The secondary resonant radiation field and resonant absorption thickness effects on the Conversion Electron Mössbauer Spectroscopy spectra are analyzed for highly enriched resonant targets. It is shown that secondary field effect is important for the thick α-Fe foil enriched in the resonant isotope. Even for the polycrystalline sample traces of the coherent resonant field have been detected as the distortion of lines. Secondary field is discussed in detail. Suitable approximations to treat spectra originating from targets with significant resonant thickness developing secondary field composed of the incoherent and coherent parts are introduced. Finally, the formalism is applied to the high quality spectrum recorded for the enriched iron foil and correlation between contribution due to the secondary field and experimental line shape is investigated.
A brief review concerned with the application of the Mössbauer spectroscopy to the investigations of iron-based superconductors is given. An introduction is devoted to the description of the most important features of the Mössbauer spectroscopy followed by the discussion of the basic properties of iron-based superconductors. Our results obtained for FeSe, LiFeP and parent compounds of the ‘122’ family, i.e., for AFe2As2 (A=Ca, Ba, Eu) are discussed later on with particular attention paid to the spin density wave (SDW) magnetism exhibited by the parents of the ‘122’ family. It is found that incommensurate SDW contain many harmonics in these layered structures and evolve from almost separated magnetic sheets through quasi-triangular forms to almost rectangular shape with the lowered temperature.
Entirely new Mössbauer spectrometer MsAa-4 is currently under design and construction. New features as compared to the basic features of the previous generation MsAa-3 spectrometer could be summarized as follows. Completely digital processing of the γ-ray detector signal beyond the Gaussian shape filter/amplifier is to be implemented. The spectrometer is going to be able to accommodate external multiple detector heads. One could collect simultaneously up to 128 γ-ray spectra in 16384 channels of 32-bit each and up to 512 Mössbauer spectra in 4096 channels of 32-bit each provided the proper external multiple detector head is used. The count-rate per single detector is limited to about 10^5 counts per second total. Improved precision of the reference function from 12-bit to 16-bit is to be provided. The reference function is stored in 8192 channels per complete cycle. Addition of the random noise to the reference corner prism of the Michelson-Morley calibration inteferometer is to be introduced to avoid spurious fringes due to the phase lock-up. Integrated universal temperature controller being able to use variety of the temperature sensors is to be interconnected with the proper spectrometer. The spectrometer is now a stand-alone network device as it is equipped with the Ethernet connection to the outside world. Modular design and use of the strict standards allows easy reconfiguration for other applications than the Mössbauer spectroscopy.
We report on ^{57}Fe-Mössbauer studies of the magnetic properties in ScFe_2 and Sc_{0.4}Ti_{0.6}Fe_2 performed as a function of pressure and temperature. Both systems crystallize in the C14-type Laves phase structure with two different Fe sites 6h and 2a. The ferromagnetic properties of ScFe_2 (T_C=540 K at ambient pressure) change around 30 GPa to antiferromagnetic order of the 6h sites with non-magnetic 2a sites. The ordering temperature is lowered to T_N=300 K at 51 GPa. This pressure-dependent behaviour of ScFe_2 resembles that observed within the Sc_{1-x}Ti_xFe_2 series as a function of x. In ferromagnetic Sc_{0.4}Ti_{0.6}Fe_2 we observe, as a function of temperature and of pressure, an abrupt high-moment to low-moment transition of the Fe band moments of the 6h sites, accompanied by a rearrangement of the spin directions. In both systems the decrease in the Fe moments is accompanied by a strong increase in the volume coefficient of the isomer shift, originating from a reduced s-electron shielding capability of the d-electrons in the low-moment state.
Iron-based superconductors Ba_{0.7}Rb_{0.3}Fe_{2}As_{2} and CaFe_{1.92}Co_{0.08}As_{2} of the `122' family have been investigated by means of the 14.41-keV Mössbauer transition in ^{57}Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe_{2}As_{2} and CaFe_{2}As_{2}. It has been found that Mössbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for superconductivity. Hence, the superconductivity occurs in the part of the sample despite the sample is single phase. This phenomenon is caused by the slight variation of the dopant concentration across the sample (crystal).
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