ALBERT

Notizen: Three samples of Nd2Fe16TiCx, with x equal to 0.0, 0.3, and 2.8, with the Th2Zn17-type rhombohedral structure, have been studied by powder x-ray and neutron diffraction, magnetic measurements, and Mössbauer spectroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction melting stoichiometric amounts of the constituent elements, whereas Nd2Fe16TiC2.8 was synthesized by methane-derived gas phase insertion of carbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction determined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transition metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition metal site and randomly occupies the other three transition metal sites. This difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by the presence of carbon in the melt. Even though the unit cell volume of Nd2Fe16TiC2.8 is larger than that of Nd2Fe17N3, the 615 K Curie temperature of Nd2Fe16TiC2.8 is much lower than the 746 K Curie temperature of Nd2Fe17N3. This is an indication that the volume expansion, which occurs upon nitrogenation of R2Fe17, is not the only factor which contributes to the increase in the Curie temperature. The Mössbauer spectra of Nd2Fe16Ti confirm the high preferential titanium occupancy of the 6c site.At 85 K the weighted average hyperfine field of Nd2Fe16Ti is approximately 263 kOe, a value which is 33 kOe smaller than that in Nd2Fe17. The 85 K Mössbauer spectrum of Nd2Fe16TiC0.3 is virtually identical to that of Nd2Fe17 and indicates an approximately random titanium occupancy of the four transition metal sites. © 1996 American Institute of Physics.

Notizen: The magnetic properties of a series of Ce2Fe17−xSix solid solutions with x equal to 0.0, 0.23, 0.4, 0.6, 0.8, 1.02, 1.98, and 3.20 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. An x-ray-diffraction study indicates that the compounds adopt the rhombohedral Th2Zn17-type structure. The substitution of silicon for iron in Ce2Fe17 leads to a contraction of the a axis by 0.2%, an expansion of the c axis by 0.2%, and a consequent reduction of the unit-cell volume by about 0.2% per substituted silicon. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 455 K for Ce2Fe14Si2. Powder neutron-diffraction results, obtained at 295 K, indicate both that the silicon atoms preferentially occupy the 18h sites and that the iron moments increase with increasing silicon content, an increase which is related to the increase in Curie temperature. The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field Hmax for an iron with zero silicon near neighbors, and a decremental field ΔH per silicon near neighbor. The compositional independence of both the weighted average maximum hyperfine field and of the decremental field indicates that the silicon acts as a magnetic hole, a hole which does not perturb the magnetic moments at the iron sites. The compositional dependence of the weighted average isomer shift is explained in terms of an interband mixing of the iron 4s and silicon 2p bands, due to the reduction of the iron 18h bond lengths. This interband mixing affects the charge but not the spin distribution at the iron sites. © 1995 American Institute of Physics.

Notizen: 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 from planar to axial with increasing gallium concentration. This change in easy magnetization direction is explained in terms of a sign reversal of the second-order crystal field parameter, A02, the most important parameter responsible for determining the terbium sublattice anisotropy. The Mössbauer effect spectra indicate a larger room-temperature average hyperfine field at the iron site in the Tb2Fe17−xGax solid solutions than in several related R2Fe17 compounds. The large observed increase in the isomer shift with increasing gallium content results from interatomic charge transfer and intraatomic s-d charge redistribution in the presence of gallium.

Notizen: Six samples of Ce2Fe17−xGax with nominal Ga content x equal to 0, 0.3, 0.5, 0.7, 1.0, 2.0 have been studied by powder neutron diffraction at room temperature. Both crystalline and magnetic refinements have been carried out. All six samples adopt the Th2Zn17-type rhombohedral structure. The only additional phase found is α-iron. Gallium atoms are found to have high affinity for the iron 18h site, and are absent from the 9d and 18f sites. The Ga substitution for Fe leads to an expansion of both the a and c axes. The Curie temperature increases from 238 K for Ce2Fe17 to 406 K for Ce2Fe15Ga2. Magnetic refinements on the samples with x=0.3, 0.5, 0.7, 1.0, and 2.0 reveal that the magnetic moments of the four Fe sites are in the basal plane and that their values increase with increasing Ga content. © 1996 American Institute of Physics.

Notizen: A neutron diffraction and Mössbauer spectral study of Y2Fe14−xSixB shows that silicon preferentially occupies the 4c, and to a lesser extent, the 8j1 sites in Y2Fe14−xSixB. The trend in the site occupancy is the same as in Nd2Fe14−xSixB. The Curie temperature of Y2Fe14−xSixB increases with increasing silicon content. Neutron diffraction data show that the increase in Curie temperature is accompanied by a contraction of the unit cell. Wigner–Seitz cell calculations, using the Y2Fe14−xSixB lattice and positional parameters obtained by neutron diffraction, show that the silicon site occupancy is correlated with the rare earth contact area of that site. The Mössbauer spectra of Y2Fe14−xSixB have been fit with a model which takes into account the distribution of near-neighbor environments of an iron atom due to the presence of silicon. The weighted average of Hmax, the average hyperfine field of the iron sites with no silicon near neighbors, decreases by 42 and 45 kOe per silicon substitution per formula unit at 85 and 295 K, respectively. The weighted average of ΔH, the average reduction in the hyperfine field caused by the addition of one silicon near neighbor to the near neighbor environment, is about 13.5 and 18 kOe at 85 and 295 K, respectively, and does not show an appreciable dependence on the silicon content. The isomer shifts obtained from these fits suggest an increase in covalency of bonding on silicon substitution, an increase which is consistent with the preference of silicon to bond with yttrium.

Notizen: The Mössbauer spectra of Th2Fe17 and Th2Fe17N2.6 have been measured at various temperatures between 85 and 295 K and analyzed with a model that is based on the Wigner–Seitz cell environment of each iron site, the orientation of the magnetization, and the magnetic moments as determined from either neutron-diffraction measurements or band-structure calculations. Upon nitrogenation of Th2Fe17, the 85 K weighted average isomer shift increases from 0.037 to 0.156 mm/s and further the isomer shifts of the four crystallographically distinct sites increase in agreement with the increase observed in their Wigner–Seitz cell volumes and the presence of a nitrogen near neighbor for two of the sites. The nonmagnetic thorium in Th2Fe17N2.6 yields an easy axis of magnetization which, in contrast to magnetic rare earths in R2Fe17 and R2Fe17Nx, is in a general basal direction and not oriented along one of the basal axes. The isomer shift and its temperature dependence for both Th2Fe17 and its nitride are very similar to those observed in related rare-earth R2Fe17 compounds and their nitrides. Upon nitrogenation of Th2Fe17, the 85 K weighted average hyperfine field increases from 256.3 to 336.5 kOe. However, the increases on the 6c and 18f sites are smaller than those observed on the 9d and 18h sites; changes which are in agreement with changes in the magnetic moments observed by neutron diffraction and calculated for the nitrogenation of Nd2Fe17, Gd2Fe17, and Y2Fe17.

Notizen: The magnetic properties of a series of Tb2Fe17−xAlx solid solutions, with nominal x compositions of 0, 2, 3, 4, 5, 6, 7, and 8, have been studied by neutron diffraction and Mössbauer spectroscopy. Neutron-diffraction data indicate that the compounds all crystallize with the Th2Zn17 structure and that the aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site only for an aluminum content greater than 6. The unit-cell volume increases by approximately 1% per aluminum atom substituted in the formula unit. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x. Mössbauer spectral results indicate that all the samples are ferromagnetically ordered at 85 K. However, at 295 K Tb2Fe9Al8 is paramagnetic and Tb2Fe10Al7 is either paramagnetic or has at most very small ferromagnetic moments. An analysis of the magnetic spectra with a basal magnetic model is successful for x values of 5 or less; however, at higher x values an axial model for the magnetization is required, indicating the presence of a spin reorientation with increasing aluminum content and decreasing temperature. The weighted average hyperfine field decreases approximately linearly by 21 kOe per substituted aluminum atom at 85 K and more rapidly at 295 K. As expected, the isomer shifts increase with increasing aluminum content as a result of interatomic charge transfer and intraatomic iron 4s-3d electronic redistribution.

Notizen: The Mössbauer spectra of Sm2Fe17 and Ho2Fe17 and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R2Fe17 and R2Fe17Nx compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare-earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic lattice, an expansion which is mainly centered on the 9d and 18h iron sites as is indicated by the increase of their Wigner–Seitz cell volumes upon nitrogenation. The 9d and 18h sites show a larger enhancement of their hyperfine fields as compared to the 6c and 18f sites as a result of improved ferromagnetic exchange between these sites and their near neighbors because of the lattice expansion and the consequent reduced iron 3d-iron 3d overlap.

Notizen: The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.0, 0.88, 2.06, 2.80, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. Neutron diffraction data indicate that the compounds all crystallize in the rhombohedral Th2Zn17-type structure. The aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site for x greater than 6. The substitution of aluminum leads to an expansion of the a and c axis by 0.5% and 0.4% per aluminum atom. The unit cell volume increases by approximately 1.4% per aluminum atom. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x, indicating that aluminum acts as a magnetic hole at the lower concentrations. The Curie temperature increases from 238 K in Ce2Fe17 to a maximum of 384 K in Ce2Fe14Al3. The Ce2Fe17−xAlx solid solutions behave as spin glasses for x greater than 7.The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron atom with zero aluminum near neighbors, and a decremental field, ΔH, per aluminum near neighbor. Mössbauer spectral results indicate both that the samples are ferromagnetically ordered in the basal plane for x values between 0.2 and 7 and that a magnetic phase transition occurs from helimagnetic in Ce2Fe17 to ferromagnetic in Ce2Fe16.8Al0.2. However, at 295 K ferromagnetic ordering is observed only for x values between 2 and 5. The average value of Hmax decreases by about 25 kOe per aluminum atom. The isomer shift increases with aluminum content, an increase which can be explained by the screening of the 4s conduction electrons by the 3d band electrons. ΔH decreases by about 1.0 kOe per aluminum atom, a decrease which can be explained by the indirect exchange interaction between iron atoms modified by the presence of non-magnetic aluminum atoms through the 4s conduction band. Note added in proof: A full account of this work has been published in J. Appl. Phys. 79, 3145 (1996). © 1996 American Institute of Physics.

Notizen: The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.00, 0.88, 2.06, 2.81, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. The compounds crystallize in the rhombohedral Th2Zn17-type structure. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 384 K for Ce2Fe14Al3 and then decreases at higher aluminum content. Powder neutron diffraction results, obtained at 295 K, indicate that aluminum avoids the 9d site for all x values and preferentially occupies the 18h site at low aluminum content. Aluminum shows a marked preference for the 6c site for x(approximately-greater-than)6. The room-temperature iron magnetic moments increase from x=0 to 2 and then decrease for x(approximately-greater-than)2. The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron with zero aluminum near neighbors, and a decremental field, ΔH, per aluminum near neighbor. The compositional dependence of the decremental field indicates the influence of aluminum on the long-range magnetic ordering in the compound.The compositional dependence of the weighted average maximum hyperfine fields and the weighted average isomer shifts in Ce2Fe17−xAlx may be understood in terms of a mixing of the 3d conduction band electrons with the 3p valence band electrons of aluminum, a mixing which is more extensive than that associated with silicon in the Ce2Fe17−xSix solid solutions. We conclude that this mixing has a larger influence on the magnetic properties of these solid solutions than does the presence of a short iron–iron bond or the expansion or contraction of the lattice parameters and unit cell volume. © 1996 American Institute of Physics.