**5.1. Dynamical experiments**

Effects of continuously changing temperature on magnetic ordering and structural transformation in MGs are demonstrated using the (Fe2.85Co<sup>1</sup> ) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup> B14 amorphous alloy. This chemical composition ensures ferromagnetic states of the as-quenched MG at room temperature. Corresponding NFS time-domain patterns are plotted in **Figure 4**.

It should be stressed that the NFS time-domain patterns in **Figure 4** are in fact raw measured data as obtained directly from the experiment. Even without any quantitative evaluation, two qualitative changes in the character of interferograms are clearly observed at ~247 and ~435 °C. They are associated with magnetic and structural transformation, respectively, characterized

**Figure 4.** Contour plot of NFS time-domain patterns accumulated during dynamic annealing of the (Fe2.85Co<sup>1</sup> ) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup> B14 MG. Heating rate is 10 K/min.

Nanocrystallization of Metallic Glasses Followed by *in situ* Nuclear Forward Scattering of Synchrotron Radiation http://dx.doi.org/10.5772/66869 17

patterns, it is refined by a sharp value of magnetic hyperfine field. The third structural component is interpreted as interface region (IF) between the former two structures. It constitutes a shell of the nanograins with undeveloped crystal symmetry. During the evaluation, it is refined by a distribution of hyperfine magnetic fields. Schematic representation of this con-

Hyperfine parameters of all fitting components evolve with temperature/time of the experiment. Eventually, at certain temperatures (e.g., Curie temperature, onset of crystallization), qualitatively different hyperfine interactions appear. Consequently, the physical model should reflect this situation by the use of appropriate type of distributions of hyperfine parameters (magnetic fields vs. quadrupole splitting). Proper type of distribution in certain temperature/

Effects of continuously changing temperature on magnetic ordering and structural trans-

chemical composition ensures ferromagnetic states of the as-quenched MG at room tempera-

It should be stressed that the NFS time-domain patterns in **Figure 4** are in fact raw measured data as obtained directly from the experiment. Even without any quantitative evaluation, two qualitative changes in the character of interferograms are clearly observed at ~247 and ~435 °C. They are associated with magnetic and structural transformation, respectively, characterized

**Figure 4.** Contour plot of NFS time-domain patterns accumulated during dynamic annealing of the (Fe2.85Co<sup>1</sup>

) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup>

B14 amorphous alloy. This

) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup> B14

time region is chosen by the help of the Hubert software [32].

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

**5. Structural transformations in metallic glasses**

formation in MGs are demonstrated using the (Fe2.85Co<sup>1</sup>

ture. Corresponding NFS time-domain patterns are plotted in **Figure 4**.

cept is presented in **Figure 3**.

**5.1. Dynamical experiments**

MG. Heating rate is 10 K/min.

**Figure 5.** NFS time-domain patterns of the (Fe2.85Co<sup>1</sup> ) <sup>77</sup>Mo<sup>8</sup> Cu<sup>1</sup> B14 MG taken at the vicinity of TC (a) and Tx1 (b) at the indicated temperatures.

by Curie TC and crystallization Tx1 temperatures. Consequently, the whole temperature region can be divided into three areas where the investigated MG exhibits magnetic amorphous, nonmagnetic amorphous, and magnetic nanocrystalline structure as it is shown in **Figure 4**.

In order to illustrate the alternations in the shapes of NFS time-domain patterns, selected individual records are plotted in **Figure 5**. It should be noted that the thickness effect of the sample has an important impact on the time-domain patterns. It results in the so-called hybrid beat character [34] which alternates their shapes.

With increasing temperature during the dynamic experiment, we can observe shift of the quantum beats maxima towards higher delayed times. Simultaneously, their intensities decrease. These effects are connected with vanishing hyperfine magnetic fields as a function of temperature. Magnetic quantum beats eventually fade away when the temperature reaches TC and the Zeeman sextet collapses. At this temperature, the six de-excitation photons have comparable energies that are moreover overlapped within the line width. The resulting interference pattern in time domain exhibits very fast time decay, and the corresponding NFS time-domain pattern nearly disappears. This situation is demonstrated in **Figure 5a** where selected records taken at the vicinity of TC~247 °C are plotted.

Formation of nanocrystalline bcc-(Fe,Co) grains during the first crystallization step resumes magnetic interactions that are identified by the corresponding quantum beats in **Figure 5b**. For comparison, NFS time-domain patterns at 435, 445, and 645 °C were chosen. At 435 °C, the the sample is still fully amorphous and paramagnetic, that is, only electric quadrupole interactions are present (compare **Figure 1d**). With the onset of nanocrystallization, more rapid oscillations, which represent hyperfine magnetic fields, appear in time region 43–83 ns. At 445 °C, they are already satisfactorily visible. Finally, at 645 °C, the NFS time-domain pattern clearly shows well-developed magnetic structure (see also **Figure 1f**) which means that the degree of crystallization is significantly high.

The temperature of the onset of the first crystallization Tx1 can be, however, accurately determined taking into consideration the whole temperature dependence of relative areas of amorphous (AM) and crystalline (CR) fitted components plotted in **Figure 6**. The total number of counts, that is, the overall area under time-domain patterns, is also provided. Here, the local minimum at ~247 °C indicates TC of the amorphous matrix. The successive abrupt increase is associated with magnetic transformation inside the amorphous matrix. Further continuous decrease in the total counts is caused by temperature dependence of the f-factor. The onset of nanocrystallization is documented in **Figure 6a** by a notable change in the slope of the curve where the inflection point represents Tx1 ~435 °C. The same Tx1 is derived from temperature dependence of relative area of the CR component in **Figure 6b**. Surface crystallization starts at somewhat lower Tx1 ~410 °C as confirmed by CEMS measurements [35].

The onset of crystallization changes the character of the beats. When new nanocrystalline grains appear, a magnetic order is established among the newly formed bcc-(Fe,Co) grains. This is indicated by an occurrence of dipole magnetic interactions that exhibit themselves by high-frequency oscillations in the corresponding NFS time-domain patterns in **Figures 4** and **5b**. The behaviour of the amorphous matrix with temperature is more complex. It can be followed by evolution of hyperfine magnetic fields in the temperature region where the system is ferromagnetic and by quadrupole splitting values at T>TC. Both parameters are plotted in **Figure 7**.

**Figure 6.** Total number of counts (spectral area) (a) and relative fractions of AM and CR components (b) of NFS timedomain patterns of the (Fe2.85Co<sup>1</sup> )77Mo<sup>8</sup> Cu<sup>1</sup> B14 MG plotted against temperature. Transition temperatures TC and Tx1 are marked with arrows.

Nanocrystallization of Metallic Glasses Followed by *in situ* Nuclear Forward Scattering of Synchrotron Radiation http://dx.doi.org/10.5772/66869 19

**Figure 7.** Average hyperfine magnetic field (a) and quadrupole splitting (b) values of the amorphous matrix in the (Fe2.85Co<sup>1</sup> )77Mo<sup>8</sup> Cu<sup>1</sup> B14 MG plotted against temperature.

For temperatures up to TC, the hyperfine magnetic fields continuously decrease till they acquire values that are comparable in strength with electric quadrupole interactions. It should be noted that both types of hyperfine interactions coexist in some temperature range. In fact, it is impossible to distinguish between them also because of low total number of counts in the NFS time-domain patterns (see **Figures 5a** and **6a**). Here, the fitting is accomplished by two qualitatively distinct models, viz. distribution of hyperfine magnetic fields and distribution of quadrupole splitting. They are applied in certain temperature regions that approach TC from bottom (i.e., T<TC) and from top (TC<T), respectively, with some small overlap [32].

Temperature evolution of quadrupole splitting provides information about bond properties and local symmetry of the iron sites in the AM phase. It shows a local minimum in **Figure 7b**. The associated inflection point indicates Tx1. Above this onset of nanocrystallization, the residual AM phase still persists, and when the CR phase is well developed (T>~500 °C), the average quadrupole splitting is stabilized.

To summarize the above discussion, we would like to note that the (Fe2.85Co<sup>1</sup> )77Mo<sup>8</sup> Cu<sup>1</sup> B14 MG was intentionally chosen to demonstrate the possibilities of dynamical NFS experiments. It is possible to follow on fly not only the evolution of its structural arrangement but also that of hyperfine magnetic interactions.

#### **5.2. Dynamical experiments in external magnetic field**

For comparison, NFS time-domain patterns at 435, 445, and 645 °C were chosen. At 435 °C, the the sample is still fully amorphous and paramagnetic, that is, only electric quadrupole interactions are present (compare **Figure 1d**). With the onset of nanocrystallization, more rapid oscillations, which represent hyperfine magnetic fields, appear in time region 43–83 ns. At 445 °C, they are already satisfactorily visible. Finally, at 645 °C, the NFS time-domain pattern clearly shows well-developed magnetic structure (see also **Figure 1f**) which means that the

The temperature of the onset of the first crystallization Tx1 can be, however, accurately determined taking into consideration the whole temperature dependence of relative areas of amorphous (AM) and crystalline (CR) fitted components plotted in **Figure 6**. The total number of counts, that is, the overall area under time-domain patterns, is also provided. Here, the local minimum at ~247 °C indicates TC of the amorphous matrix. The successive abrupt increase is associated with magnetic transformation inside the amorphous matrix. Further continuous decrease in the total counts is caused by temperature dependence of the f-factor. The onset of nanocrystallization is documented in **Figure 6a** by a notable change in the slope of the curve where the inflection point represents Tx1 ~435 °C. The same Tx1 is derived from temperature dependence of relative area of the CR component in **Figure 6b**. Surface crystallization starts

The onset of crystallization changes the character of the beats. When new nanocrystalline grains appear, a magnetic order is established among the newly formed bcc-(Fe,Co) grains. This is indicated by an occurrence of dipole magnetic interactions that exhibit themselves by high-frequency oscillations in the corresponding NFS time-domain patterns in **Figures 4** and **5b**. The behaviour of the amorphous matrix with temperature is more complex. It can be followed by evolution of hyperfine magnetic fields in the temperature region where the system is ferromagnetic and by quadrupole splitting values at T>TC. Both parameters are plotted in

**Figure 6.** Total number of counts (spectral area) (a) and relative fractions of AM and CR components (b) of NFS time-

B14 MG plotted against temperature. Transition temperatures TC and Tx1 are

at somewhat lower Tx1 ~410 °C as confirmed by CEMS measurements [35].

degree of crystallization is significantly high.

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

**Figure 7**.

domain patterns of the (Fe2.85Co<sup>1</sup>

marked with arrows.

)77Mo<sup>8</sup> Cu<sup>1</sup> In order to improve magnetic parameters of nanocrystalline alloys, crystallization of metallic glasses often takes place under external magnetic field, the so-called magnetic annealing [36]. It results in appearance of induced magnetic anisotropy in heat-treated soft MGs. Nevertheless, all studies are performed ex situ after the magnetic annealing. Naturally, a question has arisen how external magnetic field affects the progress of nanocrystallization.

Similar as in the previous experiment, in situ investigations can be effectively performed by dynamical increase of temperature without and with external magnetic field by employing the NFS technique. But now, we have selected MG that is almost purely paramagnetic at room temperature, namely Fe81Mo<sup>8</sup> Cu<sup>1</sup> B10. Only minute amounts of surface crystallization were unveiled by CEMS [37] on both sides of ribbon-shaped samples.

The reason for choosing this MG was twofold: (i) this chemical composition leads to formation of bcc-Fe nanocrystals, and iron is a calibration material for these studied; hence, its hyperfine parameters are well known, and (ii) the fitting model used for evaluation of the experimental NFS data is more simple because the residual amorphous matrix is paramagnetic, and only distributions of quadrupole splitting are used. On the other hand, formation of bcc-Fe nanograins imposes magnetic dipole interactions that are represented by single hyperfine magnetic field value. As described in the Section 4.2, iron atoms located in interface regions can be also identified via distribution of hyperfine magnetic fields. Consequently, temperature evolution of three structurally different regions in the investigated MG can be studied.

Contour plots of NFS time-domain patterns recorded during continuous increase of temperature in zero-field conditions and with applied external magnetic field (0.652 T) are shown in **Figure 8**. Dramatic impact of this rather small external magnetic field upon dynamics of the crystallization process is clearly seen. The plotted experimental data exhibit obvious modifications of hyperfine interactions that are reflected in the shapes of NFS time-domain patterns. At room temperature, the investigated Fe81Mo<sup>8</sup> Cu<sup>1</sup> B10 MG is amorphous, and in zero-field conditions, it demonstrates prevailing electric quadrupole interactions with hardly visible beatings of magnetic origin (due to surface crystallization). With the onset of nanocrystallization, contribution of magnetic dipole interactions becomes better visible in **Figure 8a** in the vicinity of ~400 °C. Appearance of hyperfine magnetic fields is remarkably accelerated when the annealing is performed in external magnetic field. In **Figure 8b**, the same character of NFS time-domain pattern is observed earlier at a temperature that is by about 100 °C lower.

Quantitative and qualitative description of the time-domain patterns is presented in **Figure 9** as derived from zero-field and in-field NFS experiments. Relative amounts of individual

**Figure 8.** Contour plots of NFS time-domain patterns of the Fe81Mo<sup>8</sup> Cu<sup>1</sup> B10 MG measured without (a) and with external magnetic field of 0.652 T (b).

components comprising relative areas of the amorphous (AM) and total nanocrystalline (CR+IF) phases are shown in **Figure 9a**. It is noteworthy that the onset of crystallization should be determined from the temperature dependences of the relative areas rather than from the contour plots in **Figure 8**. In the latter, intensities of the NFS time-domain patterns are plotted in logarithmic scale, and thus, the applied colour-coded scale is coarse to a certain extent. Consequently, fine details of the particular line shapes might not be properly seen.

room temperature, namely Fe81Mo<sup>8</sup>

At room temperature, the investigated Fe81Mo<sup>8</sup>

**Figure 8.** Contour plots of NFS time-domain patterns of the Fe81Mo<sup>8</sup>

magnetic field of 0.652 T (b).

Cu<sup>1</sup>

The reason for choosing this MG was twofold: (i) this chemical composition leads to formation of bcc-Fe nanocrystals, and iron is a calibration material for these studied; hence, its hyperfine parameters are well known, and (ii) the fitting model used for evaluation of the experimental NFS data is more simple because the residual amorphous matrix is paramagnetic, and only distributions of quadrupole splitting are used. On the other hand, formation of bcc-Fe nanograins imposes magnetic dipole interactions that are represented by single hyperfine magnetic field value. As described in the Section 4.2, iron atoms located in interface regions can be also identified via distribution of hyperfine magnetic fields. Consequently, temperature evolution of three structurally different regions in the investigated MG can be studied. Contour plots of NFS time-domain patterns recorded during continuous increase of temperature in zero-field conditions and with applied external magnetic field (0.652 T) are shown in **Figure 8**. Dramatic impact of this rather small external magnetic field upon dynamics of the crystallization process is clearly seen. The plotted experimental data exhibit obvious modifications of hyperfine interactions that are reflected in the shapes of NFS time-domain patterns.

Cu<sup>1</sup>

conditions, it demonstrates prevailing electric quadrupole interactions with hardly visible beatings of magnetic origin (due to surface crystallization). With the onset of nanocrystallization, contribution of magnetic dipole interactions becomes better visible in **Figure 8a** in the vicinity of ~400 °C. Appearance of hyperfine magnetic fields is remarkably accelerated when the annealing is performed in external magnetic field. In **Figure 8b**, the same character of NFS time-domain pattern is observed earlier at a temperature that is by about 100 °C lower.

Quantitative and qualitative description of the time-domain patterns is presented in **Figure 9** as derived from zero-field and in-field NFS experiments. Relative amounts of individual

Cu<sup>1</sup>

were unveiled by CEMS [37] on both sides of ribbon-shaped samples.

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

B10. Only minute amounts of surface crystallization

B10 MG is amorphous, and in zero-field

B10 MG measured without (a) and with external

Presence of small number of quenched-in nanocrystallites (~10 %) that were formed during the production of the ribbons was unveiled by the help of **Figure 9a**, too. They were accounted for the fitting model by introducing a component with well-defined hyperfine magnetic fields. The obtained temperature dependences are shown in **Figure 9b**. Hyperfine fields corresponding to nanocrystalline grains are compared with those of a polycrystalline bcc-Fe foil. The former exhibit systematically lower values which are caused by small amounts as well as dimensions of quenched-in nanocrystals.

At certain temperature of annealing, additional nanocrystalline grains appear, and all grains begin to grow both in number and in size. This rather abrupt onset of bulk nanocrystallization is clearly seen by a step-like increase in the hyperfine magnetic field value at ~400 °C during annealing in zero-field conditions. Under the influence of external magnetic field, this increase is rather continuous but starts at ~300 °C, that is, much earlier. For T>400 °C, both curves merge together and follow the temperature dependence of hyperfine magnetic fields that correspond to bulk bcc-Fe with almost constant difference of ~2 T. Lower hyperfine magnetic field values are due to small dimensions of the nanocrystals.

It is noteworthy that the decrease of hyperfine magnetic field with temperature as observed in **Figure 9**b is also demonstrated in **Figure 8** by shift of the maxima of time-domain patterns towards higher delayed times. With rising temperature, the magnetic ordering continuously vanishes and eventually disappears at the Curie point.

**Figure 9.** Relative amounts of amorphous (AM) (open symbols) and total nanocrystalline (NC=CR+IF) (solid symbols) components (a) and hyperfine magnetic fields of nanocrystals (b) plotted against temperature of annealing as obtained from NFS time-domain patterns of the Fe81Mo<sup>8</sup> Cu<sup>1</sup> B10. MG measured in zero magnetic field (circles) and in the field of 0.652 T (triangles). Values corresponding to bulk bcc-Fe (green curve) are given for comparison. The arrows indicate Tx1.

**Figure 10.** Contour plots of isothermal NFS time-domain patterns of the Fe90Zr7 B3 MG annealed at 470 °C (a), 480 °C (b), 510 °C (c), and at 480 °C under external magnetic field of 0.652 T (d).

In this section, we have demonstrated how NFS experiments can contribute to more detail description of structural as well as magnetic transformations that are taking place in real time upon MGs exposed to dynamically changing temperature. In particular, in situ studies of hyperfine interactions cannot be performed by any other analytical tool. Thus, a possibility to follow separately temperature evolution of individual structural components viz. AM, CR, and IF makes NFS an interesting and competitive method for the investigation of nanocrystallization in MGs *during* its progress. In the following section, other aspects of this technique are presented with a special focus at time-dependent experiments.

### **5.3. Isothermal experiments**

In the above section, we have concentrated on a dynamics of nanocrystallization. For that purpose, a continuous increase of temperature with a constant ramp was ensured. Acquisition of NFS data was accomplished in situ during ongoing progress of temperature. Thus, the investigated system was exposed to change annealing conditions. Here, we focus at the kinetics of crystallization. The studies were performed at constant annealing conditions. In this way, information on various parameters of the crystallization kinetics can be acquired.

NFS experiments were performed on Fe90Zr7 B3 MG prepared by melt spinning technique in a form of thin ribbons. After initial rapid increase of temperature with a ramp of 40 K/min, the annealing temperature was stabilized at its destination value. NFS data were recorded *in situ* with an acquisition time of 1 min. Duration of the experiments was up to 150 min. The annealing temperatures of 470, 480, and 510 °C were chosen. Isothermal experiment at 480 °C was performed also in an external magnetic field of 0.652 T.

Contour plots of NFS time-domain patterns from all experiments are shown in **Figure 10**. Time of annealing under the set conditions is given on the y-axes. The originally as-quenched sample is paramagnetic at room temperature and exhibits quantum beats typical for electric quadrupole interactions (see **Figure 1d**). Depending upon the annealing conditions, newly formed ferromagnetic nanocrystalline grains of bcc-Fe emerge from the amorphous matrix within 10–30 min after reaching the annealing temperature. They are identified through magnetic dipole interactions that give rise to hyperfine magnetic fields. The latter exhibit welldeveloped high-frequency oscillations in quantum beats similar as those in **Figure 1f**. They

Nanocrystallization of Metallic Glasses Followed by *in situ* Nuclear Forward Scattering of Synchrotron Radiation http://dx.doi.org/10.5772/66869 23

In this section, we have demonstrated how NFS experiments can contribute to more detail description of structural as well as magnetic transformations that are taking place in real time upon MGs exposed to dynamically changing temperature. In particular, in situ studies of hyperfine interactions cannot be performed by any other analytical tool. Thus, a possibility to follow separately temperature evolution of individual structural components viz. AM, CR, and IF makes NFS an interesting and competitive method for the investigation of nanocrystallization in MGs *during* its progress. In the following section, other aspects of this technique are

B3

MG annealed at 470 °C (a), 480 °C (b),

In the above section, we have concentrated on a dynamics of nanocrystallization. For that purpose, a continuous increase of temperature with a constant ramp was ensured. Acquisition of NFS data was accomplished in situ during ongoing progress of temperature. Thus, the investigated system was exposed to change annealing conditions. Here, we focus at the kinetics of crystallization. The studies were performed at constant annealing conditions. In this way,

B3

form of thin ribbons. After initial rapid increase of temperature with a ramp of 40 K/min, the annealing temperature was stabilized at its destination value. NFS data were recorded *in situ* with an acquisition time of 1 min. Duration of the experiments was up to 150 min. The annealing temperatures of 470, 480, and 510 °C were chosen. Isothermal experiment at 480 °C was

Contour plots of NFS time-domain patterns from all experiments are shown in **Figure 10**. Time of annealing under the set conditions is given on the y-axes. The originally as-quenched sample is paramagnetic at room temperature and exhibits quantum beats typical for electric quadrupole interactions (see **Figure 1d**). Depending upon the annealing conditions, newly formed ferromagnetic nanocrystalline grains of bcc-Fe emerge from the amorphous matrix within 10–30 min after reaching the annealing temperature. They are identified through magnetic dipole interactions that give rise to hyperfine magnetic fields. The latter exhibit welldeveloped high-frequency oscillations in quantum beats similar as those in **Figure 1f**. They

MG prepared by melt spinning technique in a

information on various parameters of the crystallization kinetics can be acquired.

presented with a special focus at time-dependent experiments.

**Figure 10.** Contour plots of isothermal NFS time-domain patterns of the Fe90Zr7

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

510 °C (c), and at 480 °C under external magnetic field of 0.652 T (d).

**5.3. Isothermal experiments**

NFS experiments were performed on Fe90Zr7

performed also in an external magnetic field of 0.652 T.

**Figure 11.** Relative amounts of structural components including amorphous matrix (AM, green squares), crystalline grains (CR, blue circles), and interface regions (IF, magenta triangles) plotted against time of annealing at 470 °C (a), 480 °C (b), 510 °C (c), and at 480 °C under external magnetic field of 0.652 T (d). Solid curves represent fits according to the crystallization model introduced in Ref. [18].

persist over the entire time of the experiments. Because the temperature of annealing does not change, the positions of individual beat maxima are also stable.

Time evolution of individual structural components viz. AM, CR, and IF is presented in **Figure 11**. Relative areas obtained from evaluation of NFS time-domain patterns are displayed by symbols. Subsequently, they were fitted using a crystallization model introduced recently [18]. The resulting theoretical curves are plotted by solid lines. The following observations can be noted. First, as demonstrated by the IF and CR relative contents in **Figure 11a a**nd **b**, the IF component dominates that of the CR one during the first 65 and 30 min of annealing, respectively. This indicates that the grains are rather small and thus exhibit higher contribution of the atoms located at their surfaces. Later, the grains grow in size which is documented by a higher fraction of CR than IF. The latter saturate with time which means that the grains do not grow any further, and only their number increases.

Secondly, increase in temperature of annealing from 470 to 480 °C accelerates formation of nanograins. In order to achieve ~60 % of nanograins, three times shorter time is needed for annealing temperature of 480 °C in comparison with 470 °C. Note different scales on the x-axes. Further temperature increase from 480 to 510 °C expectedly speeded-up the crystallization rate as seen in **Figure 11c**. The same effect is reached, however, by annealing at the

**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].

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 magnetic field at the same temperature of annealing.

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 magnetic interactions in comparison with the thermal energy.

**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

Nanocrystallization of Metallic Glasses Followed by *in situ* Nuclear Forward Scattering of Synchrotron Radiation http://dx.doi.org/10.5772/66869 25

**Figure 13.** Hyperfine magnetic field of the CR (a) and IF (b) components plotted against time of annealing.

in Bext= 0.652 T. In zero-field annealing at 480 °C, it exhibits only gentle increase towards the same saturation value showing slightly higher slope of 0.32.

Hyperfine magnetic fields obtained from the CR and IF components are depicted in **Figure 13** as a function of annealing time. Depending upon the temperature of annealing, BCR saturates at different values as seen in **Figure 13a**. They are by about 1.5 T smaller than those of polycrystalline bcc-Fe [38] due to nanosized dimensions of the grains. Decrease in BCR at the beginning of annealing especially at 480 °C in Bext indicates that the inner structure of bcc nanograins is still not very well developed during early stages of crystallization. Similar phenomenon is observed in **Figure 13b** where hyperfine magnetic fields of the interface components BIF are presented.

Small dimensions of nanocrystalline grains are also responsible for remarkable time evolution of BIF especially during annealing at 470 °C and to a smaller extent at 480 °C. At the beginning of grain growth, majority of the constituent Fe atoms is located at the surfaces of the nanocrystals. As a result, relative content of the IF component prevails that of the CR one as demonstrated in **Figure 11a** and **b.** Simultaneously, because of symmetry breaking in the interfacial regions, the associated hyperfine magnetic fields exhibit smaller values than the inner parts of the grains.

In this section, we have demonstrated how NFS can be used for the investigations of the kinetics of crystallization. Isothermal experiments performed under different annealing conditions, viz. temperature and presence of external magnetic field provide valuable information on the time evolution of both the content of nanocrystals and on their hyperfine magnetic fields. Moreover, these phenomena can be studied separately for structurally different regions that are found in NCAs.
