Critical behaviour of19F hyperfine fields in Ni near the Curie temperature

The temperature dependence of the magnetic hyperfine fields for¹⁹F in Ni have been studied between 550K and 640K, with the TDPAD method. The critical exponents beta, delta and gamma were determined. Beam heatup effects are also discussed.     

Mössbauer Effect Study of Eu0.88Fe4Sb12 Skutterudite

The partly filled skutterudites Eu_0.88Fe₄Sb₁₂ were investigated by ⁵⁷Fe Mössbauer spectroscopy in the temperature range 4.2 K ≤ T ≤ 295 K and in external fields up to 13.5 T. The results favour a statistical distribution of Eu and voids in the Fe near neighbour shell. Below 82 ± 1 K magnetic order is present. Debye temperatures Θ _D = 460 ± 20 K and 165 ± 30 K were obtained for Fe with completely occupied Eu sites and for Fe with vacancies in the R-neighbour shell, respectively. The temperature dependence of the quadrupole splitting reflects the thermal expansion of the lattice. The induced hyperfine fields at 4.2 K are negative and differ by roughly a factor of two for the two Fe surroundings.     

Moment compensation in NixRh1−x(Fe) from Moessbauer spectroscopy

The magnetic behaviour of very dilute ⁵⁷Fe(≈20 ppm) impurities in paramagnetic Ni_xRh_1?x (x = 0.42 and x = 0.55) alloys has been studied by Moessbauer spectroscopy in the temperature range between 11 and 0.05 K and in external fields up to 5.6 T. The magnetic moment associated with the Fe-impurity is determined via the dependence of the hyperfine magnetic field on applied magnetic field and temperature. Below 4.2 K deviations from a free spin behaviour are found. The saturation hyperfine field becomes dependent on the applied field, a behaviour which is typical for impurity spin compensation. This compensation decreases with Ni concentration.     

The temperature dependence of the57Fe hyperfine magnetic field distribution in Fe-Ni

The temperature dependence of the⁵⁷Fe hyperfine magnetic field (hmf) in Fe-Ni is stronger than the temperature dependence of the⁵⁷Fe hmf in pure Fe. By analyzing the shape of the⁵⁷Fe hmf distribution, and with the help of experiments with Si in Fe-Ni, we deduce that this anomalous temperature dependence originates from a large thermal sensitivity of the magnetic moments at those Fe atoms with more Ni nearest neighbors. A strong temperature dependence of the recoilfree fraction was also observed in Fe-Ni alloys. We suggest that a large mean square thermal displacement of Fe atoms in Fe-Ni is the cause of the anomalous temperature dependence.     

Hyperfine magnetic fields in Fe−Co alloys and their tempera ture dependences

We obtained⁵⁷Fe hyperfine field parameters from Fe₁−x-Co _x alloys (0≤x≤0.6) from 77 K to 900 K. We first discuss the origin of the low temperature hyperfine fields in terms of the 3d and 4s electrons at⁵⁷Fe atoms. The⁵⁷Fe hyperfine magnetic field (hmf) of Fe-Co alloys depends more weakly on temperature than the hmf of pure Fe. This temperature dependence occurs because the alignment of the magnetic moments at both the Fe atoms and at the Co atoms depend on temperature in the same way as the bulk magnetization of Fe-Co alloys.     

High‐pressure μSR studies of ferro‐ and antiferromagnetic metals

High‐pressure μSR experiments on ferromagnetic nickel and \alpha‐iron and antiferromagnetic chromium are reported. In Ni above 260 K B_Fermi was found to be proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly below 30 K which is presumably caused by a magnetoelastic interaction. There was no evidence for an occupation of metastable sites by the μ⁺ below the Curie temperature. By contrast, in \alpha‐Fe the temperature dependence of \curpartialB_μ/\curpartialp shows a structure which might be attributed to the occupation of excited muon states at elevated temperatures. High‐pressure zero‐field experiments on Cr performed in the temperature regime between 4.5 K and 8 K revealed a pressure dependence of B_μ as large as \curpartialB_μ/\curpartialp=-(89.15\pm 0.06)\times 10^-12 T/Pa. In terms of volume dependence a very large negative Grüneisen parameter \gamma =-27 was obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hyperfine field and diffusion of μ+ in Fe single crystals

In recent positive-muon spin rotation experiment at TRIUMF on single crystal Fe, a clear temperature dependent change has been observed, for the first time, both in frequency and depolarization rates from 300 K down to 23 K. The μ⁺ depolarization was explained by the μ⁺ diffusion through inhomogeneous dipolar fields and the diffusion constant was found to obey an Arrhenius law (activation energy 17 meV) above 70 K but surprisingly deviated from this at lower temperatures, indicating quantum diffusion. We have also found that the μ⁺ hyperfine field has a temperature dependence slightly stronger than that of the magnetization.     

Mössbauer effect study of Y(57FeMn)2

YMn₂ compound doped with⁵⁷Fe was investigated using⁵⁷Fe Mössbauer resonance in the temperature range 4.2–400 K. The magnetically split spectra were analyzed assuming two magnetically nonequivalent Fe sites with relative population dependent on iron concentration. In the transition temperature region a coexistence of the magnetic and nonmagnetic components was observed in the temperature span of about 50 K. A thermal hysteresis (of about 25 K) of the magnetic component confirms the first-order type magnetic transition. Temperature dependence of the hyperfine field of the magnetic component could be interpreted in terms of spin-fluctuation theory.     

Atom-probe study of hydrogen chemisorption on Fe and Ni

Iron and nickel, which are chemisorbing hydrogen, were mass-analyzed by the atom-probe mass spectrometer. Specimen temperature was varied from 20 to 200 K while maintaining the hydrogen pressure at 4 × 10^?6 Pa. The ratio of the number of hydrogen ions to that of Ni ions decreased sharply at the critical temperature which is low for the closely packed Fe(011) and Ni(111) planes, ~60 K, and high for other planes, ~100 K. The occurrence of hydrides was noticed only in the narrow temperature range around the critical temperature at which the largest number of hydride ions were detected. The observed results were explained as the result of the transition of the adsorption site of hydrogen atoms from the top of surface metal atoms to the sites between or under the surface atoms.     

Magnetic interaction in Tm3Fe5O12 studied by means of169Tm and57Fe Mössbauer spectroscopy

The temperature dependence of the hyperfine parameters of thulium iron garnet (Tm₃Fe₅O₁₂) powder was studied from 90 to 550 K using¹⁶⁹Tm and⁵⁷Fe Mössbauer spectroscopy (MB). The spectra were analyzed by least mean square fits to the transmission function. The temperature dependence of the magnetic fields of the thulium nuclei is well described by the mean field model. The coupling constants between the magnetic lattice occupied by the thulium atoms and the magnetic lattices occupied by the iron atoms were derived.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

Untersuchung gestörter Winkelverteilungen an F19 in ferromagnetischen Gittern

The perturbation of the 197 keV transition angular distribution in F¹⁹ was investigated by time-dependent spin rotation measurements following excitation with a pulsed beam. The recoil implantation technique was used to determine the internal magnetic fields for F¹⁹ in Fe, Co and Ni lattices. The results are:H _HF(F¹⁹ in Fe)=+(95700±500) Oe,H _HF(F¹⁹ in Co)=+(59500±1500) Oe,H _HF(F¹⁹ in Ni)=?(21830±350)Oe. The temperature and field dependence of the effective fields was studied. Strong satellite fields due to lattice perturbations were detected. The half life and the gyromagnetic ratio of the 197 keV 5/2⁺ state in F¹⁹ were redetermined asT _1/2=(80.2±0.5) nsec andg=+1.436±0.007.     

Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe₂O₄ spinel oxides. Mössbauer spectra for NiFe₂O₄ at various temperatures (79 ≤?T?≤ 900 K) are well fitted by two sextets associated with ⁵⁷Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0)[{1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.     

Mössbauer effect study of Gadolinium Iron Garnet

The Mossbauer Effect study of polycrystalline Gadolinium Iron Garnet has been undertaken between 714K and 4.2K. The temperature dependence of the sublattice magnetization has been determined at the two iron sites and the direction of the hyperfine fields at each site is [111] over the entire temperature range. The saturation fields at the octahedral and tetrahedral sites are respectively 547 kOe and 477 kOe. The ferrimagnetic Neel temperature has been found to be 562.5K and above this temperature, the quadrupole splitting at the octahedral and tetrahedral sites are –0.43 mms^–1 and ¦0.89¦mms^–1 respectively.     

Hyperfine interactions of111Cd in ion-implanted face centered cubic cobalt

The magnetic-dipole and electric-quadrupole hyperfine interactions of¹¹¹Cd probes in fcc cobalt after implantation of radioactive¹¹¹In⁺ ions have been investigated by PAC measurements with fast BaF₂ detectors. Six different sites of the probe atoms could be distinguished and characterized by their hyperfine parameters and annealing behaviour. Besides the substitutional site, three sites are assigned to In-vacancy complexes which are formed athermally in the implantation process or by thermally-activated trapping of lattice defects in annealing stage III. The remaining two sites are attributed to In located in stacking faults or hcp regions of the host. Systematic trends of impurity hyperfine fields in defect sites become evident from a comparison with other impurity-host combinations. The temperature dependence of the magnetic hyperfine fields has been measured between 20 K and 390 K. Large differences found for the various sites are discussed.     

Investigation on the undercooling and crystallization of Ni–Fe alloy coating melt by DSC

The undercooling of Ni–Fe alloy coating melt was in situ investigated by differential scanning calorimeter with flux processing technique. The highest undercooling of Ni–Fe alloy with 426 K was obtained as the thermal treatment temperature of the melt being 1904 K and the cooling rate being 50 K min^?1. When cooling rate is fixed, the undercooling depends on the melt processing temperature, and increases rapidly at the first stage. The effects of thermal treatment temperature and cooling rates on the undercooling were discussed.     

Interaction of oxygen and nitrogen with clean transition metal surfaces

The interaction of O and N with evaporated films of Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Fe, Ni, Cu, and Ag has been analysed in the range 10^?3 to 10^?10 mbar and 77 to 370 K with respect to the processes prevailing in various metal-gas systems. The shape of typical energy-reaction path diagrams estimated from thermodynamical data and from the dependence of the sticking probability on pressure, temperature, and coverage provide a suitable framework for a systematic characterization of the surface reactions observed.     

57Fe Mössbauer study of Lu2Fe3Si5 iron silicide superconductor

With the advent of Fe–As based superconductivity it has become important to study how superconductivity manifests itself in details of ⁵⁷Fe Mössbauer spectroscopy of conventional, Fe-bearing superconductors. To this end, the iron-based superconductor Lu₂Fe₃Si₅ has been studied by ⁵⁷Fe Mössbauer spectroscopy over the temperature range from 4.4 K to room temperature with particular attention to the region close to the superconducting transition temperature (T_c=6.1 K). Consistent with the two crystallographic sites for Fe in this structure, the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. The value of Debye temperature was estimated from temperature dependence of the isomer shift and the total spectral area and compared with the specific heat capacity data. Neither abnormal behavior of the hyperfine parameters at or near T_c, nor phonon softening were observed.     

57Fe M?SSBAUER STUDY ON (NdDy)TiFe11Ny COMPOUNDS

⁵⁷Fe M?ssbauer spectra have been obtained at room temperature for (NdDy)TiFe₁₁, and at 11 K and room temperature for the corresponding nitrides (NdDy)TiFe₁₁N_y. The magnetic behaviors of Fe atoms at different sites have been studied. We have found a larger increase of the hyperfine fields upon nitrogenation due to the higher nitrogen content in these compounds and got a bigger enhancement of the isomer shift in 8j site because of the nearest nitrogen environment.     

Hydrogenation of Zr2Ni

The intermetallic compound Zr₂Ni has been found to take up hydrogen on charging at room temperature. Zr₂(NiFe) H_4.7 and Zr₂(NiFe) H_4.5 show the same structure (CuAl₂ type) as the uncharged compound but with an expanded lattice.Analysis of room temperature spectra in zero and applied fields indicates that the⁵⁷Fe atoms occupy Ni sites in Zr₂(Ni ⁵⁷Fe). Volume expansion effects account for about one third of the increase in isomer shift ( +0.58 mms^–1) observed on hydrogenation. The distribution of hydrogen atoms around the probe³⁷Fe atoms also causes a decrease of 0.23 mms^–1 in the mean value for the quadrupole splitting compared Rith uncharged Zr₂Ni.