Observation of the gamma resonance of a long-lived <Superscript>109<Emphasis Type="Italic">m</Emphasis></Superscript>Ag isomer using a gravitational gamma-ray spectrometer

Experiments confirming the previous data on the small real width of the γ resonance of ^109m Ag have been performed with a gravitational gamma-ray spectrometer. The achieved resolution is eight orders of magnitude higher than that of Mössbauer spectrometers using gamma rays from a ⁵⁷Fe nuclide. The theoretically predicted broadening of the Mössbauer gamma line of ^109m Ag by five orders of magnitude (due to the dipole-dipole interaction between the magnetic moments of neighboring nuclei) has not been observed.     

First experiments on the observation of gamma resonance of a long-lived <Superscript>109<Emphasis Type="Italic">m</Emphasis></Superscript>Ag isomer using a gravitational gamma spectrometer

The first measurements of the resonant Mössbauer self-absorption of the long-lived ^109m Ag isomer gamma rays as a function of the declination angle of the gamma beams from the horizontal direction are presented for a gamma source that represents a single-crystal silver plate doped with a parent ¹⁰⁹Cd nuclide. The resonant absorption is manifested as a minimum of the counting rate ratio of the ^109m Ag and ²⁴¹Am (control gamma source) gamma-line intensities at a zero-declination angle for one of two detectors and an angle of about 1° for another detector. The magnitude of the effect is in agreement with the results of our previous experiments performed on another setup. It follows, from the data obtained, that the angular width of the gamma-resonance profile is less than two degrees, which corresponds to a broadening factor of the Mössbauer gamma line of less than five. The value of this factor calculated from the measured relative magnitude of the ^109m Ag gamma-ray resonant absorption is 6.3 _?1.9 ^+5.2 .     

Structure of the local environment and hyperfine interactions of <Superscript>57</Superscript>Fe probe atoms in DyNiO<Subscript>3</Subscript> nickelate

The local structure of DyNiO₃ nickelate at both sides of the insulator (T < T _im) ? metal (T > T _im) phase transition was studied by probe ⁵⁷Fe Mössbauer spectroscopy. The character of change in the hyperfine parameters of probe iron atoms specifically near the phase-transition temperature (T ≈ T _im) was analyzed.     

<Superscript>119</Superscript>Sn CEMS study of Sb doped SnO<Subscript>2</Subscript> film

Sb doped SnO₂ films prepared by DC sputtering and heating were characterized by ¹¹⁹Sn conversion electron Mössbauer spectrometry (CEMS). An asymmetric doublet was observed in the Mössbauer spectra of 1 %, 3 %, and 10 % Sb doped SnO₂ films. The peak ratios of doublets are considered to be due to the columnar crystal growth on the substrate. With the doping level of Sb, both the isomer shift (δ) and the quadrupole splitting (Δ) increased. After annealing, δ increased and Δ decreased for each sample. These results suggest the followings. The electron doping of the SnO₂ lattice by pentavalent Sb induces the increase of the electron density at the Sn^IV nucleus. The annealing process leads to more complete accommodation of the Sb dopant that results in more effective electron doping and therefore increasing isomer shift for tin. Simultaneously, the distortion of the lattice caused by Sb is relaxed and the quadrupole splitting decreases.     

Small broadening of the Mössbauer gamma line of the isomer <Superscript>109<Emphasis Type="Italic">m</Emphasis></Superscript>Ag as yet another indication of the protracted character of the emission of photons from nuclei and their absorption

Nine experiments performed to date by three research groups in order to observe the Mössbauer effect in the case of gamma rays emitted by the long-lived isomer ^109m Ag yielded results suggesting an anomalously small broadening of the Mössbauer gamma line of this isomer. In turn, this indicates that the emission of photons from nuclei and their resonance absorption cannot proceed within a time interval much shorter than the characteristic time of the change in the energy of hyperfine dipole—dipole interaction, because the photon energy would otherwise be determined by the sum of the instantaneous value of the energy of this interaction and the nuclear-transition energy, in which case the broadening of the Mössbauer gamma line would reach five to six orders of magnitude, as opposed to one to two orders of magnitude broadening observed experimentally.     

On the phenomenon of oscillatory decay of the <Superscript>125m</Superscript>Te isomer

The radioactive decay curve of the ^125mTe nucleus is measured in the environment of stable ¹²⁵Te nuclei representing a Mössbauer resonant screen. Thorough analysis shows that the decay of the ^125mTe nucleus corresponds to the standard exponential law with half-life T_1/2≈58 days without oscillations.     

Crystal field of Yb<Superscript>3+</Superscript> tetragonal centers in the YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> intermetallic compound

The spectra of electron paramagnetic resonance and inelastic neutron scattering in crystals of the heavy-fermion intermetallic compound YbRh₂Si₂ are interpreted. The phenomenological potentials of the crystal electric field of Yb³⁺ tetragonal centers and the parameter of the Hamiltonian for the spin-orbit interaction of electrons are determined from the experimental energy level schemes. A comparison of the results obtained from experimental data on electron paramagnetic resonance, inelastic neutron scattering, and Mössbauer spectroscopy shows that the most probable ground state of Yb³⁺ ions in the YbRh₂Si₂ crystal is the Kramers doublet Γ _t6 ^? .     

Variation of electronic density in the superconducting phase transition in Nb<Subscript>3</Subscript>Al

The variation of electronic density in the superconducting phase transition in the classical superconductor Nb₃Al with critical temperature T_c=18.6 K was studied using ⁷³Ge emission Mössbauer spectroscopy. A comparison of the results obtained and the data available for the ⁶⁷Zn isotope in the lattices of high-temperature superconductors revealed a correlation between the electronic density variation at the Mössbauer probe nuclei sites and the value of T_c. This correlation is assumed to be related to the dependence of the electronic density variation on the standard correlation length.     

Magnetic properties of single crystals of ludwigites Cu<Subscript>2</Subscript><Emphasis Type="Italic">M</Emphasis>BO<Subscript>5</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe<Superscript>3+</Superscript>, Ga<Superscript>3+</Superscript>)

High-quality single crystals of ludwigites Cu₂ MBO₅ (M = Fe³⁺, Ga³⁺) have been grown, and the magnetic, resonance, and Mössbauer studies have been performed. It is established that the Cu₂FeBO₅ and Cu₂GaBO₅ compounds are antiferromagnets with Néel temperatures of 32 and 3.4 K, respectively. A model of the magnetic structure of the compounds is proposed. It is shown that the magnetic properties of the ludwigites are substantially dependent on the degree of ion distribution over crystallographic positions.     

The <Superscript>57</Superscript>Fe hyperfine interactions in the iron-bearing phases in some LL ordinary chondrites

The study of several LL ordinary chondrites such as NWA 6286 LL6, NWA 7857 LL6 and Chelyabinsk LL5 fragments with different lithology was carried out using scanning electron microscopy with energy dispersion spectroscopy, X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy with a high velocity resolution at 295 K. Small variations in the ⁵⁷Fe hyperfine parameters were revealed for the M1 and M2 sites in olivine, orthopyroxene and clinopyroxene as well as for α-Fe(Ni, Co), α ₂-Fe(Ni, Co) and γ-Fe(Ni, Co) phases, and for troilite in different samples of studied LL ordinary chondrites.     

Experimental studies of the gamma resonances of long-lived nuclear isomers

Studying the gamma resonances of the long-lived nuclear isomers started at ITEP in the 1960s–1970s. The first experiments were conducted with silver isotopes. Its results did not contradict the existing theoretical ideas of large broadening of Mössbauer gamma lines via the interactions of nuclear magnetic moments. However, the data obtained in 11 experiments performed up until now with gamma sources made of silver metal doping by ¹⁰⁹Cd showed that there is no large broadening of ^109mAg Mössbauer gamma line with energy of 88.03 keV, that is the theoretically predicted gamma line broadening by ～10⁵ times is absent. The instrument of quite a new type—“gravitational gamma spectrometer”, designed at ITEP, allowed one to determine the form of ^109mAg gamma resonance, which proved to be ～10⁸ times narrower than that of well known nuclide ⁵⁷Fe. Some ideas are discussed as an attempt to explain this situation.     

Specific features of magnetic states of impurity iron ions in the perovskite La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript> <Superscript>57</Superscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript>

Single-phase polycrystalline La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ samples have been prepared by solidstate ceramic technology. The samples have the rhombohedral structure (space group \(R\bar 3c\)). The studies of perovskite La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ by Mössbauer spectroscopy on impurity ⁵⁷Fe nuclei in the temperature range of 5–293 K have revealed the existence of a superparamagnetic relaxation in the temperature range of 100–210 K. The parameters of hyperfine interactions (hyperfine magnetic fields, line shifts, and quadrupole shifts) and the anisotropy energy have been measured, and the frequencies of magnetic moment relaxation of iron ions have been estimated.     

Magnetic quantum beating of gamma radiation on <Superscript>181</Superscript>Ta nuclei

A frequency-time variant of gamma resonance spectroscopy is suggested with the use of quantum interference on Mössbauer transitions induced under the conditions of gamma magnetic resonance. Calculations are performed for the ¹⁸¹Ta nucleus. It has been shown that the coherent dynamics of nuclear spins is essential for the formation of the time harmonics of the absorption spectrum.     

Experimental search for laser-induced effects in <Superscript>151</Superscript>Eu and <Superscript>57</Superscript>Fe doped crystals

We studied the Mössbauer effect in ¹⁵¹Eu and ⁵⁷Fe doped crystals in the search for laser-induced effects caused by changes in the hyperfine interaction due to electronic excitation. The Mössbauer spectra observed in the presence of laser radiation demonstrated a notable change of the shape of the ¹⁵¹Eu spectrum and the appearance of an additional hyperfine pattern in the case of the ⁵⁷Fe Mössbauer resonance.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Magnetic and electrical properties of Fe<Subscript>1.91</Subscript>V<Subscript>0.09</Subscript>BO<Subscript>4</Subscript> warwickite

We have performed a complex investigation of the structure and the magnetic and electrical properties of a warwickite single crystal with the composition Fe_1.91V_0.09BO₄. The results of Mössbauer measurements at T=300 K indicate that there exist “localized” (Fe²⁺, Fe³⁺) and “delocalized” (Fe_2.5+) states distributed over two crystallographically nonequivalent positions. The results of magnetic measurements show that warwickite is a P-type ferrimagnet below T=130 K. The material exhibits hopping conductivity involving strongly interacting electrons. The experimental data are analyzed in comparison to the properties of the initial (unsubstituted) Fe₂BO₄ warwickite. The entire body of data on the electric conductivity and magnetization are interpreted on a qualitative basis.     

Experimental and <Emphasis Type="Italic">ab initio</Emphasis> study of the hyperfine parameters of ZnFe <Subscript>2</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript> with defects

We present a combined Mössbauer and ab initio study on the influence of oxygen-vacancies on the hyperfine and magnetic properties of the ZnFe ₂ O ₄ spinel ferrite. Samples with different degree of oxygen-vacancies were obtained from zinc ferrite powder that was thermally treated at different temperatures up to 650 ^°C under vacuum.Theoretical calculations of the hyperfine parameters, magnetic moments and magnetic alignment have been carried out considering different defects such as oxygen vacancies and cation inversion. We show how theoretical and experimental approaches are complementary to characterize the local structure around Fe atoms and interpret the observed changes in the hyperfine parameters as the level of defects increases.     

Diamagnetic nuclear <Superscript>119</Superscript>Sn probes in the copper chromites CuCr<Subscript>2</Subscript>X<Subscript>4</Subscript> (X = O,S, Se) with a spinel structure

The CuCr₂X₄ (X = O, S, Se) spinel system has been studied by the Mössbauer spectroscopy of the nuclear diamagnetic ¹¹⁹Sn probe at low temperatures in an external magnetic field. The hyperfine magnetic fields H ^Sn induced by paramagnetic ions at tin nuclei in the CuCr₂S₄ and CuCr₂Se₄ chalcogenides have giant values and are somewhat higher than those detected in the CuCr₂O₄ oxide. This behavior is caused by the strong covalence of the chalcogenides, which is supported by the experimentally found isomer shifts. The H ^Sn field is found to be mainly contributed by superexchange 90° interactions in the B-sublattice along the Cr[B]-X-Sn[B] bond chain, whose role increases in the series O-S-Se. In the oxygen CuCr₂O₄ spinel, the partial contributions to the H ^Sn field induced by the Cu²⁺ and Cr³⁺ ions are estimated. The local magnetic structure of the CuCr₂O₄ spinel is refined, and its total magnetization is shown to be directed along the magnetic moment of copper in the A sublattice.     

Synthesis and properties of barium ferrigermanate Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript>

The magnetic susceptibility and specific heat of single crystals of the Ba₂Fe₂GeO₇ barium ferrigermanate are investigated. It is revealed that the temperature dependence of the magnetic susceptibility exhibits a kink at a temperature T = 8.5 K. The number of nonequivalent positions of Fe³⁺ ions and their occupancies are determined using Mössbauer spectroscopy. It is shown that the Fe³⁺ ions located in tetrahedral positions T2 are ordered incompletely, which is inconsistent with the results obtained previously. An assumption is made regarding the possible ground magnetic state of the Ba₂Fe₂GeO₇ compound.