W. B. Yelon

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.

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 properties of the MnBi intermetallic compound

A MnBi alloy containing over 90 wt % low-temperature phase (LTP) has been obtained by high-temperature sintering and magnetic purification. The coercivity of the bonded MnBi magnet increases with increasing temperatures. A coercivity of 2.0 T has been achieved at 400 K. The maximum energy product (BH)max of the magnet is 7.7 MGOe (61 kJ/m3) and 4.6 MGOe (37 kJ/m3) at room temperature and 400 K, respectively. Neutron diffraction and magnetic data reveal a spin reorientation, which gives rise to low anisotropy fields and coercivity at lower temperatures for the LTP MnBi alloy.

A neutron diffraction and Mössbauer effect study of the Nd2Fe17−xSix solid solutions

Abstract The substitution of silicon for iron in Nd 2 Fe 17 strongly raises the Curie temperature but leads to a reduction in the unit cell volume. Refinement of the neutron-diffraction pattern for Nd 2 Fe 12.91 Si 4.09 indicates that silicon preferentially occupies the 18h site in the Nd 2 Fe 17 structure, the site with the most neodymium near neighbors. This occupation is surprising because conventional arguments would suggest that replacement of iron on the 6c site, which has a very short iron to near-neighbor iron bond length, would yield an increase in the Curie temperature.

Structural and magnetic properties of the novel Nd3Fe29−xTix compound from powder neutron diffraction

Abstract The structure and stoichiometry of a new rare-earth iron-based intermetallic compound Nd 3 Fe 29− x Ti x , has been determined using powder neutron diffraction on a sample with x = 1.24. The structure consists of fifteen unique transition metal sites and two rare-earth sites, in a monoclinic cell of symmetry p 21/ c . The magnetic moments are aligned along the a -axis and the Ti occupies sites with low rare-earth coordination. The structure shows a distinct stacking along the b -direction, consisting of rare-earth-Fe(Ti) layers and layers containing only Fe(Ti).

The peak in neutron powder diffraction

For the application of Rietveld profile analysis to neutron powder diffraction data a precise knowledge of the peak profile, in both shape and position, is required. The method now in use employs a Gaussian-shaped profile with a semi-empirical asymmetry correction for low-angle peaks. The integrated intensity is taken to be proportional to the classical Lorentz factor calculated for the X-ray case. In this paper an exact expression is given for the peak profile based upon the geometrical dimensions of the diffractometer. It is shown that the asymmetry of observed peaks is well reproduced by this expression. The angular displacement of the experimental profile with respect to the nominal Bragg angle value is larger than expected. Values for the correction to the classical Lorentz factor for the integrated intensity are given. The exact peak profile expression has been incorporated into a Rietveld profile analysis refinement program.

A magnetic, neutron diffraction, and Mössbauer spectral study of Nd2Fe15Ga2 and the Tb2Fe17−xGax solid solutions

An x‐ray diffraction study of the substitution of gallium in Tb2Fe17 to form the Tb2Fe17−xGax solid solutions indicates that the compounds adopt the rhombohedral Th2Zn17 structure. The unit cell volume and the a‐axis lattice parameter increase linearly with increasing gallium content. The c‐axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb2Fe17 to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd2Fe15Ga2 and Tb2Fe17−xGax, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples antiferromagnetically with the iron sublattice and that there is a change in easy magnetization direction f...

Structural and magnetic properties of Y(Mn1−xFex)12

The crystallographic and the magnetic structures of Y(Mn1−xFex)12 intermetallic compounds were investigated. They crystallize in the ThMn12 structure type. The Y atoms occupy the 2(a) sites and the transition metals are distributed on 3 nonequivalent sites 8i, 8j, and 8f. We have determined the solid solubility limit (x = 0.67) of Fe in YMn12. Neutron diffraction spectra at different temperatures have been used to study the nuclear and magnetic structure of Y(Mn0.7Fe0.3)12 and Y(Mn0.4Fe0.6)12. The Mn and Fe atoms are found to exhibit strong site preference with the i site favoring Mn atoms and the f site Fe atoms. Accordingly the instability of the RFe12 phase can be explained on the basis of the preferential atomic ordering observed in the ternary compounds. Based on the results of magnetic structure refinements using the Rietveld profiling method, antiferromagnetic, noncollinear structures are proposed for these two compounds.

Crystal structure, magnetic properties and electronic structure of the MnBi intermetallic compound

The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.