Nuclear resonance fluorescence in238U and a new approach to Doppler-shift-attenuation using high speed rotation

Using⁵⁶Co sources, a 3.254 MeV level in²³⁸U was photoexcited by aγ-line emitted in the decay of the 4.100 MeV level in⁵⁶Fe. 10γ-transitions deexciting this 3.254 MeV level were identified. Spin and parity were determined to be 1⁻. By moving the⁵⁶Co source in a high speed centrifuge the profile of the 3.254 MeV emission line was investigated. By applying a new variety of Doppler shift analysis a lifetime ofτ _Fe=55±25fs was obtained for the 4.100 MeV level in⁵⁶Fe. The cross section for nuclear resonance fluorescence yields a lifetime ofτ _U =0.33±0.12 ps for the 3.254 MeV level in²³⁸U.

     

The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting Δv_Q of the nuclear hyperfine interaction due to an electric field gradient V_zz. MAPON has successfully been applied to measure Δv_Q as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes⁵⁶Co,⁵⁷Co,⁵⁸Co and⁶⁰Co have been studied in iron single crystal hosts, yielding Δv_Q consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe<100> system. The variation of Δv_Q with direction of magnetization, measured in⁵⁸CoFe and⁶⁰CoFe single crystal samples, is also described. Further MAPON measurements are described for a⁵⁶CoFe polycrystalline sample, for which the most probable value and width of the distribution of V_zz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.     

Influence of the coordination water on the after effects of the K-capture in57Co doped FeSO4×H2O (x=0,1,4,7)

Emission Mössbauer spectroscopy is used to investigate the physico-chemical state of⁵⁷Fe atoms generated by⁵⁷Co²⁺ clectron capture in⁵⁷Co: FeSO₄xH₂O (x=0,1,4,7). The spectra of all the hydrates show at least three main components, the normal bivalent state, Fe²⁺(N), an aliovalent state, Fe³⁺, and one (or several) anomalous ferrous species in which the coordination sphere has been perturbated by the self-radiolysis. No Fe³⁺ is observed in the anhydrous compound whereas the intensity of this component increases as a function of the hydration number, reaching a saturation value of 36% for x=4 and 7.     

Mössbauer emission study of57Co: YBa2Cu3O7−y HTSC

Mössbauer emission spectroscopy is performed on⁵⁷Co:YBa₂Cu₃O_7–y oxides in a temperature range from 300 K to 77 K. The spectra show, at least two different location for the⁵⁷Co(⁵⁷Fe) impurities. The isomer shift and quadrupole splitting values of the⁵⁷Fe daughter are coincident with those observed in⁵⁷Fe absorption experiments. From the analysis of theIS andQS values as well as from the relative location of Fe impurity levels in the HTSC matrix it is argued that:a) Co impurities enters into the lattice mainly in the Cu1 sites, but some of them have a higher coordination number.b) The daughter⁵⁷Fe exists as localized Fe⁴⁺ state.c) The parent⁵⁷Co enter as a localized Co³⁺ state. These conclusions appear consistent with the observed increase ofN(0) on doping with Fe or Co ions and with the existence of localized magnetic moments as determined from paramagnetic susceptibility measurements.     

Contributions of Spin–Lattice Relaxation to the Echo Decay of Planar Cu in High-Temperature Superconductors

Measurement ofT_2G, the Gaussian component of the spin-echo envelope of planar Cu nuclei in high-temperature superconductors, gives important information about the real part of the Cu electron spin susceptibility. In the traditional picture of the planar Cu echo decay, the internuclear coupling is assumed to remain static with respect to spin–lattice relaxation and mutual exchange fluctuations. In some circumstances, however, this assumption breaks down. We calculate the internuclear corrections arising from spin–lattice relaxation to the conventional theory ofT_2Gand show thatT_2Gcan be easily corrected for these effects. We argue that mutual exchanges due to the perpendicular indirect couplings are suppressed in these materials. For YBa₂Cu₄O₈, we find a correction on the order of 10% inT_2Gand using the corrected values we find that the isotope ratio⁶³T_2G/⁶⁵T_2Gagrees with theory.     

Sr2Fe1－xCoxMoO6体系中Co的元素替代效应

在成功制备具有双钙钛矿结构Sr₂Fe_1-xCo_xMoO₆系列样品的基础上，对其结构、输运性质和磁性质进行了系统研究.结果发现，随着Co替代浓度x值的增加，样品的电阻率-温度关系由半金属行为转变为半导体行为，其室温电阻率从3.9×10^-5Ω·cm增大到6.0×10^-1Ω·cm；样品由亚铁磁体转变成反铁磁体，其磁相变温度T_N值也随之下降; Co对Fe的部分替代使其磁电阻效应受到抑制.基于对其电子结构的分析，其磁电阻效应的起源以及Co的元素替代效应也在文中进行了讨论. 关键词： 双钙钛矿结构 2FeMoO₆')" href="#">Sr₂FeMoO₆ 磁电阻     

A study of the cobalt sites in cobalt and iron molybdate catalysts by emission Mössbauer spectroscopy

Molybdates samples containing 74MBq (2mCi) of cobalt 57 have been studied by emission Mössbauer spectroscopy. Spectra of⁵⁷CoMoO₄ exhibited the three usual Fe²⁺ doublets corresponding to α and β ferrous phases, and two Fe³⁺ doublets accounting for about one third of the absorption area. The Co²⁺ sites of β⁵⁷CoMoO₄ phase exhibit higher quadrupole splittings, i.e. are more dissymmetrical than Fe²⁺ sites of βFeMoO₄ phase. Cobalt and iron molybdates Fe_x ⁵⁷Co_1?xMoO₄ provided spectra where only one Fe³⁺ doublet appeared and occupied less than 7% of the spectral area. It is concluded that a part of cobalt was present in⁵⁷CoMoO₄ as Co³⁺.     

Local states of iron in iron implanted hematite Fe2O3

Powder samples of⁵⁷Fe₂O₃ and⁵⁶Fe₂O₃ were implanted with⁵⁶Fe and⁵⁷Fe ions, respectively. By the use of Conversion Electron Mössbauer Spectroscopy it was possible to observe the local states of implanted ions (⁵⁷Fe in⁵⁶Fe₂O₃) or the states of iron atoms from the target which were displaced during implantation due to the ballistic processes (⁵⁶Fe in⁵⁷Fe₂O₃). The implanted and displaced iron atoms appear in three different states: (i) in regular substitutional positions of Fe₂O₃, (ii) as magnetite Fe₃O₄-type structures and (iii) paramagnetic FeO_1?x state. The observed fractions of each state agree rather well with the calculated values obtained from the local iron atom enrichment at the surface as well as from the analysis of the equilibrium phase diagram for the binary Fe?O system. However, in⁵⁷Fe implanted samples some enhancement of the FeO_1?x fraction was found in comparison with the⁵⁶Fe implanted hematite.     

The hyperfine magnetic field at57Fe and181Ta in Hf(Fe1−x Co x )2 system

The hyperfine magnetic field at⁵⁷Fe in the ferromagnetic Laves compounds Hf(Fe_1?x Co _x )₂ was measured by the Mössbauer effect method. The substitution of Fe atoms by Co atoms induces a gradual change ofB _hf(Fe) and a decrease in the magnetic moment values in Fe?Co sublattice. The perturbed angular correlation measurements of¹⁸¹Ta were carried out forX=0,X=0.1,X=0.55 at 300 K, andX=0.4 atT=400°C. An abrupt change ofB _hf (Ta) in 0<X<0.1 was observed.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Hyperfine field relaxations in the Fe−Co−B amorphous alloys

A systematic increase of the average hyperfine magnetic induction at⁵⁷Fe measured at room temperature during interrupted isothermal annealing was found to be inherent to the irreversible relaxation processes in the Fe−B based soft magnetic alloys at moderately elevated temperatures. Assuming superimposed asymptotic exponential field vs. time dependences. several processes can be distinguished, their relaxation times determined and from the Arrhenius-like log τ vs. 1/T plots average activation enthalpies estimated. Results on the Fe₇₀Co₁₀B₂₀ and Fe_85−x Co _x B₁₅ (x=17, 19 and 21 at. % Co) amorphous ribbons between 100 and 200°C are compared and discussed in terms of possible stress relief, free volume annihilation and short range ordering mechanisms.     

Magnetic properties of FexCo1−x/CoyFe1−yFe2O4 composite under hydrothermal condition

The metal–ferrite composites Fe_xCo_1−x/Co_yFe_1−yFe₂O₄ are synthesized by using disproportion of Fe (II) and reduction of Co (II) by Fe⁰ under hydrothermal condition. The size of the particles of the composites decreases as the [KOH] decreasing. The composites are measured by TEM and it can be deduced that when [KOH] = 0.1, the size of the alloy body-centered cubic (BCC) in composites is 20 ± 7 nm, the size of the Cobalt ferrite (spinel) is 170 ± 50 nm. The maximal value of the saturation magnetization (Ms) of the composite is about 100.14 emu/g, which is synthesized under Co (II)/Fe (II) = 0.05, [KOH] = 1 N, T = 150 °C and t = 3 h. The value of Hc of the composite synthesized under Co (II)/Fe (II) = 0.5, t = 3 h, T = 150 °C and [KOH] = 10.2 mol/L is about 2878.19 Oe. The Fe–Co alloy is synthesized through a reduction reaction of the composites in a flowing gaseous mixture. There is a maximal value (302.9 emu/g) of the Ms for the alloys generated at 1000 °C, which is the Co_0.412Fe_0.588 alloy.     

Transient fields for Ag and Pd ions IN Fe and Co;g-Factor measurements of the lowest 3/2− and 5/2− states in107,109 Ag

Transient field precessions of the 2 ₊ ¹ state of¹⁰⁸Pd were measured simultaneously with those of the 3/2 _– ¹ and 5/2 _– ¹ states of¹⁰⁷Ag and¹⁰⁹Ag as their ions traversed thin Co and Fe foils. Precessions of the 2 ₊ ¹ states of^{104,106,108,110}Pd were also measured using a natural Pd target evaporated onto 0.9 m thick Co. The results of these studies are in accord with prior predictions that the AgFe transient field might be diminished relative to that for PdFe at ion velocities appropriate for molecular orbital vacancy sharing. Transient field strengths for PdCo and PdFe were found to be consistent with scaling according to bulk magnetization of the ferromagnetic host. Gyromagnetic ratios were inferred for the first 3/2^– and 5/2^– states of^107,109Ag relative to theg factor of the 2 ₊ ¹ state of¹⁰⁸Pd. Values obtained usingg(2 ₊ ¹ ;¹⁰⁸Pd)=0.36± 0.03 are:g(3/2;¹⁰⁷Ag) =0.63±0.09,g(5/2;¹⁰⁷Ag)=0.37±0.06,g(3/2;¹⁰⁹Ag)=0.77 ±0.10,g(5/2;¹⁰⁹Ag)=0.36±0.05.     

Nuclear spin lattice relaxation of191mIr in Fe

Thermal cycling of the lattice temperature was used to determine the nuclear spin lattice relaxation of^191m IrFe in polarizing fields of 0.05 to 1.3 T. At low temperatures, the relaxation time is not very much shorter than the lifetime of^191m Ir. In the first part of the paper, the master equation formalism is extended to include a finite lifetime. Our result for the reduced relaxation constant, ² C _K =(1.48±0.11)·10¹⁴ K s^–1 T^–2 (high field limit) is in serious disagreement with that of a spin echo measurement of¹⁹³IrFe, but fits much better into the general systematics. A comparison of relaxation rates for 3d-, 4d-, and 5d-impurities in Fe is given. As a by-product, a Kapitza conductivity constant ofl _K =1.5 mW cm^–2 K^–4±30% was found between Fe and dilute³He/⁴He.     

57Fe Mössbauer and infrared studies of the system Co1−x Cd x Fe2O4

Mössbauer and infrared studies were made on samples of the ferrite system Co_1–xCd_xFe₂O₄ x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1. Mössbauer spectra were taken at room temperature. The spectra of the samples withx0.7 showed well defined Zeeman patterns and they have been analyzed with two components, one due to A-site Fe³⁺ ions, and the other due to B-site Fe³⁺ and Fe²⁺ ions. The pattern due to B-site appeared to be composite and an explanation is given. The spectra withx=0.9 and 1 showed only a quadrupole splitting. The effect of cadmium substitution on the various hyperfine interactions has been discussed and the cationic distribution has been deduced for all values ofx. Far infrared spectra of the ferrite samples in the range 200–700 cm^–1 were reported. Four bands were observed: the high frequency bandv ₁ is assigned to tetrahedral complexes, and the low frequency bandv ₂ to octahedral complexes, a small bandv ₃ is due to Co²⁺-O^2– complexes andv ₄ is assigned to the lattice vibration of the system. The splitting occurred in thev ₁ andv ₂ bands atx=0.9 and inv ₂ atx=1, indicating the presence of Fe²⁺ ions in octahedral sites.     

DPAC measurement of the hyperfine fields in100RhCo and100RhFe

Using the 85–75 keV -cascade of¹⁰⁰Rh, we report the first DPAC measurement of the hyperfine field in¹⁰⁰RhCo and¹⁰⁰RhFe. Our results were made possible by continued improvement in time resolution using BaF₂ scintillators. At room temperature we obtained fields of 420.6(1.3) kG and 540.4(1.5) kG for¹⁰⁰RhCo and¹⁰⁰RhFe, respectively. The former revises an earlier spin echo result by Kontani and Itoh by 9%; the latter is consistent with the work of Matthiaset al. using resonant destruction of angular correlations.     

Structural and 57Fe M?ssbauer study of EuCr1???x Fe x O3 nanocrystalline particles

Emission (⁵⁷Co) M?ssbauer spectra of the aspartic acid—⁵⁷CoCl₂ system were measured at T?=?80?K in frozen aqueous solution and in the form of a dried residue of this solution. The M?ssbauer spectra, besides a weak contribution from after-effects, showed two Fe^2?+?/Co^2?+? components which were ascribed to octahedrally and tetrahedrally coordinated ⁵⁷Co^II microenvironments in the Asp–cobalt(II) complex. This dual coordination mode may be due to the involvement of the second terminal carboxylic group of aspartic acid in the coordination sphere of Co.     

Microstructure evolution of undercooled Fe-Co-Cu alloys

Rapid solidification of undercooled Fe-Co-Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling ΔT_sep. When ΔT<ΔT_sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once ΔT>ΔT_sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. ΔT_sep was 222, 88 and 45 K for Fe₅₀Co₃₀Cu₂₀, Fe₂₅Co₂₅Cu₅₀ and Fe₁₅Co₁₅Cu₇₀ alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways.     

Laser radiation control of the magnetic state of multilayer nanofilms

Physical mechanisms of magnetization reversal of multilayer magnetic nanofilms by laser radiation are examined and the experiments on the effect of the magnetic field and nanosecond and picosecond laser pulses on the conductivity of the Tb₁₉Co₅Fe₇₆/Pr₆O₁₁/Tb₂₂Co₅Fe₇₃ and Co₈₀Fe₂₀/Pr₆O₁₁/Co₃₀Fe₇₀ tunnel microcontacts are carried out. It is shown that with the help of such laser pulses, magnetization reversal of magnetic nanolayers is possible in a zero external magnetic field under the action of the spin current magnetic field, induced by the photon pressure of laser radiation, or the magnetic field generated by circularly polarized picosecond laser pulses. A relative change in the resistance upon the laser magnetization reversal of one of the nanolayers in the Co₈₀Fe₂₀/Pr₆O₁₁/Co₃₀Fe₇₀ microcontacts reaches a value of ΔR/R = 0.06 for T = 300 K and ΔR/R = 0.25 for T = 80 K, in the Tb₁₉Co₅Fe₇₆/Pr₆O₁₁/Tb₂₂Co₅Fe₇₃ microcontacts, ΔR/R = 0.3 for T = 300 K and ΔR/R = 0.7 for T = 80 K.     

Observation of multiple superconducting gaps in the infrared reflectivity spectra of Ba(Fe<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe_0.9Co_0.1)₂As₂ are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s ^+/− or s ^+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe_0.9Co_0.1)₂As₂ manifest opening of several superconducting gaps at temperatures lower than critical T _c .