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Stability of amorphous structures of nearly zero magnetostriction Co-rich microwires
Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
Stability of amorphous structures of nearly zero magnetostriction Co-rich microwires G. Brekharyaa, N. Kutsevab, V. Bashevb,*, V. Larinc, A. Torcunovc a
Dnieprodzerzhinsk Technical University, Dnieprostroevskaya, 2-a, Dnieprodzerzhinsk, Ukraine b Dniepropetrovsk National University, Nauchni St., 13, Dniepropetrovsk, Ukraine c AmoTec, B-d Dacia 15, Kishinev, Moldova
Abstract The structure investigation results and magnetic properties of microwires are presented. Initial microwires Co68Fe5Ni1Si12B14 covered with glass insulation were obtained by the Taylor–Ulitovsky technique. It is shown that the crystallization of microwires occurs through the formation of the intermediate phase. The optimal soft magnetic properties are received in initial nanocrystalline state. r 2002 Published by Elsevier Science B.V. Keywords: Short-range order; Activation energy; Cooling rate
Alloys with nearly zero magnetostriction are a subject of considerable industrial interest. They have a slight modification of size in variable magnetic fields and a large magnetic permeability. These properties are of great interest in sensor applications. Usually, these alloys are obtained by the quenching from the melt (splat-quenching) method. Taylor’s technique, which is a variation of this method, is used to obtain metallic wires covered by a glass coating. According to calculations the cooling rate of microwires ranges from 105 to 106 K/s [1]. Therefore, there is a possibility to obtain different structures from crystalline to nanocrystalline/amorphous ones. The aim of this work is to investigate the phase/ structure state of nearly zero magnetostriction Corich microwires and their properties. *Corresponding author. E-mail address: [email protected] (V. Bashev).
Initial microwires Co68Fe5Ni1Si12B14 covered with glass insulation were obtained by the Taylor–Ulitovsky technique. Metallic vein diameter was 8 mm that corresponded to a cooling rate of melt of E106 K/c. The structure and properties of initial and isothermally treated microwires were investigated by X-ray analysis, the measuring of the relative electrical resistance, the differential scanning calorimetry and magnetometric measuring. X-ray data showed that the initial microwire has X-ray amorphous structure [2]. There are some diffusive halos, which are an inherent feature of disordered structures (Fig. 1). Isothermal annealing (5 min) at 3001C, 4001C, and 4501C do not alter the shape of the maxima, but the X-ray scattering intensity increases with the rise of annealing temperatures. This means that the process corresponding to a change in the structure takes place in the sample. At a temperature of 4801C there appear X-ray lines that
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 1 4 - 0
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
equilibrium value. This fact can be explained by the influence of metalloid atoms according to the data of the phase diagram [3]. The increase of the temperature to 6501C leads to the formation of equilibrium phases b-Co, Co2Si and (Co,Si)2B (structure type CuAl2). Analyzing the X-ray data one can suggest that there are relaxations and diffusive processes in the amorphous wire in the range 300–4001C. The atomic rearrangement in this temperature range is responsible of the character for the subsequent crystallization, which occurs in two steps:
o
400 C
220 200 180
o
20 C
160
I abs
140 120 100 80 o
300 C
60 40 20 0 0
20
40
60
80
100
2θ
(3 3 0 )
(1 4 0 ) (2 2 0 )
(2 3 0 )
(1 1 2 )
(2 1 1 ) (0 2 2 )
(2 1 0 )
(1 1 2 ) (1 2 0 )
Fig. 1. Mo Ka X-ray intensity of microwire at different temperatures.
(Co, Si)3 B
530°C
X-Ray Intensity
(1 1 1 ) (2 0 0 )
β -C o
480°C
40
50
60
2θ
70
(1 0 3 )
(1 1 0 )
(1 0 1 )
(1 0 0 ) (0 0 2 )
450°C
α -C 0
105
80
90
Fig. 2. Cu Ka X-ray diffraction patterns for microwires at different temperatures.
correspond to interplanar distances of hexagonal a-Co. In the temperature range from 4801C to 5301C, the structure of the microwire is a mixture of crystallites of a-,b-Co and metastable phase (Co,Si)3B (structure type Fe3C), Fig. 2. It should be noticed that the period of the lattice of b-Co solid solution is a ¼ 0:354 nm and it is less than the
(a) the formation a-,b-Co and metastable phase (Co,Si)3B; (b) the decomposition of (Co,Si)3B to the equilibrium phases Co2Si and (Co,Si)2B at T > 6501C. The plots of electrical resistivity versus temperature and the curve of enthalpy versus temperature, detected by means of a DSC-7 to validate these suppositions, were constructed. The heating rate in both methods was the same V ¼ 20 K/min. The temperature resistance coefficient (TRC) increases slightly than the lower temperatures, but there is a sharp decrease of the resistance at 440–4901C that corresponds to the X-ray data. It was mentioned above that the a-,b-Co and (Co,Si)3B phases are formed within this temperature range. The second decrease of the resistance occurs between 5801C and 6501C. As X-ray analysis showed it is associated with the formation of the equilibrium phases Co2Si and (Co,Si)2B. The results of the DSC curve correlates with the results of electrical resistance. It is seen (Fig. 3) that there is the heat evolution at Tp3601C. The next maximum is in the temperature range from 4601C to 5001C. Then the weak maximum appears at 5801C and the sharp one can be seen in the range 600–6201C. Maxima of the DSC curve at temperatures 460–5001C, 5801C and 600–6201C are consistent with the results of electrical resistance. Comparing these results with that of X-ray, which were made at different annealing temperatures, it can be considered that a-,b-Co and (Co,Si)3B are formed from amorphous phase in the range 460–5001C. The largest portion of the heat of phase transition evolves at this temperature
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
106
20°C 0.8
10000
0.6 480°C
0.4 0.2 B, T
Heat flow, a.u.
8000
6000
0.0 -0.2
4000
-0.4
320°C
-0.6
2000
-0.8 -40
0
100
200
300
400
500
600
700
-20
0 20 H, kA / m
40
Fig. 4. Hysteresis loop of microwire.
T,°C
Fig. 3. Curve of enthalpy of microwire.
(nonvariant transition corresponding to the formation of the eutectic mixture on the metastable phase diagram a-,b-Co+(Co,Si)3B). The maxima at 5801C and 600–6201C correspond to the crystallization of the equilibrium phases Co2B and (Co,Si)2B. As noted above, the shape of curves does not alter, but the intensity of these curves increases at low annealing temperatures (up to 4001C), Fig. 1. The heat evolution at To3601C and the increase of the scattering intensity suggest that the irreversible processes take place in the amorphous state. The changing of the local shortrange order of the initial matrix induces them. The alloy Co68Fe5Ni1Si12B14 may be considered as (Co,Ni,Fe)74(B,Si)26 because Fe and Ni atoms do not have the limit of solubility in the Co solution [4]. It is well known [5] that at the temperature above the eutectic point the clusters exist and their short-range order is similar to the one of the equilibrium phases, composing the eutectic. In our case, pure Co and Co2B or (Co,Si)2B are these phases. While microwires are being obtained, the competition between a-Co and Co2B or (Co,Si)2B leads to amorphous solidification. It means that
the chemical short-range order of these clusters conforms to the equilibrium phases existing in this system. On the other hand, the metastable equilibrium between a-,b-Co and (Co,Si)3B is possible in this system too. Therefore, there is an atomic rearrangement in the temperature range from 201C to 3601C, which leads to the modification of the short-range order of the initial amorphous phase into the short-range order of the amorphous phase that is responsible for the subsequent crystallization. Besides, the calculations of the activation energy of Co, (Co,Si)3B and Co2B or (Co,Si)2B made from the DSC curve showed [6] that the activation energy of Co and (Co,Si)3B were D0.3 eV, while the activation energy of Co2B or (Co,Si)2B was 1.02 eV, that is three times higher. Hence, it is energetically useful for the system to crystallize by the formation of the intermediate phase. Magnetic properties have been measured by vibrating sample magnetometer in the initial state at 3201C and at 4801C, Fig. 4. From these curves it may be obvious that the optimal magnetic properties are achieved for the initial state. Remanence and coercivity decrease slightly at 3201C, but the last increases greatly and
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
the squareness ratio decreases up to 0.2 at 4801C. This fact is connected with the formation of Co crystal into amorphous matrix. It can be concluded that the crystallization of microwires includes the following steps: (a) the formation of a-,b-Co and metastable (Co,Si)3B; (b) the decomposition of (Co,Si)3B to the equilibrium phases Co2Si and (Co,Si)2B at T > 6501C. The optimal magnetic properties are observed for the initial state. The formation of mixture a-,b-Co and (Co,Si)3B leads to degradation of magnetic properties at temperature 4801C.
107
References [1] V.F. Bashev, Analiz teplovich rezhimov ochlazhdeniya pri zakalke iz Rasplava, Visnik Dnipropetrovskogo universitetu, Dnipropetrovsk. Ukraina, Fizika, 4 (1998) 3. [2] R.W. James, The Optical Principles of the Diffraction of X-rays, London, 1950. [3] W. Person, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1958, 1039pp. [4] M. Xansen, K. Anderko, Constitution of Binary Alloys, London, 1958. [5] V.I. Danilov, Ctroenie i kristallizatsiya zhidkosti, Kiev, AN USSR, 1956, 568c. [6] H.E. Kissinger, J. Res. Natl. Bur. Stand. 57 (N4) (1956) 217.
Stability of amorphous structures of nearly zero magnetostriction Co-rich microwires G. Brekharyaa, N. Kutsevab, V. Bashevb,*, V. Larinc, A. Torcunovc a
Dnieprodzerzhinsk Technical University, Dnieprostroevskaya, 2-a, Dnieprodzerzhinsk, Ukraine b Dniepropetrovsk National University, Nauchni St., 13, Dniepropetrovsk, Ukraine c AmoTec, B-d Dacia 15, Kishinev, Moldova
Abstract The structure investigation results and magnetic properties of microwires are presented. Initial microwires Co68Fe5Ni1Si12B14 covered with glass insulation were obtained by the Taylor–Ulitovsky technique. It is shown that the crystallization of microwires occurs through the formation of the intermediate phase. The optimal soft magnetic properties are received in initial nanocrystalline state. r 2002 Published by Elsevier Science B.V. Keywords: Short-range order; Activation energy; Cooling rate
Alloys with nearly zero magnetostriction are a subject of considerable industrial interest. They have a slight modification of size in variable magnetic fields and a large magnetic permeability. These properties are of great interest in sensor applications. Usually, these alloys are obtained by the quenching from the melt (splat-quenching) method. Taylor’s technique, which is a variation of this method, is used to obtain metallic wires covered by a glass coating. According to calculations the cooling rate of microwires ranges from 105 to 106 K/s [1]. Therefore, there is a possibility to obtain different structures from crystalline to nanocrystalline/amorphous ones. The aim of this work is to investigate the phase/ structure state of nearly zero magnetostriction Corich microwires and their properties. *Corresponding author. E-mail address: [email protected] (V. Bashev).
Initial microwires Co68Fe5Ni1Si12B14 covered with glass insulation were obtained by the Taylor–Ulitovsky technique. Metallic vein diameter was 8 mm that corresponded to a cooling rate of melt of E106 K/c. The structure and properties of initial and isothermally treated microwires were investigated by X-ray analysis, the measuring of the relative electrical resistance, the differential scanning calorimetry and magnetometric measuring. X-ray data showed that the initial microwire has X-ray amorphous structure [2]. There are some diffusive halos, which are an inherent feature of disordered structures (Fig. 1). Isothermal annealing (5 min) at 3001C, 4001C, and 4501C do not alter the shape of the maxima, but the X-ray scattering intensity increases with the rise of annealing temperatures. This means that the process corresponding to a change in the structure takes place in the sample. At a temperature of 4801C there appear X-ray lines that
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 1 4 - 0
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
equilibrium value. This fact can be explained by the influence of metalloid atoms according to the data of the phase diagram [3]. The increase of the temperature to 6501C leads to the formation of equilibrium phases b-Co, Co2Si and (Co,Si)2B (structure type CuAl2). Analyzing the X-ray data one can suggest that there are relaxations and diffusive processes in the amorphous wire in the range 300–4001C. The atomic rearrangement in this temperature range is responsible of the character for the subsequent crystallization, which occurs in two steps:
o
400 C
220 200 180
o
20 C
160
I abs
140 120 100 80 o
300 C
60 40 20 0 0
20
40
60
80
100
2θ
(3 3 0 )
(1 4 0 ) (2 2 0 )
(2 3 0 )
(1 1 2 )
(2 1 1 ) (0 2 2 )
(2 1 0 )
(1 1 2 ) (1 2 0 )
Fig. 1. Mo Ka X-ray intensity of microwire at different temperatures.
(Co, Si)3 B
530°C
X-Ray Intensity
(1 1 1 ) (2 0 0 )
β -C o
480°C
40
50
60
2θ
70
(1 0 3 )
(1 1 0 )
(1 0 1 )
(1 0 0 ) (0 0 2 )
450°C
α -C 0
105
80
90
Fig. 2. Cu Ka X-ray diffraction patterns for microwires at different temperatures.
correspond to interplanar distances of hexagonal a-Co. In the temperature range from 4801C to 5301C, the structure of the microwire is a mixture of crystallites of a-,b-Co and metastable phase (Co,Si)3B (structure type Fe3C), Fig. 2. It should be noticed that the period of the lattice of b-Co solid solution is a ¼ 0:354 nm and it is less than the
(a) the formation a-,b-Co and metastable phase (Co,Si)3B; (b) the decomposition of (Co,Si)3B to the equilibrium phases Co2Si and (Co,Si)2B at T > 6501C. The plots of electrical resistivity versus temperature and the curve of enthalpy versus temperature, detected by means of a DSC-7 to validate these suppositions, were constructed. The heating rate in both methods was the same V ¼ 20 K/min. The temperature resistance coefficient (TRC) increases slightly than the lower temperatures, but there is a sharp decrease of the resistance at 440–4901C that corresponds to the X-ray data. It was mentioned above that the a-,b-Co and (Co,Si)3B phases are formed within this temperature range. The second decrease of the resistance occurs between 5801C and 6501C. As X-ray analysis showed it is associated with the formation of the equilibrium phases Co2Si and (Co,Si)2B. The results of the DSC curve correlates with the results of electrical resistance. It is seen (Fig. 3) that there is the heat evolution at Tp3601C. The next maximum is in the temperature range from 4601C to 5001C. Then the weak maximum appears at 5801C and the sharp one can be seen in the range 600–6201C. Maxima of the DSC curve at temperatures 460–5001C, 5801C and 600–6201C are consistent with the results of electrical resistance. Comparing these results with that of X-ray, which were made at different annealing temperatures, it can be considered that a-,b-Co and (Co,Si)3B are formed from amorphous phase in the range 460–5001C. The largest portion of the heat of phase transition evolves at this temperature
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
106
20°C 0.8
10000
0.6 480°C
0.4 0.2 B, T
Heat flow, a.u.
8000
6000
0.0 -0.2
4000
-0.4
320°C
-0.6
2000
-0.8 -40
0
100
200
300
400
500
600
700
-20
0 20 H, kA / m
40
Fig. 4. Hysteresis loop of microwire.
T,°C
Fig. 3. Curve of enthalpy of microwire.
(nonvariant transition corresponding to the formation of the eutectic mixture on the metastable phase diagram a-,b-Co+(Co,Si)3B). The maxima at 5801C and 600–6201C correspond to the crystallization of the equilibrium phases Co2B and (Co,Si)2B. As noted above, the shape of curves does not alter, but the intensity of these curves increases at low annealing temperatures (up to 4001C), Fig. 1. The heat evolution at To3601C and the increase of the scattering intensity suggest that the irreversible processes take place in the amorphous state. The changing of the local shortrange order of the initial matrix induces them. The alloy Co68Fe5Ni1Si12B14 may be considered as (Co,Ni,Fe)74(B,Si)26 because Fe and Ni atoms do not have the limit of solubility in the Co solution [4]. It is well known [5] that at the temperature above the eutectic point the clusters exist and their short-range order is similar to the one of the equilibrium phases, composing the eutectic. In our case, pure Co and Co2B or (Co,Si)2B are these phases. While microwires are being obtained, the competition between a-Co and Co2B or (Co,Si)2B leads to amorphous solidification. It means that
the chemical short-range order of these clusters conforms to the equilibrium phases existing in this system. On the other hand, the metastable equilibrium between a-,b-Co and (Co,Si)3B is possible in this system too. Therefore, there is an atomic rearrangement in the temperature range from 201C to 3601C, which leads to the modification of the short-range order of the initial amorphous phase into the short-range order of the amorphous phase that is responsible for the subsequent crystallization. Besides, the calculations of the activation energy of Co, (Co,Si)3B and Co2B or (Co,Si)2B made from the DSC curve showed [6] that the activation energy of Co and (Co,Si)3B were D0.3 eV, while the activation energy of Co2B or (Co,Si)2B was 1.02 eV, that is three times higher. Hence, it is energetically useful for the system to crystallize by the formation of the intermediate phase. Magnetic properties have been measured by vibrating sample magnetometer in the initial state at 3201C and at 4801C, Fig. 4. From these curves it may be obvious that the optimal magnetic properties are achieved for the initial state. Remanence and coercivity decrease slightly at 3201C, but the last increases greatly and
G. Brekharya et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 104–107
the squareness ratio decreases up to 0.2 at 4801C. This fact is connected with the formation of Co crystal into amorphous matrix. It can be concluded that the crystallization of microwires includes the following steps: (a) the formation of a-,b-Co and metastable (Co,Si)3B; (b) the decomposition of (Co,Si)3B to the equilibrium phases Co2Si and (Co,Si)2B at T > 6501C. The optimal magnetic properties are observed for the initial state. The formation of mixture a-,b-Co and (Co,Si)3B leads to degradation of magnetic properties at temperature 4801C.
107
References [1] V.F. Bashev, Analiz teplovich rezhimov ochlazhdeniya pri zakalke iz Rasplava, Visnik Dnipropetrovskogo universitetu, Dnipropetrovsk. Ukraina, Fizika, 4 (1998) 3. [2] R.W. James, The Optical Principles of the Diffraction of X-rays, London, 1950. [3] W. Person, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1958, 1039pp. [4] M. Xansen, K. Anderko, Constitution of Binary Alloys, London, 1958. [5] V.I. Danilov, Ctroenie i kristallizatsiya zhidkosti, Kiev, AN USSR, 1956, 568c. [6] H.E. Kissinger, J. Res. Natl. Bur. Stand. 57 (N4) (1956) 217.