Mossbauer Isotope Research Articles

Роль замещающих марганец ионов и кислородной нестехиометрии в формировании свойств манганитов

Structural and magnetic characteristics of La0.7Sr0.3Mn0.957Fe0.05Zn0.05O3+γ and La0.7Sr0.3Mn0.957Fe0.05Mg0.05O3+γ manganites, in which the iron is Mossbauer isotope 57Fe, are investigated and compared. Ceramic samples were sintered in air at 1473 K. They were then exposed to heat treatments at 1223 K and various partial pressure of oxygen in the gas phase, which ensured the production of manganites with following values of non-stoichiometry index: γ = −0.005; 0.000; 0.007; 0.008 for Zn-containing manganites (ZnM), and γ = −0.022; 0.000; 0.002; 0.005 for Mg-containing manganites (MgM). All synthesized manganites have rhombohedral crystal structure. Mössbauer spectroscopy data correspond to Fe3+ (3d5) ions. Curie point and magnetization depend on γ nonmonotonically. ZnM have essentially higher values of magnetic parameters and narrowest temperature interval of “ferromagnetic-paramagnetic” transition as compared to MgM. Their structure is considered as more homogeneous, which corresponds to lower value of quadrupole splitting of Mössbauer spectra. The results obtained indicate that different effect of Zn2+ and Mg2+ ions on electromagnetic characteristics of manganites is mainly determined by the configuration of their electron shells (3d10 and 2p6, correspondingly).

Methods of Coherent Control of Spectral and Temporal Properties of Gamma Photons and Their Potential Applications

Promising methods for controlling the spectral and temporal properties of gamma photons, which are important for developing quantum informatics and improving the accuracy of Mossbauer spectroscopy measurements, are discussed. The specific experimental implementations of gamma photon control achieved to date are discussed, as well as ways for the further development of this direction. The results obtained using the new generation of ETNA piezoelectric transducers are presented. They can significantly increase the vibration frequency of the source or absorber, generate nanosecond gamma-radiation pulses that significantly increase the information capacity of single-photon wave packets, and bring the properties of gamma-radiation sources based on the decay of unstable nuclei closer to those of synchrotron radiation. These piezo-transducers use thin quartz plates with a polished surface. They are coated (using sputtering technique) with an absorber containing 57Fe Mossbauer isotopes, which, due to the vibrations of a thin quartz plate, convert a single line of source radiation into a comb structure. A study of the spectrum of modified radiation makes it possible to determine the amplitude of vibrations of a thin quartz plate with sub-angstrom accuracy.

Mössbauer Probe Diagnostics of the Properties of Quasi-Liquid Water Layer on the Aluminosilicate Surface of Natural Origin

The structure and properties of the quasi-liquid water layer in the “frozen water–clay mineral” system have been studied using a probe technique developed on the basis of the 57Fe Mossbauer isotope in two forms (Fe2+ and Fe3+). The elasticity parameters (shear modulus, Poisson’s ratio, and Gruneisen parameter) of the surface ice are estimated within the fractal geometry. The “Menger sponge"–"Cantor dust” transition with an increase in the layer thickness is established.

Mössbauer Method for Studying Vibrations in a Granular Medium Excited by Ultrasound

Vibrations of a granular medium immersed in epoxy resin without a hardener have been studied by Mossbauer spectroscopy. The granular medium consists of small potassium ferrocyanide particles enriched in the 57Fe Mossbauer isotope and having a mean size of 1.25 µm. Vibrations of particles in resin at a frequency of 12.72 MHz have been excited by means of a polymer piezoelectric transducer. The spectrum of 57Fe nuclei at rest in potassium ferrocyanide consists of a single line. Ultrasonic irradiation results in the splitting of the line into a comb structure with a frequency period equal to the frequency of vibrations. The analysis of the spectrum makes it possible to estimate the amplitude of vibrations of granules and damping of sound in such a medium. The proposed method is unique because it allows measuring subangstrom displacements of particles from their equilibrium positions.

The sapphire backscattering monochromator at the Dynamics beamline P01 of PETRA III

We report on a high resolution sapphire backscattering monochromator installed at the Dynamics beamline P01 of PETRA III. The device enables nuclear resonance scattering experiments on Mossbauer isotopes with transition energies between 20 and 60 keV with sub-meV to meV resolution. In a first performance test with 119Sn nuclear resonance at a X-ray energy of 23.88 keV an energy resolution of 1.34 meV was achieved. The device extends the field of nuclear resonance scattering at the PETRA III synchrotron light source to many further isotopes like 151Eu, 149Sm, 161Dy, 125Te and 121Sb.

Prospects for emission Mössbauer spectroscopy in chemical investigations

Emission Mossbauer spectroscopy is a radiochemical method for investigating materials and the consequences of nuclear transformations taking place in them. Isotopes are traditionally used as structural probes, and the sensitivity of the method is 2–3 orders of magnitude higher than that of absorption Mossbauer spectroscopy. The elements of Mossbauer isotopes with parent nuclei that undergo electron capture or a converted isomeric transition (i.e., lead to high Auger ionization) are the best-studied elements. The electron processes that accompany ionization and their effect on the state of daughter Mossbauer atoms in qualitatively different compounds, from elementary oxides, superconductors, insulators and magnetics to sophisticated bioorganic complexes, are considered.

Concept of “broadening” of a Mössbauer γ photon

The previously found anomalies of radioactive decay of Mossbauer isotopes have been successively interpreted on the basis of the model of a “wave field” in a source saturated with its own decay products.

Defects in semiconductors—results from Mössbauer spectroscopy

Progress in the development of Mossbauer techniques with ion-implanted, radioactive precursors to a Mossbauer isotope is discussed. Results obtained for elemental group IV semiconductors and their alloys as well as for III–V and II–VI compound semiconductors are presented. Emphasis is put on Mossbauer emission spectroscopy with radioactive probe atoms where the recoil energy in the nuclear decay is sufficient to expel the daughter atoms from a (substitutional) lattice site. The interactions of such (interstitial) atoms have been studied for 119Sb →119Sn in III–V compounds and for 57Fe in silicon in particular. Finally, preliminary results, contributing to the question of the origin and nature of the magnetism in the Fe-doped, dilute magnetic semiconductor ZnO, are given.

Observation of Nuclear Excitation of 19F by Synchrotron Radiation

Fluorine (F) has the lowest atomic number and atomic weight of the members of the halogen family. Only the isotope with atomic weight 19 is stable. Fluorine is the most electronegative element and the most chemically energetic of the nonmetallic elements. The reactivity of the element fluorine is so high that chemical processes take place at about 500 C; a good sample is the reaction of fluorine and uranium dioxide leading to the production of uranium hexafluoride. F nuclei with a nuclear spin of 1/2 and a 100% natural abundance presents favourable nuclear magnetic resonance (NMR) characteristics. The application of the F NMR to metabolic studies of fluoropyrimidine drugs in clinical use; such as the anticancer drug 5-fluorouracil in human biofluids, have emphasized the usefulness of the technique for the quantitative detection of novel and unexpected metabolites. The use of synchrotron radiation as a source for nuclear resonant scattering experiments has led to many new studies of physical problems such as nuclear Bragg scattering, nuclear forward scattering, and nuclear inelastic scattering that can not be investigated satisfactorily with radioactive sources in the conventional Mossbauer spectroscopy. The unique characteristics of synchrotron radiation are the collimation, the polarization, the pulsed nature, and the high brilliance. The discrimination between the scattering from electrons and the scattering from nuclei has been solved by taking advantage of the instantaneous character of the atomic prompt nonresonant scattering compared with the relatively long lifetime of the nuclear excited states. Avalanche photodiodes allow the observation of delayed events after a few nanoseconds, even in the presence of a huge prompt background. High-resolution monochromators allow us to select energy and tune by energy bands of the order of 1meV width. The third generation synchrotron radiation storage rings, ESRF, APS, and SPring-8, were constructed and equipped with beamlines dedicated to nuclear resonance scattering experiments. The nuclear resonance scattering of synchrotron radiation has been experimentally observed in various nuclei, i.e., Ta (6.21 keV), Tm (8.41 keV), Kr (9.4 keV), Fe (14.41 keV), Eu (21.54 keV), Sm (22.49 keV), Sn (23.88 keV), Dy (25.65 keV), K (29.83 keV), and Sb (37.13 keV). The use of parentless Mossbauer isotopes, e.g., K, has become a reality. Other isotopes such Ni (67.41 keV), Au (77.35 keV), and Gd (79.51 keV) with high-transition energies above 50 keV have been excited and their fluorescence decay has been detected. Excitation energies above 100 keV of nuclei have not yet been reported. In this paper, we report the observation of the F nuclear excitation (109.89 keV) by the use of synchrotron radiation. The experiment was performed at the undulator beamline (BL09XU) at SPring-8. The undulator gap was tuned such that the seventeenth harmonic of the undulator radiation corresponds to the F resonance energy for the transition from the 1=2þ ground state to the 109.89 keV 1=2 excited state. Figure 1 shows our optics and detecting system for the F nuclear excitation and deexcitation. The double-crystal monochromator operates with diamond (333) reflection, while the X-rays from diamond (111) reflection were eliminated using a Ge(111) crystal. X-ray pulses with a time width of 50 ps [full width of half maximum (FWHM)] excited the F nuclei, using a several-bunch operation of the storage ring. The system with a silicon avalanche photodiode (APD) recorded the time spectra of -rays emitted from the 109.89 keV 1=2 state of the F nuclei. The details of the fast counting system with the APD detector were reported by Kishimoto. The APD has a 4 2 array of 3 5 0:15mm pixels and a depletion layer 150 mm thick. An efficiency of 0.6% at 100 keV was estimated, corresponding to absorption in 150-mm-thick silicon. The time resolution is 1.3 ns (FWHM). The APD was mounted under the sample. We used a fast amplifier to obtain fast negative pulses of the APD outputs, which were processed with a constant fraction discriminator (CFD). The outputs from the CFD were used as the start signals into a time-toamplitude converter (TAC). The veto signals into the CFD and the stop signals into the TAC were supplied through some divider and delay modules by 508.58MHz signals from the accelerator. The time spectra were recorded by the TAC and a multichannel analyzer. The prompt radiation outputs were inhibited during the first 5 ns by veto signals. Delayed signals started to be detected after the first 5 ns of the prompt radiation. For the study of the first excited level of the F nuclei, a lithium fluoride sample (LiF, 20 20 10mm) was used as a scattering sample. Figure 2 shows the time dependences of

Nuclear Inelastic Scattering on Ferrocene-Based Rotator Phases: Theoryvs.Experiment

Nuclear inelastic scattering provides the unique opportunity to probe vibrational densities of states projected onto a selected Mossbauer isotope. This property can be used to monitor local changes of bonding strengths for particular modes, e.g. across phase transitions. Moreover, the partial vibrational DOS can be simulated in a straightforward way using standard quantum chemical methods. We present a comparative study of the two ferrocene-based plastic crystals octamethyl ferrocene (OMF) and octamethyl ethinyl ferrocene (OMFA), with particular emphasis on the solid-solid order-disorder transition in OMFA at 240 K. Using nuclear inelastic scattering (NIS), significant changes were observed when approaching and crossing the phase transition. Molecular NIS spectra were calculated using standard hybrid density-functional methods. The relative importance of intra- vs. intermolecular dynamics for the orientational phase transition is discussed.

A complete solution to the Mössbauer problem, all in one place

We present a full solution to the general combined interactions static Mossbauer problem that is easily generalized to any Mossbauer isotope, and applies for M1, E1, and E2 transitions as well as combined M1-E2 transitions. Explicit expressions are given for both powder and single crystal samples.

Calculation of absolute concentrations and probability of resonant absorption for iron-bearing precipitates in zirconium alloys

In order to find the absolute concentrations and the probability of resonant absorption, the theoretical dependence of effective thickness from Mossbauer absorption line area has been obtained. Calculations of absolute concentrations of secondary phase precipitate in zirconium alloys with natural iron and with iron enriched with Mossbauer isotopes were carried out.

Nuclear Resonant Scattering of Synchrotron Radiation by158Gd

An energy tunable synchrotron radiation (SR) source has a high capability to perform the nuclear resonant scattering experiments with various Mossbauer elements. So far, most of studies of nuclear resonant scattering have been conducted by using several isotopes having a Mossbauer level below 50 keV. When the SR source extends over the X-rays of higher energy region, the number of available Mossbauer isotopes increases remarkably. In this view point, the third generation SR facility such as SPring-8 (Hyogo, Japan) is very suitable as the X-ray source for high-energy nuclear resonant scattering experiments. Actually, the standard undulator beamline of SPring-8 can realize a high photon flux even for over 50 keV. It is due to the high electronic acceleration energy of 8.0 GeV. In this paper, we report the nuclear excitation of a gadolinium (Gd) isotope by SR in order to explore the prospective implementability of high energy nuclear resonant scattering using SR. As is shown in Table I, gadolinium is a unique Mossbauer element which has nine nuclear resonance energies in six isotopes, and all observable Mossbauer levels lie in the high energy region over 50 keV [in Table I, 2.18(3) signifies 2:18 0:03 etc.]. As for the four lighter isotopes of Gd, Gd, Gd, and Gd, Europium parent isotopes can be used as the radioactive Mossbauer source. In particular, the Mossbauer level of Gd can also be populated by electron capture (EC) decay of Tb. As for the two heavier isotopes of Gd and Gd, they have no appropriate radioactive parent nuclide. Therefore, the coulomb excitation technique for Gd and Gd or the in-beam Gd(n, ) neutron capture reactions for Gd has been used. In contrast, if the SR is used as a Mossbauer source for Gd, all available Mossbauer levels can be excited selectively by adjusting the incident X-ray energy. In present experiment, we have observed the excitation of the 79.5-keV nuclear level of Gd. The experiments were performed at the JAERI undulator beamline (BL11XU) of SPring-8. The experimental optics is shown in Fig. 1. The electron current of the storage ring was constantly kept up 100mA at 8GeV by the ‘‘top-up’’ operation. The ‘‘84 (4-bunch train)’’ operation mode of the storage ring was used for the measurement. As is shown in the upper left side of Fig. 1, this mode consists of 84 bunch-trains equally separated by a 51.1-ns interval, and each bunch-train is composed of consecutive 4 bunches. The undulator gap was tuned so that the thirteen harmonic of the undulator radiation corresponds to the resonance energy of Gd (79.51 keV). The X-rays from the undulator were monochromatized around the resonance energy of Gd by the liquid-N2-cooled double-crystal Si(333) reflections, 8,9) whose angle had been calibrated precisely by measuring the Sn nuclear resonance energy [23.879478(18) keV] with SnO2 powder. Adding to Si(333) reflection, Si(111) reflection, whose energy is 26.5 keV, also passes through the monochromator. Moreover, the flux intensity is so strong that the Si avalanche photodiode (APD) detector is saturated. Therefore, an Al (thickness is 25mm) absorber was inserted in order to cut the X-rays from Si(111) reflection. With this thickness of Al, the 26.5-keV X-rays were reduced to 0.0043%, while 25.9% of the 79.51-keV X-rays were transmitted. The incident beam was limited to 0:5 2mm with a slit, and was directed to a non-enriched Gd2O3 powder sample. A Si-APD was placed beneath the sample to observe the incoherent nuclear resonant scattering. The APD element (fabricated by Hamamatsu Photonics) had a detection area of 7.1mm (3mm in diameter) and a depletion layer of 30 mm. The X-ray detection efficiency was about 0.15% at 79.51 keV. To find the resonance excitation of Gd, delayed signals after 9 ns from the first prompt incident X-ray pulses were counted. By changing the Bragg angle of Si(333) monochromator, the incident X-ray energy was changed and a resonance peak was searched at room temperature (298K). The result of the energy scan is shown in Fig. 2. The nuclear resonant scattering from Gd was clearly observed, and the average count rate of the resonance peak was 0.4 count/s. Observed delayed counts were normalized with the incident X-ray flux on the sample. From the result of the Gaussian curve fitting, the measured nuclear resonance energy of Gd was evaluated to be Table I. Mossbauer isotope characteristics of gadolinium.

Electronic and Magnetic Transitions in Europium Compounds Studied by Nuclear Forward Scattering of Synchrotron Radiation

Since its observation in 1985, nuclear resonance scattering of synchrotron radiation has become an excellent tool to study hyperfine interactions as well as dynamical effects in solids. It has proven to be a complementary method to Mossbauer spectroscopy. Nuclear resonance scattering combines the advantages of both local probe experiments and scattering techniques. It gives valuable information as well on electronic and magnetic structures and on dynamics in solids. Experiments benefit from the high beam quality of third-generation synchrotron radiation sources, as the small beam size and divergence. Besides the “standard isotope” 57Fe, other Mossbauer isotopes have become more important in nuclear resonant scattering of synchrotron radiation. This article concentrates on the 151Eu isotope.

Narrow spectral lines of modulation harmonics of gamma radiation in unordered magnetic materials

The spectra of gamma-resonance lines excited using the amplitude modulation of the pumping field of the Mossbauer effect were studied in unordered magnetic materials with a strong spread of magnetic hyperfine fields at nuclei of Mossbauer isotopes. The analysis demonstrates that the spectra of modulation harmonics consist of narrow lines, and their form and intensity are determined by the contribution of magnetic local hyperfine fields, which are excited at a pumping frequency. The inversion of intensities for odd harmonics with the change in sign of detuning of the pumping frequency relative to the average Larmor frequency of the Mossbauer nucleus was noted. The application of this method to the spectral analysis of inhomogeneous magnetic materials is discussed.

High-Energy Ball Milling of Some Intermetallics

High-energy ball milling of metallic powders has been used in recent years for the synthesis of alloys through reactions mainly occurring in solid state. The diffusive phenomena accompanying and promoting the reactions of formation are related to the microstructure acquired by the powders as a consequence of the intense mechanical deformations. The process induces a remarkable comminution of powder particles, inside of which domains of nanometric size are formed and compositional variations often occur. Several analytical techniques are suitable for following the structural evolution of the powders during milling. Among them, Mossbauer spectroscopy is suitable for obtaining detailed local information on the atomic arrangement of the treated materials, if one of the constituents is a Mossbauer isotope, and for detecting little changes occurring at an atomic scale. For these reasons Mossbauer spectroscopy is more sensitive than other analytical techniques especially in the early stages of the process. Some recent results are presented regarding in particular the Fe–Cu, Fe–Al, Fe–Al–Cu, NiAl(Fe) and Fe–Mn systems.

Mössbauer Studies on Magnetic Multilayers

Mossbauer spectroscopy is a useful tool to study the magnetic properties of multilayers from microscopic viewpoints. Recent experimental results relating to the issues on giant magnetoresistance (GMR) and interlayer coupling are briefly described. For the study of magnetic layers, 57Fe is a suitable Mossbauer isotope. On the other hand, in order to estimate spin polarization in non-magnetic layers, 119Sn and 197Au are used as microscopic probes. It is also introduced that the antiferromagnetic order of ultrathin Cr layers is investigated from Mossbauer spectra of inserted 119Sn probe layers with a monatomic thickness.

Mössbauer optics of synchrotron radiation at an isotopic boundary

The inelastic coherent Mossbauer scattering (ICMS) of synchrotron radiation at an isotopic boundary—a flat interface between two regions of matter which have different concentrations of the Mo ssbauer isotope—is investigated theoretically. Attention is focused primarily on the ICMS component for which the absorption of a synchrotron radiation photon by a nucleus occurs with recoil, i.e., with the creation or annihilation of lattice phonons, and the subsequent process of reemission of a photon by the Mossbauer nucleus occurs without recoil, as a result of which radiation is pumped from the wide synchrotron radiation line into the narrow Mo ssbauer line. Formulas similar to the Fresnel formulas, well known in optics, for the transmission and reflection of light at a dielectric boundary are obtained for ICMS at an isotopic boundary. Specifically, it is shown that the angle of reflection for ICMS at an isotopic boundary is different from the angle of mirror reflection of a synchrotron radiation beam, and the direction of the ICMS transmitted through the isotopic boundary depends on the deviation of its frequency from the exact value of the Mossbauer resonance frequency and in general is different from the direction of propagation of the synchrotron radiation beam. The suppression of ICMS at grazing angles of incidence of the synchrotron radiation beam is analyzed. A similar problem is solved for a plate-shaped sample containing a Mossbauer isotope. It is shown that the specific nature of the ICMS at an isotopic boundary could be helpful in the problem of Mo ssbauer filtering of synchrotron radiation.

Mössbauer spectroscopy at ISOLDE

Applications of radioactive ion beams produced at the ISOLDE facility for Mossbauer studies of probe atoms in solids are presented. Examples are given for a site-selective incorporation on different substitutional sites in compound semiconductors by ion implantation and thermal annealing of the radiation damage resulting from the implantation. The interactions of the probe atoms with lattice defects created in the implantation process have been studied to elucidate likely causes for the site-selective implantation mechanism. The technique has enabled to determine the electronic densities at electrically active substitutional probe atoms, having shallow donor or acceptor states as well as states deeper in the band gap. The results are in good agreement with theoretical results from local density calculations. Methodological aspects of the Mossbauer emission techniques employed at ISOLDE are compared to alternative accelerator based techniques and the consequences of the application of different precursor isotopes to the 57Fe Mossbauer isotope are treated in detail for 57Fe in silicon. Finally, results obtained for the magnetic hyperfine interactions of 5 sp impurities associated with vacancies in ferromagnetic metals are discussed.

Recent developments in Mossbauer spectroscopy

The Mossbauer effect found by a young nuclear physicist forty years ago has become a useful tool of spectroscopy to investigate solid materials. Through the hyperfine structures of the nuclear levels of Mossbauer isotopes, we can analyse various kinds of structures at the atomic and electronic scale both qualitatively and quantitatively. In this article, recent methodological developments in the Mossbauer spectroscopy with some examples of materials research are introduced. They are the use of the synchrotron orbital radiation and the Coulomb excitation instead of using radio-isotope sources, a high temperature measurement capable of seeing the phase transitions and diffusion processes above 1000 K and the high pressure measurement reaching the level of 100 GPa. Another new technique, the simultaneous detection of Mossbauer gamma-rays, internal conversion electrons and X-rays from different depths of one specimen, is also described. These new developments will stimulate the wider and newer applications of Mossbauer spectroscopy in science and technology related to solid materials.