**6. Conclusions**

lower temperature of 480 °C in external magnetic field of 0.652 T as demonstrated in **Figure 11d**. The time needed for crystallization is by a factor of ~3 less than that without the external

**Figure 12.** Relative amount of nanograins ACR+IF plotted against time of annealing (a) and relative content of nanograins obtained at 470 °C (b). Black solid curves in (a) represent fit according to the crystallization model introduced in Ref. [18].

Relative content of nanocrystals given as a sum of the corresponding crystalline (CR) and interface (IF) components ACR+IF is depicted in **Figure 12**. Formation of nanograins under different annealing conditions is mutually compared in **Figure 12a** with respect to the annealing time. As discussed above, small increase in the annealing temperature from 470 to 480 °C, that is, only by 10 °C causes notable increase in the amount of nanograins that are formed during the same time period. This is due to proximity of the first crystallization temperature. When the annealing temperature is elevated further to 510 °C (i.e., by 30 °C), the change in the character of the ACR+IF dependence is not so dramatic. We stress again, that almost the same contents of nanograins is obtained when annealing at 480 °C proceeds under weak magnetic field. Presumably, this is caused by huge influence of small energetic perturbations of mag-

**Figure 12a** shows absolute values of the nanocrystalline content, and as it is more closely discussed elsewhere [19], all processes behave identically from a qualitative point of view. Dramatic differences among them are revealed, however, by the help of **Figure 12b**. Here, the total amount of nanograins ACR+IF is plotted against this parameter obtained from the slowest isothermal experiment performed at 470 °C. Consequently, the experimentally acquired data are distributed along a straight line with the slope equal 1. Dramatic changes in the slopes are observed during annealing at 480, 510, and 480 °C in external magnetic field. The associated rates of nanocrystallization are by a factor of ~30 higher that at 470 °C. They are indicated in **Figure 12b** by straight almost vertical lines. In order to visualize these rapid processes more clearly, the x-axis is reduced to the equivalent of the first ~65 min of the isothermal experiments.

After the initial rapid onset of nanocrystals formation, further increase in ACR+IF is not so steep. In fact, it almost saturates with the slope of 0.1 for annealing at 510 °C and at 480 °C

magnetic field at the same temperature of annealing.

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

netic interactions in comparison with the thermal energy.

Nanocrystallization of metallic glasses was followed by in situ experiments of nuclear forward scattering (NFS) of synchrotron radiation to fine details that are completely hidden when conventional analytical tools are employed. Detailed analyses of NFS time-domain patterns that were decomposed into contributions stemming from the amorphous residual phase and newly formed nanocrystallites provided an opportunity to study independently the role of structurally different regions. Moreover, using this approach, it was possible to further differentiate between contributions from the surfaces and the inner parts of nanograins. Different amounts of iron atoms located at the grains' surfaces and in their bulk were observed when different crystallization conditions, viz. temperature and/or external magnetic field, were applied.

The application of in situ NFS experiments has a huge potential for observations of the evolution of phase transformations in real time performed on fly during short time intervals. This was documented by two types of in situ NFS experiments. Namely, dynamical temperature increase and isothermal annealing under constant conditions were applied. It was possible to follow not only structural transformations, but, at the same time, also changes in magnetic arrangement were revealed. The latter is feasible owing to rapid screening of the corresponding hyperfine interactions. In this way, detail information about the nearest neighbourhoods of the resonant atoms is experimentally achievable. Moreover, the local arrangements can be checked in real time thus enabling structural and/or magnetic transformation to be followed on fly. In addition, a possibility of comparing the experimental results with those obtained from simulations and/or theoretical calculations is offered. Such experiments are unique and open new horizons in materials research by employing the technique of nuclear forward scattering of synchrotron radiation.
