Er. Girt

Structural and magnetic properties of Nd2Fe17−δCrδ (δ=0, 0.5, 1, 1.9)

In Nd2Fe17−δCrδ (δ=0, 0.5, 1, 1.9), Cr substitutes for Fe atoms without changing the crystal structure. Neutron diffraction measurements show that for all values of δ, Cr preferentially substitutes for Fe in the 6 c site and randomly substitutes for Fe in the 18 f site. Cr substitution can be explained by considering the enthalphy of mixing using Miedema’s model. The Nd–Cr enthalphy of mixing is positive and this causes preferential substitution of Cr in those sites that have the lowest number of Nd nearest neighbors, i.e., the 6c site. Upon Cr substitution the Curie temperature increases from 327 K up to 374 K for δ=1 and then decreases to a value of 356 K for δ=1.9. This anomalous behavior can be qualitatively explained by two competing mechanisms arising from the decrease in the number of 6c-6c Fe pairs with increasing Cr concentration and from the decrease in volume of Nd2Fe17−δCrδ with increasing δ which affects the 6c-6c interatomic Fe distances.

Fe-substitution in Nd2Fe17-δXδC0.3 (X = Al, Ti, V, W and δ = 0, 0.5)

Abstract Neutron diffraction results on Nd 2 Fe 17-δ X δ C 0.3 (δ = 0, 0.5) show that C occupies exclusively the 9e site in agreement with the an elastic continuum model calculation which shows that the elastic energy required for C to access the 9e and 18g sites is 1.7 and 14.0 kJ/mol, respectively. Al preferentially substitutes for Fe in the 18h site while Ti, V and W preferentially substitutes for Fe in the 6c site in agreement with the results obtained for the carbon-free compounds. Al has a higher affinity towards Nd than Fe thus occupies the Fe site which has the highest fraction of its Wigner-Seitz cell surface shared with Nd. However, Ti, V and W have a higher affinity towards Fe than Nd and those elements substitute for Fe sites with the highest fraction of the Wigner-Seitz cell surface shared with Fe. Due to the high Ti affinity towards C we find that C does not enter the 2–17 structure preferring to form TiC.

The influence of the enthalpy of mixing on the Fe-substitution in Nd2Fe16.5X0.5 (X = Al, Ti, Nb, W)

Abstract Neutron diffraction results on Nd 2 Fe 16.5 X 0.5 (X = Al, Ti, Nb, W) show that Al preferentially substitutes for Fe in the 18h site while Ti, Nb and W preferentially substitute for Fe and the 6c site. This substitution can be explained by considering the enthalpy of mixing using Miedemas model. We find that the enthalpy of mixing for AlNd is much larger and more negative than for AlFe. On the other hand, Ti, Nb and W have positive enthalpies of mixing with Nd while their enthalpy of mixing is negative or zero with Fe. This drives the Al atoms to substitute for Fe in sites with the largest number, 3, of Nd nearest neighbours (i.e. the 18h site) and Ti, Nd and W to substitute for Fe in sites with the smallest number, 1, of Nd nearest neighbours (i.e. the 6c site). From the size similarity of the X elements, we conclude that the volume of the Fe sites does not play a role in the substitution mechanism.

X‐ray structural studies of nitrogen diffusion in Dy2Fe17

The reaction between N2 and Dy2Fe17 has been studied by thermopiezic analysis on 20–25 μm sized powders in the temperature range 400–500 °C. Partially nitrided powders were analyzed using CuKα x‐ray diffraction and thermomagnetic techniques. Both high angle x‐ray and thermomagnetic data show only the presence of Dy2Fe17 and Dy2Fe17N3−δ (δ<0.3) with no evidence of intermediate compositions. The results of the x‐ray diffraction experiments at several Bragg peaks were simulated using a two phase model structure: a Dy2Fe17 core with a Dy2Fe17N3 surface layer. The results show that at low temperatures the nitride layer is too thin to account for all of the nitrogen absorbed by the sample, indicating that a significant amount of the nitrogen diffused into the core of the particles, presumably along grain boundaries.

The effects of group IV B/V B/VI B additions on the magnetic properties of Sm2+δ Fe17 carbonitrides

Curie temperature, anisotropy field, and lattice constants are measured for the Sm2+δ Fe17M0.4 (δ≤0.6) carbonitrides for M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. It is found that minor additions of M reduce the Curie temperature, with the largest decrease of 28 K for M=Mo, and that the fluctuations in Curie temperature are not related to the changes in unit cell volumes. Compared with Sm2+δ Fe17NxCy, most of the M additives reduce the anisotropy field with the largest decrease of about 10 kOe for M=Zr. The V addition has almost no effect on the anisotropy field and the Ti addition gives a 4–5 kOe enhancement of the anisotropy field.

Formation of Nd(Fe1−yCoy)2 in rapidly quenched Nd13.75(Fe1−xCox)80.25B6 (x=0−0.5) alloys

X‐ray diffraction patterns of overquenched Nd13.75(Fe1−xCox)80.25B6 (x=0−0.5) ribbons show fine grains of the 2–14–1 phase in an amorphous matrix. The ribbons, subsequently annealed at 975 K for 5 min, were investigated using quantitative x‐ray structural analysis. The results indicate that in addition to the main 2–14–1 phase, all the samples have approximately 1 vol % of the γ‐Nd phase. The existence of the Nd(Fe1−yCoy)2 phase is established for x≥0.25 and its amount reaches up to 5.4 vol % for higher Co concentrations. From an interpolation of the measured lattice parameters of the 1–2 phase, the Co content is estimated to vary from y=0.38 to y=0.58 for this series of alloys. Using differential scanning calorimetry (DSC) the enthalpy of formation of the 1–2 phase in overquenched Nd13.75(Fe0.7Co0.3)80.25B6 ribbons is found to be about 3 kJ/mol. The activation energy necessary for the formation of the 1–2 phase, for the same composition, as determined from the Kissinger relation is 2.2±0.1 eV.

Hydrogen-induced changes in temperature dependence of the resistivity in ZrFe metallic glasses

Abstract We present the results for the temperature dependence of the electrical resistivity in hydrogen-doped ZrFe metallic glasses in the range 2 K to 290 K for various dopant concentrations. We show that the data provide further evidence for the role of quantum corrections to the electronic transport properties in highly disordered 3-D conductors. In the paramagnetic Zr 0.68 Fe 0.32 system the hydrogen produces a pronounced positive anomaly in the resistivity below about 25 K leading to a maximum in ϱ(T) before the curve resumes a monotonic decrease with increasing temperature. This is ascribed to the increase of the screening parameter for the Coulomb interaction F ∗ associated with the enhancement of the spin fluctuations with hydrogen concentration.

Structural properties of Nd2Fe14−δAlδB (δ = 0,0.5)

Abstract Neutron diffraction studies on Nd2Fe14−δAlδB (δ = 0, 0.5) show that Al preferentially substitutes for Fe in the 4c and 8j2 sites. This subsitution can be understood by considering a model which incldues the contributions from the enthalpy of solution of the Al Atom in each Fe site and the elastic energy due to the differences in the volume of the site and the volume of the Al atom. High affinity of Al towards Nd drives the Al atoms to substitutes for Fe in sites with the largest number, 4, of Nd nearest neighbours, the 4c sites. However, due to the size effect Al atoms tend to substitute preferentially for Fe in the largest Fe-sites, the 8j2 sites as well.

Anisotropic sintered Sm2(Fe,M)17Nx magnets made by rotational alignment

Anisotropic bulk Sm2(Fe,M)17Nx magnets were prepared by rotation aligning easy plane Sm2(Fe,M)17 powders followed by sintering and nitriding. Vanadium substitution for iron can help the c-axis alignment compared to the binary Sm2Fe17 compound. Magnets with an energy product of 15 MG Oe and an intrinsic coercivity of 7 kOe were obtained on a single phase Sm2Fe16.15V0.85Nx compound with a density of 91% of theoretical density.Anisotropic bulk Sm2(Fe,M)17Nx magnets were prepared by rotation aligning easy plane Sm2(Fe,M)17 powders followed by sintering and nitriding. Vanadium substitution for iron can help the c-axis alignment compared to the binary Sm2Fe17 compound. Magnets with an energy product of 15 MG Oe and an intrinsic coercivity of 7 kOe were obtained on a single phase Sm2Fe16.15V0.85Nx compound with a density of 91% of theoretical density.