Mössbauer study of the intermetallic compound Nd2Fe14B. II. Temperature dependence and spin reorientation

The ⁵⁷Fe Mössbauer effects of Nd₂Fe₁₄B were measured in a temperature range of 4.2−300 K. Below the spin reorientation transition temperature T_sc = 148 K, the spectra were satisfactorily analyzed with twelve Zeeman sextuplets due to splitting of six crystallographic Fe-sites into twelve non-equivalent sites. It was shown that the magnetic moments of the Fe and the Nd atoms are non-collinearly coupled in the magnetic structure with canted moments below T_sc. The directions of the moments at 4.2 K are inclined at 27° for Fe and at 58° for Nd from the c-axis to the [110] direction. The average moments are 2.27μ_B for Fe and 3.3μ_B for Nd at 4.2 K. The increase of the average hyperfine field with decreasing temperature is suppressed below T_sc, and its value at 4.2 K is reduced by 1% from the value of 337 kOe which is observed in Y₂Fe₁₄B and also estimated for Nd₂Fe₁₄B by extrapolating the values above T_sc. On the other hand, the Nd moment increases abruptly around T_sc as the temperature decreases. The directions of the principal axes of electric field gradients on the six distinct Fe-sites were also obtained. The anomalous temperature dependence of quadrupole splittings and isomer shifts was observed around T_sc. They were discussed in a framework of the changes in the band structure and the lattice parameters incidental to the spin reorientation transition.     

57Fe Mössbauer study of amorphous and nanocrystalline Fe73.5Nb3Cu1Si13.5B9 after neutron irradiation

⁵⁷Fe Mössbauer spectroscopy is used to study neutron irradiation induced changes in the short-range order of Fe_73.5Nb₃Cu₁Si_13.5B₉ alloy. The samples are investigated in both amorphous and nanocrystalline states. Neutron irradiation leads to an increase of the standard deviation of a hyperfine field distribution (HFD), implying rearrangement of the atoms towards disordering. Simultaneously, changes in the average value of the hyperfine field and a net magnetic moment position occur as a consequence of a spin reorientation, atom mixing and microscopic stress centres which are introduced by neutron irradiation.     

The temperature dependence of the Mössbauer hyperfine parameters in Nd2Fe14B

We have determined the temperature dependence of the internal hyperfine field, isomer shift, and quadrupole shift at each of the six iron sites in Nd₂Fe₁₄B. The hyperfine parameters are consistent with the local iron site environments. The quadrupole and isomer shifts and their temperature dependences support our assignments of the relative ordering of the internal hyperfine fields as j₂>k₂>ck₁>j₁>c. We obtain a Mössbauer temperature of 390 K for Nd₂Fe₁₄B, which compares well with the Debye temperature of 420K for pure iron.     

Temperature dependence of the hyperfine parameters of the iron bearing phases in the Mössbauer spectra collected by the Mars Exploration Rover Spirit

This contribution focuses on the Mössbauer spectra acquired by the Mars Exploration Rover Spirit which carried a MIMOS II Mössbauer spectrometer. Only those spectra which present a reasonable statistical quality were selected to for this study. Twenty five Mössbauer spectra have been considered. Common phases identified from the temperature dependent hyperfine parameters are olivine, pyroxene, hematite and magnetite. It is believed that the applied analysis method has provided accurate values for the various hyperfine data averaged over single 10 K temperature intervals in the range 210–260 K. The obtained results, to some extent forced to evolve consistently over the various ΔT intervals considered for a given soil/rock target, are in many cases different from previously published data. Possible reasons for these differences will be discussed.     

Mössbauer effect studies of the Dy12Fe82B6 system

The Mössbauer measurements of both⁵⁷Fe and¹⁶¹Dy in Dy₁₂Fe₈₂B₆ system are reported. The presence of B exceeds the magnetic hyperfine fields at both⁵⁷Fe and¹⁶¹Dy nuclei as compared to corresponding pure metals Fe and Dy respectively. The only small excess of the hyperfine field at⁵⁷Fe nucleus is observed. The excess at¹⁶¹Dy nucleus is about 12% of the free ion Dy³⁺ value.     

Mössbauer study of spin states in amorphous Fe100−x Zr x alloys

Spin states in amorphous Fe_100–x Zr_x (7x12) are investigated using hyperfine field distributions (HFDs) as derived from⁵⁷Fe transmission Mössbauer experiments. The existence of low-, medium- and high-spin states is demonstrated by three-dimensional projections of HFDs. Temperature dependences of the corresponding average values revealed anomalous behaviour in the vicinity ofT _fc. An increase in the hyperfine magnetic fields observed towards low temperatures is discussed in terms of a ferromagnet-to-spin-glass transition. Utilizing also theT _c values, which were obtained from a thermal scanning Mössbauer technique, we propose a diagram of spin states in amorphous Fe-Zr.     

Mössbauer study of amorphous (Fe1−x Ga x )84B16

Using the⁵⁷Fe Mössbauer effect the influence of the Ga content in amorphous (Fe_1?x Ga _x )₈₄B₁₆ on the average hyperfine fields \(\bar H\) and isomer shift has been studied. For the sample (Fe_0.98Ga_0.02)₈₄B₁₆ the \(\bar H\) , as well as the recoilless fraction,f _a were measured as functions of temperature ranging from 12 K to 300 K. The experimental results show a linear correlation between Inf _a and δ, and well as between δ andx. In the temperature range \(\bar H(T)\) can be described by the Brillouin function and the second-order Doppler shift is appreciable. The characteristic temperature for such an amorphous alloy is 372 K. the effective vibrating massM _eff=79 a.u.     

The method of invariants applied to the analysis of 57Fe Mössbauer spectra

The performance of proposed antihydrogen spectroscopy or gravity experiments will crucially depend on the temperature of the initial antihydrogen sample. Measurements by ATRAP and ATHENA have shown that antihydrogen produced with the nested-trap technique is much hotter than the temperature of the surrounding trap. Therefore, novel schemes for antihydrogen recombination as well as for the pre-cooling of antiprotons are being considered. We are investigating a possible antiproton cooling technique based on the laser cooling of negative osmium ions. If demonstrated to be successful, it will allow the sympathetic cooling of antiprotons—or any negatively charged particles—to microkelvin temperatures. As a first milestone toward the laser cooling of negative ions, we have performed collinear laser spectroscopy on negative osmium and determined the transition frequency and the cross-section of the relevant bound–bound electric-dipole transition.     

Broadening of the Mössbauer line for Gaussian spectra

A procedure described byBykov andHien is employed to obtain an approximation formula for the line broadening expected in a Mössbauer experiment for the case where the emission and absorption spectra have Gaussian shapes. For identical emission and absorption spectra, the broadening as given by this formula has an error of less than 4·5% for effective absorber thicknesses up toT _A =10. The case where the emission and absorption spectra have different half widths is also considered.     

Mössbauer and PAC studies of nanocrystalline Fe

High-purity Fe powder was mechanically milled under argon at ambient temperature using an SPEX 8000 mill. The local atomic and magnetic structure was studied using⁵⁷Co/Fe Mössbauer and¹¹¹In/Cd perturbed angular correlations (PAC) spectroscopies. After 32 hours of milling, X-ray diffraction revealed effective grain diameters of 18 nm and energy-dispersive X-ray analysis indicated a Cr impurity concentration of 5%, presumably introduced by mechanical attrition of steel ball bearings used for milling. In addition to a spectral component very similar to bulk iron metal, the Mössbauer spectra exhibited hyperfine field shifts attributed to the Cr impurities. PAC spectra on Fe milled for 5 h, with no contamination, exhibited two components: (1) A slightly broadened magnetic interaction attributed to interior, defect-free sites of In/Cd probes with a mean hyperfine field slightly greater than in macroscopic grains. The defect-free site fraction grew appreciably during milling, even though In is essentially insoluble in Fe. (2) An indistinct signal due to mixed magnetic and quadrupole interactions attributed to probes at surface or other defect sites.     

Spreadsheet based analysis of Mössbauer spectra

Using spreadsheet programs to analyse spectral data opens up new possibilities in data analysis. The spreadsheet program contains all the functionality needed for graphical support, fitting and post processing of the results. Unconventional restrictions between fitting parameters can be set up freely, and simultaneous analysis i.e. analysis of many spectra simultaneously in terms of model parameters is straightforward. The free program package Vinda – used for analysing Mössbauer spectra – is described. The package contains support for reading data, calibration, and common functions of particular importance for Mössbauer spectroscopy (f-factors, second order Doppler shift etc.). Methods to create spectral series and support for error analysis is included. Different types of fitting models are included, ranging from simple Lorentzian models to complex distribution models.     

Approximation of Mössbauer spectra of metallic glasses

Mössbauer spectra of iron-rich metallic glasses are approximated by means of six broadened lines which have line position relations similar to those of -Fe. It was shown via the results of the DISPA lineshape analysis that each spectral peak is broadened owing to a sum of Lorentzian lines weighted by a Gaussian distribution in the peak position. This paper presents the Mössbauer parameters of amorphous metallic Fe₈₃B₁₇ and Fe₄₀Ni₄₀B₂₀ alloys derived from the fitted spectra.We would like to thank Dr. P. Duhaj for the preparation of amorphous samples.     

In-plane magnetic texture of sputtered amorphous Fe60B40 detected by Mössbauer spectroscopy

Angular investigation of Mössbauer line intensities has been performed on a sputtered sample of amorphous Fe₆₀B₄₀. Accurate data, obtained using linear combination of standard spectra, reveal in-plane magnetic texture of the material.     

A Mössbauer effect study of the Sb impurity site hyperfine field in amorphous Fe80B20

Mössbauer effect was used to determine the Sb site hyperfine field in the amorphous ferromagnet Fe₇₉SbB₂₀. The¹²¹Sb Mössbauer spectrum at 77 K showed a distribution of hyperfine fields with a most probable value of 219 kOe. This result is discussed in terms of the systematics of non-magnetic site hyperfine fields in ferromagnets.⁵⁷Fe Mössbauer effect was also used to determine the most probable value of the Fe site field, , and the width of the field distribution, H. Our measurements yield 280 kOe and H=104 kOe. These results are consistent with previous results on the amorphous ferromagnet Fe₈₀B₂₀.     

Site analysis and calculation of the quadrupole splitting of Prussian Blue Mössbauer spectra

The insoluble Prussian Blue (IPB) structure, Fe\(^{\mathrm {3+}}_{~~~4}\)[Fe ^II(CN)₆]₃.14H₂O, is shown to have ten distinctly different ferric sites, as well as the single ferrous site. The statistical probability of these sites was determined and the point charge model used to calculate the relative quadrupole splitting values. Previous Mössbauer spectra of IPB have fitted the ferric site with one doublet. Our results show that there is a large difference between the quadrupole splittings of the different ferric sites and hence it is not a reasonable approximation to fit them using a single doublet.     

Mössbauer and susceptibility studies of FeMoVO7

The oxidic phase FeMoVO₇ of the Fe₂O₃-V₂O₅-MoO₃ system has been synthesized. Susceptibility measurements from 4.2 K to room temperature display an antiferromagnetic behaviour with an effective magnetic moment of 5.80 _b . This value, and the measured Mössbauer isomer shift of 0.40±0.02 mm/s, are characteristic of high-spin Fe(III). The lowT _n value of 15 K suggests a weak interaction among the Fe-O polyhedra. The relative small QS=0.28±0.02 mm/s found in this compound shows that the Fe-O polyhedra symmetry is close to octahedral and almost undistorted.