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Neutron powder diffraction study of the RFe11.5Ta0.5 (R Ξ Lu, Er, Ho, Dy and Tb) compounds
Journal of Physics: Condensed Matter
Abstract
We report a systematic study of the crystallographic and magnetic structures of the RFe11.5
Ta0.5
(R
Lu, Er, Ho, Dy and Tb) compounds carried out by means of neutron powder diffraction. Thermal dependencies of lattice parameters, magnetic moments and magnetization directions have been determined. The hierarchy of the Fe magnetic moments at the 8i, 8j and 8f sites was found to be µ8i
>µ8j
µ8f
for all compounds at all temperatures. The influence of the atomic environments on the strength of the Fe local moments at each of the crystallographic sites is discussed. The results of the magnetic refinement are compared to those previously obtained from magnetic measurements on the same compounds.
... Therefore a great deal is now known about the mixing of T and M elements on the 8i, 8j and 8f sites. Examples which have appeared in the literature are Y-Fe-Ti [6], Y-Fe-V [7], Y-Fe-Al [8], R-Mn-Al [9], R-Co-V [10,11], Y-Co-Ti [12], Y-Co-Mo [12], Y-Fe-Cr [13], R-Co-Mo [14], R-Fe-Ga [15], R-Fe-Mo [16], R-Fe-Ta [17,18], R-Fe-Mn [19]. ...
Intermetallic compounds of the type RFe(10)Si(2) and RCo(10)Si(2) crystallize in the ThMn(12) structure (space group I4/mmm) whilst the heavy rare earth series RNi(10)Si(2) crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi(10)Si(2) and ErNi(10)Si(2) which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 °C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients α(11), α(33) and α(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order-disorder and displacive transitions, β = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.
The structural and magnetic properties of DyFe12−xTax compounds (with x = 0.5, 0.6, 0.65 and 0.7) have been investigated in detail using dc magnetization, differential scanning calorimetry, Mössbauer effect spectroscopy (5–300 K) and high-resolution synchrotron x-ray diffraction (10–700 K). The easy magnetization directions at room temperature are along the c-axis. With decreasing temperature, the magneto-crystalline anisotropy changes from easy axis at room temperature to easy cone at Tsr2 for all compounds, then to easy plane at Tsr1. Both Tsr1 and Tsr2 change significantly over the ranges 158–216 K and 239–259 K with increasing Ta content (x = 0.5–0.7) respectively, while the Curie temperature is found to remain essentially unchanged with a value of ∼540 K. Pronounced magneto volume effects have been detected at the Curie temperature while no clear anomalies are observed around the spin reorientation temperatures. Large spontaneous magnetostriction has been detected below TC for all of the compounds.
Refinements of the X-ray diffraction patterns show that DyFe12−x
Ta
x
compounds with x = 0.5–0.7 crystallise in the ThMn12-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility
for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T
sr1, then to easy plane at T
sr2. Both T
sr1 and T
sr2 increase with increasing Ta content up to x = 0.65 before decreasing with further increase in Ta content. Analyses of the Mssbauer spectra indicate that the individual
Fe site hyperfine fields derived at 4.5 K for DyFe11.35Ta0.65 are B
hf = 37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.
An investigation of the structural and vibrational properties of RFe12−xTax (R=Tb, Dy, Ho) compounds has been performed by a series of interatomic pair potentials obtained through the lattice inversion method. Calculated results show Ta atoms can stabilize RFe12−xTax with ThMn12-type structure, and Ta atoms substitute for Fe with a strong preference for the 8i sites. The phase stability of the intermetallics RFe12−xTax is tested by many means including random atom shift, global deformation and high temperature disturbance under the control of the pair potentials. The calculated lattice parameters are corresponding to the experimental results. In particular, the phonon densities of states for RFe11.5Ta0.5 are first evaluated. A qualitative analysis is carried out with the relevant potentials for the vibrational modes, which makes it possible to predict some properties related to lattice vibration.
The paper presents an iron-57 Mössbauer spectral study of RFe11.5Ta0.5 , with R=Tb,Dy,Ho,Er,Lu , and an evaluation of the different contributions to the hyperfine magnetic fields. The Mössbauer spectra have been analyzed with a model that considers both the distribution of the tantalum atoms in the near-neighbor environment of the iron atoms and the relative orientation of the hyperfine field and the principal axis of the electric field gradient. Their possible directions in the ThMn12 structure have been determined from a close examination of the point symmetry of each iron site. A local model for the hyperfine field which enables to determine their components from experimental data, has been developed and a calculation of the lattice dipolar hyperfine field in RFe11.5Ta0.5 has been performed. We have investigated in detail the origin and influence of the contributions to the hyperfine field coming from self 3d polarization, the iron and rare earth transferred fields and the orbital and dipolar hyperfine fields. The iron and rare earth transferred fields have been analyzed for RFe11.5Ta0.5 and other rare earth-iron intermetallic compounds. From this analysis it is shown that the iron transferred fields are different at each crystallographic site, and comparable to the self 3d polarization contributions, and that the rare earth transferred field is mainly originated by the indirect exchange between the rare earth 4f and iron 3d electrons.
Results of x-ray powder diffraction and magnetic measurements performed on RFe11.3W0.7 compounds, where R=Dy, Ho, Er, and Lu, are presented. The Curie temperature, saturation magnetization, and spin reorientation transitions are discussed in relation to several RFe12-xTx systems, where T=Ta, Nb, Mo, Ti, and the x-substituent concentration is within a narrow range x=0.5-1.0. In these systems, the dependence of the Curie temperature versus the unit cell volume, for a given R partner, suggests combined size and electronic effects produced by the T atom. The strengths of the rare-earth-iron and iron-iron exchange interactions are evaluated in the mean-field framework, and analyzed with reference to RFe12-xTx compounds, with T=Ta, Re, Nb, Mo, Ti, Cr, and V. Based on (i) the phenomenological mean-field model accounting for the sublattice of conduction band electrons, (ii) Brooks' reinterpretation of Campbell's formulation of the intersublattice exchange, and (iii) Wigner-Seitz cell analysis, we explore the role played by the conduction band electrons, particularly the nd-type electrons (n=3, 4, 5), on the intersublattice exchange coupling in the iron-rich RFe12-xTx class of compounds. It is proposed that the variation in the conduction-electron density, by varying the T element type and concentration, leads to a modification of the 5d band occupancy number and consequently, of the R-Fe intersublattice exchange coupling.
We present a 57Fe Mössbauer spectral study of the RFe11.5Ta0.5 compounds, with R = Tb, Dy, Ho, Er and Lu. The Mössbauer spectra have been analyzed with a model that considers both the distribution of the tantalum atom in the near-neighbor environment of the iron atoms and the influence of the easy magnetization direction. The assignment of the hyperfine fields and isomer shifts are in agreement with the iron magnetic moments measured by neutron diffraction and with the Wigner-Seitz cell analysis of the three iron sites in RFe12-xMx compounds, respectively.
The structural and magnetic properties of ErFe12−xNbx compounds (x = 0.6, 0.7 and 0.8) have been investigated by x-ray diffraction, ac susceptibility and dc magnetization measurements and 57Fe Mössbauer spectroscopy. Refinements of the x-ray diffraction patterns show that the Nb atoms preferentially occupy the 8i sites; this can be understood in the terms of enthalpy effects and differences in the metallic radii. The average Fe–Fe distance at the different sites is found to behave as dFe−Fe(8i)> dFe−Fe(8j)> dFe−Fe(8f). The unit cell volume increases slightly with increasing Nb content, consistent with the larger radius of Nb compared with Fe. A spin reorientation from easy-axis at room temperature to easy-cone at low temperatures has been detected for all compounds. The spin reorientation temperatures Tsr in ErFe12−xNbx compounds remain essentially unchanged (Tsr~42–44 K) with increasing Nb concentration, whereas a significant decrease in Tsr (Tsr 1~236–204 K; Tsr 2~154–94 K) is obtained in DyFe12−xNbx from x = 0.6 to 0.8. This can be understood by taking the different crystal-field terms responsible for the spin reorientation in the two systems into account. We find that the spin-reorientation process is particularly sensitive to the sixth-order term B60O60 of the crystal field acting on the Er3+ ion, due to its large and positive value of γJ. 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j and 8f sites from the Mössbauer spectra. The weighted average 57Fe hyperfine field values were found to follow a T2 dependence; this suggests that a single-particle excitation mechanism is responsible for reduction of the 3d-sublattice magnetization with increasing temperature.
We have measured the electrical resistivity of RFe11.5Ta0.5 and RFe11.3W0.7 (R = Tb, Dy, Ho, Er, and Lu) polycrystalline systems over a temperature range of 4–700 K. The overall behaviour of the resistivity is determined mainly by the transition-metal ions. However, some interesting features are related to rare-earth atoms. At low temperatures, the resistivity, ρ, increases with temperature as AT2, where the coefficient A varies systematically with the total spin of rare-earth ions. We also find that ρ depends on the M element in our RFe12−xMx iron-rich compounds. Close to the Curie temperature, the resistivities of the alloys studied clearly show a change in the slope of ρ versus T. Saturation values of the high temperature magnetic resistivity follow the prediction by de Gennes and Friedel for spin disorder scattering.
Refinements of the X-ray diffraction patterns show that DyFe12−x
Tax
compounds with x=0.5−0.7 crystallise in the ThMn12-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T
srl, then to easy plane at T
sr2. Both T
srl and T
sr2 increase with increasing Ta content up to x=0.65 before decreasing with further increase in Ta content. Analyses of the Mössbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFell.35TaO.65 are B
hf=37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.
A series of compounds with the ThMn12-type structure, RFe11.35Nb0.65 (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Lu), were synthesized. The corresponding nitrides, obtained by gas-solid reactions, retained the same structure as their parent compounds, but with a relative volume expansion of 3%. The Nb atoms occupy the 8i sites in the ThMn12 structure. The highest Curie temperatures are 597 K and 773 K for GdFe11.35Nb0.65 and its nitride, respectively. The average iron magnetic moment μFe at T=1.5 K is 1.9μB for parent compounds and 2.1μB for the nitrides. By the introduction of nitrogen, the Fe-Fe exchange interaction is much strengthened while the R-Fe exchange interaction is slightly weakened. The enhancement of μFe by the influence of interstitial nitrogen is about 11%. The Fe sublattices of RFe11.35Nb0.65 compounds and their nitrides have an easy c-axis anisotropy. At T=1.5 K, the anisotropy constant K1(Fe) is 25.7 K/f.u. for the parent compounds and 21.9 K/f.u. for the nitrides. The reduction of K1(Fe) by the influence of interstitial nitrogen is about 17%. Spin-reorientation transitions were observed in DyFe11.35Nb0.65 and ErFe11.35Nb0.65 compounds. The complex magnetic behavior of RFe11.35Nb0.65 at low temperatures can be explained by exchange and crystal-field-interaction models. The SmFe11.35Nb0.65 compound has an anisotropy field of 10.0 T at room temperature. Its high Curie temperature and magnetization make it a potential candidate for applications as a permanent magnet.
Mössbauer spectra, magnetization measurements, and self-consistent spin-polarized electronic structures of YFe12-xMox, where x=0.5, 1.0, 2.0, 3.0, and 4.0, are reported. The ternary compounds YFe12-xMox have the crystalline tetragonal ThMn12 structure. Analyses of the Mössbauer spectra show that Mo atoms occupy the 8i Fe sites of the ThMn12 structure, in agreement with previous observations. Room-temperature magnetic and Mössbauer measurements show that the compounds with x≤2.0 are ferromagnetic and with x≥3.0 are paramagnetic. Measurements at 25 K show that all the samples are magnetically ordered. The magnetic hyperfine field is found to decrease with increasing Mo concentration, which is in qualitative agreement with the calculated magnetic moments. The calculated magnetization decreases less rapidly with increasing x than the experimental data. In general the data suggest that with increasing Mo concentration there is an increase of antiferromagnetic coupling among the Fe moments, which leads to cluster-glass or spin-glass-like phenomena. The measured isomer shift relative to α-iron is found to decrease linearly with x.
The structural and magnetic properties of hydrides with -type structure have been studied by means of magnetic measurements, the neutron powder diffraction technique and self-consistent spin-polarized band calculations (LMTO-ASA). We found that the hydrides retain the -type structure, but with an increase of unit-cell volume. The neutron diffraction results indicate that hydrogen atoms occupy the interstitial 2b sites. Magnetic measurements carried out on , and their hydrides show that both the Curie temperature and the saturation magnetization can be enhanced by introducing interstitial hydrogen atoms. Band-structure calculations and spin-fluctuation theory give a fair description of the enhancement of the magnetization and Curie temperature.
Thermomagnetic measurements and X-ray diffraction analysis on oriented powders were carried out to study the magnetic properties of RETa≈0.5Fe≈11.5 (RE = Tb, Dy, Ho, Er and Lu). All the compounds had a spontaneous magnetization at room temperature, with Curie points ranging from 576 K for RE = Tb to 499 K for Lu. The easy magnetization direction lies along the c-axis for RE = Dy, Ho, Er and Lu, and is parallel to the basal plane for Tb. The thermal variation of the magnetic susceptibility exhibits strong similarities to that observed in isotypic compounds with M = Nb, suggesting the existence of spin reorientation transitions at 40 K for Er, and 185 and 285 K for Dy.
The magnetic anisotropy of the DyFe11Ti intermetallic has been studied by measuring the angular dependence of the components of the magnetization parallel and perpendicular to the applied magnetic field. The measurements were performed on two plate-shaped samples with the surfaces parallel to the (110) and (001) planes, respectively. From the measurements in the (110) plane the thermal dependence of the spin reorientation angle has been obtained. The magnetic field dependence of M/sub /// and Mperpendicular to showed a first-order magnetization process (FOMP). A single-ion model for the crystal-electric-field interaction and a mean-field model for the exchange interaction have been used to explain theoretically the experimental results. An in-depth study of the character of the spin reorientation transition from easy cone to easy plane has been undertaken. From our study we show that for temperatures near this spin reorientation temperature, two magnetic phases (planar and conical phases) coexist in the sample, giving rise to an average angle that evolves continuously with temperature. A similar behaviour has been also observed for magnetic fields near the critical field of the FOMP, producing a continuous change of the magnetization instead of a jump at the critical field.
Magnetisation measurements, in particular the angular dependences of the components of the magnetisation vector, parallel and perpendicular to the applied field were measured on oriented powder samples of the HoFe11.4Ta0.6 alloy and its hydride and carbide. These experiments allow a study of the magnetocrystalline anisotropy and an accurate determination of the spin reorientation process occurring in the hydride. Moreover powder neutron diffraction experiments led to a determination of the crystallographic and magnetic structures of these materials, in particular the values of the iron and holmium moments on the different sites. Analysis of the results allows an estimate of the magnetocrystalline anisotropy and of the exchange interactions. Accordingly their changes after hydrogen or carbon insertion in the starting alloy are determined.
Iron-rich ternary intermetallics YFe12−xMox (0.5⩽x⩽4.0) with the ThMn12 tetragonal structure and their nitrides YFe12−xMoxNy(y≈1) were prepared. The lattice parameters increased linearly with Mo concentration (5.06 Å3/Mo) The values a = 8.468 Å and c = 4.752 Å were extrapolated for the hypothetical YFe12. Upon nitrogenation, the cell volume expanded more than 3%, where the a-axis mainly lengthens. The Fe-Fe exchange interactions were greatly enhanced by interstitial N atoms, resulting in a Curie temperature increase of about 200 K on average. Saturation magnetization was increased by nitrogenation. While saturation magnetization on the host compounds could be explained by the ‘magnetic valence model’, the data on the nitrides did not fit in the same model. Both nitrides with x = 0.5 and 1.0 have fairly high TC (708 K and 664 K) and saturation magnetization at room temperature (167 Am2/kg and 151 Am2/kg), being interesting for applications.
The absorption of hydrogen in ternary RFe12−xMx (R=Y or rare earth, M=Ti, Mo) alloys has been investigated by neutron diffraction. These compounds crystallise in the tetragonal (I4/mmm) structure of ThMn12 type. In this work we report structural and magnetic investigations undertaken by neutron diffraction for some hydrides with high hydrogen contents and we focus on the location of hydrogen in Ti and Mo containing materials. The effects of hydrogenation on the intrinsic properties such as enhancement of Curie temperature, lattice parameters, Fe moment as determined by neutron diffraction, magnetic and Mössbauer measurements, are discussed.
A new series of compounds with the 0953-8984/10/48/025/img11-type structure 0953-8984/10/48/025/img12 (0953-8984/10/48/025/img13, Dy, Ho, Er and Lu) has been synthesized and studied by x-ray diffraction, ac susceptibility and magnetization versus temperature and field measurements. The maximum Curie temperature is for the Tb compound 0953-8984/10/48/025/img14. Spin reorientation transitions have been observed for the compounds 0953-8984/10/48/025/img15 and 0953-8984/10/48/025/img16. A first order magnetization process has been observed at low temperature in 0953-8984/10/48/025/img15 and 0953-8984/10/48/025/img18. The importance of the metallic radius of M in the magnetic properties of 0953-8984/10/48/025/img19 (for the same rate of M substitution) is discussed.
The 57Fe Mössbauer spectra of various compounds of the type RFe12-xTx (T = V, Cr, Mo) have been measured. They were analysed in terms of the different nearest neighbour coordinations of the Fe atoms, arising from a statistical occupation of the T atoms on exclusively the 8(i) site in the tetragonal ThMn12 type of structure. An assignment of the hyperfine fields to the 3 crystallographic sites 8(i), 8(j) and 8(f) has been made on the basis of the results of recent band structure calculations.
YFe12-xMox (x=1 and 3) and full-charged nitrides were prepared. Neutron-powder-diffraction experiments were performed at 10 K and room temperature on both the host compounds and their nitrides. The Rietveld technique was used for data analysis. Mo was found to reside mainly on the 8i site in the tetragonal ThMn12 structure (space group I4/mmm), with a small fraction on the 8f site. The nitrides retain the crystallographic symmetry of the host compounds. Nitrogen atoms are located on the 2b site with nearly 100% occupancy. The nitrogenation-induced modification of the rare-earth environment and in turn the magnetocrystalline anisotropy properties are discussed. The refined moments are comparable with those obtained by magnetization measurement.
A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.
Magnetization measurements along the main symmetry directions of the tetragonal structure, [100], [110] and [001], have been performed on several single crystals (R = Y, Tb and Ho), at applied high magnetic fields up to 12 Tesla and temperatures ranging from 4.2 to 300 K. From the measurements on the yttrium compound the second- and fourth-order anisotropy constants and the spontaneous magnetization of the Fe sublattice have been determined. The complex magnetic behaviour has been explained using a single-ion model for the crystal electric field (CEF) interaction and a mean-field model for the exchange interaction. A reliable set of CEF parameters and mean exchange field have been obtained for the and intermetallic compounds. All the experimental features observed, the first-order magnetization process under magnetic field in and spin reorientation transition (SRT) in , have been explained using such CEF and exchange parameters. A study of the character of the SRT observed in the intermetallic compound has been undertaken by measuring the parallel and perpendicular components of the magnetization to the applied magnetic field. The experimental results obtained have been explained considering the coexistence of two magnetic phases for temperatures close to the spin reorientation one.
Neutron powder diffraction experiments and magnetic measurements were performed on compounds of the series (R = Y, Dy, Ho and Er). The influence of the R element on both the structural and the magnetic properties of the different compounds is discussed, as well as the possible correlation between the iron environments and the local moments. Comparison is made with a previous Mössbauer study on the same compounds.
Magnetic properties of the series of ThMn12-structure intermetallic compounds R(Fe11Ti) have been determined for rare earths from Nd to Lu plus Y. The highest Curie temperature (607 K) is for R=Gd, and R-Fe exchange interactions are much stronger for light rare earths than for heavy ones. The temperature dependence of the iron sublattice magnetisation and anisotropy are determined for the Y and Lu compounds. Spin reorientation transitions are found as a function of temperature for the rare earths with a negative second-order Stevens coefficient alpha J(Nd, Tb, Dy), and a set of crystal-field parameters is derived to account for the transitions in a consistent way. A sharp increase in magnetisation observed for Sm(Fe11Ti) below 130 K in a field of about 10 T applied perpendicular to the easy direction indicates that J-mixing may be important for Sm3+. Compared with R2Fe14B, the iron anisotropy in R(Fe11Ti) is greater, and the rare-earth anisotropy is much weaker at low temperature, with the opposite sign for the rare-earth crystal-field coefficient A20. The average iron moment is 1.7 mu B in R(Fe11Ti) at 4.2 K; Mossbauer spectra are analysed to yield the average moments on each site. Limits set by the intrinsic magnetic properties on the performance of magnets made from these families of alloys are discussed.
New RFe12 - xMx alloys as well as parent hydrides and carbides have been synthesised with M = Ta and for rare earth metal R belonging to the series Tb to Lu. The range of stability of the compounds was found rather narrow, i.e. 0.5 less than or equal to x less than or equal to 0.7 making this system very similar to that recently reported for M = Nb. The crystal cell parameters and the Curie temperature of the compounds and their parent interstitial have been measured, the latter being particularly high.
The structural and magnetic properties of RFe10.5Mo1.5 alloys and their hydrides were studied using X-ray diffraction and magnetic measurements. Hydrogen occupies the octahedral interstitial 2b site of the tetragonal ThMn12-type structure (space group I4/mmm, Z = 2), which allows a maximum uptake of one hydrogen atom per formula unit. The unit cell volume expands about 1% and the Curie temperature is increased about 10% by the hydrogen uptake. The mean magnetic moment of the iron sublattices is increased. On hydrogenation, (i) the magnetocrystalline anisotropy of the 3d sublattice is enhanced, (ii) in the compounds containing the R elements with a positive second-order Stevens factor αJ, the c-axis is the easy axis, and (iii) for the compounds with R having a negative second-order Stevens factor, the easy direction lies in the ab-plane. The Dy and Tb alloys show temperature induced spin reorientation transitions (SRT). On hydrogen uptake, their hydrides have a basal plane behaviour. This change in the series can be understood by the donor character of H together with a narrowing of the 3d metal bands induced by the volume expansion.
We report on the relationship between the local magnetic moments and crystal sites, mainly in iron-rich intermetallics. The topology of the atomic environment is analysed in terms of numbers of neighbours, disclination lines and atomic domains. The wide range of compounds studied includes rare earth-iron alloys, such as R6Fe23, RFe3, R(FeM)12, R2Fe17, ternary rare earth-iron-metalloid intermetallics such as R2Fe17Hx, R2Fe17N3, R2Fe17Cx, R2Fe14B, ThFe11Cx, R(FeM)12Nx and R(FeM)12Cx, as well as binary iron nitrides (Fe3N, Fe4N, Fe16N2), carbides (Fe5C2, Fe3C) or borides (Fe2B and Fe3B).The sites having major ligand lines are observed to carry the larger magnetic moments encountered in the structures. In the ternary compounds strong bonds are found between iron sites and a neighbouring metalloid atom, thus leading to a lower iron magnetic moment. Finally, a close relationship between the local moment and the volume of the iron site has been observed: a large atomic domain is found to favour a high moment.
Mössbauer spectroscopy and neutron diffraction experiments were performed on the series of compounds ErFe10.4−xCoxMo1.6 (x = 0, 2, 5, 7 and 10.4). The occupation of Co at the three different crystallographic sites has been determined. Preferential occupation is observed for the 8j and 8f sites, while the 8i sites show unfavoured occupation. The complex Mössbauer spectra were analysed with a set of subspectra which take into account the disorder of the Mo atoms. The site assignment from the Mössbauer spectra is discussed and is supported by the neutron diffraction results.
Self-consistent ab initio band-structure calculations using the augmented-spherical-wave method were performed for the hypothetical compounds YFe12 and YFe8M4 (M=Ti, V, Cr, Mn, Mo, and W) with the ThMn12 structure, in which the M atoms occupy the 8(i) crystallographic sites. We found that YFe12 is a weak ferromagnet: For none of the three Fe sites is the majority-spin 3d band completely occupied. Using extrapolated experimental lattice parameters, the calculated total magnetization (24.2μB/formula-unit) and the calculated moment reduction after replacement of the Fe(i) atoms by an M atom are in good agreement with experimental data on YFe12-xMx (1≤x≤3) compounds. The calculated local magnetic moments are compared with the results of neutron-diffraction and Mössbauer-spectroscopy experiments, as well as with the results of band-structure calculations on some structurally related compounds.
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