A. I. Chumakov

Wave theory of focusing monochromator of synchrotron radiation.

The concept of a focusing monochromator with high energy resolution is presented. Different from conventional optical schemes, the device exploits not the angular but the spatial dispersion of synchrotron radiation. The wave theory of the monochromator is developed; it shows that the monochromator can reach an energy resolution of about 0.1 meV without significant loss of the spectral density of synchrotron radiation.

Fast Switching of Nuclear Bragg Scattering of Synchrotron Radiation by a Pulsed Magnetic Field

The time dependence of perturbed nuclear Bragg scattering of synchrotron radiation by a 57FeBO3 crystal was studied. The perturbation was an abrupt rotation by 90° of the hyperfine fields in the crystal effectuated by fast switching of an external magnetic field. This perturbation did not destroy the coherence. It transformed the nuclear transitions and caused, therefore, dramatic changes in the time evolution. A new quantum beat pattern of different frequency was immediately established after switching. The intensity and decay rate of the re-emitted radiation sensitively depended on the moment of switching. The frequency of the original beat pattern was restored by switching back.

Nuclear Bragg Diffraction of Synchrotron Radiation in the Presence of Acoustic Vibrations

The time evolution of nuclear Bragg diffraction of 14.4 keV synchrotron radiation in a 57FeBO3 crystal was studied when the crystal was excited to magnetoelastic vibrations by a r.f. field of 2.93 MHz. The vibrations did not modify the time evolution of the diffraction. This result is due to the fact that after the excitation by the SR pulse the nuclear vibrations cause only a phase modulation of the re-emitted radiation. Energy spectra, by contrast, are extremely sensitive to vibrations. Thus the present case is an instructive example of complementarity between energy and time domain Mossbauer spectroscopy. Time domain spectroscopy, in particular, will allow to reveal phenomena, which are blurred in the energy spectra by vibrations.

Nuclear resonance scattering of synchrotron radiation by two non-equivalent crystal sublattices in57Fe3BO6

Pure nuclear Bragg reflections off a single crystal of57Fe3BO6 consisting of two antiferromagnetic sublattices were studied using Mössbauer and synchrotron radiation. Energy and time spectra were measured of different reflections (h00), when either only one sublattice was reflecting, (300), or both sublattices at comparable strength either in antiphase, (500), or in phase, (700). Characteristic line shapes and quantum beat modulations revealed the interference of the scattering by the two sublattices.

Quantum beat dilation due to magnetisation breakdown

The temperature dependence of the magnetisation of a nearly perfect single crystal57FeBO3 was studied in between room temperature and Néel temperature by measuring the time dependence of the nuclear response to a flashlike excitation by pulsed synchrotron radiation

Shaping of Nuclear Bragg Reflected SR-Pulses by Broad Resonance Absorbers

Time and energy spectra have been studied of Mossbauer filter systems consisting of a pure nuclear reflection 3FeBO57(333) in connection with a broad resonance absorber. The energy spectra show how much of each line of the multiplet has been absorbed. The corresponding time spectra show the modified quantum beats to be expected from this, and also display risetime effects revealing the absorbers action. Part of the motivation is to produce simpler wave packets suitable for subsequent analysis of 57Fe-containing materials.

Nuclear Diffraction of Synchrotron Radiation in Laue Geometry: Quantum Beat Shift

Time spectra of nuclear resonant diffraction of synchrotron radiation in nearly perfect single crystals of 57FeBO3 and 57Fe3BO6 have been measured in Laue geometry. Characteristic shifts of the quantum beat pattern proportional to the crystal thickness were observed. This shifts originate from the thickness dependence of the relative phases of the interfering hyperfine components of the scattered radiation. The phase velocities of the components are different because of a complicated dispersion law where all 14.4 keV nuclear hyperfine transitions of 57Fe contribute to the scattering amplitudes. A kind of interferometry employing beam splitting in energy was demonstrated in the experiment.

The giant effect of magnetic ordering on a sound velocity in a σ-Fe55Cr45 alloy

We studied atomic dynamics of (x = 45 and 49.5) alloys using nuclear inelastic scattering of synchrotron radiation. For the σ-Fe55Cr45 alloy, the derived reduced iron partial density of phonon states reveals a huge difference in the low-energy region between magnetic and paramagnetic states. The latter implies a ~36% increase of the sound velocity in the magnetic phase, which testifies to a magnetically induced hardening of the lattice. Estimations of the longitudinal and transverse components of the sound velocity based on theoretically calculated elastic constants for the σ-CoMo alloys imply that both components are affected by magnetism.

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.

Nuclear Diffraction of Mössbauer Radiation Under the Influence of Magnetoelastic Excitation

Mossbauer absorption and diffraction spectra were studied on a nearly perfect 57FeBO3 single-crystal platelet exposed to a magnetic r.f. field with a frequency of about 1 MHz. In the Mossbauer absorption spectra the r.f. field causes a strong broadening of the absorption lines which is mainly due to lattice vibrations polarized along the surface of the platelet. These vibrations have a strong influence on the pure nuclear Bragg reflection 57FeBO3(333), where they produce a Gaussian broadening of the four lines of the Mossbauer diffraction spectrum to such an extent that the spectrum forms a region of high reflectivity about 100 ÷ 200 natural linewidths wide.