more Si into Fe lattice delays the dissolution of Co. Note that such delayed process would enhance the density of dislocations and fraction of grain boundaries at higher Co content. This is in good agreement with the results shown in Figure 5.12. Therefore, the variations of D, ρ and fGB are also strongly depend on the alloying process, which are expected to play a considerable role in the magnetic properties of these alloyed powders.
Figure 5.14. The representation of M as a function of H-1 and H-2 for 40 hours milled Fe85- yCoySi15 alloy powders.
However, a careful observation of the loops reveals that the substitution of Co fine tunes the slope of the curves towards lower field as observed in Fe90-yCoySi10 alloy powders. In order to determine the variation of MS and Keff, the field dependence of magnetization data were fitted to the power series using eqn.(4.09) after verifying the applicability of power series to analyse the experimental magnetization data as shown in Figure 5.14. The determined values of MS
and Keff and the extracted values of HC are plotted as a function of Co content in Figure 5.15.
MS increases with increasing Co content up to 10 at.% and then decreases slightly for higher Co substitution. This behaviour is somewhat different from the behaviour observed for Fe90- yCoySi10 alloy powders (see Figure 5.09(a)) where the values of MS increase continuously with increasing Co content. This could be correlated to the change in the structural parameters due to the difference in the alloying process as a result of delay in the dissolution of Co into Fe lattice in the presence of more Si content. Nevertheless, the increase in MS could be attributed to the atomic ordering as discussed earlier. On the other hand, HC increases at a faster rate initially for low Co substitution and the rate of increase of HC decreases for high Co content.
Figure 5.15. Variations of MS, HC and Keff of 40 hours milled Fe85-yCoySi15 alloy powders.
It is well known that HC is one of the structural sensitive properties of the magnetic materials, which depends strongly on the internal defects such as reduction in grain size, grain boundaries, dislocations and inclusion of additional materials. The increase in HC can be correlated to the atomic pairing which induces additional magnetic anisotropy and change in the grain boundaries, undissolved solute atoms acts as inclusions and hinder the domain wall motion [KHAJ2011]. To understand the variation of HC with Co content, the determined values of Keff are also plotted in Figure 5.15(c) for comparison. The variation of Keff shows almost similar behaviour of coercivity up to 15 at.% and then decreases for the samples with 20 at.%
Co. This suggests that the effective magnetic anisotropy induced by the internal strain through magnetoelastic coupling and by the atomic pairing by Co substitution plays a major role on the HC behaviour of the Fe85-yCoySi15 alloy powders.
Figure 5.16. Variation of Curie temperature (TC) of 40 hours milled Fe85-yCoySi15 alloy powders. Inset: Normalized M-T curves as a function of temperature close to magnetic phase transition temperatures.
It has been reported that the substitution of Co in Fe based amorphous and nanocrystalline alloys increases TC considerably [MISH2010]. To understand the effect of Co on the behaviour of TC of the as-milled nanocrystalline Fe85-yCoySi15 alloy powders, high temperature M-T measurements were performed at a heating rate of 5 K per minute in the temperature range 300 K to 1123 K under the applied magnetic field of 100 Oe. Figure 5.16 depicts the variation of TC of as-milled Fe85-yCoySi15 alloy powders and the normalized thermomagnetization data around the magnetic phase transition temperature (inset). In order to show the transition temperature clearly, M-T data for each sample was normalized with respect to their room temperature magnetization value. A close observation of the magnetization data reveals that for Fe85-yCoySi15 alloy powders with Co content less than 10 at.% exhibit a clear magnetic phase transition from ferromagnetic to paramagnetic state. As a result, the magnetization becomes zero after completing the magnetic phase transition. On further increasing the Co content, we observed an increase in the magnetization above 1050 K and the magnitude of increase in the magnetization increases continuously. This could be attributed to the commencement of crystallization process, which results in growth of the nanocrystalline phase resulting enhanced magnetization. This confirms that the substitution of Co in Fe85-yCoySi15
alloy powders reduces the crystallization temperature. This is in good agreement with the earlier reports on similar systems [IDZI2005, UMCY2004, MISH2010, MISH2011]. TC values were determined from the thermal derivative of M-T data and plotted as a function of Co content. TC increases at a rate of nearly 4 K per at.% Co for the Co substitution up to 10 at.%, but the rate of increase in TC drops down largely to 1.4 K per at.% Co for Co substitution above 10 at.%. The rate of increase in TC with Co content in the presently investigated alloy is substantially low as compared to the values reported in Co substituted amorphous Fe-based alloys: ~14 K per at.% Co in amorphous Fe82-xCoxNb3Ta1Mo1B13 alloys [UMCY2004] and 12 – 15 K per at.% Co in amorphous Fe89-x-yZr11BxCoy alloys [MISH2009]. This could be mainly due to the sample preparation techniques involving different kinematics on the formation of final alloys. In the presently investigated samples, the formation of nanocrystalline solid solution strongly depends on the milling parameters, solute and the solvents.