Sanjay R. Mishra

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.

Comparative Mössbauer effect study of several R2Fe17 and R2Fe17Nx compounds

The Mossbauer 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.

Neutron‐diffraction and Mössbauer effect study of the Tb2Fe17−xAlx solid solutions

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 Mossbauer 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. Mossbauer 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 magneti...

A Mössbauer Effect Study of the Interstitial Hydrides and Nitride of Nd 2 Fe 17

The Mössbauer effect spectra of Nd2Fe17H3, Nd2Fe17H5, and Nd2Fe17N3 have been measured at several temperatures between 85 and 295 K and fitted with a model which is similar to that used for the analysis of the Mössbauer effect spectra of Nd2Fe17 and Nd2Fe17N2.6. The weighted average isomer shift increases in going from Nd2Fe17 to its hydrides and nitride, an increase which results mainly from the lattice expansion. The changes in the individual isomer shifts upon hydrogenation may be understood in terms of the expansion of the Wigner-Seitz cell volume and the presence of hydrogen or nitrogen near neighbors at a specific site. The 295 K weighted average hyperfine field increases from 157 kOe in Nd2Fe17 to 243 kOe in Nd2Fe17H3, 280 kOe in Nd2Fe17H5, and 318 kOe in Nd2Fe17N3, a sequence determined by the Curie temperatures. In contrast, the weighted average hyperfine field at 85 K and the saturation magnetization at 5 K for Nd2Fe17H3 are lower than those of Nd2Fe17, presumably because of thec-axis lattice contraction which occurs upon hydrogenation of this compound. The main difference between the effect of hydrogenation and nitrogenation resides in the substantial increase observed for the 9d and 18h hyperfine fields upon nitrogenation.

A magnetic, neutron diffraction, and Mössbauer spectral study of the Ce2Fe17 − xGax solid solutions

Abstract The magnetic properties of the rhombohedral Ce 2 Fe 17 − x Ga x solid solutions, with x equal to 0.3, 0.5, 0.7, 1.0, and 2.0, have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. Magnetization studies indicate that, as the unit cell volume expands with Ga addition, the Curie temperature increases from 238 K for Ce 2 Fe 17 to 406 K for Ce 2 Fe 15 Ga 2 . Powder neutron diffraction measurements at 295 and 14 K indicate that the Fe moments are both oriented in the basal plane and increase with increasing Ga content. The Mossbauer spectra, which have been fit with a binomial distribution of the near-neighbor environments, indicate that the maximum hyperfine field, H max , increases with increasing Ga content and is largest for the 6c site. The decremental field, ΔH , per added Ga atom is virtually independent of temperature and Ga content, indicating that the next-nearest-neighbor interactions are very small. The compositional dependence of the isomer shifts for R 2 Fe 17 − x M x , where R is Ce or Tb and M is Al, Si, or Ga, is analyzed in terms of volume and chemical terms, the second of which reveals the importance of the electronic configuration of M in understanding its influence on the magnetic properties of these solid solutions. Finally, the similarities and differences in the changes in crystallographic, magnetic, and Mossbauer spectral properties of R 2 Fe 17 − x M x , at low x values, indicate that electronic rather than structural factors are more important in modifying the magnetic properties and indicate the need for extensive band structure calculations on these solid solutions.

Neutron diffraction structural study of Ce2Fe17−xGax

Six samples of Ce2Fe17−x Ga x with nominal Ga content x equal to 0, 0.3, 0.5, 0.7, 1.0, 2.0 have been studied by powderneutron 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.

A magnetic and Mössbauer spectral study of the Tb2Fe17−xAlx solid solutions

Abstract The magnetic properties of a series of Tb 2 Fe 17− x Al x solid solutions, with x equal to 0.00, 1.00, 1.98, 3.14, 4.08, 5.10, 6.06, 7.16, and 8.12, have been studied by magnetic measurements and Mossbauer spectroscopy. Magnetization studies indicate that the Curie temperature increases from 420 K in Tb 2 Fe 17 to a maximum of 475 K in Tb 2 Fe 13.86 Al 3.14 . The magnetization results indicate an antiferromagnetic coupling of the terbium sublattice with the iron sublattice at low temperature. The magnetically ordered Mossbauer spectra have been fit with a binomial distribution of near neigbors. The weighted average maximum hyperfine field, H max , decreases by 12 kOe per aluminum at 85 K. The decrease in hyperfine field is due to the dilution of the magnetic moments by aluminum. The weighted average decremental field, Δ H , decreases by 2 and 1 kOe per aluminum at 85 and 295 K, respectively. The compositional dependence of the decremental field indicates the presence of RKKY type spatial spin oscillation in the 4s conduction band, an oscillation which is modified by the presence of aluminum. The weighted average isomer shift increases by 0.016 and 0.010 mm/s per aluminum at 85 and 295 K, respectively, because of both the screening of the 4s electrons by the 3d electrons due to interband mixing of the 3d band with the valence band of aluminum, and the unit cell volume expansion upon aluminum substitution.

A Mössbauer spectral study of the Sm2Fe17-xAlx solid solutions

Abstract The Mossbauer spectra of the Sm 2 Fe 17− x Al x solid solutions, where x is 0, 0.5, 0.75, 1, 3, and 4, which crystallize in the rhombohedral Th 2 Zn 17 structure, have been measured between 85 and 295xa0K. The spectra have been successfully analyzed with a model based on the binomial distribution of aluminum near neighbors calculated from the aluminum site occupancies derived from neutron diffraction measurements on Nd 2 Fe 17− x Al x solid solutions. This analysis indicates that the Mossbauer spectra are sensitive to the spin reorientation occurring between x values of one and three, confirms that the aluminum site occupancies are independent of the rare-earth element in the R 2 Fe 17− x Al x solid solutions, gives values of the iron site magnetic moments, and reveals an important covalent bonding between iron and its aluminum near neighbors.

Structural and magnetic studies on PrBa2Fe3O8

Abstract It is known that Fe moments order antiferromagnetically with Neel temperature, TN, above 700 K in RBa2Fe3O7 − δ with R = Y and Eu, and Pr moments order antiferromagnetically in PrBa2Cu3O7 − δ, with TN of 17 K. We have synthesized PrBa2Fe3O8 to study the ordering of Pr and Fe moments. From X-ray and neutron diffraction studies, this compound is found to crystallize in the tetragonal structure (space group P4/mmm). 57Fe Mossbauer studies show a complex hyperfine split pattern at 77 K, but a doublet superimposed on a six-line pattern at 300 K implying that some of the Fe moments order magnetically between 77 and 300 K while other may have ordering temperature of more than 300 K. Magnetization measurements reveal discontinuities in the susceptibility data around 80 and 20 K, probably an indication of magnetic ordering of the Fe and Pr moments. Neutron diffraction studies at 300 K suggest the presence of additional oxygen in the Pr plane and a magnetic structure very different from that of YBa2Fe3O8.

A magnetic, neutron diffraction, and Mȯssbauer spectral study of the Ce2Fe17−xAlx solid solutions (abstract)

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 Mossbauer 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 Ce2Fe14Al...