**2. Metallic glasses and nanocrystalline alloys**

Fe-based ferromagnetic MGs possess interesting physical properties that are superior to those of their crystalline counterparts. This is mainly due to lack of any long-range order, that is, periodic atomic arrangement that is particularly important for their magnetic performance [1, 2]. They are often employed as magnetic shielding, transformer cores, sensors, recording media [3] as well as in other technical applications [4].

Structural changes that can occur in MGs when they are exposed to high enough temperatures for extended operational times degrade their working parameters. This becomes particularly important with the onset of crystallization. On the other hand, suitable chemical compositions of some MGs assure formation of crystalline grains with typical sizes of up to several tens of nanometres. The resulting NCAs represent a novel type of materials whose physical properties can be tailored not only by appropriate chemical elements but also by varying the size of the nanocrystalline grains, their morphology, and the composition of nanograins. NCAs all can be prepared from MGs by annealing under well-defined conditions (temperature and time) which ensure controlled temperature treatment and trigger partial crystallization. In opposite to MGs, the magnetic parameters of NCAs do not substantially deteriorate at elevated temperatures [5]. Therefore, a comprehensive understanding of the evolution of nanograins during nanocrystallization is essential in order to understand, optimize, and conserve the unique magnetic properties exhibited by metallic glasses and/or their nanocrystalline counterparts.

and performance of devices based on these materials. Because of their amorphous nature, these metallic alloys are often referred to as metallic glasses (MGs). Their suitable chemical composition ensures formation of crystallites that grow inside the amorphous matrix during thermal annealing and measure only several nanometres in size. Due to these dimensions, they provide beneficial magnetic properties in these the so-called nanocrystalline alloys (NCAs). At the same time, formation of nanograins stabilizes the whole structure against

Changes in microstructure, crystallization behaviour, and magnetic states of NCAs have suggested that interface regions between nanocrystalline grains and the surrounding amorphous matrix play a significant role in propagation of ferromagnetic exchange interactions between the nanograins through the residual amorphous matrix. In order to understand the process of nanocrystallization, it is inevitable to study it in situ, that is, *during* annealing. For this purpose, we use in situ nuclear forward scattering (NFS) of synchrotron radiation. NFS provides information on changes in structural arrangement via hyperfine interactions in real time. In

This contribution aims at providing insight into the studies of structural transformations that are taking place in iron-based metallic glasses exposed to elevated temperatures. Evolution of nanocrystalline grains during dynamical increase of temperature and isothermal annealing is discussed by the help of NFS technique. Before doing that, we provide brief description of MGs and NCAs. In addition, a short review of the methods used for their structural characterization is also offered. Prior to introducing the results of NFS investigations, basic principles

Fe-based ferromagnetic MGs possess interesting physical properties that are superior to those of their crystalline counterparts. This is mainly due to lack of any long-range order, that is, periodic atomic arrangement that is particularly important for their magnetic performance [1, 2]. They are often employed as magnetic shielding, transformer cores, sensors, recording

Structural changes that can occur in MGs when they are exposed to high enough temperatures for extended operational times degrade their working parameters. This becomes particularly important with the onset of crystallization. On the other hand, suitable chemical compositions of some MGs assure formation of crystalline grains with typical sizes of up to several tens of nanometres. The resulting NCAs represent a novel type of materials whose physical properties can be tailored not only by appropriate chemical elements but also by varying the size of the nanocrystalline grains, their morphology, and the composition of nanograins. NCAs all can be prepared from MGs by annealing under well-defined conditions (temperature and time) which ensure controlled temperature treatment and trigger partial crystallization. In opposite to MGs, the magnetic parameters of NCAs do not substantially deteriorate at elevated temperatures [5]. Therefore, a comprehensive understanding of the

further thermal deterioration.

of this method are presented, too.

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

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

**2. Metallic glasses and nanocrystalline alloys**

media [3] as well as in other technical applications [4].

Though studied already for decades, MGs still attract the interest of researchers due to their unique physical properties [6]. A central problem seems to be the understanding of structureto-properties relationship namely when MGs are transformed into NCAs. This is essential for tailoring the functionalities, efficiency, and performance of these near-future materials. These phenomena are routinely studied in a steady state, that is, once the particular structural arrangement is achieved, it is correlated with the resulting physical properties. Less attention is paid to the investigation of transient states that temporarily exist *for the period of* a structural transformation. Such an approach is, however, a demanding experimental task.

Broad arsenal of diagnostic techniques is applied in order to understand the process of transformation from MG into NCA structural arrangement, that is, the crystallization of MGs [7–9]. Yet, majority of these techniques provide only *ex situ* information as the time needed for acquisition of sufficiently good statistics of the experimental data frequently extends over several tens of minutes or even hours. In addition, some of the imaging techniques require special treatment for sample preparation which can substantially affect their structure. Subsequently, in situ investigation of the induced structural transformations using these techniques is not possible in real time, and the study of dynamics and/or kinetics of crystallization process is not so straightforward.

Methods like DSC or magnetic measurements can examine materials in real time and, thus, provide in situ investigations. However, they scan the whole bulk of the investigated systems. As a result, the obtained information is averaged over all structurally different regions which are present in the studied system. That is why in situ characterization of structural transformations during crystallization of MGs is an experimental challenge.

Along with conventional analytical tools, also sophisticated and advanced techniques like atom probe tomography are employed [10]. The use of in situ characterization techniques is, however, still limited to diffraction of synchrotron radiation [11–13]. Recently, more sophisticated techniques of real-time in situ synchrotron X-ray tomographic microscopy [14] and combination of time-resolved X-ray photon correlation spectroscopy and high-energy XRD [15] were applied. All these studies which make use of very up-to-date synchrotron-based approaches provide valuable information of amorphous structures by revealing complex atomic rearrangements even though no well-defined structural positions exist in MGs.

Though X-ray diffraction based techniques are capable of in situ investigations, they do not provide site-specific information as the signal is averaged over the distribution of electron densities. From this point of view, use of local probe techniques such as Mössbauer spectroscopy offers unique opportunities to access both magnetic properties and structural states of the investigated material [16]. The relatively long acquisition times of a conventional Mössbauer spectrum (up to several hours), however, limit the application of this technique only to samples in steady equilibrium conditions. This substantially disables monitoring of the crystallization process itself.
