**4.1. NFS experiments**

rupole interaction at the resonating nuclei splits the excited level into two degenerating ones. Consequently, in the case of <sup>57</sup>Fe nuclei (nuclear spin 3/2 in the excited state and 1/2 in the ground state), two transitions are possible. Hence, a doublet of absorption Mössbauer lines is formed as seen in **Figure 1c**. Zeeman-split sextet is observed in **Figure 1e** when magnetic dipole interactions act upon the 57Fe resonant nuclei. Presence of sextets in Mössbauer spectra indicates that the corresponding fraction of iron atoms (represented by a spectral area under the absorption lines) is ferro-, ferri-, or antiferromagnetically ordered. In real samples, any

With the development of monochromators, synchrotron radiation turned out to be suitable candidate for replacing conventional radionuclide sources of photons. As schematically depicted in the upper part of **Figure 2**, bunches of accelerated particles (electrons) produce flashes of synchrotron radiation when they pass through undulators. Pulses of photons have typical duration of ~50 ps and repetition rate ~200 ns. Their energy is tuned to the requested Mössbauer transition using high-resolution monochromator that provides energies of photons within a bandwidth (ΔEγ) of several meV. The pulse contains wider range of energies than is needed for excitation of available nuclear levels in the studied sample. It is drawn in the bottom part of **Figure 2** as a broad (blue) arrow and ensures immediate excitation of all nuclear transitions. Energy separation of nuclear levels due to hyperfine interactions is of the order of several hundreds of neV. Thus, not only the different transitions of the same nucleus but also all transitions of different nuclei are excited simultaneously at the *same time* upon an impingement of the synchrotron radiation pulse upon the sample. Let us remind that in Mössbauer spectrometry, nuclear transitions are excited sequentially one by one as the energy

In the time slot between two subsequent pulses, all excited nuclei emit the excess energy in a form of resonance delayed photons that are registered with a fast detector. The decay of the

**Figure 2.** Basic layout of a typical NFS beamline with the major components (upper part). In the bottom part, magnetically split nuclear levels are simultaneously excited by a single pulse of incident synchrotron radiation with an energy spread

–E<sup>6</sup>

) that sum up to the NFS time

ΔEγ≈meV. Subsequent de-excitation provides scattered photons of different energies (E<sup>1</sup>

combination of the three basic spectral shapes is possible.

12 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

of photons varies over specific values.

domain pattern.

NFS experiments were performed at The European Synchrotron (ESRF), Grenoble using the Nuclear Resonance ID22N and ID18 stations. Excitation of the 57Fe nuclear levels was accom-

<sup>1</sup> In a literature, one can find the expression 'NFS time spectrum'. However, the term 'spectrum' implies a dependence of counts upon energy. Because of the interference nature of NFS data, we find it physically inconsistent.

plished by a photon beam with the energy of 14.413 keV, flux of ~109 photons/s, and bandwidth of ~3 meV. The spot size of the synchrotron beam was of 0.7×0.3 mm2 . The estimated heat load at the sample was ~2 μW. Samples of the investigated MGs were placed in a vacuum furnace. Metallic glasses of Fe90Zr7 B3 , Fe81Mo<sup>8</sup> Cu<sup>1</sup> B10, and (Fe2.85Co<sup>1</sup> ) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup> B14 were prepared by the method of rapid quenching on a rotating wheel. The chosen compositions of MGs ensure formation of nanocrystalline alloys in which crystalline grains with the size of several nanometres are formed in the early stage of structural transformation. The obtained ribbons of MGs were ~1–2 mm wide and ~20 μm thick.

The maximum annealing temperature (up to ~700 °C) was limited by Kapton windows of the furnace. Nevertheless, this temperature was far behind the first crystallization step of the investigated MGs.

Two types of studies were performed: (i) dynamical time-dependent temperature increase/ decrease and (ii) isothermal heat treatment. During the first type of experiments, temperature at the sample was continuously increasing with a ramp of 10 K/min. In the second type of experiments, the set temperature was reached with a ramp of 40 K/min and then maintained for up to 3 h. In both cases, NFS time-domain patterns were continuously recorded every minute during the whole duration of the experiment performed in transmission geometry. Thus, information on the bulk of the samples was obtained.

For the sake of more clear presentation of a high quantity of experimentally acquired timedomain patterns within a single experiment, we use contour plots to display the obtained NFS patterns. The latter are stacked with respect to the duration time of the experiment which constitutes the vertical axes of the contour plots. For dynamical experiments, this axis' scale is eventually converted into temperature values assuming a constant ramp of 10 K/min. The elapsed time is then given on the horizontal axes, and the counts of the registered photons (intensities) are colour coded in a logarithmic scale.
