High-pressure Mössbauer spectroscopy using synchrotron radiation and radioactive sources

A high-pressure ⁵⁷Fe Mössbauer study of SrFeO₃ up to 74 GPa has been performed with diamond-anvil-cell (DAC) using synchrotron radiation and a radioactive point source of ⁵⁷Co in Rh. SrFeO₃ is known as a typical cubic perovskite with a high-valence state of Fe⁴⁺ and shows metallic conductivity at 0.1 MPa down to 4.2 K. Applying an external high pressure, SrFeO₃ has not shown any structural transformation up to 74 GPa keeping an Fe⁴⁺ state but the Néel temperature increases up to 300 K at 18 GPa. The external high pressure may induce the ferromagnetism in SrFeO₃ by a decrease of the interatomic distance of Fe or an increase of the d-band width. ⁵⁷Fe Mössbauer measurements under externally applied longitudinal magnetic field using radioactive ⁵⁷Co in Rh source and also nuclear forward scattering measurements with a linearly polarized synchrotron radiation under external magnetic field indicate the existence of the pressure induced ferromagnetism in SrFeO₃. In this work we compare high-pressure Mössbauer spectroscopy using synchrotron and radioactive sources and summarize the advantages and disadvantages of each method.     

High-pressure Mössbauer spectroscopy with nuclear forward scattering of synchrotron radiation

Abstract

Using a diamond anvil cell, high-pressure ⁵⁷Fe Mössbauer spectroscopy has been performed with the nuclear forward scattering of synchrotron radiation. A pressure-induced magnetic hyperfine interaction at ⁵⁷Fe in SrFeO_{2, 97} has been detected at 44 GPa and 300 K for a first time by a quantum-beat modulation of the decay rate after collective nuclear excitation by the synchrotron pulse. The basic concept and method used to detect nuclear forward scattering with synchrotron radiation are discussed.     

Grazing‐incidence synchrotron‐radiation 57Fe‐Mössbauer spectroscopy using a nuclear Bragg monochromator and its application to the study of magnetic thin films

Energy‐domain grazing‐incidence ⁵⁷Fe‐Mössbauer spectroscopy (E‐GIMS) with synchrotron radiation (SR) has been developed to study surface and interface structures of thin films. Highly brilliant ⁵⁷Fe‐Mössbauer radiation, filtered from SR by a ⁵⁷FeBO₃ single‐crystal nuclear Bragg monochromator, allows conventional Mössbauer spectroscopy to be performed for dilute ⁵⁷Fe in a mirror‐like film in any bunch‐mode operation of SR. A theoretical and experimental study of the specular reflections from isotope‐enriched (⁵⁷Fe: 95%) and natural‐abundance (⁵⁷Fe: ～2%) iron thin films has been carried out to clarify the basic features of the coherent interference between electronic and nuclear resonant scattering of ⁵⁷Fe‐Mössbauer radiation in thin films. Moreover, a new surface‐ and interface‐sensitive method has been developed by the combination of SR‐based E‐GIMS and the ⁵⁷Fe‐probe layer technique, which enables us to probe interfacial complex magnetic structures in thin films with atomic‐scale depth resolution.     

Development of an energy‐domain 57Fe‐Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh‐pressure studies with a diamond anvil cell

An energy‐domain ⁵⁷Fe‐Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh‐pressure measurements. The main optical system consists of a single‐line pure nuclear Bragg reflection from an oscillating ⁵⁷FeBO₃ single crystal near the Néel temperature and an X‐ray focusing device. The developed spectrometer can filter the Doppler‐shifted single‐line ⁵⁷Fe‐Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X‐rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR ⁵⁷Fe‐Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure‐induced magnetic phase transition of polycrystalline ⁵⁷Fe₃BO₆ and an unknown high‐pressure phase of Gd⁵⁷Fe₂ alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earth's core.     

57Fe Mössbauer study of SrFeO3 under ultra-high pressure

⁵⁷Fe Mössbauer absorption spectra under ultra-high pressure up to 53 GPa have been measured using a diamond anvil cell for SrFeO_2.97 which is one of the typical Fe⁴⁺ oxides having a cubic perovskite structure. External high pressure up to 53 GPa makes no indication of structural transformation and does not show any change in valence state of iron, however the Néel temperature of 131 K at 0 GP increases to 300 K and the⁵⁷Fe magnetic hyperfine field decreases from 32.9 T at 0 GPa and 6.5 K to 23.3 T at 53 GPa and 300 K.     

High-Pressure Mössbauer Spectroscopy of Perovskite Iron Oxides

Using a diamond anvil cell and a low-temperature cryostat with a superconducting solenoid, high-pressure ⁵⁷Fe Mössbauer spectroscopy with a radioactive source at 4.5 K under external magnetic fields up to 7 T has been developed. Pressure-temperature magnetic phase-diagrams for perovskite iron oxides, SrFeO₃, CaFeO₃ and Sr₂LaFe₃O₉ are presented up to about 70 GPa. High-pressure Mössbauer measurements under external magnetic fields proved that the electronic ground states of these oxides switch to uniform-charge and ferromagnetic (and most probably metallic) states under extremely high pressure.

     

Mössbauer study of CaFeO3 under external high—Pressure

⁵⁷Fe Mössbauer and X-ray diffraction measurements have been performed on a perovskite CaFeO₃ under external high pressure upto 50 GPa at room temperature using a diamond anvil cell. Above 29 GPa the⁵⁷Fe magnetic hyperfine splitting appears superimposing with usual paramagnetic pattern of CaFeO₃. Magnitude of hyperfine field is 16 T and much smaller than 33 T of typical Fe⁴⁺ in SrFeO₃ suggesting a transition from high-spin S=2 to low-spin S=1 state in CaFeO₃.     

Synchrotron Mössbauer reflectometry using stroboscopic detection

The concept of the heterodyne/stroboscopic detection of nuclear resonance scattering of synchrotron radiation is extended to the grazing incidence geometry. Model calculations for an antiferromagnetic [⁵⁷Fe/Cr]₂₀ multilayer are shown and discussed. Principles and methodological aspects of stroboscopic synchrotron Mössbauer reflectometry are briefly reviewed.     

Nuclear resonant scattering and molecular orbital calculations on an iron(II) spin‐crossover complex

Nuclear resonant scattering of synchrotron radiation was applied to investigate the spin‐crossover complex Fe(tpa)(NCS)₂ (tpa=tris(2‐pyridylmethyl)amine). The nuclear forward scattering experiments are compared with conventional Mössbauer experiments, and the nuclear inelastic scattering experiments are compared with the results from a theoretical normal mode analysis based on molecular orbital calculations.     

The 57Fe Synchrotron Mössbauer Source at the ESRF

The design of a ⁵⁷Fe Synchrotron Mössbauer Source (SMS) for energy‐domain Mössbauer spectroscopy using synchrotron radiation at the Nuclear Resonance beamline (ID18) at the European Synchrotron Radiation Facility is described. The SMS is based on a nuclear resonant monochromator employing pure nuclear reflections of an iron borate (⁵⁷FeBO₃) crystal. The source provides ⁵⁷Fe resonant radiation at 14.4 keV within a bandwidth of 15 neV which is tunable in energy over a range of about ±0.6 µeV. In contrast to radioactive sources, the beam of γ‐radiation emitted by the SMS is almost fully resonant and fully polarized, has high brilliance and can be focused to a 10 µm × 5 µm spot size. Applications include, among others, the study of very small samples under extreme conditions, for example at ultrahigh pressure or combined high pressure and high temperature, and thin films under ultrahigh vacuum. The small cross section of the beam and its high intensity allow for rapid collection of Mössbauer data. For example, the measuring time of a spectrum for a sample in a diamond anvil cell at ～100 GPa is around 10 min, whereas such an experiment with a radioactive point source would take more than one week and the data quality would be considerably less. The SMS is optimized for highest intensity and best energy resolution, which is achieved by collimation of the incident synchrotron radiation beam and thus illumination of the high‐quality iron borate crystal within a narrow angular range around an optimal position of the rocking curve. The SMS is permanently located in an optics hutch and is operational immediately after moving it into the incident beam. The SMS is an in‐line monochromator, i.e. the beam emitted by the SMS is directed almost exactly along the incident synchrotron radiation beam. Thus, the SMS can be easily utilized with all existing sample environments in the experimental hutches of the beamline. Owing to a very strong suppression of electronic scattering for pure nuclear reflections (～10^?9), SMS operation does not required any gating of the prompt electronic scattering. Thus, the SMS can be utilized in any mode of storage ring operation.     

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [⁵⁷Fe/Cr]₃₀ periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent ⁵⁷Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π‐polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.     

Mössbauer spectroscopy in the time domain applied to the study of single‐crystalline guanidinium nitroprusside

Guanidinium nitroprusside GNP, (CN₃H₆)₂[Fe(CN)₅NO] has been investigated in single‐crystalline form by nuclear resonant forward scattering (NFS) using synchrotron radiation (Mössbauer spectroscopy in the time domain). This method provides a direct measure of effective absorber thickness and therefore also of the Lamb–Mössbauer factor f _LM. GNP has the advantage that all [Fe(CN₅)NO]^2- anions are practically aligned within the crystal. For the two different crystal orientations, with the crystallographic a- and c-direction parallel to the synchrotron beam, respectively, we have obtained f _LM ^(a)= 0.122(10) and f _LM ^(c)= 0.206(10), i.e., GNP exhibits significant anisotropic vibrational behavior. The quantum beat pattern of the NFS spectra obtained for the two different crystal orientations is discussed on the basis of radiation characteristics of the polarized synchrotron beam and the multipole transitions of oriented ⁵⁷Fe nuclei.     

High pressure Mössbauer spectroscopy using a diamond anvil cell

Recent applications of high pressure Mössbauer spectroscopy using a diamond anvil cell are presented. High pressure Mössbauer studies of two perovskite-related iron oxides SrFeO_2.97 and CaFeO₃, magnetite Fe₃O₄, and wüstite Fe_1– O have been carried out at 300 K at pressures of up to 74 GPa. A preliminary result by the resonant forward scattering of synchrotron radiation for high pressure Mössbauer spectroscopy using a diamond anvil cell is also presented.     

Comparison of nuclear Bragg scattering with Mössbauer spectroscopy for the measurement of hyperfine interactions

Fourier analysis of the time evolution of the nuclear Bragg scattering from the (7, 7, 7) reflection in α-hematite (⁵⁷Fe) excited by synchrotron radiation was used to extract values of Zeeman splitting of the ground and 14.413-keV states of⁵⁷Fe in the crystal magnetic field. The results so obtained apparently do not agree with the corresponding values obtained from Mössbauer spectroscopy on α-hematite. Possible reasons for the discrepancy are discussed.     

High-pressure Mössbauer studies of magnetism in RFe2 Laves phases and Eu-chalcogenides

A review is given on current high-pressure studies of magnetism employing the new method of nuclear forward scattering of synchrotron radiation as well as the conventional Mössbauer effect. Comparative studies of the magnetic properties of intermetallic RFe₂ Laves phases and Eu(II)-chalcogenides are described. We present as examples the pressure induced changes in YFe₂ and ScFe₂ at pressures up to 100 GPa as well as studies of EuTe in the NaCl-type and the CsCl-type high-pressure phase. Future high-pressure applications of nuclear resonant scattering will be discussed.     

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the⁵⁷Fe Mössbauer spectra of pure iron foil and α-Fe₂O₃ powder under high pressure have been measured at room temperature.⁵⁷Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.⁵⁷Fe Mössbauer spectra of Fe₂O₃ under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.     

A Mössbauer effect investigation of the magnetic behaviour of (Fe,Co)S1 + x solid solutions

The Mössbauer spectra of (Fe, Co)S_{1 + x} were recorded at room temperature and 4.2 K for samples of varying composition to study the magnetic behaviour of the solid solutions. The Mössbauer spectra are split magnetically at iron concentrations above 16% Fe. For samples with less than 16%Fe, the Mössbauer spectra show no evidence of magnetic splitting down to 4.2 K. The room temperature centre shift data appear to vary continuously with composition and the hyperfine magnetic field decreases with decreasing Fe²⁺ concentration. A Mössbauer spectrum of ⁵⁷Fe:CoS at 4.2 K in an external field of 25 kOe showed no evidence of magnetic splitting beyond that caused by the applied field, indicating a net zero internal field.A high spin to low spin transition in Fe²⁺ is ruled out as being responsible for the observed magnetic behaviour on the basis of the centre shift data. The Mössbauer data are interpreted to indicate a substantial increase in electron delocalization towards the ligands as the 〈M-S〉 distance decreases with decreasing Fe²⁺concentration. This causes a reduction in the magnitude of the internal magnetic field contributions as well as a decrease of shielding of the nucleus, giving rise to the observed Mössbauer parameters.The Mössbauer spectrum of ⁵⁷Fe:CoS at room temperature is compared with the spectrum of FeS above the 6.7 GPa phase transition at room temperature. The similarities of the centre shift and the 〈M-S〉 distance in the two phases indicate that covalency may also be responsible for the observed high pressure behaviour of FeS, and not the presence of Fe³⁺ as was originally suggested.     

High-spin-low-spin transition and the sequence of the phase transformations in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The transition of Fe³⁺ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO₃ multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mössbauer spectroscopy) and FeK _β high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO₃ insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover.     

Nuclear Bragg diffraction of synchrotron radiation from the a-sites of YIG

Time differential measurements of nuclear Bragg diffraction of synchrotron radiation were applied to⁵⁷Fe nuclei at the a-sites of yttrium iron garnet (YIG). The hyperfine parameters were determined by evaluation of the time spectra using the dynamical theory of Mössbauer optics. The observation of nuclear Bragg diffraction allows site selective spectroscopy.