The Crystallization Kinetics, Structural and Magnetic Properties of Fe72.5Ag2Nb3Si13.5B9 Amorphous Ribbons as Affected by Annealing

The amorphous ribbon of composition Fe72.5Ag2Nb3Si13.5B9 has been prepared by rapid solidification technique under an atmosphere of pure argon and the amorphous nature has been confirmed by X-ray diffraction (XRD). The crystallization behavior and the nanocrystal formation have been studied by Differential Thermal Analysis (DTA) and XRD. The effect of annealing has been explained on the basis of XRD spectra. Magnetization measurements have been carried out using vibrating sample magnetometer (VSM). The activation energy for crystallization is evaluated by Kissinger’s plot. The peak temperature is found to be shifted towards the higher value with heating rate. The peak shift indicates the change of the values of Si-content of nanograins and therefore, the change of the lattice parameter of nanograins. At higher annealing temperature (Ta) the crystallization peak becomes smaller and displays diffused character meaning that substantial amount of crystallization of α-Fe (Si) phase has already been completed. The activation energy for α-Fe-(Si) phase is found to be 5.78 eV and 0.164 eV for before and after annealing respectively. The saturation magnetization (Ms) and Curie temperature (Tc) were found 114 emu/g and 305°C respectively. The sharp fall of magnetization at Tc is obtained which is an indication of homogeneity of the material.


Introduction
It has been well established by the time through extensive research that the addition of Cu and Nb, simultaneously with Fe-Si-B based amorphous alloys is the necessary condition for the extraordinary soft magnetic properties called FINEMET having composition Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 , developed in 1988 by Yoshizawa, Oguma and Yamauchi at Hitachi Metals Ltd. [1]. The Cu additives play a key role in the formation of the nucleation centres and Nb inhibits the grain growth [2]. This addition extends the temperature range between the primary crystallization of α-Fe(Si) phase and secondary crystallization of Fe 2 B phase for achieving superior magnetic properties [3]. It should be stressed again that good soft magnetic properties require not only a small grain size but at the same time the absence of boron compounds. The separation between the primary crystallization of bcc α-Fe(Si) and the precipitation of Fe 2 B compounds is not only determined by Cu and Nb addition but also decrease with increasing content. This put a further constraint on the alloy composition namely that boron content should be kept at a low or moderate level in order to obtain an optimum nanoscaled structure. Amorphous alloys provide an extremely convenient precursor material for preparation of nanocrystals through the crystallization process controlled by thermal treatments [4][5][6][7]. Müller et. al. [8] studied the influence of Cu/ Nb content and annealing condition on the microstructure and the magnetic properties of FINEMET alloys. Grain size, phase composition and transition temperature were observed, depend on the Cu/ Nb content. These represent a new family of excellent soft magnetic core materials and have stimulated an enormous research activity due to their potential applications [9][10][11][12].
Investigations have been carried out on the effect of substitution of Au for Cu in the FINEMET on the crystallization behavior and magnetic properties [13]. It has been found that Au behaves similarly as Cu on crystallization behavior and magnetic properties. This paper focuses on the experimental investigation of crystallization behavior, nanocrystalline structure formation and magnetic properties of Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 alloys in the amorphous and annealed states.

Materials and Methods
The amorphous ribbon of composition Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 was prepared from high purity Fe (99.9 %), Ag (99.9%), Nb (99.9 %), Si (99.9 %) and B (99.9 %). The ribbons were produced in an arc furnace on a water-cooled copper hearth by a single roller melt-spinning technique under an atmosphere of pure argon at the Centre of Materials Science, National University of Hanoi, Vietnam. The wheel velocity was about 34 m/s. The ribbons were annealed in a vacuum heat treatment furnace at 550, 600, 650, 700 and 750°C respectively for constant time 30 minutes and then cooled down to the room temperature. Crystallization phase analysis was carried out by DTA. The activation energy for crystallization of primary and secondary phases have been calculated using Kissinger's equation [14]: ln , where β is the heating rate, T p is the crystallization peak temperature, E is the activation energy and k is the Boltzman's constant. Amorphousity of the ribbon and nanocrystalline structure have been observed by XRD (Philips X 'Pert PRO XRD) with Cu-Kα radiation. Lattice radiation. Grain size (D g ) of all annealed samples of the alloy composition has been determined using Scherrer method. Si contents were calculated using the equation: where X is at.% Si in the nanograins. Magnetic properties such as field dependent specific magnetization and temperature dependent magnetization were performed by using VSM.

DTA
DTA traces of as-cast amorphous ribbons Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 with heating at the rate of 10-60°C/min at the step of 10°C with continuous heating from room temperature to 800°C are shown in the figure 1. It is clear from Figure 1(a) to 1(f) that one exothermic peak is found. The soft magnetic properties correspond to the primary crystallization of α-Fe-(Si) phase initiated at 1 x T . The peak temperature, 1 P T displays exothermic peak, i.e., release of heat during the crystallization of α-Fe-(Si) phase. It is observed that the crystallization of the phase has occurred over a wide range of temperatures. It is also observed that the peak temperature shifted towards the higher value and the crystallization temperature range increases with the increase of heating rate. That means it requires more heat energy for the formation of crystalline phase with increasing heating rate. The heat consumption is observed during the primary crystallization. The peak is shifting in the range from 579°C-607°C is evident from the figure1. From the Figure 2(a) and Fig. 2(b) the activation energy for the α-Fe-(Si) phase is found to be 5.78 eV and 0.164 eV for before and after annealing respectively. The values of crystallization onset temperature, peak temperature with respect to heating rate and activation energy are listed in the Table 1.  The DTA traces of Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 alloy in the as cast state and annealed at different temperatures for 2 hours are shown in Figure 3(a) to 3(d) respectively. It is observed from the DTA scan that the onset temperature for the sample Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 annealed at 550°C is almost unchanged with respect to its amorphous precursor which is quite logical since 550°C is still lower than 1 x T = 579°C. But the same sample when annealed at T a = 600 o C and 650 o C which are higher than the onset of crystallization temperature of 1 x T = 579°C, the crystallization peak is becoming smaller and display diffused character meaning that substantial amount of crystallization, phase has already been completed when annealed at 600°C and 650 o C for 2 hours.   The XRD spectra of as cast and sample annealed from 550 to 750°C have been presented in the figure 4. In the case of as cast state, there has a broadened peak which could be the evidence of amorphous nature. When the sample annealed at 550 o C, it exhibited small peak around 2θ = 45 o at the position of d 110 reflection which is generally known as diffuse hallow. This diffuse hallow indicates the amorphous nature of the sample. It means at this temperature, no crystallization peak has been detected. With the increasing of T a , (110) peak becomes sharper which means the grains are growing bigger. The value of full width half maxima (FWHM) of the peak annealed at 550 o C was not detected due to the lack of sharp peak. For the higher T a , the FWHM value is getting smaller. It shows that the crystallization occurs to a good extent at the higher T a . The crystallization onset temperatures from DTA experiment for different heating rates were found in the range of 569 to 590°C, which shows a good consistency with the XRD results. The lattice parameter, the silicon content in bcc nanograins and grain size of α-Fe(Si) grain can easily be calculated from the fundamental peak of (110) reflections. All results are shown in Table 2. Figure 5 presents the inverse relationship between lattice parameter and silicon content with T a . At higher T a , with increasing T a , Si-content is observed to rise, explained by the fact that at higher temperatures silicon diffuses out of nanograins due to crystallization which is consistent with the result of other FINEMET's [15]. Si having a smaller atomic size compared to Fe, diffuses in the α-Fe(Si) lattice during annealing at different temperatures which results in a contraction of α-Fe(Si) lattice. So the more diffusion of Si, there should be more contraction of the α-Fe(Si) lattice and thereby, the decrease of a 0 . However the decrease in lattice parameter is evident at higher annealing temperature when the diffusion of Si became easier at that temperature due to stress-relief in microstructure caused by heat treatment. Figure 6 shows the change of grain size (D g ) with T a. The increase of T a initiates partitioning α-Fe(Si) phase and thus grain growth due to formation of nanocrystalline α-Fe(Si) grains. In the range of T a from 600 o C to 750 o C, the D g remains in the range of 50 to 69 nm corresponding to soft magnetic α-Fe(Si) phases. The grain size increasing with increasing T a up to 650 o C but with further increasing T a , the D g increases which may be resulted due to higher physical distortion and internal strain. These facts reveal that heat treatment temperature should be limited with in 600 o C as D g remains 50 nm to obtain optimum soft magnetic behavior.    The magnetization process of the nanocrystalline amorphous ribbon of composition Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 is shown in Figure 7. From the magnetization curve it is clearly evidenced that the M s is found114 emu/g at room temperature. The small loop area indicates the soft magnetic performance of the ribbon.

Temperature Dependence of Specific Magnetization
The variation of magnetization (M) as a function of temperature in the range 0°C to 400°C with constant applied field of 10 kOe in the amorphous state for the nanocrystalline amorphous sample with composition Fe 72.5 Ag 2 Nb 3 Si 13.5 B 9 is shown in Figure 8. versus temperature curve of amorphous nanocrystalline ribbon with composition Fe72.5Ag2Nb3Si13.5B9.