**1. Introduction**

Understanding the structure-to-properties relationship is one of the most important problems in materials research. Thorough knowledge on structural arrangement namely in disordered systems like amorphous metallic alloys is essential for tailoring the functionalities, efficiency

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 further thermal deterioration.

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 respect, it is superior to other in situ techniques.

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 of this method are presented, too.
