C. D. Fuerst

High‐remanence rapidly solidified Nd‐Fe‐B: Die‐upset magnets (invited)

The best starting materials for die‐upset magnets are moderately overquenched melt‐spun ribbons of Nd‐Fe‐B alloys with ∼20 at. % more rare earth than stoichiometric Nd2Fe14B. Remanence increases with increasing die‐upset (DU) level, reaching a maximum of 13.5 kG at 70% DU for ternary Nd‐Fe‐B magnets, beyond which cracking limits performance. Remanence is also limited by nonuniform deformation, particularly for moderately die‐upset magnets. Heat treatments, dysprosium substitutions, or low‐level additives can be used to compensate for the decreases in coercivity which accompany increases in die‐upset level. By die upsetting in stages and removing misaligned surfaces, a large Nd‐Fe‐Co‐B‐Ga magnet (∼30 g) has been produced with both a high remanence (14.2 kG) and a high‐energy product (48.5 MG Oe).

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.

Steric variation of the cerium valence in Ce2Fe14B and related compounds

Through measurements of the near‐edge Ce L3 x‐ray absorption structure we have investigated the influence of site volume on the cerium valence in Ce2Fe14B, Ce2Fe14BHx, and Ce2Fe17. The crystal structures of these compounds are isomorphic with those of technologically important permanent magnet materials. Confirming other experiments, we find that the Ce ion in Ce2Fe14B is in a strongly mixed‐valent α‐like state which is incompatible with a local 4f moment. Comparison of the spectroscopically determined valences of the three materials demonstrates a shift toward a γ‐like cerium state, which supports a magnetic moment, as the steric volume of the Ce site(s) increases. Our results strongly suggest that the volume of the rare‐earth site is the principal factor controlling the Ce chemical valence in these systems.

Magnetically hard Sm2Fe17Nx prepared by nitriding melt‐spun ribbons

We have obtained magnetically hard Sm‐Fe‐N ribbons with a room‐temperature intrinsic coercivity Hci=23 kOe by nitriding melt‐spun Sm‐Fe precursor ribbons. Best results are obtained by grinding the ribbons to a <25 μm powder, then heat treating the powder in vacuum for 1 h at 700 °C prior to nitriding in N2 gas at 450–480 °C. X‐ray diffraction shows that the primary phase is rhombohedral Sm2Fe17Nx, or possibly the disordered hexagonal modification of this phase having the TbCu7 structure. Higher coercivities will be obtainable if the volume fraction of soft magnetic impurity phases, such as α‐Fe, can be reduced by optimizing the composition and processing.

Enhanced coercivities in die-upset Nd-Fe-B magnets with diffusion-alloyed additives (Zn, Cu, and Ni)

Rapidly solidified Nd‐Fe‐B ribbons were crushed and mixed with a fine powder (10–100μm) of a pure element before hot working into fully dense anisotropic magnets. At the high temperatures and pressures required for densification some additives diffused throughout the ribbon matrix and presumably into the neodymium‐rich phase surrounding the Nd2Fe14B grains. In small concentrations (0.5–0.8 wt%), zinc, copper, and nickel additions enhanced the coercivities of die‐upset magnets by 100%, 77%, and 53%, respectively. Additives which did not diffuse thoroughly, such as manganese, had no measurable effect on the coercivities.Rapidly solidified Nd‐Fe‐B ribbons were crushed and mixed with a fine powder (10–100μm) of a pure element before hot working into fully dense anisotropic magnets. At the high temperatures and pressures required for densification some additives diffused throughout the ribbon matrix and presumably into the neodymium‐rich phase surrounding the Nd2Fe14B grains. In small concentrations (0.5–0.8 wt%), zinc, copper, and nickel additions enhanced the coercivities of die‐upset magnets by 100%, 77%, and 53%, respectively. Additives which did not diffuse thoroughly, such as manganese, had no measurable effect on the coercivities.

On the formation of NdFe9.5−xTx compounds (T = Ti, Cr, Mn)

Stable, isostructural, ternary compounds having compositions specified by NdFe9.5−xTx with T = Ti (x = 0.5), Cr (x = 1.5) and Mn (x ⩾ 3.5) have been discovered to form. We report magnetization and Curie temperature measurements for these phases and we tentatively identify the lattice symmetry and space group of their crystal structure from analyses of X-ray powder diffraction data.

Diffusion-alloyed additives in die-upset Nd-Fe-B magnets

In general, the stoichiometry of melt‐spun ribbons and subsequent hot‐worked Nd‐Fe‐B magnets derives from the composition of the starting ingot. We have shown, however, that it is possible to introduce small amounts of powdered metals into the ribbons after the ingot has been melt spun. Many elements, when mixed with the ribbons as a fine powder and then hot pressed, have been observed to diffuse into nearby ribbons, and in some cases (Cd, Cu, Au, Ir, Mg, Ni, Pd, Pt, Ru, Ag, and Zn) the elements diffused fairly evenly throughout the Nd‐Fe‐B ribbon matrix. These eleven elements significantly enhanced the coercivity of die‐upset magnets, with the highest coercivity observed with the addition of zinc (∼16 kOe).

Hard magnetic properties of melt‐spun Nd‐Co‐Fe‐B materials

The effects of cobalt substitution on the hard magnetic properties of melt‐spun neodymium‐iron‐boron materials are reported. Samples having the starting composition Nd0.135[(CoxFe1−x)0.95B0.05 ]0.865 with x=0, 0.05, 0.10, 0.15, 0.20, 0.35, 0.50, 0.75, and 1 were prepared by melt‐spinning over a range of substrate surface velocities (i.e., quench rates) and their room‐temperature magnetization characteristics measured. Choosing samples having maximal energy products, it was found that in the 0≤x≤0.5 interval the coercivity and remanence show modest overall diminution with x while the energy product declines more rapidly. Above x=0.5, the remanence, energy product, and especially the coercivity, severely deteriorate. X‐ray diffraction studies indicate that for x>0.5 formation of the Nd2Fe14B‐type structure is suppressed, and the samples contain progressively greater amounts of Nd2(Co,Fe)17.

Structural, magnetic, and magnetocaloric properties of (Hf0.83Ta0.17)Fe2+x materials

We have investigated the sensitivity of structural, magnetic, and magnetocaloric properties of (Hf0.83Ta0.17)Fe2+x alloys (x=−0.18, −0.09, −0.02, 0.00, 0.09, 0.26) to Fe content. As‐cast samples for all x consist essentially exclusively of hexagonal MgZn2‐type material. Via magnetization and differential scanning calorimetry measurements we find that all the alloys except x=0.26 feature both a ferromagnetic→antiferromagnetic (FM→AFM) transition at temperature T0 and a Neel transition. The most abrupt FM→AFM transition occurs in the x=−0.02 material which also exhibits the lowest T0 and largest low‐temperature moment. We report a magnetocaloric property, the field‐induced entropy change ΔS, for the x=0.00 and x=±0.09 alloys.

Nitriding of melt‐spun Nd‐Fe‐Mo alloys

The hard magnetic properties of nitrided melt‐spun Nd‐Fe‐Mo ribbons have been investigated as functions of composition, quench rate, and nitriding time. Ingots having the compositions Nd1Fe10Mo2, Nd1.15Fe10Mo2, Nd1.3Fe10Mo2, and Nd1.45Fe10Mo2 were melt spun over a range of quench rates specified by substrate wheel velocities vs in the 5 m/s≤vs≤40 m/s interval. Ribbons prepared with vs≤17.5 m/s from the Nd1.15Fe10Mo2 alloy consist only of stoichiometric NdFe10Mo2 with the ThMn12 crystal structure; at higher quench rates the ThMn12 structure is suppressed. It is found that all ribbons prior to nitriding are characterized by intrinsic coercivity Hci<0.4 kOe. For ribbons direct quenched at wheel speeds between 12.5 and 17.5 m/s, nitriding produces technologically significant coercivity (Hci∼6 kOe). The maxima in the technical magnetic properties as functions of vs resemble those observed in direct‐quenched Nd‐Fe‐B ribbons.