**3. Nuclear forward scattering of synchrotron radiation**

Progress in synchrotron sources of radiation has introduced the method of nuclear forward scattering of synchrotron radiation [17]. This method uses <sup>57</sup>Fe resonant nuclei as probes of the local magnetic and electronic properties in the investigated samples. Thus, information on hyperfine interactions similar to Mössbauer spectrometry is readily available. Application of NFS is helpful in revealing the mutual relation between the magnetic arrangement and the structure of the studied materials. Due to extremely high brilliance of the latest synchrotron sources, studies can be performed in dynamic in situ regimes. Rapid recording of experimental data allows for direct observation of dynamical processes that are taking place *during* heat treatment [18–22].

In this contribution, we discuss time aspect of NFS which enables time-dependent processes to be followed in real time. We demonstrate them using dynamical and kinetics processes that are taking place during nanocrystallization of selected MGs. Before doing so, let us mention another important technique which exploits nuclear properties of 57Fe resonant nuclei that are activated by synchrotron radiation. It is the so-called nuclear inelastic scattering (NIS) of synchrotron radiation. It enables studies of the dynamics of NCAs via atomic vibrations and densities of phonon states [23, 24]. Because the time needed for acquisition of experimental data is still rather long, time-dependent NIS investigations are only evolving.

NFS belongs to the family of nuclear resonant scattering processes [25]. This technique can be considered as a full analogue of Mössbauer spectrometry [26]. It is especially useful under extreme conditions including high temperature, pressure, and/or magnetic fields when the space with such an environment is very limited, and hence, the sample can be as small as several tens of micrometers. High brilliance of synchrotron sources enables sufficient data counts even from such spatially limited regions. NFS permits on fly inspection of structural and/or magnetic arrangement that continuously evolves with changing temperature/time. In this respect, it is superior to other in situ techniques.

Energetic levels of atomic nuclei are exposed to the so-called hyperfine interactions. The latter are due to an effective field that originates from the presence of surrounding atoms, their electronic shells, and/or external fields. Consequently, nuclear levels are shifted and/ or split, and in this way, they sensitively reflect chemical and topological states of the resonant atoms. Effect of the three main types of hyperfine interactions, viz. electric monopole, electric quadrupole, and magnetic dipole interaction upon nuclear levels, is schematically drawn in the upper part of **Figure 1**. Possible transitions among nuclear levels are indicated by arrows.

Electric monopole interaction is proportional to charge density at the nucleus and provides information about valence and spin states of the resonant atom, about its electronegativity and chemical bonding. Electric quadrupole interaction is governed by electric field gradient acting upon the nucleus and reflects the charge distribution. It is related to oxidation state, spin state as well as symmetry of the positions of resonant atoms. Magnetic dipole interaction

Nanocrystallization of Metallic Glasses Followed by *in situ* Nuclear Forward Scattering of Synchrotron Radiation http://dx.doi.org/10.5772/66869 11

**3. Nuclear forward scattering of synchrotron radiation**

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

treatment [18–22].

only evolving.

by arrows.

this respect, it is superior to other in situ techniques.

Progress in synchrotron sources of radiation has introduced the method of nuclear forward scattering of synchrotron radiation [17]. This method uses <sup>57</sup>Fe resonant nuclei as probes of the local magnetic and electronic properties in the investigated samples. Thus, information on hyperfine interactions similar to Mössbauer spectrometry is readily available. Application of NFS is helpful in revealing the mutual relation between the magnetic arrangement and the structure of the studied materials. Due to extremely high brilliance of the latest synchrotron sources, studies can be performed in dynamic in situ regimes. Rapid recording of experimental data allows for direct observation of dynamical processes that are taking place *during* heat

In this contribution, we discuss time aspect of NFS which enables time-dependent processes to be followed in real time. We demonstrate them using dynamical and kinetics processes that are taking place during nanocrystallization of selected MGs. Before doing so, let us mention another important technique which exploits nuclear properties of 57Fe resonant nuclei that are activated by synchrotron radiation. It is the so-called nuclear inelastic scattering (NIS) of synchrotron radiation. It enables studies of the dynamics of NCAs via atomic vibrations and densities of phonon states [23, 24]. Because the time needed for acquisition of experimental data is still rather long, time-dependent NIS investigations are

NFS belongs to the family of nuclear resonant scattering processes [25]. This technique can be considered as a full analogue of Mössbauer spectrometry [26]. It is especially useful under extreme conditions including high temperature, pressure, and/or magnetic fields when the space with such an environment is very limited, and hence, the sample can be as small as several tens of micrometers. High brilliance of synchrotron sources enables sufficient data counts even from such spatially limited regions. NFS permits on fly inspection of structural and/or magnetic arrangement that continuously evolves with changing temperature/time. In

Energetic levels of atomic nuclei are exposed to the so-called hyperfine interactions. The latter are due to an effective field that originates from the presence of surrounding atoms, their electronic shells, and/or external fields. Consequently, nuclear levels are shifted and/ or split, and in this way, they sensitively reflect chemical and topological states of the resonant atoms. Effect of the three main types of hyperfine interactions, viz. electric monopole, electric quadrupole, and magnetic dipole interaction upon nuclear levels, is schematically drawn in the upper part of **Figure 1**. Possible transitions among nuclear levels are indicated

Electric monopole interaction is proportional to charge density at the nucleus and provides information about valence and spin states of the resonant atom, about its electronegativity and chemical bonding. Electric quadrupole interaction is governed by electric field gradient acting upon the nucleus and reflects the charge distribution. It is related to oxidation state, spin state as well as symmetry of the positions of resonant atoms. Magnetic dipole interaction

**Figure 1.** Typical representatives of the basic shapes of Mössbauer spectra recorded in energy domain (middle row) and the corresponding NFS time-domain patterns (bottom row). They demonstrate presence of electric monopole (a and b), electric quadrupole (c and d), and magnetic dipole (e and f) hyperfine interactions that cause the shift/splitting of nuclear levels as drawn in the top row.

originates from coupling between nuclear magnetic moment and effective magnetic field at the nucleus due to spin polarization. Thus, information on magnetic states of the resonant atoms, which is, moreover, temperature-dependent, is readily available. It is noteworthy that resonant atoms located in defined structural positions (e.g., in a crystalline lattice) feature individual set of hyperfine interactions and, hence, corresponding spectral parameters. The latter are like fingerprints which uniquely identify these particular atomic sites and can be derived either from Mössbauer spectra or from NFS experiments. In addition, relative fractions of such structurally different positions in the investigated samples are related to the contribution of the particular spectral components.

Transitions between ground and excited states of the resonant nuclei are accompanied by absorption and emission of photons with precise energy that is typically several tens of keV. In conventional Mössbauer spectrometry, resonance absorption of the emitted gamma photons by a particular absorber (i.e., the investigated sample) is achieved by fine tuning of their energy through a Doppler effect [27]. As a source of radiation, suitable radioactive nuclides are used. Splitting of nuclear energy levels is of the order of several hundreds of neV and is reflected via corresponding hyperfine parameters in the associated Mössbauer spectra that are recorded in energy domain. The resulting basic shapes of Mössbauer spectra are depicted in the middle row of **Figure 1**.

If only electric monopole hyperfine interaction occurs, the corresponding Mössbauer spectrum shows only one line, the so-called singlet as seen in **Figure 1a**. Presence of electric quadrupole 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 combination of the three basic spectral shapes is possible.

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 of photons varies over specific values.

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 ΔEγ≈meV. Subsequent de-excitation provides scattered photons of different energies (E<sup>1</sup> –E<sup>6</sup> ) that sum up to the NFS time domain pattern.

nuclear excited states reflects hyperfine interactions of the resonant nuclei. All de-excitation photons sum up and give rise to interference patterns in time domain as shown in the bottom row of **Figure 1**. In the following, we will call them *NFS time-domain patterns*. 1 The prompt excitation pulse sets the time zero. Due to its extremely high intensity the detector starts to count when only delayed photons are present which usually takes about 10–20 ns after the excitation.

The counts of delayed photons are registered as a function of time that has elapsed after the excitation. That is why NFS is sometimes referred to as Mössbauer spectrometry in time domain. Single transition is characterized by an exponentially decaying signal (linear in semi logarithmic scale, see **Figure 1b**). Multiple photons originating from multiple transitions exhibit characteristic beating of intensities called quantum beats. Their character is unique for particular hyperfine structure as demonstrated in **Figure 1d** and **f** for electric quadrupole and magnetic dipole interactions, respectively.

In general, any NFS time-domain pattern can be represented by some of the basic patterns plotted in the bottom row of **Figure 1** and/or a combination of them. In any case, they carry information on hyperfine interactions that are unique for individual atomic sites of the resonant atoms. Evaluation of experimental NFS data is performed by their fit to a suitably chosen theoretically calculated model. Each model consists of several sets of hyperfine parameters that are each ascribed to one particular atomic site. The obtained resulting parameters identify valence states, symmetry of charge distribution, and magnetic ordering. Phase composition of the material under study can be identified, and relative amount of each phase can be determined. Due to high site selectivity, we can study local electronic arrangement, and its fine distortions can be revealed. Some parameters like, for example, electric field gradient can be compared with the results of ab initio calculations. We can also easily follow magnetization of individual magnetic structures via hyperfine magnetic fields within magnetically active materials. So far, NFS technique was successfully applied to the study of different problems of materials research [28].

In this contribution, we present in situ NFS experiments that provide important information on the early stages of crystallization in MGs. This process can be monitored starting from formation of nucleation centres, their growth, and continuation towards equilibrium nanocrystalline state. Using this approach, the obtained results are not affected by a cooling process which is the case when ex situ experiments are employed.
