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Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5−xAlxB9 alloy
Journal of Non-Crystalline Solids 353 (2007) 1577–1581 www.elsevier.com/locate/jnoncrysol
Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5xAlxB9 alloy D. Prabhu a
a,b
, A. Narayanasamy
a,*
, K. Chattopadhyay
b
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India b Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India Received 16 June 2006; received in revised form 19 January 2007 Available online 19 March 2007
Abstract We have synthesized FINEMET type amorphous Fe73.5Cu1Mo3Si13.5xAlxB9 alloy by the single wheel melt spinning technique. The effect of Al substitution on the magnetic properties has been studied using a vibrating sample magnetometer, SQUID and Mo¨ssbauer spectroscopy. Magnetization and Curie temperature of the amorphous phase of the alloys were found to decrease with Al concentration. The results are attributed to the dilution effect of Al on the magnetic moment of Fe and to the increase in Fe–Fe interaction distance resulting in the weakening of exchange interaction. 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Bb; 75.50.Kj Keywords: Amorphous metals, metallic glasses; Alloys; Magnetic properties; Mossbauer effect and spectroscopy
1. Introduction FINEMET alloy of the composition FeCuMSiB (M = Mo, Nb, etc.) is one of the state of the art magnetic materials which has been extensively studied in the literature [1–4]. These materials have found applications in transformer cores, choke coils, and EMI filters, for example. Ever since the discovery of magnetism in amorphous materials, tremendous interest has been shown by researchers due to the potential application of these amorphous ferromagnetic alloys and also the light it can shed on understanding the effect of structural disorder on the solid state properties in general and magnetic properties in particular [5]. The effect of substitution of aluminum for iron on magnetic and crystallization properties of FINEMET alloys has been studied by various researchers. Tate et al. [6] have observed an improvement in the dc coercivity
*
Corresponding author. Tel.: +91 44 2235 1444; fax: +91 44 2235 2870. E-mail address: ansjourn@rediffmail.com (A. Narayanasamy).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.030
and attributed it to the decrease in the effective anisotropy of the alloy due to the substitution of Al. Takahashi et al. [7] have proved that the ternary FeSiAl alloys have lower anisotropy constant compared to binary FeSi thereby opening up the possibility to enhance the magnetic properties of the existing FINEMET alloys. Zelanakova et al. [8] have concluded from their studies that the Al has no significant influence on the crystallization temperature or on the average grain size in these classes of alloys. However, their studies show that there is a decrease in the Curie temperature of the crystalline phase above 3% Al substitution. In this paper, we have reported the effect of substitution of Al for Si on the magnetic behavior in the amorphous state of the FINEMET type Fe73.5Cu1Mo3Si13.5xAlxB9 alloy. The report also presents a discussion on the observed systematic variation in magnetization and Curie temperature of the alloy as a function of Al concentration. An attempt has been made to explain the observed results by analyzing the hyperfine field distribution obtained from Mo¨ssbauer spectroscopy and low temperature thermomagnetization data obtained from SQUID.
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
2. Experiment The Fe73.5Cu1Mo3Si13.5xAlxB9 (x = 0, 1, 3 and 5) FINEMET type alloys, in the form of ribbons, have been synthesized using the single wheel melt spinning technique. High purity metals in the elemental form and in suitable weight ratios were melted in an arc melting unit and the ingot so obtained was used for melt spinning. The ingot was melted in a quartz nozzle having an orifice of 1 mm diameter and the molten alloy (1300 C) was ejected out with a positive argon pressure on to a copper wheel rotating at a velocity of 55 m/s. The ribbons obtained were of 2– 3 mm in width and 20–30 lm in thickness. The amorphous nature of the ribbons was confirmed using XRD [Huber, ˚ ) radiation. The Curie temGermany] with Fe Ka (1.9606 A peratures of the as-spun ribbons were measured using a vibrating sample magnetometer (VSM) [EG & G PARC 4500] with an applied field of 5 mT. The temperature was increased at the rate of 5 K/min for the thermomagnetization measurements. The 57Fe Mo¨ssbauer spectra of the specimens were recorded at 298 K with a constant acceleration Mo¨ssbauer spectrometer [Wissel, MDU 1200]. The magnetization measurements from 10 to 300 K were performed using a SQUID magnetometer. 3. Results
recorded for all the specimens. The value of Curie temperature (measured with an accuracy of ±3 K) has been determined by the tangent method as shown in the inset for Fig. 1 for the composition with 0% Al sample and it is found to be 617 K. Similarly, the values of TC have been determined for the other alloys. One could make the following observations from the M vs T curves for all the samples. The magnetization value of all the samples falls to zero at the Curie temperature of the amorphous phase confirming the amorphous nature of the sample, which is also evident from the X-ray diffraction patterns in Fig. 2. The TC of the amorphous phase is found to decrease with increase in Al concentration as shown in Fig. 3.
Fe73.5Al5Si8.5Mo3B9Cu1
Counts (arb. units)
1578
Fe73.5Al3Si10.5Mo3B9Cu1
Fe73.5Al1Si12.5Mo3B9Cu1
3.1. Thermomagnetic studies Fe73.5Si13.5Mo3B9Cu1
The Curie temperatures (TC) of the amorphous phase for the as-spun ribbons for all the alloys have been determined by measuring the magnetization (M) as a function of temperature. Fig. 1 shows the thermomagnetic curves
30
40
50
60
70
80
2θ (degrees)
Fig. 2. The XRD pattern for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 (x = 0, 1, 3 and 5) ribbons. 50
Applied field 50 Oe 0% Al 1% Al 3% Al 5% Al
615
30 50
TC (˚K)
30
2
20
600
40
M (Am /kg)
M (Am2kg-1)
40
10
20 10
585
0 500
550
600
650
700
T (K)
0 450
500
550
600
650
Temp (°K)
570 0
Fig. 1. The magnetization as a function of temperature for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 alloys with the inset showing the tangent method of determining the Curie temperature for the as-spun Fe73.5Cu1Mo3Si13.5B9 specimen.
1
2
3
4
5
Al concentration (%) Fig. 3. The plot showing the variation of TC as a function of Al concentration (the line joining data points is a guide to the eye).
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
1579
3.2. Magnetization measurements
5%
3% 3.9%
1%
P (H)
Absorption (%)
The variation of magnetization (measured with an accuracy of ±1 emu/g) with Al concentration is given in Fig. 4. The magnetization reduced from 145 emu/g to 116 emu/g falling approximately at the rate of 6 emu/g per 1% Al addition. In order to confirm the trend in the variation of the magnetization value with the Al concentration, the measurement was carried out at a low temperature of 10 K. The trend was very similar to the one observed at room temperature. The magnetization value for the 0% Al sample increased to 165 emu/g at 10 K from 145 emu/g at 300 K, but its variation with Al concentration was very similar, to that at 300 K. Goscianska et al. [9] have observed a similar decrease in magnetization in Fe73.5xAlxCu1Nb3Si13.5B9 thin films.
3.8%
4.2%
P(1) 4.4%
0%
P(2) -8
-4
0
4
8
0
150
Velocity (mm/s)
300
450
H (T)
3.3. Mo¨ssbauer effect studies
Fig. 5. The least-squares fitting of the Mo¨ssbauer spectra recorded at 298 K and the corresponding P(H) distributions for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 ribbons.
In order to further understand the effect of Al on the magnetic behavior of FINEMET alloy, we have carried out Mo¨ssbauer measurements. The spectra were recorded at 298 K for all the above specimens. The instrument was calibrated using a-Fe as the standard. The spectra were analyzed using the Window method [10] to extract the probability distribution P(H) of the magnetic hyperfine fields (Fig. 5). The as-spun sample with 0% Al showed a characteristic bimodal distribution [4,11–14]. Miglierini et al. [12] have made similar observation regarding the hyperfine field distribution of FINEMET alloys and attributed the bimodal distribution to the presence of two distinct types of surroundings for the resonant atom. The most probable peak in the distribution is attributed to Fe atoms having Fe, Si and B as their nearest neighbors and the low-field peak is
attributed to Fe atoms having non-magnetic components like Mo and Cu as their nearest neighbors. The following observations could be made from the hyperfine field distributions for the various alloys. The most probable peak (P1) and the peak at lower field (P2) shift to the left with increasing Al concentration. The values of peak hyperfine field and the low-field peak field reduced from 24.2 to 22.3 T and from 13.5 to 10 T, respectively, as the Al concentration was increased from 0% to 5%. The variation of the two peak hyperfine fields and average hyperfine field with Al concentration is given in Fig. 6. It should be noted that the decrease in the peak field is more in the case of P2 than for P1. Also the intensity of P2 appears to reduce with Al concentration.
170
14
25
M (10 K) M (300 K)
(T)
22.0
160
21.6
150
140
130
120
21.2
24
20.8
12
0
1
2
3
4
5
Al concentration (%)
11 23 10
High-field peak position(T)
Low-field peakposition (T)
Ms ( Am2kg-1 )
B
avg
13
9 22 0
0
1
2
3
4
5
Al concentration (%) Fig. 4. The plot showing the variation of magnetization measured at 300 and 10 K as a function of Al concentration (the line joining data points is a guide to the eye).
1
2
3
4
5
Al concentration (%)
Fig. 6. The plot showing the variation of the peak position of the two peaks in the hyperfine field distribution as a function of Al concentration with the inset giving the variation of average hyperfine field. (the line joining the data points is a guide to the eye).
1580
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
3.4. Spin wave analysis
of about 0.18 for all the alloys, typical of amorphous ferromagnets.
The existence of long wavelength magnetic excitations in amorphous ferromagnets has been reported in the literature [15–19]. The relative change of magnetization in ferromagnetic materials obeys the expression DM ¼ BT 3=2 Mð0Þ
ð1Þ
at relatively low temperatures of about 0.3TC, where B is related to the spin wave stiffness constant D, as given by the relation, namely, 3=2 glB kB ð2Þ B ¼ fð3=2Þ Mð0Þ 4pD with f(3/2) (=2.612) is the Riemann zeta function, g the Lande g-factor, lB the Bohr magneton (=9.27 · 1024 J/T), M(0) the magnetization at 0 K obtained by extrapolation and kB is the Boltzmann constant (=1.38 · 1023 J/K). The low temperature thermomagnetic curves from 10 K to 300 K for the as-spun alloy with various Al concentrations were recorded in an applied field of 0.5 T. DM/M(0) was plotted as a function of T3/2 which had a linear trend (Fig. 7). The data were fitted with a straight line passing through the origin by a linear regression, whose slope is B, which was used to calculate D from Eq. (2). It was found that the value of the exchange stiffness constant decreased ˚ 2 for 0% Al to 107 ± 1 meV A ˚ 2 for the from 116 ± 1 meV A alloy substituted with 5% Al. The value of D is very typical of non-crystalline solids and is in good agreement with the values reported earlier in literature [20,21]. The D/TC for non-crystalline ferromagnets is typically smaller in comparison to the value of 0.25–0.27 for crystalline ferromagnets which is indicative of the short range nature of interactions in these classes of materials [20]. We have obtained a D/TC 0.06
ΔM/ M(0)
0.04
4. Discussion The lowering of the most probable peak field with increasing Al concentration can be attributed to the larger dilution effect of Al on the magnetic moment of Fe than by Si. The reduction in the peak field for the low-field peak is more rapid falling approximately at the rate of 0.7 T per 1% Al in comparison to the fall of the most probable field peak at 0.4 T per 1% Al. Aluminum being a non-magnetic atom its dilution effect is, therefore, more pronounced on the Fe atom in non-magnetic surroundings than on the Fe atom present in magnetic atom surroundings. As a result the shift in the peak position of the low-field peak is larger than that of the most probable field peak. This would mean that the overlapping of the profiles is reduced resulting in the observed lowering of intensity of the lowfield peak. The average hyperfine field also decreases with increasing Al concentration which is in accordance with the magnetization measurements (Fig. 6). The reduction in the value of D with Al concentration could probably be explained by considering the atomic radius of the atoms involved. The substitution of Al atoms with a higher atomic radius than that of Si [22] results in an increase in the separation between Fe atoms. This leads to the weakening of exchange interaction between Fe atoms resulting in the lowering of Curie temperature. 5. Conclusion Ribbons of the Fe73.5Cu1Mo3Si13.5xAlxB9 alloy have been prepared using melt spinning technique and their amorphicity has been confirmed by X-ray diffraction. The decrease in magnetization is attributed to the dilution effect of Al on the magnetic moment of Fe as is evident from the decrease of the hyperfine field values. The decrease in Curie temperature of the amorphous phase has been attributed to the weakening of exchange interaction due to an increase in the Fe–Fe distance by the substitution of Al atoms having an atomic radius greater than Si. Studies on the structural and magnetic properties of these alloys for various thermal treatments are being pursued and will be reported as a separate communication.
0.02
0% Al 1% Al 3% Al 5% Al 0.00 0
1000 3/2
T
2000 3/2
(K )
Fig. 7. The DM/M(0) data fitted with a straight line by linear regression using the Bloch’s relation DM/M(0) = BT3/2 in the low temperature region for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 ribbons.
Acknowledgements The financial assistance through the DRDO Major Research Project (ERIP/Er/003284/M/01) sanctioned to the University of Madras, Chennai and IISc, Bangalore is gratefully acknowledged. The financial grant from UGCSAP (Phase III) to the University of Madras is acknowledged. One of the authors D.P would like to thank the CSIR, Government of India for the award of Senior Research Fellowship.
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] Y. Yoshizawa, K. Yamauchi, T. Yamanae, H. Sugihara, J. Appl. Phys. 64 (1988) 6047. [3] N. Kataoka, A. Inoue, T. Masumoto, Y. Yoshizawa, K. Yamauchi, Jpn. J. Appl. Phys. 28 (1989) L1820. [4] N. Ponpandian, A. Naryanasamy, K. Chattopadhyay, M. Manivel Raja, K. Ganesan, C.N. Chinnasamy, B. Jeyadevan, J. Appl. Phys. 93 (2003) 6182. [5] S.N. Kaul, Phys. Rev. B 24 (1981) 6550. [6] B.J. Tate, B.S. Parmar, I. Todd, H.A. Davies, M.R.J. Gibbs, R.V. Major, J. Appl. Phys. 83 (1988) 6335. [7] M. Takahashi, S. Nishimaki, T. Wakiyama, J. Magn. Magn. Mater. 66 (1987) 55. [8] A. Zelenakova, M. Kuzminski, J. Fuzer, P. Kollar, T. Svec, J. Magn. Magn. Mater. 215&216 (2000) 459. [9] I. Goscianska, J. Dubowik, H. Ratajczak, M. Konc, P. Sovak, J. Magn. Magn. Mater. 242–245 (2002) 177.
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[10] B. Window, J. Phys. E: Sci. Instrum. 4 (1971) 401. [11] E.V. Kaevitser, S. D Kaloshkin, B.V. Jalnin, V.V. Tcherdyntsev, V. Baldokhin, V. Yu, Mater. Sci. Eng. A 375–377 (2004) 683. [12] M. Miglierini, J. Phys.: Condens. Mater 4 (1992) 10295. [13] G. Rixecker, P. Schaaf, U. Conser, J. Phys.: Condens. Mater 4 (1992) 10295. [14] G. LeGer, J.M. Dubois, J. Phys. E: Sci. Instrum. 12 (1979) 1083. [15] R.W. Cochrane, G.S. Cargill, Phys. Rev. Lett. 32 (1974) 476. [16] C.C. Tsuei, H. Lilienthal, Phys. Rev. B 13 (1976) 4899. [17] C.L. Chien, R. Hasegawa, Phys. Rev. B 16 (1977) 2115. [18] R. Hasegawa, Ranjan Ray, Phys. Rev. B 20 (1979) 211. [19] F.E. Luborsky, J.L. Walter, H.H. Libermann, E.P. Wohlfarth, J. Magn. Magn. Mater. 15–18 (1980) 1351. [20] S.M. Bhagat, M.L. Spano, K.V. Rao, J. Appl. Phys. 50 (1979) 1580. [21] E. Pulido, P. Crespo, A. Hernando, IEEE Trans. Magn. 28 (1992) 3189. [22] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New York, 1972, p. 135.
Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5xAlxB9 alloy D. Prabhu a
a,b
, A. Narayanasamy
a,*
, K. Chattopadhyay
b
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India b Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India Received 16 June 2006; received in revised form 19 January 2007 Available online 19 March 2007
Abstract We have synthesized FINEMET type amorphous Fe73.5Cu1Mo3Si13.5xAlxB9 alloy by the single wheel melt spinning technique. The effect of Al substitution on the magnetic properties has been studied using a vibrating sample magnetometer, SQUID and Mo¨ssbauer spectroscopy. Magnetization and Curie temperature of the amorphous phase of the alloys were found to decrease with Al concentration. The results are attributed to the dilution effect of Al on the magnetic moment of Fe and to the increase in Fe–Fe interaction distance resulting in the weakening of exchange interaction. 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Bb; 75.50.Kj Keywords: Amorphous metals, metallic glasses; Alloys; Magnetic properties; Mossbauer effect and spectroscopy
1. Introduction FINEMET alloy of the composition FeCuMSiB (M = Mo, Nb, etc.) is one of the state of the art magnetic materials which has been extensively studied in the literature [1–4]. These materials have found applications in transformer cores, choke coils, and EMI filters, for example. Ever since the discovery of magnetism in amorphous materials, tremendous interest has been shown by researchers due to the potential application of these amorphous ferromagnetic alloys and also the light it can shed on understanding the effect of structural disorder on the solid state properties in general and magnetic properties in particular [5]. The effect of substitution of aluminum for iron on magnetic and crystallization properties of FINEMET alloys has been studied by various researchers. Tate et al. [6] have observed an improvement in the dc coercivity
*
Corresponding author. Tel.: +91 44 2235 1444; fax: +91 44 2235 2870. E-mail address: ansjourn@rediffmail.com (A. Narayanasamy).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.030
and attributed it to the decrease in the effective anisotropy of the alloy due to the substitution of Al. Takahashi et al. [7] have proved that the ternary FeSiAl alloys have lower anisotropy constant compared to binary FeSi thereby opening up the possibility to enhance the magnetic properties of the existing FINEMET alloys. Zelanakova et al. [8] have concluded from their studies that the Al has no significant influence on the crystallization temperature or on the average grain size in these classes of alloys. However, their studies show that there is a decrease in the Curie temperature of the crystalline phase above 3% Al substitution. In this paper, we have reported the effect of substitution of Al for Si on the magnetic behavior in the amorphous state of the FINEMET type Fe73.5Cu1Mo3Si13.5xAlxB9 alloy. The report also presents a discussion on the observed systematic variation in magnetization and Curie temperature of the alloy as a function of Al concentration. An attempt has been made to explain the observed results by analyzing the hyperfine field distribution obtained from Mo¨ssbauer spectroscopy and low temperature thermomagnetization data obtained from SQUID.
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
2. Experiment The Fe73.5Cu1Mo3Si13.5xAlxB9 (x = 0, 1, 3 and 5) FINEMET type alloys, in the form of ribbons, have been synthesized using the single wheel melt spinning technique. High purity metals in the elemental form and in suitable weight ratios were melted in an arc melting unit and the ingot so obtained was used for melt spinning. The ingot was melted in a quartz nozzle having an orifice of 1 mm diameter and the molten alloy (1300 C) was ejected out with a positive argon pressure on to a copper wheel rotating at a velocity of 55 m/s. The ribbons obtained were of 2– 3 mm in width and 20–30 lm in thickness. The amorphous nature of the ribbons was confirmed using XRD [Huber, ˚ ) radiation. The Curie temGermany] with Fe Ka (1.9606 A peratures of the as-spun ribbons were measured using a vibrating sample magnetometer (VSM) [EG & G PARC 4500] with an applied field of 5 mT. The temperature was increased at the rate of 5 K/min for the thermomagnetization measurements. The 57Fe Mo¨ssbauer spectra of the specimens were recorded at 298 K with a constant acceleration Mo¨ssbauer spectrometer [Wissel, MDU 1200]. The magnetization measurements from 10 to 300 K were performed using a SQUID magnetometer. 3. Results
recorded for all the specimens. The value of Curie temperature (measured with an accuracy of ±3 K) has been determined by the tangent method as shown in the inset for Fig. 1 for the composition with 0% Al sample and it is found to be 617 K. Similarly, the values of TC have been determined for the other alloys. One could make the following observations from the M vs T curves for all the samples. The magnetization value of all the samples falls to zero at the Curie temperature of the amorphous phase confirming the amorphous nature of the sample, which is also evident from the X-ray diffraction patterns in Fig. 2. The TC of the amorphous phase is found to decrease with increase in Al concentration as shown in Fig. 3.
Fe73.5Al5Si8.5Mo3B9Cu1
Counts (arb. units)
1578
Fe73.5Al3Si10.5Mo3B9Cu1
Fe73.5Al1Si12.5Mo3B9Cu1
3.1. Thermomagnetic studies Fe73.5Si13.5Mo3B9Cu1
The Curie temperatures (TC) of the amorphous phase for the as-spun ribbons for all the alloys have been determined by measuring the magnetization (M) as a function of temperature. Fig. 1 shows the thermomagnetic curves
30
40
50
60
70
80
2θ (degrees)
Fig. 2. The XRD pattern for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 (x = 0, 1, 3 and 5) ribbons. 50
Applied field 50 Oe 0% Al 1% Al 3% Al 5% Al
615
30 50
TC (˚K)
30
2
20
600
40
M (Am /kg)
M (Am2kg-1)
40
10
20 10
585
0 500
550
600
650
700
T (K)
0 450
500
550
600
650
Temp (°K)
570 0
Fig. 1. The magnetization as a function of temperature for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 alloys with the inset showing the tangent method of determining the Curie temperature for the as-spun Fe73.5Cu1Mo3Si13.5B9 specimen.
1
2
3
4
5
Al concentration (%) Fig. 3. The plot showing the variation of TC as a function of Al concentration (the line joining data points is a guide to the eye).
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
1579
3.2. Magnetization measurements
5%
3% 3.9%
1%
P (H)
Absorption (%)
The variation of magnetization (measured with an accuracy of ±1 emu/g) with Al concentration is given in Fig. 4. The magnetization reduced from 145 emu/g to 116 emu/g falling approximately at the rate of 6 emu/g per 1% Al addition. In order to confirm the trend in the variation of the magnetization value with the Al concentration, the measurement was carried out at a low temperature of 10 K. The trend was very similar to the one observed at room temperature. The magnetization value for the 0% Al sample increased to 165 emu/g at 10 K from 145 emu/g at 300 K, but its variation with Al concentration was very similar, to that at 300 K. Goscianska et al. [9] have observed a similar decrease in magnetization in Fe73.5xAlxCu1Nb3Si13.5B9 thin films.
3.8%
4.2%
P(1) 4.4%
0%
P(2) -8
-4
0
4
8
0
150
Velocity (mm/s)
300
450
H (T)
3.3. Mo¨ssbauer effect studies
Fig. 5. The least-squares fitting of the Mo¨ssbauer spectra recorded at 298 K and the corresponding P(H) distributions for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 ribbons.
In order to further understand the effect of Al on the magnetic behavior of FINEMET alloy, we have carried out Mo¨ssbauer measurements. The spectra were recorded at 298 K for all the above specimens. The instrument was calibrated using a-Fe as the standard. The spectra were analyzed using the Window method [10] to extract the probability distribution P(H) of the magnetic hyperfine fields (Fig. 5). The as-spun sample with 0% Al showed a characteristic bimodal distribution [4,11–14]. Miglierini et al. [12] have made similar observation regarding the hyperfine field distribution of FINEMET alloys and attributed the bimodal distribution to the presence of two distinct types of surroundings for the resonant atom. The most probable peak in the distribution is attributed to Fe atoms having Fe, Si and B as their nearest neighbors and the low-field peak is
attributed to Fe atoms having non-magnetic components like Mo and Cu as their nearest neighbors. The following observations could be made from the hyperfine field distributions for the various alloys. The most probable peak (P1) and the peak at lower field (P2) shift to the left with increasing Al concentration. The values of peak hyperfine field and the low-field peak field reduced from 24.2 to 22.3 T and from 13.5 to 10 T, respectively, as the Al concentration was increased from 0% to 5%. The variation of the two peak hyperfine fields and average hyperfine field with Al concentration is given in Fig. 6. It should be noted that the decrease in the peak field is more in the case of P2 than for P1. Also the intensity of P2 appears to reduce with Al concentration.
170
14
25
M (10 K) M (300 K)
(T)
22.0
160
21.6
150
140
130
120
21.2
24
20.8
12
0
1
2
3
4
5
Al concentration (%)
11 23 10
High-field peak position(T)
Low-field peakposition (T)
Ms ( Am2kg-1 )
B
avg
13
9 22 0
0
1
2
3
4
5
Al concentration (%) Fig. 4. The plot showing the variation of magnetization measured at 300 and 10 K as a function of Al concentration (the line joining data points is a guide to the eye).
1
2
3
4
5
Al concentration (%)
Fig. 6. The plot showing the variation of the peak position of the two peaks in the hyperfine field distribution as a function of Al concentration with the inset giving the variation of average hyperfine field. (the line joining the data points is a guide to the eye).
1580
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
3.4. Spin wave analysis
of about 0.18 for all the alloys, typical of amorphous ferromagnets.
The existence of long wavelength magnetic excitations in amorphous ferromagnets has been reported in the literature [15–19]. The relative change of magnetization in ferromagnetic materials obeys the expression DM ¼ BT 3=2 Mð0Þ
ð1Þ
at relatively low temperatures of about 0.3TC, where B is related to the spin wave stiffness constant D, as given by the relation, namely, 3=2 glB kB ð2Þ B ¼ fð3=2Þ Mð0Þ 4pD with f(3/2) (=2.612) is the Riemann zeta function, g the Lande g-factor, lB the Bohr magneton (=9.27 · 1024 J/T), M(0) the magnetization at 0 K obtained by extrapolation and kB is the Boltzmann constant (=1.38 · 1023 J/K). The low temperature thermomagnetic curves from 10 K to 300 K for the as-spun alloy with various Al concentrations were recorded in an applied field of 0.5 T. DM/M(0) was plotted as a function of T3/2 which had a linear trend (Fig. 7). The data were fitted with a straight line passing through the origin by a linear regression, whose slope is B, which was used to calculate D from Eq. (2). It was found that the value of the exchange stiffness constant decreased ˚ 2 for 0% Al to 107 ± 1 meV A ˚ 2 for the from 116 ± 1 meV A alloy substituted with 5% Al. The value of D is very typical of non-crystalline solids and is in good agreement with the values reported earlier in literature [20,21]. The D/TC for non-crystalline ferromagnets is typically smaller in comparison to the value of 0.25–0.27 for crystalline ferromagnets which is indicative of the short range nature of interactions in these classes of materials [20]. We have obtained a D/TC 0.06
ΔM/ M(0)
0.04
4. Discussion The lowering of the most probable peak field with increasing Al concentration can be attributed to the larger dilution effect of Al on the magnetic moment of Fe than by Si. The reduction in the peak field for the low-field peak is more rapid falling approximately at the rate of 0.7 T per 1% Al in comparison to the fall of the most probable field peak at 0.4 T per 1% Al. Aluminum being a non-magnetic atom its dilution effect is, therefore, more pronounced on the Fe atom in non-magnetic surroundings than on the Fe atom present in magnetic atom surroundings. As a result the shift in the peak position of the low-field peak is larger than that of the most probable field peak. This would mean that the overlapping of the profiles is reduced resulting in the observed lowering of intensity of the lowfield peak. The average hyperfine field also decreases with increasing Al concentration which is in accordance with the magnetization measurements (Fig. 6). The reduction in the value of D with Al concentration could probably be explained by considering the atomic radius of the atoms involved. The substitution of Al atoms with a higher atomic radius than that of Si [22] results in an increase in the separation between Fe atoms. This leads to the weakening of exchange interaction between Fe atoms resulting in the lowering of Curie temperature. 5. Conclusion Ribbons of the Fe73.5Cu1Mo3Si13.5xAlxB9 alloy have been prepared using melt spinning technique and their amorphicity has been confirmed by X-ray diffraction. The decrease in magnetization is attributed to the dilution effect of Al on the magnetic moment of Fe as is evident from the decrease of the hyperfine field values. The decrease in Curie temperature of the amorphous phase has been attributed to the weakening of exchange interaction due to an increase in the Fe–Fe distance by the substitution of Al atoms having an atomic radius greater than Si. Studies on the structural and magnetic properties of these alloys for various thermal treatments are being pursued and will be reported as a separate communication.
0.02
0% Al 1% Al 3% Al 5% Al 0.00 0
1000 3/2
T
2000 3/2
(K )
Fig. 7. The DM/M(0) data fitted with a straight line by linear regression using the Bloch’s relation DM/M(0) = BT3/2 in the low temperature region for the as-spun Fe73.5Cu1Mo3Si13.5xAlxB9 ribbons.
Acknowledgements The financial assistance through the DRDO Major Research Project (ERIP/Er/003284/M/01) sanctioned to the University of Madras, Chennai and IISc, Bangalore is gratefully acknowledged. The financial grant from UGCSAP (Phase III) to the University of Madras is acknowledged. One of the authors D.P would like to thank the CSIR, Government of India for the award of Senior Research Fellowship.
D. Prabhu et al. / Journal of Non-Crystalline Solids 353 (2007) 1577–1581
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