and are antiparallel in Dy2Fe14B (ferrimagnetic).

Structural and magnetic properties of R3(Fe,T)29 compounds

We report the formation of several members of the recently discovered class of R3(Fe,T)29 compounds and some of their intrinsic properties. Focusing on materials with T=Ti and Cr, we have prepared R3(Fe,Ti)29 (R=Ce, Pr, Nd, Sm) and R3(Fe,Cr)29 (R=Ce, Nd, Sm) compounds which all feature the novel monoclinic crystal structure first established for Nd3(Fe,Ti)29. In each of the Ti‐ and Cr‐containing groups the Ce member has the smallest magnetization, Curie temperature, and unit cell, suggesting that the Ce ion is tetravalent, or nearly so, in this family of materials. We observed no evidence of R3(Fe,Ti)29 phase formation for R=Y, Gd, Dy, and Er. Our magnetization measurements indicate that only Nd3(Fe,Ti)29 and Nd3(Fe,Cr)29 exhibit spin reorientations, at temperatures of 235 and 145 K, respectively.

ESTIMATING THE EFFECTIVE MAGNETOSTRICTION OF A COMPOSITE: A SIMPLE MODEL

For a composite consisting of magnetostrictive particles dispersed in a nonmagnetostrictive matrix, a simple, approximate model for the effective magnetostriction, defined as the ratio of the magnetostriction of the composite to that of the magnetostrictive component, is described. The predictions of the model compare well with measurements on SmFe2-based composites having Al and Fe matrices.

MAGNETOSTRICTIVE SMFE2/METAL COMPOSITES

We have fabricated novel magnetostrictive composites consisting of SmFe2 embedded in an Fe or Al matrix which feature high magnetostriction and good mechanical strength. Hot pressed composites of 50% SmFe2/50% Fe or Al by volume have magnetostrictive strains parallel to the applied magnetic field, λ∥, between −280 and −440 ppm when consolidated at 610 (Fe) or 540 °C (Al). One unique feature of this work is the production of the magnetostrictive SmFe2 component by melt spinning; ribbon-based composites with Al combine significant magnetic coercivity, Hci=3 kOe, with high magnetostriction (λ∥=−280 ppm). Unlike their brittle SmFe2 parent materials, the composites are easily machinable.