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Influence of quenching rate on the structural, soft magnetic and magnetoimpedance properties of melt-spun Co69Fe7Si14B10 alloy
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 3084–3089
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of quenching rate on the structural, soft magnetic and magnetoimpedance properties of melt-spun Co69Fe7Si14B10 alloy D. Arvindha Babu, J. Arout Chelvane , D. Akhtar, K. Satya Prasad, V. Chandrasekaran Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500 058, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 19 March 2009 Received in revised form 30 April 2009 Available online 12 May 2009
Melt-spun ribbons of Co69Fe7Si14B10 alloy have been prepared at different wheel speeds viz. 47, 34 and 17 m/s and investigated for structural and magnetic properties. Degree of amorphicity in the as-spun ribbons is found to increase with wheel speed. Amorphous phase crystallizes in two stages producing Co2Si, Co2B and CoSi phases on annealing. Increase in wheel speed improves soft magnetic and magnetoimpedance properties due to decrease in perpendicular anisotropy which is associated with stripe domain formation. On annealing soft magnetic properties and magnetoimpedance deteriorate due to the formation of crystalline phases. & 2009 Elsevier B.V. All rights reserved.
Keywords: Rapid solidification Amorphous soft magnet Magnetic anisotropy magnetoimpedance
1. Introduction Amorphous soft magnetic materials have been extensively explored by several researchers due to their potential applications as transformer cores, inductive devices, magneto impedance (MI) sensors, etc. Soft magnetic properties such as high permeability and low coercivity exhibited by amorphous alloys make them a promising candidate for these applications. The soft magnetic properties arise in these materials due to very low magnetocrystalline anisotropy. Soon after the recent discovery of giant magnetoimpedance (GMI) effect in soft magnetic alloys there has been a tremendous thrust to develop sensor devices such as magnetic field sensor, magnetic heads for recording, etc [1–4]. GMI sensors have magnetic field resolution comparable with the flux-gate sensors, without a need for exciting and sensing coils. Additionally, GMI sensors have been found to be more field sensitive than the present giant magneto-resistance (GMR) sensors [5]. The GMR materials generally require large fields to obtain a response of a few percent, whereas the GMI materials yield a large change in impedance upon application of very low fields. Fe–Co based soft magnetic alloy ribbons exhibit large MI due to a large change in the complex permeability of the material upon application of an external magnetic field. Among the several Fe–Co based soft magnetic alloys investigated, Co–Fe–Si–B alloys show large magnetoimpedance. The effect of various parameters such as composition, alloying addition, length of the sample, etc. on magnetoimpedance in Co–Fe–Si–B ribbons has been reported
Corresponding author. Tel.: +91 40 24586339; fax: +91 40 24340884.
E-mail address: [email protected] (J.A. Chelvane). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.05.010
by various researchers [6–10]. Although there are several reports on the quenching rate dependence of soft magnetic properties, the effect of rapid quenching on MI and its correlation with the related soft magnetic properties is still elusive [11–13]. Hence a study has been undertaken to understand the GMI effect at different quenching rates to evolve a structure-property correlation.
2. Experimental An alloy of nominal composition Co69Fe7Si14B10 was prepared by melting the pure elements such as Co, Fe, B and Si (purity 99.95%) in a vacuum arc melting furnace. Ribbons with different wheel speeds namely 47, 34 and 17 m/s have been prepared in a single roller vacuum melt spinning unit under argon atmosphere. The as-spun ribbon samples were sealed in evacuated quartz tube (vacuum level of 2 10 5 mbar) and annealed at 450, 520 and 570 1C for 1 h. Structural characterization of the as-spun and heattreated ribbons was carried out using X-ray diffraction (XRD) employing Cu–Ka radiation (Model: Philips-PW1830). Crystallization studies were performed using a differential scanning calorimeter (DSC) (TA instruments –DSC 910S) at a constant heating rate of 20 1C/min. A transmission electron microscope (TEM) (FEI Tecnai-20T) equipped with energy dispersive X-ray analysis (EDAX), was used to observe the microstructural features and elemental compositions of the melt-spun ribbons. The samples were thinned from both sides of ribbon using precision ion polishing system by bombarding with Ar ion. Magnetization measurements were carried out using a vibrating sample magnetometer (Model: ADE EV9) upto a magnetic field of 20 kOe in the temperature range 30–460 1C. The magnetic field
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was applied perpendicular to the plane of the ribbon (on rectangular strips cut from the ribbons of approximately 2 mm 10 mm) for anisotropy measurements. The demagnetization field was assumed to be constant in all the samples. Room temperature coercivity and permeability measurements were carried out using a B–H loop recorder (Model: Ferrites India) on the ribbon samples wound in the form of torroids, upto an applied magnetic field of 5 Oe at 50 Hz. Room temperature magnetoimpedance measurements were carried out on both as-spun and annealed ribbons (5 cm long) in the frequency interval 2–10 MHz employing an impedance analyzer (Model: HP 4192A) and a Helmoltz coil upto a DC magnetic field of 80 Oe.
3. Results and discussion 3.1. Structural characterization Fig. 1 shows the XRD patterns of the as-spun ribbons prepared at different wheel speeds. It is found that ribbons prepared at 47 and 34 m/s were amorphous while a crystalline Co2Si phase appears for the ribbon prepared at 17 m/s. The Co2Si crystalline phase formation is due to the less cooling rate achieved in case of 17 m/s ribbons. TEM bright field (BF) images and corresponding selected area electron diffraction (SAD) patterns of as-spun ribbons prepared at different wheel speeds are shown in Fig. 2. The ribbon prepared at 47 m/s shows featureless contrast in BF image (Fig. 2(a)) and corresponding bright halo in the SAD pattern (inset) reveals the formation of completely amorphous phase. Fig. 2(b) shows the BF image and the inset shows the bright halo SAD pattern obtained from contrastless area (region I) and the single crystal diffraction pattern (Zone axis /2 0 1S of orthorhombic phase) from a large grain (region II) of 34 m/s ribbon. It is observed that the microstructure exhibits a predominantly amorphous phase with a small fraction of crystalline phase (Co2Si orthorhombic phase) with cellular structure within the amorphous matrix. The compositions obtained from EDAX analyses for the amorphous region and the cells reveal the formation of solid solution. However, it is to be noted that boron could not be detected through EDAX probe. Fig. 2(c) shows the BF image and
Fig. 1. XRD patterns of as-spun ribbons prepared at different wheel speeds.
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corresponding SAD pattern (inset) of 17 m/s wheel speed ribbon. It is interesting to note the formation of nanocrystalline Co2Si orthorhombic phase with the grain diameter ranging from 10 to 80 nm. Fig. 3 shows the DSC thermograms for the ribbons prepared at different wheel speeds. Two exothermic peaks are observed with the onset of crystallization Tx1 and Tx2 for all the ribbons. The values of Tx1 and Tx2 are found to be constant with wheel speed (Table 1). In order to obtain the degree of amorphicity, the energy of crystallization (DH (J/g)) was calculated from the area of the first exothermic peak and found to be 139, 93 and 73 J/g for 47, 34 and 17 m/s ribbons, respectively. It can be seen that the energy of crystallization decreases with decreasing wheel speed indicating a decrease in the degree of amorphicity with wheel speed [10]. In addition to these phase transformations, a slope change could be observed before the first crystallization peak in DSC thermogram of 47 m/s wheel speed which is attributed to glass transition temperature (Tg) [14]. Since ribbon prepared at the wheel speed of 47 m/s exhibit complete amorphous phase, it has been considered for crystallization studies. Fig. 4 shows the XRD patterns of samples annealed at 450, 520 and 570 1C. It can be seen that the ribbon annealed at 450 1C exhibits Co2Si phase while ribbons annealed at 520 1C forms additional Co2B along with Co2Si phase. Annealing at 570 1C forms CoSi phase along with Co2Si and Co2B phases. The first exothermic peak which appears in DSC (Fig. 3) corresponds to crystallization of Co2Si and Co2B phases while the second exothermic peak correspond to the crystallization of CoSi phase, respectively.
3.2. Magnetic properties Fig. 5 shows the thermomagnetic plots of as-spun ribbons prepared at different wheel speeds. The thermomagnetic plots for 47 and 34 m/s wheel speed ribbons show a ferromagnetic to paramagnetic transition temperature, Tc of the amorphous phase around 400 1C. The Tc of amorphous phase in the ribbon prepared at 17 m/s is 374 1C which may be attributed to the change in the amorphous phase composition due to simultaneous formation of crystalline phases. The increase in magnetization beyond 415 1C is due to the formation of a high Tc CoSi phase as discussed later [15]. Fig. 6 shows the thermomagnetic plots of annealed ribbons prepared at 47 m/s wheel speed. The Tc of amorphous phase was found to be 367 1C for the ribbon annealed at 450 1C. The decrease in Tc is again attributed to the change in amorphous phase composition due to the formation of crystalline Co2Si phase. It can also be seen that a ferro to paramagnetic transition of Co2B phase was observed around 167 1C for the ribbons annealed at 520 and 570 1C. The residual magnetization observed for the ribbons annealed at 520 and 570 1C corresponds to the magnetization of the high temperature CoSi phase. This is in conformity with the observed increase in magnetization (Fig. 5) beyond 415 1C in all the ribbons prepared at different wheel speeds due to the formation of high temperature CoSi phase. The Tc of Co2Si phase was not observed as it is not ferromagnetic above room temperature [15]. Fig. 7 shows the room temperature magnetization curves of asspun ribbons prepared at different wheel speeds. The magnetization curves were found to exhibit two-step-like behaviour in all samples. This is attributed to the formation of stripe domains associated with perpendicular anisotropy [16–19]. The shearing of the hysteresis loop towards the higher magnetic field indicates the increase in perpendicular anisotropy with decreasing wheel speed. This is attributed to the progressive formation of Co2Si orthorhombic phase with decreasing wheel
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Fig. 2. TEM bright field (BF) images and corresponding selected area electron diffraction (SAD) patterns of as-spun ribbons prepared at wheel speeds of (a) 47 m/s, (b) 34 m/ s and (c) 17 m/s.
Fig. 3. DSC thermograms of as-spun ribbons at a heating rate of 20 1C/min.
speed (Fig. 2). Inset in Fig. 7 shows the room temperature magnetization curves for annealed ribbons prepared at the wheel speed of 47 m/s. It is evident from the plot that the perpendicular anisotropy increases with annealing temperature. It is ascribed to
the formation of the crystalline phases (Co2Si, CoSi and Co2B) with annealing temperature (Fig. 4). Fig. 8 shows the hysteresis loops for as-spun ribbons prepared at different wheel speeds. Soft magnetic properties of as-spun and
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Table 1 Crystallization parameters (Tx1, Tx2 and DH) for ribbons spun at different wheel speed. Wheel speed (m/s)
Tx1 (1C)
Tx2 (1C)
DH (J/g)
47 34 17
500 500 500
557 557 556
139 93 73
Fig. 6. Thermomagnetic curves (500 Oe field) of 47 m/s ribbon annealed at 450, 520 and 570 1C.
Fig. 4. XRD patterns of as-spun and annealed ribbons prepared at a wheel speed of 47 m/s.
Fig. 7. Magnetization curves of as-spun ribbons prepared at different wheel speeds. Inset shows the magnetization curves of 47 m/s ribbon annealed at 450, 520 and 570 1C.
(mi) increase with increasing wheel speed. The decrease in coercivity and increase in permeability of higher wheel speed ribbons is attributed to the increase in degree of amorphicity and decrease in perpendicular anisotropy [10–12,16]. 3.3. Magneto impedance studies
Fig. 5. Thermomagnetic curves (500 Oe field) of as-spun ribbons prepared at different wheel speeds.
annealed samples derived from B–H loops are listed in Table 2. The coercivity values are found to decrease with increasing wheel speed while the peak permeability (mp) and initial permeability
Fig. 9 shows the variation of MI with applied magnetic field at various frequencies for the as-spun ribbons prepared at the wheel speed of 47 m/s. A two-peak behaviour observed at all frequencies indicates the presence of transverse anisotropy. The variation of MI values with magnetic field is found to increase with increasing frequency upto 5 MHz, with further increase in frequency the MI value decreases. The initial increase in the MI values is attributed to decrease in skin effect which causes the transverse permeability to increase resulting in a net increase in
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Fig. 8. B–H loops of as-spun ribbons prepared at the wheel speeds of 47, 34 and 17 m/s.
Table 2 Soft magnetic properties of as-spun and annealed ribbons. Sample description
Wheel speed/ annealing temperature
As-spun
47 m/s 34 m/s 17 m/s
47 m/s annealed
450 1C 520 1C 570 1C
Initial permeability (mi)
Peak permeability (mp)
0.07 0.08 0.2
37555 22900 3457
51780 44895 7377
14.35 542 878
776 53 47
1913 75 69
Coercivity, Hc (Oe)
Fig. 10. Magnetoimpedance as a function of magnetic field measured at 5 MHz for as-spun ribbons prepared at the wheel speed of 47, 34 and 17 m/s. Inset shows the magnetoimpedance as a function of magnetic field for ribbon annealed at 4501C at 5 MHz.
It can be observed that the peak at which maximum MI value observed increases with decreasing wheel speed revealing the increase in transverse anisotropy. The highest MI value of about 48% is observed for the ribbon prepared at 47 m/s and it decreases with wheel speed. The observed decrease in MI with wheel speed is attributed to the deterioration of soft magnetic properties as discussed earlier which modifies the MI response through change in skin effect [21]. Inset in Fig. 10 shows the variation of MI with applied magnetic field measured at 5 MHz for the ribbons annealed at 450 1C. It is evident that the MI value decreases drastically to 4% on annealing at 450 1C. Further, MI was not observed for the ribbons annealed at 520 and 570 1C. This is attributed to deterioration of soft magnetic properties in annealed samples due to the formation of Co2Si, Co2B and CoSi phases.
4. Conclusions Melt-spun ribbons of Co69Fe7Si14B10 alloy have been prepared at different wheel speeds of 47, 34 and 17 m/s. Degree of amorphicity increases with wheel speed due to enhanced cooling rate. Amorphous ribbons crystallize to Co2Si, Co2B and CoSi phases on annealing. Ferro to para transition temperature of amorphous phase is around 400 1C for 47 and 34 m/s and it decreases for 17 m/s ribbons possibly due to change in amorphous phase composition on crystalline phase formation. Soft magnetic properties improve with increasing wheel speed which is attributed to increase in degree of amorphicity and decrease in perpendicular anisotropy. Improved soft magnetic properties lead to enhanced GMI properties. Annealing of the ribbons deteriorates the soft magnetic properties and GMI due to formation of crystalline phases. Fig. 9. Magnetoimpedance as a function of frequency for as-spun ribbon prepared at 47 m/s.
Acknowledgements magnetoimpedance. The decrease in MI values at higher frequencies is due to the damping of domain wall motion as a result of eddy currents around domain walls [20]. Fig. 10 shows the variation of MI with applied magnetic field at 5 MHz for the as-spun ribbon prepared at different wheel speeds.
This work was supported by DRDO, Govt. of India. The authors are thankful to Prof. Markandeyulu, IIT Madras for fruitful discussion and Dr. G. Malakondaiah, Director, DMRL for continued support and permission to publish this work.
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[11] M. Knobel, J.P. Sinnecker, R. Sato Turtelli, H.R. Rechenberg, R. Gro¨ssinger, J. Appl. Phys. 73 (1993) 6603. [12] V. Madurga, J.M. Barandiara´n, M. Va´zquez, J. Appl. Phys. 61 (1987) 3228. [13] P. Allia, M. Coisson, P. Tiberto, F. Vinai, L. Lanotte, J.Magn. Magn. Mater. 215–216 (2000) 346. [14] S.R. Elliott, Physics of Amorphous Materials, Longman Publishers, New York, 1984. [15] T. Kulik, H. Matyja, B. Lisowski, Mater. Sci. Eng. 99 (1988) 77–80. [16] C.B. Craus, A.R. Chezan, M.H. Siekman, J.C. Lodder, D.O. Boerma, L. Niesen, J. Magn. Magn. Mater. 240 (2002) 423. [17] A.R. Chezan, C.B. Craus, N.G. Chechenin, T. Vystavel, L. Niesen, J.Th.M. De Hosson, D.O. Boerma, J. Magn. Magn. Mater. 299 (2006) 219. [18] D. Arvindha Babu, B. Majumdar, R. Sarkar, D. Akhtar, V. Chandrasekharan, J. Phys. D. Appl. Phys. 41 (2008) 195002. [19] M. Knobel, K.R. Pirota, J. Magn. Magn. Mater. 242–245 (2002) 33. [20] J. He, H.Q. Guo, B.G. Shen, K.Y. He, H. Kronmuller, J. Appl. Phys. 86 (1999) 3873. [21] J.S. Blazquez, V. Franco, A. Conde, L.F. Kiss, J. Appl. Phys. 93 (2003) 2172.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of quenching rate on the structural, soft magnetic and magnetoimpedance properties of melt-spun Co69Fe7Si14B10 alloy D. Arvindha Babu, J. Arout Chelvane , D. Akhtar, K. Satya Prasad, V. Chandrasekaran Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500 058, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 19 March 2009 Received in revised form 30 April 2009 Available online 12 May 2009
Melt-spun ribbons of Co69Fe7Si14B10 alloy have been prepared at different wheel speeds viz. 47, 34 and 17 m/s and investigated for structural and magnetic properties. Degree of amorphicity in the as-spun ribbons is found to increase with wheel speed. Amorphous phase crystallizes in two stages producing Co2Si, Co2B and CoSi phases on annealing. Increase in wheel speed improves soft magnetic and magnetoimpedance properties due to decrease in perpendicular anisotropy which is associated with stripe domain formation. On annealing soft magnetic properties and magnetoimpedance deteriorate due to the formation of crystalline phases. & 2009 Elsevier B.V. All rights reserved.
Keywords: Rapid solidification Amorphous soft magnet Magnetic anisotropy magnetoimpedance
1. Introduction Amorphous soft magnetic materials have been extensively explored by several researchers due to their potential applications as transformer cores, inductive devices, magneto impedance (MI) sensors, etc. Soft magnetic properties such as high permeability and low coercivity exhibited by amorphous alloys make them a promising candidate for these applications. The soft magnetic properties arise in these materials due to very low magnetocrystalline anisotropy. Soon after the recent discovery of giant magnetoimpedance (GMI) effect in soft magnetic alloys there has been a tremendous thrust to develop sensor devices such as magnetic field sensor, magnetic heads for recording, etc [1–4]. GMI sensors have magnetic field resolution comparable with the flux-gate sensors, without a need for exciting and sensing coils. Additionally, GMI sensors have been found to be more field sensitive than the present giant magneto-resistance (GMR) sensors [5]. The GMR materials generally require large fields to obtain a response of a few percent, whereas the GMI materials yield a large change in impedance upon application of very low fields. Fe–Co based soft magnetic alloy ribbons exhibit large MI due to a large change in the complex permeability of the material upon application of an external magnetic field. Among the several Fe–Co based soft magnetic alloys investigated, Co–Fe–Si–B alloys show large magnetoimpedance. The effect of various parameters such as composition, alloying addition, length of the sample, etc. on magnetoimpedance in Co–Fe–Si–B ribbons has been reported
Corresponding author. Tel.: +91 40 24586339; fax: +91 40 24340884.
E-mail address: [email protected] (J.A. Chelvane). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.05.010
by various researchers [6–10]. Although there are several reports on the quenching rate dependence of soft magnetic properties, the effect of rapid quenching on MI and its correlation with the related soft magnetic properties is still elusive [11–13]. Hence a study has been undertaken to understand the GMI effect at different quenching rates to evolve a structure-property correlation.
2. Experimental An alloy of nominal composition Co69Fe7Si14B10 was prepared by melting the pure elements such as Co, Fe, B and Si (purity 99.95%) in a vacuum arc melting furnace. Ribbons with different wheel speeds namely 47, 34 and 17 m/s have been prepared in a single roller vacuum melt spinning unit under argon atmosphere. The as-spun ribbon samples were sealed in evacuated quartz tube (vacuum level of 2 10 5 mbar) and annealed at 450, 520 and 570 1C for 1 h. Structural characterization of the as-spun and heattreated ribbons was carried out using X-ray diffraction (XRD) employing Cu–Ka radiation (Model: Philips-PW1830). Crystallization studies were performed using a differential scanning calorimeter (DSC) (TA instruments –DSC 910S) at a constant heating rate of 20 1C/min. A transmission electron microscope (TEM) (FEI Tecnai-20T) equipped with energy dispersive X-ray analysis (EDAX), was used to observe the microstructural features and elemental compositions of the melt-spun ribbons. The samples were thinned from both sides of ribbon using precision ion polishing system by bombarding with Ar ion. Magnetization measurements were carried out using a vibrating sample magnetometer (Model: ADE EV9) upto a magnetic field of 20 kOe in the temperature range 30–460 1C. The magnetic field
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was applied perpendicular to the plane of the ribbon (on rectangular strips cut from the ribbons of approximately 2 mm 10 mm) for anisotropy measurements. The demagnetization field was assumed to be constant in all the samples. Room temperature coercivity and permeability measurements were carried out using a B–H loop recorder (Model: Ferrites India) on the ribbon samples wound in the form of torroids, upto an applied magnetic field of 5 Oe at 50 Hz. Room temperature magnetoimpedance measurements were carried out on both as-spun and annealed ribbons (5 cm long) in the frequency interval 2–10 MHz employing an impedance analyzer (Model: HP 4192A) and a Helmoltz coil upto a DC magnetic field of 80 Oe.
3. Results and discussion 3.1. Structural characterization Fig. 1 shows the XRD patterns of the as-spun ribbons prepared at different wheel speeds. It is found that ribbons prepared at 47 and 34 m/s were amorphous while a crystalline Co2Si phase appears for the ribbon prepared at 17 m/s. The Co2Si crystalline phase formation is due to the less cooling rate achieved in case of 17 m/s ribbons. TEM bright field (BF) images and corresponding selected area electron diffraction (SAD) patterns of as-spun ribbons prepared at different wheel speeds are shown in Fig. 2. The ribbon prepared at 47 m/s shows featureless contrast in BF image (Fig. 2(a)) and corresponding bright halo in the SAD pattern (inset) reveals the formation of completely amorphous phase. Fig. 2(b) shows the BF image and the inset shows the bright halo SAD pattern obtained from contrastless area (region I) and the single crystal diffraction pattern (Zone axis /2 0 1S of orthorhombic phase) from a large grain (region II) of 34 m/s ribbon. It is observed that the microstructure exhibits a predominantly amorphous phase with a small fraction of crystalline phase (Co2Si orthorhombic phase) with cellular structure within the amorphous matrix. The compositions obtained from EDAX analyses for the amorphous region and the cells reveal the formation of solid solution. However, it is to be noted that boron could not be detected through EDAX probe. Fig. 2(c) shows the BF image and
Fig. 1. XRD patterns of as-spun ribbons prepared at different wheel speeds.
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corresponding SAD pattern (inset) of 17 m/s wheel speed ribbon. It is interesting to note the formation of nanocrystalline Co2Si orthorhombic phase with the grain diameter ranging from 10 to 80 nm. Fig. 3 shows the DSC thermograms for the ribbons prepared at different wheel speeds. Two exothermic peaks are observed with the onset of crystallization Tx1 and Tx2 for all the ribbons. The values of Tx1 and Tx2 are found to be constant with wheel speed (Table 1). In order to obtain the degree of amorphicity, the energy of crystallization (DH (J/g)) was calculated from the area of the first exothermic peak and found to be 139, 93 and 73 J/g for 47, 34 and 17 m/s ribbons, respectively. It can be seen that the energy of crystallization decreases with decreasing wheel speed indicating a decrease in the degree of amorphicity with wheel speed [10]. In addition to these phase transformations, a slope change could be observed before the first crystallization peak in DSC thermogram of 47 m/s wheel speed which is attributed to glass transition temperature (Tg) [14]. Since ribbon prepared at the wheel speed of 47 m/s exhibit complete amorphous phase, it has been considered for crystallization studies. Fig. 4 shows the XRD patterns of samples annealed at 450, 520 and 570 1C. It can be seen that the ribbon annealed at 450 1C exhibits Co2Si phase while ribbons annealed at 520 1C forms additional Co2B along with Co2Si phase. Annealing at 570 1C forms CoSi phase along with Co2Si and Co2B phases. The first exothermic peak which appears in DSC (Fig. 3) corresponds to crystallization of Co2Si and Co2B phases while the second exothermic peak correspond to the crystallization of CoSi phase, respectively.
3.2. Magnetic properties Fig. 5 shows the thermomagnetic plots of as-spun ribbons prepared at different wheel speeds. The thermomagnetic plots for 47 and 34 m/s wheel speed ribbons show a ferromagnetic to paramagnetic transition temperature, Tc of the amorphous phase around 400 1C. The Tc of amorphous phase in the ribbon prepared at 17 m/s is 374 1C which may be attributed to the change in the amorphous phase composition due to simultaneous formation of crystalline phases. The increase in magnetization beyond 415 1C is due to the formation of a high Tc CoSi phase as discussed later [15]. Fig. 6 shows the thermomagnetic plots of annealed ribbons prepared at 47 m/s wheel speed. The Tc of amorphous phase was found to be 367 1C for the ribbon annealed at 450 1C. The decrease in Tc is again attributed to the change in amorphous phase composition due to the formation of crystalline Co2Si phase. It can also be seen that a ferro to paramagnetic transition of Co2B phase was observed around 167 1C for the ribbons annealed at 520 and 570 1C. The residual magnetization observed for the ribbons annealed at 520 and 570 1C corresponds to the magnetization of the high temperature CoSi phase. This is in conformity with the observed increase in magnetization (Fig. 5) beyond 415 1C in all the ribbons prepared at different wheel speeds due to the formation of high temperature CoSi phase. The Tc of Co2Si phase was not observed as it is not ferromagnetic above room temperature [15]. Fig. 7 shows the room temperature magnetization curves of asspun ribbons prepared at different wheel speeds. The magnetization curves were found to exhibit two-step-like behaviour in all samples. This is attributed to the formation of stripe domains associated with perpendicular anisotropy [16–19]. The shearing of the hysteresis loop towards the higher magnetic field indicates the increase in perpendicular anisotropy with decreasing wheel speed. This is attributed to the progressive formation of Co2Si orthorhombic phase with decreasing wheel
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Fig. 2. TEM bright field (BF) images and corresponding selected area electron diffraction (SAD) patterns of as-spun ribbons prepared at wheel speeds of (a) 47 m/s, (b) 34 m/ s and (c) 17 m/s.
Fig. 3. DSC thermograms of as-spun ribbons at a heating rate of 20 1C/min.
speed (Fig. 2). Inset in Fig. 7 shows the room temperature magnetization curves for annealed ribbons prepared at the wheel speed of 47 m/s. It is evident from the plot that the perpendicular anisotropy increases with annealing temperature. It is ascribed to
the formation of the crystalline phases (Co2Si, CoSi and Co2B) with annealing temperature (Fig. 4). Fig. 8 shows the hysteresis loops for as-spun ribbons prepared at different wheel speeds. Soft magnetic properties of as-spun and
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Table 1 Crystallization parameters (Tx1, Tx2 and DH) for ribbons spun at different wheel speed. Wheel speed (m/s)
Tx1 (1C)
Tx2 (1C)
DH (J/g)
47 34 17
500 500 500
557 557 556
139 93 73
Fig. 6. Thermomagnetic curves (500 Oe field) of 47 m/s ribbon annealed at 450, 520 and 570 1C.
Fig. 4. XRD patterns of as-spun and annealed ribbons prepared at a wheel speed of 47 m/s.
Fig. 7. Magnetization curves of as-spun ribbons prepared at different wheel speeds. Inset shows the magnetization curves of 47 m/s ribbon annealed at 450, 520 and 570 1C.
(mi) increase with increasing wheel speed. The decrease in coercivity and increase in permeability of higher wheel speed ribbons is attributed to the increase in degree of amorphicity and decrease in perpendicular anisotropy [10–12,16]. 3.3. Magneto impedance studies
Fig. 5. Thermomagnetic curves (500 Oe field) of as-spun ribbons prepared at different wheel speeds.
annealed samples derived from B–H loops are listed in Table 2. The coercivity values are found to decrease with increasing wheel speed while the peak permeability (mp) and initial permeability
Fig. 9 shows the variation of MI with applied magnetic field at various frequencies for the as-spun ribbons prepared at the wheel speed of 47 m/s. A two-peak behaviour observed at all frequencies indicates the presence of transverse anisotropy. The variation of MI values with magnetic field is found to increase with increasing frequency upto 5 MHz, with further increase in frequency the MI value decreases. The initial increase in the MI values is attributed to decrease in skin effect which causes the transverse permeability to increase resulting in a net increase in
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Fig. 8. B–H loops of as-spun ribbons prepared at the wheel speeds of 47, 34 and 17 m/s.
Table 2 Soft magnetic properties of as-spun and annealed ribbons. Sample description
Wheel speed/ annealing temperature
As-spun
47 m/s 34 m/s 17 m/s
47 m/s annealed
450 1C 520 1C 570 1C
Initial permeability (mi)
Peak permeability (mp)
0.07 0.08 0.2
37555 22900 3457
51780 44895 7377
14.35 542 878
776 53 47
1913 75 69
Coercivity, Hc (Oe)
Fig. 10. Magnetoimpedance as a function of magnetic field measured at 5 MHz for as-spun ribbons prepared at the wheel speed of 47, 34 and 17 m/s. Inset shows the magnetoimpedance as a function of magnetic field for ribbon annealed at 4501C at 5 MHz.
It can be observed that the peak at which maximum MI value observed increases with decreasing wheel speed revealing the increase in transverse anisotropy. The highest MI value of about 48% is observed for the ribbon prepared at 47 m/s and it decreases with wheel speed. The observed decrease in MI with wheel speed is attributed to the deterioration of soft magnetic properties as discussed earlier which modifies the MI response through change in skin effect [21]. Inset in Fig. 10 shows the variation of MI with applied magnetic field measured at 5 MHz for the ribbons annealed at 450 1C. It is evident that the MI value decreases drastically to 4% on annealing at 450 1C. Further, MI was not observed for the ribbons annealed at 520 and 570 1C. This is attributed to deterioration of soft magnetic properties in annealed samples due to the formation of Co2Si, Co2B and CoSi phases.
4. Conclusions Melt-spun ribbons of Co69Fe7Si14B10 alloy have been prepared at different wheel speeds of 47, 34 and 17 m/s. Degree of amorphicity increases with wheel speed due to enhanced cooling rate. Amorphous ribbons crystallize to Co2Si, Co2B and CoSi phases on annealing. Ferro to para transition temperature of amorphous phase is around 400 1C for 47 and 34 m/s and it decreases for 17 m/s ribbons possibly due to change in amorphous phase composition on crystalline phase formation. Soft magnetic properties improve with increasing wheel speed which is attributed to increase in degree of amorphicity and decrease in perpendicular anisotropy. Improved soft magnetic properties lead to enhanced GMI properties. Annealing of the ribbons deteriorates the soft magnetic properties and GMI due to formation of crystalline phases. Fig. 9. Magnetoimpedance as a function of frequency for as-spun ribbon prepared at 47 m/s.
Acknowledgements magnetoimpedance. The decrease in MI values at higher frequencies is due to the damping of domain wall motion as a result of eddy currents around domain walls [20]. Fig. 10 shows the variation of MI with applied magnetic field at 5 MHz for the as-spun ribbon prepared at different wheel speeds.
This work was supported by DRDO, Govt. of India. The authors are thankful to Prof. Markandeyulu, IIT Madras for fruitful discussion and Dr. G. Malakondaiah, Director, DMRL for continued support and permission to publish this work.
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