ALBERT

Notes: 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.

Notes: 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.

Notes: The magnetic properties of a series of Nd2Fe17−xAlx solid solutions, with x equal to 2.04, 4.01, 5.97, 7.94, and 9.06, have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. Magnetization studies indicate that the Curie temperature increases from 330 K in Nd2Fe17 to a maximum of ∼470 K at an x of 3.5. The compounds crystallize in the Th2Zn17 structure with lattice parameters and unit cell volumes which increase linearly with increasing aluminum content. The neutron diffraction results indicate that aluminum atoms are excluded from the 9d site, prefer the 18h site at low aluminum content, and prefer the 6c and 18f sites at high aluminum content. At 10 K the magnetic moments of the iron and neodymium atoms are collinear and take up a basal orientation at all aluminum contents. The moments decrease with increasing aluminum content and the magnetic moments per unit cell at 10 K are in excellent agreement with the 4.2 K saturation magnetization values. At 295 K the Nd2Fe17−xAlx solid solutions for x equal to 7.94 and 9.06 are paramagnetic. The magnetic Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments. The weighted average isomer shift increases, as expected, with increasing aluminum content as a result of interatomic charge transfer and intraatomic iron 4s–3d charge redistribution. The weighted average maximum hyperfine field at 295 K shows a maximum of 221 kOe at x equal to 2.04 but at 85 K it decreases uniformly with increasing aluminum content. The weighted average decremental field, ΔH, the change in the hyperfine field per aluminum near-neighbor, decreases with increasing aluminum content. It is proposed that, as a consequence of the increase in the average distance between an iron atom and its next near-neighbor shell with increasing aluminum content, the wavelength of the Friedel oscillation increases and the ratio of this wavelength and the shell distance becomes more favorable for ferromagnetic exchange.

Notes: We have investigated the effect of substitution and nitrogenation on the hyperfine parameters in Gd2Fe17−x(Ga,Al)x (x=1–10) and Gd(Fe,Co)12−xMox(Ny) (x=1.5–3.0 and y≈1) by means of 155Gd Mössbauer spectroscopy. We have observed marked changes in the values of the electric field gradient Vzz, the effective Gd-hyperfine field, and isomer shift. Al substitution in Gd2Fe17 leads to a sign reversal of the electric field gradient. No sign reversal of Vzz has been observed in the Gd2Fe17−xGax compounds. The effective hyperfine fields of the Gd2Fe17−x(Ga,Al)x compounds are decreasing with increasing x, due to the reduction of the 6s electron polarization resulting from the decreasing Fe moments. The ultimate value of the isomer shift based on a simple model on chemical effects that would be reached for x=17 is 0.52 mm s−1 for Al and 0.64 mm s−1 for Ga substitution in Gd2Fe17, respectively. The electric field gradient of the GdFe12−xMox compounds were obtained for x=1.5 and x=3.0. Nitrogenation is shown to lead to a sign reversal of the 4f and Fe sublattice anisotropy. Mo substitution in GdCo12−xMox shows a decreasing negative value of Vzz with increasing Mo concentration x. The values ||Heff|| in GdCo12−xMox increase with increasing Mo concentration. This can be attributed to a different near neighbor Gd and 3d-metal contribution to the effective hyperfine field compared to those of GdFe12−xMox. © 1996 American Institute of Physics.

Notes: 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.

Notes: 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.

Notes: 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.

Notes: 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.