J. J. Croat

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.

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.

Neutron diffraction studies of Nd2(CoxFe1−x)17 alloys: Preferential site occupation and magnetic structure

Rietveld analyses of room‐temperature neutron powder diffraction data for seven Nd2(CoxFe1−x)17 alloys (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. In the ternary systems we find that the c‐type transition metal sites are preferentially occupied by Fe ions; concomitantly, the d and h sites have Co occupations larger than those predicted by stoichiometry, while the f sites deviate only slightly from random occupation. The magnetic moments obtained from the data refinements indicate a transition from a low to a high spin state on the d,  f, and h sites as Fe is incorporated into the structure; a reverse trend is exhibited by the moments on the c site.

Magnetic hardening of Pr‐Fe and Nd‐Fe alloys by melt‐spinning

The dependence of the hard magnetic properties of melt‐spun Nd1−xFex (0.5⩽x⩽0.7) and Pr1−xFex (0.4⩽x⩽0.7) alloys on quench rate is reported; the latter was controlled by varying the substrate surface velocity (vs ) of the melt‐spinner. All of the alloys show an appreciable maximum in coercivity (Hci ) as a function of quench rate. For Nd1−xFex , a peak room temperature Hci of 8.65 kOe was found for a Nd0.5Fe0.5 alloy. Room‐temperature coercivities of ∼7.5 kOe were found in Pr1−xFex over the interval 0.6⩽x⩽0.7. The temperature dependence of the coercivity changed from a 1/T dependence for the most amorphous alloys to exponential for the alloys exhibiting maximum coercivity; coercivities as high as 78 kOe were found at 20 K. X‐ray data indicate that the coercive force results from an amorphous and/or very finely crystalline microstructure whose average particle size and/or intrinsic anisotropy varies as a function of quench rate. Crystallization studies, using differential scanning calorimetry and x‐ray dif...

Permanent magnet properties of rapidly quenched rare earth-iron alloys

Recent studies have demonstrated the potential of producing rare earth-iron permanent magnets by rapid quench processing. High coercivity, unachievable by traditional powder-metallurgy methods, has been obtained either by crystallization of an amorphous or rapidly quenched precursor or by direct-quenching. Results obtained by both techniques on a variety of rare earth-iron alloys are discussed. In particular, melt-spun Nd-Fe and Pr-Fe alloys develop an appreciable maximum (7-9 kOe) in room temperature coercivity (H ci ) as a function of quench rate, which is controlled by varying the surface velocity of the melt-spinner substrate. Even higher H ci (>20 kOe) has been observed in Sm-Fe. Magnetic and crystallization properties suggest that the coercive force of these materials is related to the formation of one or more metastable rare earth-iron phases.

Observation of large room‐temperature coercivity in melt‐spun Nd0.4Fe0.6

An intrinsic room‐temperature coercivity of 7.45 kOe has been found in melt‐spun Nd0.4Fe0.6; this value is the largest ever reported for a rare‐earth–iron alloy. A significant correlation was found between quench rate and coercivitiy, suggesting that the quenched alloys consist of fine crystallites or clusters, whose average particle size can be matched to the single‐domain optimum by the appropriate quench rate.

Preparation and coercive force of melt‐spun Pr‐Fe alloys

Amorphous Pr1−x‐Fex (0.45⩽x⩽0.90) alloys were prepared over a wide compositional range by melt spinning. Significant room‐temperature intrinsic coercivities were found; a maximum of 2.8 kOe was found at composition x=0.6. At reduced temperature (20 K) coercivities as high as 60 kOe were observed. High‐purity constituents were required to obtain amorphous samples over the composition interval 0.66⩽x⩽0.90.

Magnetic properties of melt‐spun Pr‐Fe alloys

The magnetic properties of a series of amorphous Pr1−xFex alloys (.45⩽x⩽ .90) prepared by spin melting are reported. Use of high purity constituents significantly increases the range over which Roentgen amorphous alloys can be obtained by this technique. The ratio of spontaneous to saturation magnetization (at 95 kOe) indicates that, over the composition interval 0.45⩽x⩽ .75, these alloys have sperimagnetic structures in which the Fe is ferromagnetic while the Pr moments are randomly oriented. At high applied field these alloys form collinear ferromagnetic structures. At reduced temperatures (20K) coercivities of over 4800 kA/m (60 kOe) were observed. Although exhibiting the pronounced temperature dependent behavior characteristic of non S‐state rare earth‐transition metal alloys, room temperature coercivities were still appreciable; a maximum room temperature value of 224 kA/m (2.8 kOe) was found at composition x = 0.6.

The microstructure of hot formed neodymium-iron-boron magnets with energy product 48 MG Oe

Melt‐spun Nd–Fe–B ribbons containing small amounts of Co, Ga, and C were die upset to 60% and 70% reduction in height in an argon atmosphere between 750 and 800 °C. The magnet, which was die upset 70%, has a remanence of 14.2 kG, Hci of 14.8 kOe, and (BH)max of 48 MG Oe. The interior of the magnet consists of well‐aligned Nd2Fe14B grains separated by a Nd‐rich intergranular phase. Intermixed with such aligned grains, we observed zones of unaligned fine‐grained material without any intergranular phase. Since the intergranular phase is a key factor for deformation and alignment and it is uniformly distributed in the hot pressed precursor, it will be possible to enhance the alignment further by controlling the redistribution of the intergranular phase during die upsetting.