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Magnetization of amorphous and crystalline CoSiB alloys
Materials Science and Engineering, 99 (1988) 77-80
77
Magnetization of Amorphous and Crystalline Co-Si-B Alloys* T. KULIK and H. MATYJA Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw (Poland)
B. L1SOWSKI Institute of Physics, Polish Academy of Sciences, AI. Lomik6w 32/46, 02-669 Warsaw (Poland)
Abstract Studies were made on the influence o f composition on the Curie temperature and magnetization o f C o - S i B alloys in three states: (i) as-quenched (fully amorphous), (ii) after the first stage of crystallization and (iiO after complete crystallization. Two new ferromagnetic phases were obtained and characterized. It was found that increase in metalloid content in COlo0 x(Sio sBo.5)~ glasses. (x = 22-32at. %) causes a decrease in the Curie temperature T c and in the effective magnetic moment # per cobalt atom. Glasses of intermediate metalloid content (28 <~x <<,30at. %) crystallized polymorphously forming the metastable C017n(Sio.sBo.5)Tn ferromagnetic phase with Tc "~ 500 K and magnetization at OK of M(O) = 6.33 x 10 -8 Wb m g ~ (for its amorphous counterpart T,.= 4 8 6 K and M(O) = 7.89× 10 -8 Wb m g - l ) . For the second new phase Co2Si(B), appearing during the first stage o f crystallization o f metalloid-rich glasses (x > 30at. %), the following values were obtained: To= 1 0 7 K a n d M(O)= l . O x lO 8 W b m g - i . It was found that for glasses containing up to 30 at. % metalloid the amorphous state is more favourable as far as the room-temperature magnetization is concerned.
1. Introduction Magnetization of Co-B and Co-Si-B glasses has already been reported in many works [I-8], but no systematic study has been made on the influence of crystallization on the magnetization of these alloys. So far, there is only one report [5] concerning the magnetic properties of Co-Si-B alloy in the amorphous state and the single-phase crystalline state of the same composition. In previous studies [9, I0] it was shown that Co-SiB glasses can be divided into three groups depending
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3-7, 1987. 0025-5416/88/$3.50
on the course of their crystallization. Glasses containing 29-30 at.% metalloid crystallize polymorphously, and during the first stage of crystallization the amorphous phase transforms entirely into a single metastable phase of the same composition COiTn(Sio.sBo.5)7n. However, for glasses rich in metal the primary crystallization is of the Co(Si) phase and for those rich in metalloid the C02Si(B) phase is observed. It is of interest that both COjTn(Sio.sBo.5)7. and C02Si(B) are ferromagnetic phases and, so far, there is no report of their magnetic properties. The purpose of the present work is to determine the influence of alloy composition on the magnetization of Co-Si-B alloys in three states: (I) as-quenched (fully amorphous), (2) after the first stage of crystallization and (3) after complete crystallization. Moreover, the new ferromagnetic phases obtained were characterized. 2. Experimental details A m o r p h o u s COloo_x(Sio.sBo.5) ~ ribbons (x---2232 at.%) were prepared by rapid quenching, using a single roller. The ribbons obtained, typically 0.03 m m × 5 mm in cross-section, did not reveal longrange crystalline order, as proved by X-ray diffractometry and transmission electron microscopy (TEM). The temperature dependence of the saturation magnetization Ms(T) was measured with an applied magnetic field of 6 × 105 A m-1(7.4 kOe), using a magnetic balance with a resolution of about 10 13 Wb m (10 -4 e.m.u.) in the temperature range from 75 K to 1100 K, and using a vibrating sample magnetometer in the temperature range from 4.2 K to room temperature. The samples weights were about 20 mg. The magnetization Ms(0) at 0 K was determined by extrapolation on the T 3/2 scale from the values obtained at temperatures above 4.2 K or above 75 K. The Curie temperature was determined by the point of intersection of the steepest tangent to the Ms(T) curve with the T axis.
© Elsevier Sequoia/Printed in The Netherlands
78 The controlled crystallization of glasses was performed using a Perkin-Elmer DSC-2 calorimeter. The samples were continuously heated (]" = 20 K m i n - ~) to the temperature corresponding to the end of the first crystallization stage (the first exothermal peak on the thermogram) and subsequently cooled to room temperature at the maximum obtainable cooling rate (T ~< 320 K min-~).
CI ) 8 : , . .
O
3. Resultsanddiscussion 0
3.1. Alloys in the amorphous state The temperature dependence of saturation magnetization M+(T) of some selected COloo_x(Sio.sBos)x glasses is shown in Figs. 1-3. Curves (a) on these figures represent alloys in the amorphous state. At low temperatures the magnetization of glasses can be described by the Bloch T 3/2 law what can be seen in Fig. 4.
M(T)=M(O)
(
1 - B 3 / 2 (zyq,] ~Tj
(1)
The influence of the metalloid content on saturation magnetization M(0) at 0 K and at room temperature M,(300), the Curie temperature T¢ and the effective magnetic moment per cobalt atom g in the glasses studied are presented in Fig. 5 and Table 1. All these quantities decrease with the increase of metalloid content. The decrease in/~ can be explained on the basis of a rigid band model, i.e. by electron transfer from the metalloid atoms to the cobalt 3d band [1]. The average rate of decrease in # is about 0.053/~B a t . % per atomic per cent of metalloid. This value is a little
l
l
L
2oo
l
400
L.
.
.
+oo
.
.
I
8oo
L
T[K]
~iI
Fig. 2. Temperature dependence of saturation magnetization Ms for Co~o.5Sil4.~5B14.75alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K min- ~).
}
b'-N,
5 -+ ~
' ~ .-. -.
•
...
\\ I 1
"-,, ',
\
21
o
T: +K/m,n
',,
X~ \I
,
t+ +oo
+<,,m,+
_~
"I 4oo
'"
600
+ T [ K+ ]
'~goo
Fig. 3. Temperature dependence of saturation magnetization Ms for C068Si16B16alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K rain-~).
I
2 '
'
M(T)
3 T[IO2K
'
.9
'+I+++++,.
4
.,
+
"-,,,
?",,.0 T. . . .
~
o
°o~
2 o
+x:,,.s o
T[K
Fig. 1. Temperature dependence of saturation magnetization Ms for Co76Si12B12alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K rain-~).
i
:,
,+
+
T+,[IO+K+]:,]
Fig. 4. Reduced magnetization M(T)/M(O) plotted against T3/2 for C%oo_ x(Sio.sBo.5)xglasses.
79 TABLE 1 Values of Curie temperature T¢, saturation magnetization (M,(0) and M,(300)), magnetic moment per cobalt atom p, Bloch's law coefficient B3/z and maximum temperature of Bloch's law applicability T~,,~ for Con0o_~(Sio.sBo.5)~ alloys in amorphous and crystalline state x
Phase
(at.%)
Tc
Ms(O)
M~(300)
( × 1 0 - S W b m g -1)
( x 1 0 - S W b m g -l)
/~ ~a)
B3/2
(K)
T,.~ (K)
24 27 29.5 32
Amorphous Amorphous Amorphous Amorphous
740 585 486 355
11.175 9.591 7.894 6.071
10.245 8.296 5.933 2.351
1.037 0.911 0.755 0.589
0.26 0.35 0.38 0.46
0.38 0.53 0.39 0.54
29.5 32
Co|Tn(Si,B)7n
500 107
6.329 ~ 1.0
4.752
0.606
0.27 1.3
0.26
Co2B(Si)
bigger than that found for Co-B alloys (0.047) [2], and is probably caused by the larger charge transfer from silicon than from boron [6]. The rate of Tc decrease is about 5 0 K ( a t . % metalloid) l, and is slightly lower than for Co-B glasses for which this value is about 56 K (at.% B) -1 [3]. However the influence of silicon content on Tc value is negative and the T~ values of Co-Si-B glasses are about 100 K lower than those for Co-B glasses with the same metalloid content.
3.2. Alloys after the first stage of crystallization The curves (b) in Figs. 1-3 show the temperature dependence of saturation magnetization Ms(T) after the first stage of crystallization for glasses representing three groups characterized by different course of crystallization. After the first stage of crystallization the metal-rich glasses (x < 29 at.% metalloid) are composed of primary crystals of Co(Si) and amorphous matrix (Am 1) enriched in metalloid. The curve (b) in Fig. ! apparently reflects superposition of magnetization of these two phases. Depletion of the amorphous matrix in cobalt resulted in substantial decrease in the Curie temperature of the amorphous phase and a decrease in the overall magnetization of the alloy in this state. The glasses of intermediate metalloid content (x = 29-30 at.%) crystallize polymorphously and are fully crystalline after the first stage of crystallization, being composed of a single Col7n(Sio.sBo.5)7n phase. The temperature dependence of the magnetization of this phase is presented as curve (b) in Fig. 2. The values of the Curie temperature of this alloy in the crystalline and amorphous states are not much different from each other (see Table 1). Applicability of the Bloch law for the description of the magnetization M(T) of the Cot7n(Si0 5Bo.5)7,, phase is limited to temperatures up to Tmax~0.26T¢ (about 130 K). This temperature range is appreciably lower than that for the same alloy in the amorphous state, Tm~x~ 0.31T¢ (see Table 1). These observations of the amorphous
~(o)
b
•
~]1) t
i
I
I'1
II
\\° 5
0" 2'2 ' 2? '26 ' 2'8' 3'0' ~2 X [ a t . % ]
Fig. 5. Dependences of saturation magnetization (at 0 K and room temperature, M(0) and M(300) (O) respectively),Curie temperature T¢ and effectivemagnetic moment per cobalt atom # on the composition of metallic glasses Coloo_x(Sio.sBos)x. (Dependence of saturation magnetization in room temperature Mcr(300) (@) on the composition of the alloys after complete crystallization is included.)
and crystalline phase of the C07o.sSi14.7sBla.75 alloy studied are consistent with those reported for C075Si15B1o [5]. The only difference between the results obtained for these two alloys concerns the value of magnetic moment/~ per cobalt atom (and, correspondingly, the magnetization M). The value of/~ (and M(0)) in the present work is 20% lower for the alloy in crystalline state than for the amorphous alloy. For C075Si]sBi0 almost the same value of p (and M(0)) was found in the amorphous and the crystalline states [5]. The difference between these two alloys can be explained as being due to different final phases of glass crystallization. After the first stage of crystallization the metalloidrich glasses (x > 30 at.%) are composed of primary crystals of the C02Si(B) phase and the amorphous matrix (Am 1) enriched in cobalt and, perhaps, in
80
boron. Curve (b) in Fig. 3 shows summary magnetization of these two phases in the Co6sSil6B16 alloy. The enrichment of the amorphous matrix in cobalt resulted in a large increase in the Curie temperature and an increase in the magnetization at temperatures above 150K. The silicon-rich Co2Si(B) phase appeared to be a low-temperature ferromagnetic phase with Tc ~ 107 K and M(0) ~ 0.289 x 10 -s Wb m g-l (assuming mass fraction of the phase in the alloy to be equal to 30%).
3.3. Alloys after complete crystallization from glassy state The alloys studied after heating to temperatures over I000 K were completely crystalline and composed of three stable phases: Co(Si), Co2B and Co2Si. The last phase is paramagnetic. The curves (c) in Figs. 1-3 show the temperature dependence of saturation magnetization Ms(T) after complete crystallization of the Co-Si-B alloys for three groups characterized by a different course of crystallization. These curves represent summary magnetization of two ferromagnetic phases (Co(Si) and Co2B) and are qualitatively identical. Figure 5 shows, among other parameters, the room temperature saturation magnetization of the alloys studied in the amorphous M(300) state and the crystalline state Mcr(300). It is seen that only for the metalloid-rich glasses (group III) do the values of magnetization in the crystalline state exceed those in the amorphous state (Mcr(300)> M(300)). For the remaining glasses the amorphous state is more favourable as far as the room temperature magnetization is concerned. 4. Summary and conclusions (i) The increase in metalloid content in Co~oo-x(SiosBo.5)x glasses causes a decrease in the Curie temperature Tc and in the effective magnetic moment per cobalt atom/~ (and, correspondingly, in the magnetization M). (ii) Replacement of half of the boron content (in Co-B glasses) with silicon results in (a) decrease in Tc (about 100 K), (b) lower (about 10%) rate of decrease in T~ with metalloid content x (dTJdx ,~ 50 K
(at.% metalloid)-1), (c) higher (about 12%) rate of decrease in /~ with metalloid content (d/z/dx 0.053/~B (at. % metalloid) - 1). (iii) Amorphous Co7o.5Sil4.75B14.75 alloy exhibits comparable values of To, and a higher (about 20%) value of/~ (and M(0)) than its single-phase crystalline counterpart. (iv) The new metastable ferromagnetic phases Col7,(Si0.sBo.5)7n and Co2B(Si) are characterized by the following values of the Curie temperature T c ~ 5 0 0 K and 107K respectively, and magnetization M(0) =6.33 x 1 0 - S W b m g -I and 1.0 x I0 8 Wb m g-~ respectively. (v) For Co-Si-B alloys containing up to 30 at.% metalloid the amorphous state is more favourable as far as the magnetization in the range to 400 K is concerned.
Acknowledgments The work was supported by grants from the Ministry of Science, Academic Education and Technology under contract MR.I.21 and from the Government under contract PR.3.19.
References I R. C. O'Handley, R. Hasegawa, R. Ray and C.-P. Chou, Appl. Phys. Lett., 29 (1976) 330. 2 H. Watanabe, H. Morita and H. Yamauchi, IEEE Trans. Magn., 14 (1978) 944. 3 M. Takahashi, C. O. Kim, M. Koshimura and T. Suzuki, Jpn. J. Appl. Phys., 17(1978) 1911. 4 R. Hasegawa and R. Ray, J. Appl. Phys., 50(1979) 1586. 5 H. Watanabe, T. Masumoto, M. Kameda, N. Kazama and H. Yamauchi, Physica B, 86-88 (1977) 801. 6 J. van der Borst, F. J. A. den Broeder and T. Scheffers, J. Appl. Phys., 48 (1977) 1724. 7 L. T. Baczewski and M. Maszkiewicz, in C. Hargitai, I. Bakonyi and T. Kemeny (eds.), Proc. Conf. on Metallic Glasses, Vol. 2, Central Research Institute for Physics, Budapest, 1980, p. 3. 8 M. Maszkiewicz, J. Appl, Phys., 53 (1982) 7765. 9 T. Kulik and H. Matyja, in C. Hargitai, I. Bakonyi, and T. Kemeny (eds.), Proc. Conf. on Metallic Glasses, Vol. 2, Central Research Institute for Physics, Budapest, 1980, p. 267. 10 T. Kulik and H. Matyja, in T. Masumoto and K. Suzuki (eds.), Proc. Int. Conf. on Rapidly Quenched Metals, Vol. 1, The Japan Institute of Metals, Sendai, 1982, p. 659.
77
Magnetization of Amorphous and Crystalline Co-Si-B Alloys* T. KULIK and H. MATYJA Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw (Poland)
B. L1SOWSKI Institute of Physics, Polish Academy of Sciences, AI. Lomik6w 32/46, 02-669 Warsaw (Poland)
Abstract Studies were made on the influence o f composition on the Curie temperature and magnetization o f C o - S i B alloys in three states: (i) as-quenched (fully amorphous), (ii) after the first stage of crystallization and (iiO after complete crystallization. Two new ferromagnetic phases were obtained and characterized. It was found that increase in metalloid content in COlo0 x(Sio sBo.5)~ glasses. (x = 22-32at. %) causes a decrease in the Curie temperature T c and in the effective magnetic moment # per cobalt atom. Glasses of intermediate metalloid content (28 <~x <<,30at. %) crystallized polymorphously forming the metastable C017n(Sio.sBo.5)Tn ferromagnetic phase with Tc "~ 500 K and magnetization at OK of M(O) = 6.33 x 10 -8 Wb m g ~ (for its amorphous counterpart T,.= 4 8 6 K and M(O) = 7.89× 10 -8 Wb m g - l ) . For the second new phase Co2Si(B), appearing during the first stage o f crystallization o f metalloid-rich glasses (x > 30at. %), the following values were obtained: To= 1 0 7 K a n d M(O)= l . O x lO 8 W b m g - i . It was found that for glasses containing up to 30 at. % metalloid the amorphous state is more favourable as far as the room-temperature magnetization is concerned.
1. Introduction Magnetization of Co-B and Co-Si-B glasses has already been reported in many works [I-8], but no systematic study has been made on the influence of crystallization on the magnetization of these alloys. So far, there is only one report [5] concerning the magnetic properties of Co-Si-B alloy in the amorphous state and the single-phase crystalline state of the same composition. In previous studies [9, I0] it was shown that Co-SiB glasses can be divided into three groups depending
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3-7, 1987. 0025-5416/88/$3.50
on the course of their crystallization. Glasses containing 29-30 at.% metalloid crystallize polymorphously, and during the first stage of crystallization the amorphous phase transforms entirely into a single metastable phase of the same composition COiTn(Sio.sBo.5)7n. However, for glasses rich in metal the primary crystallization is of the Co(Si) phase and for those rich in metalloid the C02Si(B) phase is observed. It is of interest that both COjTn(Sio.sBo.5)7. and C02Si(B) are ferromagnetic phases and, so far, there is no report of their magnetic properties. The purpose of the present work is to determine the influence of alloy composition on the magnetization of Co-Si-B alloys in three states: (I) as-quenched (fully amorphous), (2) after the first stage of crystallization and (3) after complete crystallization. Moreover, the new ferromagnetic phases obtained were characterized. 2. Experimental details A m o r p h o u s COloo_x(Sio.sBo.5) ~ ribbons (x---2232 at.%) were prepared by rapid quenching, using a single roller. The ribbons obtained, typically 0.03 m m × 5 mm in cross-section, did not reveal longrange crystalline order, as proved by X-ray diffractometry and transmission electron microscopy (TEM). The temperature dependence of the saturation magnetization Ms(T) was measured with an applied magnetic field of 6 × 105 A m-1(7.4 kOe), using a magnetic balance with a resolution of about 10 13 Wb m (10 -4 e.m.u.) in the temperature range from 75 K to 1100 K, and using a vibrating sample magnetometer in the temperature range from 4.2 K to room temperature. The samples weights were about 20 mg. The magnetization Ms(0) at 0 K was determined by extrapolation on the T 3/2 scale from the values obtained at temperatures above 4.2 K or above 75 K. The Curie temperature was determined by the point of intersection of the steepest tangent to the Ms(T) curve with the T axis.
© Elsevier Sequoia/Printed in The Netherlands
78 The controlled crystallization of glasses was performed using a Perkin-Elmer DSC-2 calorimeter. The samples were continuously heated (]" = 20 K m i n - ~) to the temperature corresponding to the end of the first crystallization stage (the first exothermal peak on the thermogram) and subsequently cooled to room temperature at the maximum obtainable cooling rate (T ~< 320 K min-~).
CI ) 8 : , . .
O
3. Resultsanddiscussion 0
3.1. Alloys in the amorphous state The temperature dependence of saturation magnetization M+(T) of some selected COloo_x(Sio.sBos)x glasses is shown in Figs. 1-3. Curves (a) on these figures represent alloys in the amorphous state. At low temperatures the magnetization of glasses can be described by the Bloch T 3/2 law what can be seen in Fig. 4.
M(T)=M(O)
(
1 - B 3 / 2 (zyq,] ~Tj
(1)
The influence of the metalloid content on saturation magnetization M(0) at 0 K and at room temperature M,(300), the Curie temperature T¢ and the effective magnetic moment per cobalt atom g in the glasses studied are presented in Fig. 5 and Table 1. All these quantities decrease with the increase of metalloid content. The decrease in/~ can be explained on the basis of a rigid band model, i.e. by electron transfer from the metalloid atoms to the cobalt 3d band [1]. The average rate of decrease in # is about 0.053/~B a t . % per atomic per cent of metalloid. This value is a little
l
l
L
2oo
l
400
L.
.
.
+oo
.
.
I
8oo
L
T[K]
~iI
Fig. 2. Temperature dependence of saturation magnetization Ms for Co~o.5Sil4.~5B14.75alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K min- ~).
}
b'-N,
5 -+ ~
' ~ .-. -.
•
...
\\ I 1
"-,, ',
\
21
o
T: +K/m,n
',,
X~ \I
,
t+ +oo
+<,,m,+
_~
"I 4oo
'"
600
+ T [ K+ ]
'~goo
Fig. 3. Temperature dependence of saturation magnetization Ms for C068Si16B16alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K rain-~).
I
2 '
'
M(T)
3 T[IO2K
'
.9
'+I+++++,.
4
.,
+
"-,,,
?",,.0 T. . . .
~
o
°o~
2 o
+x:,,.s o
T[K
Fig. 1. Temperature dependence of saturation magnetization Ms for Co76Si12B12alloy in (a) "as-quenched" (fully amorphous) state, (b) after the first stage of crystallization and (c) after complete crystallization (heating rate, 5 K rain-~).
i
:,
,+
+
T+,[IO+K+]:,]
Fig. 4. Reduced magnetization M(T)/M(O) plotted against T3/2 for C%oo_ x(Sio.sBo.5)xglasses.
79 TABLE 1 Values of Curie temperature T¢, saturation magnetization (M,(0) and M,(300)), magnetic moment per cobalt atom p, Bloch's law coefficient B3/z and maximum temperature of Bloch's law applicability T~,,~ for Con0o_~(Sio.sBo.5)~ alloys in amorphous and crystalline state x
Phase
(at.%)
Tc
Ms(O)
M~(300)
( × 1 0 - S W b m g -1)
( x 1 0 - S W b m g -l)
/~ ~a)
B3/2
(K)
T,.~ (K)
24 27 29.5 32
Amorphous Amorphous Amorphous Amorphous
740 585 486 355
11.175 9.591 7.894 6.071
10.245 8.296 5.933 2.351
1.037 0.911 0.755 0.589
0.26 0.35 0.38 0.46
0.38 0.53 0.39 0.54
29.5 32
Co|Tn(Si,B)7n
500 107
6.329 ~ 1.0
4.752
0.606
0.27 1.3
0.26
Co2B(Si)
bigger than that found for Co-B alloys (0.047) [2], and is probably caused by the larger charge transfer from silicon than from boron [6]. The rate of Tc decrease is about 5 0 K ( a t . % metalloid) l, and is slightly lower than for Co-B glasses for which this value is about 56 K (at.% B) -1 [3]. However the influence of silicon content on Tc value is negative and the T~ values of Co-Si-B glasses are about 100 K lower than those for Co-B glasses with the same metalloid content.
3.2. Alloys after the first stage of crystallization The curves (b) in Figs. 1-3 show the temperature dependence of saturation magnetization Ms(T) after the first stage of crystallization for glasses representing three groups characterized by different course of crystallization. After the first stage of crystallization the metal-rich glasses (x < 29 at.% metalloid) are composed of primary crystals of Co(Si) and amorphous matrix (Am 1) enriched in metalloid. The curve (b) in Fig. ! apparently reflects superposition of magnetization of these two phases. Depletion of the amorphous matrix in cobalt resulted in substantial decrease in the Curie temperature of the amorphous phase and a decrease in the overall magnetization of the alloy in this state. The glasses of intermediate metalloid content (x = 29-30 at.%) crystallize polymorphously and are fully crystalline after the first stage of crystallization, being composed of a single Col7n(Sio.sBo.5)7n phase. The temperature dependence of the magnetization of this phase is presented as curve (b) in Fig. 2. The values of the Curie temperature of this alloy in the crystalline and amorphous states are not much different from each other (see Table 1). Applicability of the Bloch law for the description of the magnetization M(T) of the Cot7n(Si0 5Bo.5)7,, phase is limited to temperatures up to Tmax~0.26T¢ (about 130 K). This temperature range is appreciably lower than that for the same alloy in the amorphous state, Tm~x~ 0.31T¢ (see Table 1). These observations of the amorphous
~(o)
b
•
~]1) t
i
I
I'1
II
\\° 5
0" 2'2 ' 2? '26 ' 2'8' 3'0' ~2 X [ a t . % ]
Fig. 5. Dependences of saturation magnetization (at 0 K and room temperature, M(0) and M(300) (O) respectively),Curie temperature T¢ and effectivemagnetic moment per cobalt atom # on the composition of metallic glasses Coloo_x(Sio.sBos)x. (Dependence of saturation magnetization in room temperature Mcr(300) (@) on the composition of the alloys after complete crystallization is included.)
and crystalline phase of the C07o.sSi14.7sBla.75 alloy studied are consistent with those reported for C075Si15B1o [5]. The only difference between the results obtained for these two alloys concerns the value of magnetic moment/~ per cobalt atom (and, correspondingly, the magnetization M). The value of/~ (and M(0)) in the present work is 20% lower for the alloy in crystalline state than for the amorphous alloy. For C075Si]sBi0 almost the same value of p (and M(0)) was found in the amorphous and the crystalline states [5]. The difference between these two alloys can be explained as being due to different final phases of glass crystallization. After the first stage of crystallization the metalloidrich glasses (x > 30 at.%) are composed of primary crystals of the C02Si(B) phase and the amorphous matrix (Am 1) enriched in cobalt and, perhaps, in
80
boron. Curve (b) in Fig. 3 shows summary magnetization of these two phases in the Co6sSil6B16 alloy. The enrichment of the amorphous matrix in cobalt resulted in a large increase in the Curie temperature and an increase in the magnetization at temperatures above 150K. The silicon-rich Co2Si(B) phase appeared to be a low-temperature ferromagnetic phase with Tc ~ 107 K and M(0) ~ 0.289 x 10 -s Wb m g-l (assuming mass fraction of the phase in the alloy to be equal to 30%).
3.3. Alloys after complete crystallization from glassy state The alloys studied after heating to temperatures over I000 K were completely crystalline and composed of three stable phases: Co(Si), Co2B and Co2Si. The last phase is paramagnetic. The curves (c) in Figs. 1-3 show the temperature dependence of saturation magnetization Ms(T) after complete crystallization of the Co-Si-B alloys for three groups characterized by a different course of crystallization. These curves represent summary magnetization of two ferromagnetic phases (Co(Si) and Co2B) and are qualitatively identical. Figure 5 shows, among other parameters, the room temperature saturation magnetization of the alloys studied in the amorphous M(300) state and the crystalline state Mcr(300). It is seen that only for the metalloid-rich glasses (group III) do the values of magnetization in the crystalline state exceed those in the amorphous state (Mcr(300)> M(300)). For the remaining glasses the amorphous state is more favourable as far as the room temperature magnetization is concerned. 4. Summary and conclusions (i) The increase in metalloid content in Co~oo-x(SiosBo.5)x glasses causes a decrease in the Curie temperature Tc and in the effective magnetic moment per cobalt atom/~ (and, correspondingly, in the magnetization M). (ii) Replacement of half of the boron content (in Co-B glasses) with silicon results in (a) decrease in Tc (about 100 K), (b) lower (about 10%) rate of decrease in T~ with metalloid content x (dTJdx ,~ 50 K
(at.% metalloid)-1), (c) higher (about 12%) rate of decrease in /~ with metalloid content (d/z/dx 0.053/~B (at. % metalloid) - 1). (iii) Amorphous Co7o.5Sil4.75B14.75 alloy exhibits comparable values of To, and a higher (about 20%) value of/~ (and M(0)) than its single-phase crystalline counterpart. (iv) The new metastable ferromagnetic phases Col7,(Si0.sBo.5)7n and Co2B(Si) are characterized by the following values of the Curie temperature T c ~ 5 0 0 K and 107K respectively, and magnetization M(0) =6.33 x 1 0 - S W b m g -I and 1.0 x I0 8 Wb m g-~ respectively. (v) For Co-Si-B alloys containing up to 30 at.% metalloid the amorphous state is more favourable as far as the magnetization in the range to 400 K is concerned.
Acknowledgments The work was supported by grants from the Ministry of Science, Academic Education and Technology under contract MR.I.21 and from the Government under contract PR.3.19.
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