ABSTRACT

The amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ (0 ≤ x ≤ 0.30) alloys were prepared by the standard melt spinning technique and investigated their crystallization, thermal, transport and magnetic properties. Crystallization was observed from 400˚C to 650˚C with an interval 50˚C within 30 minutes annealing time by XRD. The as-cast samples were amorphous in nature. Annealing 400˚C to 450˚C samples showed the mixed bcc Fe and amorphous structures. The lattice parameter “a” was varied from 2.855 to 2.859 Å but above 450˚C, samples contained hexagonal, FeP and FeC structures. The lattice parameters “a” and “c” were varied from (5.016 - 5.036) Å and (13.575 - 13.820) Å, respectively. Average crystallite size was found to vary from 8 to 48 nm. Crystallization temperature and weight change were observed by differential thermal analysis and thermogravimetric analysis, respectively. Crystallization temperature was increased with increasing Mn content. Resistivity was increased above and bellows the Curie temperature. Real permeability remained almost constant upto around 10⁶ Hz for of all samples after that it was decreased with increasing frequency and it was also decreased with Mn, whereas imaginary permeability was increased sharply above frequency 10⁷ Hz. The value of saturation magnetization was found to decrease with increment Mn.

1. Introduction

Metallic glasses consist of 3d transition metal elements (Cr, Mn, Fe, Co, Ni) and high valence elements (B, Al, Si, P, C, etc.) which are opaque, ductile and good conductors of electricity and heat. Metallic glasses show amorphous structures with a non equilibrium state, so the amorphous phase can be transfer to a stable equilibrium state by the rise of temperature. There are a number of experimental evidences which indicate that the amorphous structure is crystallized above a certain temperature [1,2]. Crystallization involves a change in properties such as structural, heat capacity, electrical resistivity and magnetic, etc. [3]. Amorphous and nanocrystalline Fe base metal alloys have been attracted both for experimental and theoretical researchers in the fields of solid state physics, electronics and electrical engineering [4-8]. The (Fe_1–xMn_x)₇₅P₁₅C₁₀ alloys are soft magnetic which have been used as a transformer cores, modern power electronic devices, thermal transport properties of nanometric devices, temperature independent resistors, etc. [8]. Several authors have been reported on the different properties of the (Fe_1–xMn_x)₇₅P₁₅C₁₀ alloys such as magnetization [9], transport & magnetic [10] and thermoelectric power [11]. But there is no such experimental report on the crystallization process, thermogravimetric analysis and the permeability of the amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ alloys. Here is shown that the details crystallization process, thermogravimetric analysis and the permeability of the amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ alloys. Besides these low temperatures resistivity and magnetization measurements carried out for the purpose of making a comprehensive analysis of the amorphous (Fe_1–x Mn_x)₇₅P₁₅C₁₀ alloys system which gaining its important in practical.

2. Experimental

The amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ (x = 0.00, 0.10, 0.20 & 0.30) metallic ribbons (0.75 - 1.2 mm in width and 10 - 25 µm in thickness) have been prepared from the appropriate ratio of the pure elements (Fe = Mn = P = C = 99.95%) by standard melt-spinning technique with a wheel speed of 25 m/sec and cooling rate 1.8 × 10⁶˚C/sec. The amorphous nature of as-cast samples was examined by X-ray diffraction (XRD) experiment using a PW 3040- X’Pert PRO (Phillips) CuK_α radiation (λ = 1.54178 Å).

Some as-cast ribbons of the same concentration were taken into an aluminium crucible and were ground carefully until they became fine power and then the fine powder was put in an aluminium foil (1.5 cm × 1.5 cm) and folded. The foil along with the folded sample was subjected in a digital furnace (Carbolite, Sheffield, England) to the heat treatment at 400˚C for 30 minutes. After heating 30 minutes at 400˚C, the furnace was allowed to cool to room temperature (RT). When the temperature of the furnace was reached to RT, the sample was taken out from the furnace and ground until it became fine powder. Then the XRD were taken from this fine powder for 2θ values in the range 30˚ to 75˚. The heating and cooling rate of the sample was 10˚C/min and 5˚C/min, respectively. The samples were annealed at 450˚C, 500˚C, 550˚C, 600˚C and 650˚C for 30 minutes and XRD were taken in every case. The whole procedure was repeated for the other concentrations.

The crystallization temperatures (T_x) and the mass loss and/or gain of all the samples were investigated by differential thermal analysis (DTA) and thermogravimetric analysis (TGA) (Seiko-Ex-STAR-6300, Japan), respectively. Resistivity, ρ (T) was measured by the four point probe method in the temperature range (93 - 298) K with an interval 5 K. Complex permeability was measured as a function of frequency from 100 Hz to 100 MHz of ascast samples using LF Impedance Analyzer (4192A, 100 Hz - 110 MHz). Room temperature magnetization was measured with the vibrating sample magnetometer (VSM, Model 7400) at the department of Physics and Astronomy, Delaware University, Newark, DE19716 USA, in the range of (0 - 2.0) T magnetic field.  

3. Results and Discussion

3.1. X-Ray Diffraction Analysis (XRD)

The XRD patterns of all as-cast and annealed samples as shown Figure 1 and Figures 2(a)-(d), respectively. Each as-cast sample contains a small broad peak that confirms the samples are amorphous in nature. Annealing from

400˚C to 450˚C, each sample contains a few peaks which are identified as (301), (231) and (110) Millar planes. Besides these, there have also some tiny peaks that indicate residual amorphous nature. The lattice parameter “a” of the samples was calculated for (110) plane using the following relation (1).

(1)

The lattice parameter “a” for x = 0.00 is 2.856 Å which is consistent with a bcc Fe structure [12] and the value of “a” increases slightly with increasing Mn content (as shown in Table 1). Increasing lattice parameter and hence the unit cell volume with Mn content is due to the volume expansion effect, as Mn has the larger ionic radius (0.89 Å) compared to Fe (0.74 Å) [13]. Linear increasing of the lattice parameter of these alloys with Mn content suggests a simple dilution process. Annealing

from 500˚C to 650˚C height of the (110) plane diminishes and simultaneously some new sharp peaks with very low intensity arise in the spectra which are identified as (104), (222), (116), (211), (214) and (200) Millar planes respectively. This result may be yield from the crystallization of the residual amorphous phase and also the structural change of the samples from 500˚C to higher temperatures. Structure of these alloys is consistent with hexagonal, FeP, FeC structure (JCPDS card No. 80-0042) and with the XRD machine code reference 76-0182 [12]. The lattice parameters (a and c) were calculated for the (116) and (214) plane using the following relation (2). 

(2)

The lattice parameters “a” & “c” varies from 5.016 to 5.036 Å and 13.575 to 13.820 Å, respectively. The lattice parameters, crystallite size and volume of all the samples with temperature as shown in Table 1. The average crystallite size of the samples was estimated from the broadening of the corresponding X-ray spectral peaks using the Scherrer formula.

(3)

where D_g is the average grain size, λ is the wavelength of the CuK_α radiation, 0.05 is the instrumentation broadening, B is the full width at half-maximum (FWHM) of a diffraction peak expressed in radians and θ is the Bragg angle. The estimated average crystallite size of the samples lies in between 16 nm to 47 nm for temperature 400˚C to 450^ºC and 8 nm to 48 nm in the temperature range 500˚C to 650˚C. The crystallization process of the amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ alloys proceeds through the whole annealing temperature is shown in the following way:

3.2. DTA, TG% and DTG

Some thermal traces such as DTA, TG% and DTG of the samples is shown in Figures 3(a)-(d). From DTA trace of Figures 3(a)-(d) we see that crystallization temperature (T_x) increases with increasing Mn. With the increasing temperature the micro voids which formed during the growth process of the ribbon during melt spinning are gradually eliminated. At higher temperature both Fe and Mn ions are oxidized by the environment and hence the mass could be slightly enhanced (fig. TG%). It is assumed that the volume/grain may be increased as with Mn because of the ionic radius of Mn (0.89 Å) is larger than the ionic radius of Fe (0.74 Å). In the DTG curves some small peaks are found for x = 0.00 & 0.10 whereas for x = 0.20 & 0.30 some broad peaks are generated in the trace which indicate that there is a change of mass within a small change of heating temperature or time.

3.3. Temperature Dependent Resistivity

Variation of resistant within temperature 93 K to 298 K is shown in Figure 4. From Figure 4 it is seen that ρ (T) for x = 0.00 decreases almost linearly with decreasing temperature. Although there is no contribution of Mn but the incoherent electron-magnon scattering contribute to the resistivity and this scattering may be decreased with decreasing temperature. But for x = 0.10 & 0.20, the resistivity increases with the decrease of temperature. Since the Curie temperature of x = 0.10 is 420 K and of x = 0.20 is 350 K [11,14]. Resistivity increasing below the Curie temperature is due to the structural topological scattering. Whereas for x = 0.30 resistivity decreases nearly up to the Curie temperature (T_c = 166 K) and the resistivity shallow minima also observed which has been assigned to a competition of structural and spin scattering [10,15].

3.4. Permeability

The dynamic response of the magnetic domains to the external magnetic field determines the complex permeability. The real, µ^/ and imaginary, µ^// permeability were calculated using the following relations (4) and (5)

(4)

(5)

where, L_s is the self-inductance of the sample core and is derived geometrically. Here L₀ is the inductance of the winding coil without the sample core, N is the number of turns of the coil (N = 5), S is the area of cross section of the toroidal sample as given by,

, where, here d₁ & d₂ are inner and outer diameters, height and is the mean diameter of the toroidal sample is given as:

The real part, µ^/ and imaginary part, µ^// of the permeability of as-cast samples are presented separately as a function of frequency in Figures 5(a) and (b).

The real permeability for x = 0.00 is of the order of 10² and this value decreases with the increase of Mn content (Figure 5(a)). With the increase of Mn both the randomization and antiferromagnetic interaction increases as a result the permeability decreases. In the lower frequency, permeability originates from the Bloch wall (BW) motion. Since the real permeability does not drop to zero at the lower frequency, so the BW’s are not very mobile from the start of the frequency. That in turn suggests very broad BW’s. When the frequency gets higher than 10⁶ Hz, the BW cannot follow the applied force. At higher frequency (above 10⁷ Hz) imaginary permeability increases sharply that means at higher frequency, the Bloch wall oscillations could yield a very strong oscillator with a high loss (as shown in Figure 5(b)) developing near to the end of the BW-motion regime.

3.5. Magnetization

The magnetic effect of Mn content into the amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ system was investigated by measuring the room temperature magnetization (Figure 6). Room temperature saturation magnetization decreases with the

step-up of Mn content. As with the increment of Mn content into the (Fe_1–xMn_x)₇₅P₁₅C₁₀ system it is assumed that the isolated antiferromagnetic (AF) coupled sites simply order antiparallel to the majority ferromagnetic (FM) order. These magnetic properties lead to the suggestion that the antiferromagnetic interactions introduced by Mn atoms cause deviations from a pure ferromagnetic structure and may reducing the total magnetization.

4. Conclusion

Crystallization experiments were done on the amorphous (Fe_1–xMn_x)₇₅P₁₅C₁₀ ribbons. The bcc Fe and amorphous structures were observed in the temperature range 400˚C to 450˚C and the hexagonal, FeP, FeC structures were found between the temperature 500˚C and 650˚C. The crystallite size of the samples lies in between 16 nm to 47 nm for temperature 400˚C to 450˚C and whereas 8 nm to 48 nm in the temperature range 500˚C to 650˚C. The crystallization temperatures were detected from DTA trace 418.9˚C, 436.3˚C, 449.7˚C and 472.4˚C for x = 0.00, 0.10, 0.20 and 0.30, respectively. Resistivity increases above and below the Curie temperature. Real permeability remain constant up to 10⁶ Hz where as imaginary permeability sharply increases above 10⁷ Hz. Room temperature saturation magnetization gradually decreases with the increase of Mn content. From these comprehensive analyses we may suggests that these alloys are soft magnetic materials and may be used as a transformer core, thermal transport properties of nanometric devices, temperature independent resistors, etc.

5. Acknowledgements

The authors would like to express their thanks to Professor K. Bärner, Department of Physics (4 physik), University of Göttingen, Germany, for preparing and providing the samples used in this study and his valuable discussion regarding the results. The authors would also like to express their thanks to Professor G. C. Hadjipanayis, Department of Physics and Astronomy, University of Delaware, USA for magnetization measurement. The authors would like to thankful to Bangladesh University of Engineering and Technology (BUET) for assistance.

REFERENCES

NOTES

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