Chapter-Ill Theoretical Background
4.7 Magnetization Measurement Techniques
5.2.4 Grain Size Determination
Fig.-5.7(b) presents the inverse relationship between lattice parameter and silicon content. This kinds of relation between lattice parameter and Si content was observed in the reports [5.23], which are verified in present work effectively. Fig.-5.7(a) shows the initial stage of crystallization lattice parameter first decreases with increase of silicon content, because silicon diffuses in the bce FeCo phase, to form the soft nanocrystalline a-FeCo(Si) phases. Such decrease of lattice parameter with increase of silicon content prevails up to750 °C, beyond which the lattice parameter with increase of silicon content.
This is the sequence of recrystallization when silicon diffuses out initiating formation of boride phases.
Where X = 1.54178
A
is the wavelength of Cu-Ka radiation. 0 is the diffraction angle and is the full width at half maximum (FWIIM) of diffraction peak in radian for different steps of annealing temperature.Instrumental broadening of the system was determined from 0 - 20 scan of standard Si. At the position of (110) reflection, the value of instrumental broadening was found to be 0.070. This value of instrumental broadening was subtracted FWHM value of each peak. Asymmetrical broadening of the peak due to stacking fault of bcc crystal was corrected negligible in the present case. All determined grain size was values for every steps of heat treatment are listed in Table-5.3. In Fig.-5.4 it is clear that at lower annealing temperature 550 °C, the FWHM of the peak is large and with the increase of annealing temperature, the value of FWHM are getting smaller. The peaks are, therefore becoming sharper with the shifting of peak position towards higher 20 value. The peak shifts indicate the change of the values of silicon content of nanograins and therefore. the change of the values of lattice parameter of nanograins.
From Fig.-5.8 and Table-5.3 that grain size increases with annealing temperature from a value of Dg = 9nm for Ta = 550 °C to Dg = 26nm for the sample annealed at Ta = 750 °C while Si-content decrease with annealing temperature. The increasing of annealing temperature initiates partitioning of Si in the bce FeCo phase and thus grain growth due to formation of nanocrystalline bcc FeCo(Si) grains. In the range of annealing teniperature 550 °C to 675 °C. the grain size remains in the range of 9 to 122 nm corresponding to soft magnetic bcc FeCo(Si) phases. Above 675 °C, grain grow rapidly and attain value of 19nm at 700 °C indicating formation of FeB and or FeCoB phases.
Formation of boride phase is detrimental to soft magnetic properties. These factsa reveals that heat treatment temperature should be limited within 550 °C to 650 °C to obtain optimum soft magnetic behavior, which will be clear that constant grain size.
The formation of the nanometric microstructure corresponding to the grain growth with the increase of annealing temperature is ascribed to combined effects of Cu and Nb and their low solubility in iron. Cu which is insoluble in a-FeCo, segregates prior to at the very beginning of nanocrystallization forming Cu-rich clusters and the nucleation of
FeCo(Si) grains is thought to be multiplied by clustering of Cu which stands as the reason for the grain growth at the initial stage of crystallization. On the other hand the rejection of Nb at the crystal interface causes hindrance to grain growth t'or which the change in grain size is not so obvious constant up to 650 °C. The increase of nucleation density caused by Cu as well as inhibition of grain growth by Nb results in homogeneous distribution of nano grains in the surrounding amorphous matrix. Our results corresponds well with the reported results of Rubinstein et. al. [5.281.
28
24
E
Cl 20
16
12
8
550 600 650 YOU (SU
Annealing temperature T0(°C)
Fig.-5.8 Variation of grain size with annealing temperature
As mentioned earlier in the simple Scherrer formula, the width of given reflection is used for grain size determination which under estimates the grain size as the strains evid&nfly present in the nanostructures are not taken into account. However, the short coming regarding evaluation of structural parameters does not affect much in correct estimation of the parameters and still XRD is a widely used experiment for investigation of microstructure of crystals. It was observed that grain size 9nm for the sample annealed at 550 °C for 30 minutes to a limiting values of 10 - 13nm between annealing temperature 600 °C to 675 °C.
5.3 Dynamic Magnetic Properties of (Fe0.95Coo.o5)73.Cu1 Nb3Si135B9 Alloy
Dynamic magnetic properties of as-quenched nanocrystallinc samples with composition (Fe095Co0.05)73.5Cui Nb3Sii3.5B9 has been measured as a function of frequency in the range 1kHz to 13MHz. Permeability measurements were performed on toroidal samples at frequency of 1kHz to 13MHz and an applied ac driving field 0.4AIm to ensure the measurements of initial permeability. Frequency spectrum of real and imaginary parts of initial permeability, loss factor and relative quality factor are analyzed. The measurements have been done for as cast sample and samples annealed at different temperatures with constant annealing time 30 minutes. In order to avoid experimental error due to fluctuation in ribbon thickness and thermal treatment, just one piece of each ribbon has been measured at room temperature after subsequent annealing temperatures at constant annealing temperature (30 minutes) magnetic properties of amorphous nanocrystalline magnetic materials are strongly dependent on its annealing temperature 5.2J. In the present work, initial permeability of the toroidal shaped samples annealed at different temperatures are measured to understand their soft magnetic properties and correlation with the micro structural features which are obtained from XRD analysis.
5.3.1 Frequency Dependence of Initial Permeability of