- Email: [email protected]
Influence of Co content on the structure and magnetic permeability of nanocrystalline (Fe1−xCox)73.5Cu1Nb3Si13.5B9 alloys
Materials Science and Engineering B 156 (2009) 57–61
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Influence of Co content on the structure and magnetic permeability of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys Ye-mei Han a , Zhi Wang a,∗ , Xiang-hui Che a , Xue-gang Chen a , Wen-run Li a , Ya-li Li b a b
School of Science, Tianjin University, Tianjin 300072, PR China Key Laboratory of Advanced Ceramics and Machining Technology (Tianjin University), Ministry of Education, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 28 October 2008 Accepted 5 November 2008 Keywords: Nanocrystalline FeCo-based alloy Initial permeability i –T curve Exchange coupling
a b s t r a c t Structural and soft magnetic response of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys were investigated. Influences of heat treatment and Co content on the crystallization were analyzed through estimation of the crystalline volume fraction Vcr and thickness of the intergranular amorphous layer from XRD patterns. High-temperature soft magnetic response was analyzed by the temperature evolution of the magnetic permeability from room temperature to 780 ◦ C. It was found that the high-temperature soft magnetic behavior is improved with increasing Co content for x ≤ 0.5, which is ascribed to reinforcing magnetic coupling between the crystals, however, a decrease of i was observed for the Co richest sample. The observed i behavior as a function of Co content is interpreted in terms of variations in the degree of exchange coupling. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline Fe–Cu–Nb–Si–B soft magnetic alloys show attractive magnetic properties such as high saturation magnetization, high permeability, and low core loss, simultaneously [1–3]. However, due to the relatively low Curie point of the residual amorphous matrix [4], they cannot be used at high temperature. Partial substitution of Co for Fe was carried out to meet the desire for high-temperature magnetic properties. When Co is added to the FeSiBNbCu alloys, crystalline phase with higher magnetic moment and Curie temperature determines their magnetic response at high temperatures [5–9]. Softness is mostly related to the exchange coupling between nanocrystalline grains through the amorphous matrix. The exchange coupling will be reinforced by increasing the saturation magnetization of the crystalline phases. As is shown [9], Co atoms selectively substitute Fe atoms into the nonequivalent Fe sites on the crystallization process and this substitution could affect the saturation magnetization of the crystalline phases and thus affect the magnetic interaction between crystals. Up to now, little attention has been paid to the discrepancy of magnetic moment of Co and Fe atoms and its effect on the magnetic exchange coupling and the soft magnetic response. In this paper, structural and soft magnetic response of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys is analyzed by means of measuring i –T curves [4,10–12], influence of variations in the degree of exchange
∗ Corresponding author. Tel.: +86 22 87892075; fax: +86 22 27890681. E-mail address: [email protected] (Z. Wang). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.006
coupling on the i behavior as a function of Co content will be analyzed from the viewpoint of magnetic moment. In addition, changes in the crystalline volume fraction with Co content affecting the exchange coupling will also be discussed. 2. Experimental Amorphous ribbons 5–10 mm wide, about 30 m thick, were obtained by melt spinning technique with nominal composition (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys (x = 0, 0.25, 0.5, 0.75). The toroidal samples with an outer diameter of about 22 mm and inner diameter of about 18 mm were fabricated by winding the ribbons into toroidal cores. In order to obtain the characteristic nanocrystalline structure, the samples were submitted to isothermal treatments (30 min) under vacuum atmosphere (10−3 Pa) in a tubular furnace. The phase structure of the ribbons was examined by X-ray diffraction (XRD) using D/max-2500/PC with Co K␣ radiation ( = 1.7889 Å). Differential scanning calorimetry (DSC) with a heating rate of 20 ◦ C/min was used to determine the thermal processes of alloy studied. The permeability was in situ measured in a furnace with Ar atmosphere protection by using an HP4194A impedance analyzer at H = 0.4 A/m and f = 10 kHz and the heating rate was 10 ◦ C/min. 3. Results and discussion The nanocrystalline structure, mainly composed of (FeCo)3 Si and ␣-FeCo(Si) crystals embedded in amorphous matrix, was obtained in these alloys after annealing at temperatures of
58
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61 Table 1 The average grain size, (D/nm), of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C. Ta (◦ C) 470 500 530 560 590 620
Fig. 1. XRD patterns for the 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys.
450–620 ◦ C. The structure was monitored by X-ray diffraction. Fig. 1 shows the XRD patterns for the 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys. As presented, patterns of all the annealed samples show the characteristic diffraction peaks corresponded to crystalline precipitates. It is indicated that all the annealed samples partially crystallize after annealing and their microstructure consists of the residual amorphous phase and the nanocrystalline phase. The crystalline volume fraction, Vcr , was calculated from the XRD spectra according to Vcr =
Icr Icr + Iam
where Icr and Iam are the integral intensities of diffraction peaks of crystalline phase and amorphous phase, respectively. These integral intensities are determined from peak areas [11,13,14] and the results are shown in Fig. 2. For the sample of x = 0.5, Vcr increases with Ta and the value of Vcr is relatively low for Ta = 470 ◦ C because of the incomplete occurrence of the first stage of crystallization during the annealing treatment. Whereas, in the case of the alloy for x = 0.75, a higher Vcr of about 54% is obtained for the 470 ◦ C-annealed sample and the Vcr has no significant increase with Ta in the range from 530 to 620 ◦ C. This suggests that annealing at 530 ◦ C for 30 min ensures the formation of stable structure for the sample of x = 0.75. In fact, Vcr values were obtained by numerous computations and a difference close to 5% is introduced as an error bar in data displayed in Fig. 2. It is noteworthy that the highest Vcr was obtained
Fig. 2. The crystalline volume fraction (Vcr /%) for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C.
x=0
x = 0.25
x = 0.5
x = 0.75
7 11 12 12 14
11 11 12 11 13 14
12 13 14 13 14 15
15 16 16 17 17 16
for the sample of x = 0.5 when (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys were sufficiently crystallized. In addition, the obtained Vcr values are basically consistent with what were presented by Gercsi et al. [15]. The average grain size, D, was calculated from the X-ray diffraction patterns by means of the Scherrer formula. As is shown in Table 1, in principle, D increases with Co content. The calculated D values agree with the tendency of D upon Co content reported by Mazaleyrat et al. [14]. In addition, D increases slightly with Ta for x ≤ 0.5, while it keeps nearly unchangeable with Ta from 470 to 620 ◦ C for the sample of x = 0.75. Additionally, the thickness of the amorphous layer between the surfaces of two adjacent nanocrystallites, , can be approximately estimated as [16]: =D
1/3 1 Vcr
−1
The calculated results were presented in Fig. 3. It can be seen that cobalt content in the alloys clearly affects values, examined within the annealing temperature range of 470–620 ◦ C. Taking into account the error in the employed method, a maximum deviation of 12% is computed from errors of Vcr and D according to the theory of errors. Such a maximum deviation is introduced as an error bar in data shown in Fig. 3. However, even within this range of estimated error, it can be clearly seen that the corresponding distance between the crystallites, , becomes smaller as Ta increases, the lowest value was obtained for the alloy with Co content of x = 0.5. The analysis of the above graphs leads to conclusions that nanocrystalline structure formed during heat treatment is influenced by annealing temperature Ta , and also by the Co content. The addition of a larger amount of Co may lower the nanocrystallization temperature. With the optimal annealing conditions, the highest Vcr values and the lowest values were found to be x = 0.5 for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys. With respect to the
Fig. 3. The thickness of amorphous layer between the surfaces of two adjacent nanocrystallites, (/nm), for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C.
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
Fig. 5. i –T curve of heating (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloy.
59
and
cooling
for
as-quenched
Fig. 4. i –T curves for nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
influence of the detected changes in structure on the magnetic properties of the samples, we measured the temperature dependence of initial permeability i (i –T curves) for nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys, as shown in Fig. 4. A sharp Hopkinson peak [17] was observed at the Curie point of amorphous phase, TCA , from which we can get TCA = 335, 425, 460 and 460 ◦ C for the samples of x = 0, 0.25, 0.5 and 0.75, respectively. When the measuring temperature T approaches TCA , i decreased steeply to practically zero, which is attributed to the transition from ferromagnetic to paramagnetic state of the amorphous alloy. With increasing temperature of T > TCA , crystallization of amorphous phase gives rise to the rise of i . Finally, a magnetic hardening occurs at around 700 ◦ C, which is reflected by a dramatic drop of i . The magnetic hardening can be attributed either to magnetic transition from ferromagnetic to paramagnetic state of the precipitated crystalline phase or to formation of a hard magnetic phase. If the former controls the drop of i , a parallel peak of i can be detected on the cooling i –T curve. However, the value of i remains unchangeable when the alloy is cooled down from 700 ◦ C to room temperature. Consequently, magnetic softness is lost, as is presented in Fig. 5; accordingly, this magnetic hardening at high temperatures should be attributed to the formation of hard magnetic phase. Apart from TCA , the detectable crystallization temperature, T1 , associated to the first obvious rise of i , can be reflected by i –T curves for as-quenched (Fe0.5 Co0.5 )73.5 Cu1 Nb3 Si13.5 B9 alloys. Dependence of TCA and T1 on Co content for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys are shown in Fig. 6. As is shown, TCA increases with Co content before x < 0.5, after x ≥ 0.5, TCA is unchangeable. However, T1 continuously decreases with increasing Co content in the range of 0 ≤ x ≤ 0.75. This decrease is also confirmed through the DSC analysis presented in Fig. 7. In addition, we observed that the temperature gap between TCA and T1 becomes narrow with Co content increasing in the alloys. In particular, TCA = T1 for (Fe0.25 Co0.75 )73.5 Cu1 Nb3 Si13.5 B9 alloy, which means that the amorphous precursor will nanocrystallize as soon as it transforms from ferromagnetic to paramagnetic state. Fig. 7 shows the DSC curves performed at 20 ◦ C/min for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys. As is shown, the first crystal-
Fig. 6. Dependence of TCA and T1 on Co content for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
lization onset temperature, Tx1 , decreases with the increase of Co content. In order to analyze the effect of Co content on i behavior of the alloys, i –T curves for 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys were measured as is shown in Fig. 8. The optimum soft magnetic state at room temperature is obtained for the annealed sample with x = 0. However, a rapid deterioration of soft magnetic properties is observed with increasing temperature, close to the Curie point of the amorphous material. Thus, Co-free nanocrystalline alloys do not exhibit good
Fig. 7. DSC curves for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
60
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
where K1 is the local anisotropy of the crystallites, v and D are the volume fraction and the mean grain size of crystals, respectively. The Aeff should be related to the strength of the exchange interactions between any pair of adjacent nanocrystals, which depends on the crystalline volume fraction and influence of the grain’s magnetization [19]. The combination of the above equations readily gives i = p
Ms2 A3eff
(3)
0 v2 K14 D6
As mentioned above, Ms denotes the average saturation magnetization of materials consisting of the saturation magnetization of cryst residual amorphous phases (Msam ) and crystalline phases (Ms ). Then in a duel phase system i can be written as i = p
cryst 2 3 ) Aeff 0 v2 K14 D6
(va Msam + vMs
where va and v are the volume fraction of residual amorphous phase and crystalline phase, respectively. When taking into consideration
Fig. 8. i –T curves for 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
2 ∝ (1 − (T/T A )) of the term Mam C
high-temperature soft magnetic properties. With respect to the sample for x = 0.75, the i was found to vary little up to 650 ◦ C during the heating process from room temperature to 700 ◦ C, but the value of i is relatively low. For the four samples, the best combination of high permeability i and magnetic stability in the measuring temperature range of 300–700 ◦ C is obtained for the sample of x = 0.5. For all i –T curves in Fig. 8, the rapid fall of i at 710 ◦ C is due to the formation of the boride phases. The detected behavior of i with x must be correlated with the dependence of Vcr and on Co content as is reflected by previous X-ray diffraction analysis. An improvement in the soft magnetic behavior is attributed to magnetic coupling between the crystalline grains through the intergranular phase. High-temperature magnetic behavior is strongly dependent on the volumetric fraction of crystalline grains. As the crystalline fraction increases and the intergranular distances decrease, the magnetic coupling between the grains becomes effective, causing a magnetic softening. For the (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys, the highest Vcr values and the lowest values were obtained for the alloys with Co content of x = 0.5; thus, the magnetic coupling is the most effective which results in the highest i value at high temperatures. As shown in above results, addition of Co can shift the Curie temperature to higher values. i at high temperatures increases with increasing Co content in the presented nanocrystalline alloys (x ≤ 0.5). However, TCA does not exhibit significant change and a decrease in i is observed as increasing Co content for higher Co content alloys (from x = 0.5 to x = 0.75). As is reported, the magnetic characteristics of the precipitated crystalline phase play a dominant role in the evolution of the magnetic properties of the FeCo-based nanocrystalline alloys [9]. The initial permeability can be expressed as [18]: i = p
Ms2 0 K
(1)
where Ms is average saturation magnetization of the materials, p is a dimensionless prefactor and 0 is the vacuum magnetic permeability. According to Hernando et al. [19], the effective magnetic anisotropy K of nanostructured soft magnetic alloy may be expressed as K =
v2 K14 D6 A3eff
(4)
(2)
2ˇ
[19], where ˇ is the Heisenberg cryst
exponent, obviously, when T ≥ TCA , Msam << Ms , Eq. (4) can lead to the following expression for magnetic permeability: i = p
cryst 2 3 ) Aeff 4 0 K1 D6
(Ms
(5)
In the Fe-based nanocrystalline alloys, the crystal phase consisting mainly of the Fe3 Si with fcc DO3 -type superstructure has been extensively studied [20]. In the alloys of Fe partial substitution by Co, there is a selective substitution for Fe in the crystallization process [21,22]. At elevated temperature the magnetic exchange coupling between nanocrystalline grains is associated with the change in the magnetization of the precipitated crystalline phase. In general, Co [(Co) = 1.7B ] substitutes FeII [(FeII , in (1/2, 1/2, 1/2) positions) = 1.35B ] [9,23] can improve soft magnetic behavior by increasing the magnetic moment of the precipitated crystalline phase. However, when successive increasing Co concentration in the alloys, the superfluous Co may substitute FeI [(FeI , in (1/4, 1/4, 1/4) positions) = 2.2B ] decreasing magnetization of the crystalline phase and gives rise to a diminution of the strength of the magnetic coupling, Aeff . Hence, reduction in both the strength of the exchange cryst interactions Aeff and saturation magnetization Ms , coupled with the highly anisotropy of the precipitated Co-rich phases K1 [9,14], give rise to a decrease of i for the studied alloys when x > 0.5, according to Eq. (5). 4. Conclusions (1) The nanocrystalline structures of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys formed during annealing treatments are influenced by annealing temperature Ta and Co content. The highest Vcr values and the lowest values were found to be x = 0.5 for the alloys. (2) Structural evolutions of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys in heating processes can be reflected by means of i –T curves. TCA and T1 can be directly indicated by the sharp Hopkinson peak and the obvious rise of i after TCA , respectively; furthermore, the formation of hard magnetic phase can also be detected associated with abrupt declines of i above 700 ◦ C. The TCA increases with Co content in the range of x ≤ 0.5, however, T1 decreases with increasing Co content. (3) The high-temperature magnetic permeability increase with Co content (x ≤ 0.5) in the alloys. A noticeable decrease of i was observed for the Co-richest alloys (x = 0.75). The behavior of i as a function of Co content depends on the changes in the
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
crystalline volume fraction and saturation magnetization of the precipitated crystalline phase. The selective substitutions of Co into the nonequivalent Fe sites contribute to variations in the saturation magnetization of the crystalline phases. Considerable i values at high temperature for the samples for x = 0.5 is attributed to the combination of the increment of saturation magnetization and high crystalline volume fraction, which reinforces the magnetic exchange coupling. Acknowledgment This work was supported by National Natural Science Foundation of China, under grant no. 50871073. References [1] Y. Yoshizawa, S. Oguma, Y. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] Y. Yoshizawa, K. Yamauchi, Mater. Trans. JIM 31 (1990) 307. [3] Y. Yoshizawa, K. Yamauchi, Mater. Sci. Eng. A 133 (176) (1991).
61
[4] Z. Wang, K.Y. He, J. Jin, J. He, L. Zhang, H.W. Zhang, B.G. Shen, Mater. Sci. Eng. A 304–306 (2001) 1046. [5] M. Müller, et al., J. Magn. Magn. Mater. 160 (1996) 284. [6] J. Zbroszczyk, et al., J. Magn. Magn. Mater. 160 (1996) 281. [7] L. Pascual, et al., J. Magn. Magn. Mater. 203 (1999) 79. [8] J.M. Borrego, et al., J. Non-Cryst. Solids 287 (2001) 120. [9] C. Gómez-Polo, et al., Phys. Rev. B 65 (2002) 024433. [10] M.S. Leu, I.C. Tung, T.S. Chin, Mater. Chem. Phys. 57 (1998) 117–124. [11] X.H. Ma, Z. Wang, X.T. Han, X. Yin, B.W. Wang, Mater. Sci. Eng. A 448 (2007) 216–220. [12] C.F. Conde, V. Franco, A. Conde, L.F. Kiss, J. Magn. Magn. Mater. 272–276 (2004) 1430–1432. [13] X.B. Liang, et al., J. Magn. Magn. Mater. 308 (2007) 227–232. [14] F. Mazaleyrat, Zs. Gercsi, et al., Mater. Sci. Eng. A 375–377 (2004) 1110–1115. [15] Zs. Gercsi, S.N. Kane, et al., Phys. Stat. Sol. C 1 (12) (2004) 3603–3607. [16] A. Hernando, I. Navarro, P. Gorría, Phys. Rev. B 51 (1995) 3281. [17] Q.Z. Zhi, B.S. Dong, W.Z. Chen, K.Y. He, Sci. Eng. A 448 (2007) 249–252. [18] G. Herzer, Scripta Metall. Mater. 33 (1995) 1741. [19] A. Hernando, et al., Phys. Rev. B 58 (1998) 366. [20] A. Hines, A.H. Menotti, J.I. Budnick, et al., Phys. Rev. B 13 (1976) 4060. [21] V.A. Niculescu, et al., J. Magn. Magn. Mater. 39 (1983) 223. [22] C. Gómez-Polo, J.I. Pérez-Landazabal, V. Recarte, Phys. Rev. B 66 (2002) 012401. [23] A. Hernando, A. González, J. Non-Cryst. Solids 287 (2001) 256–257.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Influence of Co content on the structure and magnetic permeability of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys Ye-mei Han a , Zhi Wang a,∗ , Xiang-hui Che a , Xue-gang Chen a , Wen-run Li a , Ya-li Li b a b
School of Science, Tianjin University, Tianjin 300072, PR China Key Laboratory of Advanced Ceramics and Machining Technology (Tianjin University), Ministry of Education, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2008 Received in revised form 28 October 2008 Accepted 5 November 2008 Keywords: Nanocrystalline FeCo-based alloy Initial permeability i –T curve Exchange coupling
a b s t r a c t Structural and soft magnetic response of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys were investigated. Influences of heat treatment and Co content on the crystallization were analyzed through estimation of the crystalline volume fraction Vcr and thickness of the intergranular amorphous layer from XRD patterns. High-temperature soft magnetic response was analyzed by the temperature evolution of the magnetic permeability from room temperature to 780 ◦ C. It was found that the high-temperature soft magnetic behavior is improved with increasing Co content for x ≤ 0.5, which is ascribed to reinforcing magnetic coupling between the crystals, however, a decrease of i was observed for the Co richest sample. The observed i behavior as a function of Co content is interpreted in terms of variations in the degree of exchange coupling. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline Fe–Cu–Nb–Si–B soft magnetic alloys show attractive magnetic properties such as high saturation magnetization, high permeability, and low core loss, simultaneously [1–3]. However, due to the relatively low Curie point of the residual amorphous matrix [4], they cannot be used at high temperature. Partial substitution of Co for Fe was carried out to meet the desire for high-temperature magnetic properties. When Co is added to the FeSiBNbCu alloys, crystalline phase with higher magnetic moment and Curie temperature determines their magnetic response at high temperatures [5–9]. Softness is mostly related to the exchange coupling between nanocrystalline grains through the amorphous matrix. The exchange coupling will be reinforced by increasing the saturation magnetization of the crystalline phases. As is shown [9], Co atoms selectively substitute Fe atoms into the nonequivalent Fe sites on the crystallization process and this substitution could affect the saturation magnetization of the crystalline phases and thus affect the magnetic interaction between crystals. Up to now, little attention has been paid to the discrepancy of magnetic moment of Co and Fe atoms and its effect on the magnetic exchange coupling and the soft magnetic response. In this paper, structural and soft magnetic response of nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys is analyzed by means of measuring i –T curves [4,10–12], influence of variations in the degree of exchange
∗ Corresponding author. Tel.: +86 22 87892075; fax: +86 22 27890681. E-mail address: [email protected] (Z. Wang). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.006
coupling on the i behavior as a function of Co content will be analyzed from the viewpoint of magnetic moment. In addition, changes in the crystalline volume fraction with Co content affecting the exchange coupling will also be discussed. 2. Experimental Amorphous ribbons 5–10 mm wide, about 30 m thick, were obtained by melt spinning technique with nominal composition (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys (x = 0, 0.25, 0.5, 0.75). The toroidal samples with an outer diameter of about 22 mm and inner diameter of about 18 mm were fabricated by winding the ribbons into toroidal cores. In order to obtain the characteristic nanocrystalline structure, the samples were submitted to isothermal treatments (30 min) under vacuum atmosphere (10−3 Pa) in a tubular furnace. The phase structure of the ribbons was examined by X-ray diffraction (XRD) using D/max-2500/PC with Co K␣ radiation ( = 1.7889 Å). Differential scanning calorimetry (DSC) with a heating rate of 20 ◦ C/min was used to determine the thermal processes of alloy studied. The permeability was in situ measured in a furnace with Ar atmosphere protection by using an HP4194A impedance analyzer at H = 0.4 A/m and f = 10 kHz and the heating rate was 10 ◦ C/min. 3. Results and discussion The nanocrystalline structure, mainly composed of (FeCo)3 Si and ␣-FeCo(Si) crystals embedded in amorphous matrix, was obtained in these alloys after annealing at temperatures of
58
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61 Table 1 The average grain size, (D/nm), of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C. Ta (◦ C) 470 500 530 560 590 620
Fig. 1. XRD patterns for the 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys.
450–620 ◦ C. The structure was monitored by X-ray diffraction. Fig. 1 shows the XRD patterns for the 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys. As presented, patterns of all the annealed samples show the characteristic diffraction peaks corresponded to crystalline precipitates. It is indicated that all the annealed samples partially crystallize after annealing and their microstructure consists of the residual amorphous phase and the nanocrystalline phase. The crystalline volume fraction, Vcr , was calculated from the XRD spectra according to Vcr =
Icr Icr + Iam
where Icr and Iam are the integral intensities of diffraction peaks of crystalline phase and amorphous phase, respectively. These integral intensities are determined from peak areas [11,13,14] and the results are shown in Fig. 2. For the sample of x = 0.5, Vcr increases with Ta and the value of Vcr is relatively low for Ta = 470 ◦ C because of the incomplete occurrence of the first stage of crystallization during the annealing treatment. Whereas, in the case of the alloy for x = 0.75, a higher Vcr of about 54% is obtained for the 470 ◦ C-annealed sample and the Vcr has no significant increase with Ta in the range from 530 to 620 ◦ C. This suggests that annealing at 530 ◦ C for 30 min ensures the formation of stable structure for the sample of x = 0.75. In fact, Vcr values were obtained by numerous computations and a difference close to 5% is introduced as an error bar in data displayed in Fig. 2. It is noteworthy that the highest Vcr was obtained
Fig. 2. The crystalline volume fraction (Vcr /%) for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C.
x=0
x = 0.25
x = 0.5
x = 0.75
7 11 12 12 14
11 11 12 11 13 14
12 13 14 13 14 15
15 16 16 17 17 16
for the sample of x = 0.5 when (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys were sufficiently crystallized. In addition, the obtained Vcr values are basically consistent with what were presented by Gercsi et al. [15]. The average grain size, D, was calculated from the X-ray diffraction patterns by means of the Scherrer formula. As is shown in Table 1, in principle, D increases with Co content. The calculated D values agree with the tendency of D upon Co content reported by Mazaleyrat et al. [14]. In addition, D increases slightly with Ta for x ≤ 0.5, while it keeps nearly unchangeable with Ta from 470 to 620 ◦ C for the sample of x = 0.75. Additionally, the thickness of the amorphous layer between the surfaces of two adjacent nanocrystallites, , can be approximately estimated as [16]: =D
1/3 1 Vcr
−1
The calculated results were presented in Fig. 3. It can be seen that cobalt content in the alloys clearly affects values, examined within the annealing temperature range of 470–620 ◦ C. Taking into account the error in the employed method, a maximum deviation of 12% is computed from errors of Vcr and D according to the theory of errors. Such a maximum deviation is introduced as an error bar in data shown in Fig. 3. However, even within this range of estimated error, it can be clearly seen that the corresponding distance between the crystallites, , becomes smaller as Ta increases, the lowest value was obtained for the alloy with Co content of x = 0.5. The analysis of the above graphs leads to conclusions that nanocrystalline structure formed during heat treatment is influenced by annealing temperature Ta , and also by the Co content. The addition of a larger amount of Co may lower the nanocrystallization temperature. With the optimal annealing conditions, the highest Vcr values and the lowest values were found to be x = 0.5 for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys. With respect to the
Fig. 3. The thickness of amorphous layer between the surfaces of two adjacent nanocrystallites, (/nm), for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys annealed at 470–620 ◦ C.
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
Fig. 5. i –T curve of heating (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloy.
59
and
cooling
for
as-quenched
Fig. 4. i –T curves for nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
influence of the detected changes in structure on the magnetic properties of the samples, we measured the temperature dependence of initial permeability i (i –T curves) for nanocrystalline (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys, as shown in Fig. 4. A sharp Hopkinson peak [17] was observed at the Curie point of amorphous phase, TCA , from which we can get TCA = 335, 425, 460 and 460 ◦ C for the samples of x = 0, 0.25, 0.5 and 0.75, respectively. When the measuring temperature T approaches TCA , i decreased steeply to practically zero, which is attributed to the transition from ferromagnetic to paramagnetic state of the amorphous alloy. With increasing temperature of T > TCA , crystallization of amorphous phase gives rise to the rise of i . Finally, a magnetic hardening occurs at around 700 ◦ C, which is reflected by a dramatic drop of i . The magnetic hardening can be attributed either to magnetic transition from ferromagnetic to paramagnetic state of the precipitated crystalline phase or to formation of a hard magnetic phase. If the former controls the drop of i , a parallel peak of i can be detected on the cooling i –T curve. However, the value of i remains unchangeable when the alloy is cooled down from 700 ◦ C to room temperature. Consequently, magnetic softness is lost, as is presented in Fig. 5; accordingly, this magnetic hardening at high temperatures should be attributed to the formation of hard magnetic phase. Apart from TCA , the detectable crystallization temperature, T1 , associated to the first obvious rise of i , can be reflected by i –T curves for as-quenched (Fe0.5 Co0.5 )73.5 Cu1 Nb3 Si13.5 B9 alloys. Dependence of TCA and T1 on Co content for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys are shown in Fig. 6. As is shown, TCA increases with Co content before x < 0.5, after x ≥ 0.5, TCA is unchangeable. However, T1 continuously decreases with increasing Co content in the range of 0 ≤ x ≤ 0.75. This decrease is also confirmed through the DSC analysis presented in Fig. 7. In addition, we observed that the temperature gap between TCA and T1 becomes narrow with Co content increasing in the alloys. In particular, TCA = T1 for (Fe0.25 Co0.75 )73.5 Cu1 Nb3 Si13.5 B9 alloy, which means that the amorphous precursor will nanocrystallize as soon as it transforms from ferromagnetic to paramagnetic state. Fig. 7 shows the DSC curves performed at 20 ◦ C/min for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys. As is shown, the first crystal-
Fig. 6. Dependence of TCA and T1 on Co content for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
lization onset temperature, Tx1 , decreases with the increase of Co content. In order to analyze the effect of Co content on i behavior of the alloys, i –T curves for 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys were measured as is shown in Fig. 8. The optimum soft magnetic state at room temperature is obtained for the annealed sample with x = 0. However, a rapid deterioration of soft magnetic properties is observed with increasing temperature, close to the Curie point of the amorphous material. Thus, Co-free nanocrystalline alloys do not exhibit good
Fig. 7. DSC curves for (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
60
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
where K1 is the local anisotropy of the crystallites, v and D are the volume fraction and the mean grain size of crystals, respectively. The Aeff should be related to the strength of the exchange interactions between any pair of adjacent nanocrystals, which depends on the crystalline volume fraction and influence of the grain’s magnetization [19]. The combination of the above equations readily gives i = p
Ms2 A3eff
(3)
0 v2 K14 D6
As mentioned above, Ms denotes the average saturation magnetization of materials consisting of the saturation magnetization of cryst residual amorphous phases (Msam ) and crystalline phases (Ms ). Then in a duel phase system i can be written as i = p
cryst 2 3 ) Aeff 0 v2 K14 D6
(va Msam + vMs
where va and v are the volume fraction of residual amorphous phase and crystalline phase, respectively. When taking into consideration
Fig. 8. i –T curves for 560 ◦ C-annealed (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 alloys.
2 ∝ (1 − (T/T A )) of the term Mam C
high-temperature soft magnetic properties. With respect to the sample for x = 0.75, the i was found to vary little up to 650 ◦ C during the heating process from room temperature to 700 ◦ C, but the value of i is relatively low. For the four samples, the best combination of high permeability i and magnetic stability in the measuring temperature range of 300–700 ◦ C is obtained for the sample of x = 0.5. For all i –T curves in Fig. 8, the rapid fall of i at 710 ◦ C is due to the formation of the boride phases. The detected behavior of i with x must be correlated with the dependence of Vcr and on Co content as is reflected by previous X-ray diffraction analysis. An improvement in the soft magnetic behavior is attributed to magnetic coupling between the crystalline grains through the intergranular phase. High-temperature magnetic behavior is strongly dependent on the volumetric fraction of crystalline grains. As the crystalline fraction increases and the intergranular distances decrease, the magnetic coupling between the grains becomes effective, causing a magnetic softening. For the (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys, the highest Vcr values and the lowest values were obtained for the alloys with Co content of x = 0.5; thus, the magnetic coupling is the most effective which results in the highest i value at high temperatures. As shown in above results, addition of Co can shift the Curie temperature to higher values. i at high temperatures increases with increasing Co content in the presented nanocrystalline alloys (x ≤ 0.5). However, TCA does not exhibit significant change and a decrease in i is observed as increasing Co content for higher Co content alloys (from x = 0.5 to x = 0.75). As is reported, the magnetic characteristics of the precipitated crystalline phase play a dominant role in the evolution of the magnetic properties of the FeCo-based nanocrystalline alloys [9]. The initial permeability can be expressed as [18]: i = p
Ms2 0 K
(1)
where Ms is average saturation magnetization of the materials, p is a dimensionless prefactor and 0 is the vacuum magnetic permeability. According to Hernando et al. [19], the effective magnetic anisotropy K of nanostructured soft magnetic alloy may be expressed as K =
v2 K14 D6 A3eff
(4)
(2)
2ˇ
[19], where ˇ is the Heisenberg cryst
exponent, obviously, when T ≥ TCA , Msam << Ms , Eq. (4) can lead to the following expression for magnetic permeability: i = p
cryst 2 3 ) Aeff 4 0 K1 D6
(Ms
(5)
In the Fe-based nanocrystalline alloys, the crystal phase consisting mainly of the Fe3 Si with fcc DO3 -type superstructure has been extensively studied [20]. In the alloys of Fe partial substitution by Co, there is a selective substitution for Fe in the crystallization process [21,22]. At elevated temperature the magnetic exchange coupling between nanocrystalline grains is associated with the change in the magnetization of the precipitated crystalline phase. In general, Co [(Co) = 1.7B ] substitutes FeII [(FeII , in (1/2, 1/2, 1/2) positions) = 1.35B ] [9,23] can improve soft magnetic behavior by increasing the magnetic moment of the precipitated crystalline phase. However, when successive increasing Co concentration in the alloys, the superfluous Co may substitute FeI [(FeI , in (1/4, 1/4, 1/4) positions) = 2.2B ] decreasing magnetization of the crystalline phase and gives rise to a diminution of the strength of the magnetic coupling, Aeff . Hence, reduction in both the strength of the exchange cryst interactions Aeff and saturation magnetization Ms , coupled with the highly anisotropy of the precipitated Co-rich phases K1 [9,14], give rise to a decrease of i for the studied alloys when x > 0.5, according to Eq. (5). 4. Conclusions (1) The nanocrystalline structures of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys formed during annealing treatments are influenced by annealing temperature Ta and Co content. The highest Vcr values and the lowest values were found to be x = 0.5 for the alloys. (2) Structural evolutions of (Fe1−x Cox )73.5 Cu1 Nb3 Si13.5 B9 (x = 0, 0.25, 0.5, 0.75) alloys in heating processes can be reflected by means of i –T curves. TCA and T1 can be directly indicated by the sharp Hopkinson peak and the obvious rise of i after TCA , respectively; furthermore, the formation of hard magnetic phase can also be detected associated with abrupt declines of i above 700 ◦ C. The TCA increases with Co content in the range of x ≤ 0.5, however, T1 decreases with increasing Co content. (3) The high-temperature magnetic permeability increase with Co content (x ≤ 0.5) in the alloys. A noticeable decrease of i was observed for the Co-richest alloys (x = 0.75). The behavior of i as a function of Co content depends on the changes in the
Y.-m. Han et al. / Materials Science and Engineering B 156 (2009) 57–61
crystalline volume fraction and saturation magnetization of the precipitated crystalline phase. The selective substitutions of Co into the nonequivalent Fe sites contribute to variations in the saturation magnetization of the crystalline phases. Considerable i values at high temperature for the samples for x = 0.5 is attributed to the combination of the increment of saturation magnetization and high crystalline volume fraction, which reinforces the magnetic exchange coupling. Acknowledgment This work was supported by National Natural Science Foundation of China, under grant no. 50871073. References [1] Y. Yoshizawa, S. Oguma, Y. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] Y. Yoshizawa, K. Yamauchi, Mater. Trans. JIM 31 (1990) 307. [3] Y. Yoshizawa, K. Yamauchi, Mater. Sci. Eng. A 133 (176) (1991).
61
[4] Z. Wang, K.Y. He, J. Jin, J. He, L. Zhang, H.W. Zhang, B.G. Shen, Mater. Sci. Eng. A 304–306 (2001) 1046. [5] M. Müller, et al., J. Magn. Magn. Mater. 160 (1996) 284. [6] J. Zbroszczyk, et al., J. Magn. Magn. Mater. 160 (1996) 281. [7] L. Pascual, et al., J. Magn. Magn. Mater. 203 (1999) 79. [8] J.M. Borrego, et al., J. Non-Cryst. Solids 287 (2001) 120. [9] C. Gómez-Polo, et al., Phys. Rev. B 65 (2002) 024433. [10] M.S. Leu, I.C. Tung, T.S. Chin, Mater. Chem. Phys. 57 (1998) 117–124. [11] X.H. Ma, Z. Wang, X.T. Han, X. Yin, B.W. Wang, Mater. Sci. Eng. A 448 (2007) 216–220. [12] C.F. Conde, V. Franco, A. Conde, L.F. Kiss, J. Magn. Magn. Mater. 272–276 (2004) 1430–1432. [13] X.B. Liang, et al., J. Magn. Magn. Mater. 308 (2007) 227–232. [14] F. Mazaleyrat, Zs. Gercsi, et al., Mater. Sci. Eng. A 375–377 (2004) 1110–1115. [15] Zs. Gercsi, S.N. Kane, et al., Phys. Stat. Sol. C 1 (12) (2004) 3603–3607. [16] A. Hernando, I. Navarro, P. Gorría, Phys. Rev. B 51 (1995) 3281. [17] Q.Z. Zhi, B.S. Dong, W.Z. Chen, K.Y. He, Sci. Eng. A 448 (2007) 249–252. [18] G. Herzer, Scripta Metall. Mater. 33 (1995) 1741. [19] A. Hernando, et al., Phys. Rev. B 58 (1998) 366. [20] A. Hines, A.H. Menotti, J.I. Budnick, et al., Phys. Rev. B 13 (1976) 4060. [21] V.A. Niculescu, et al., J. Magn. Magn. Mater. 39 (1983) 223. [22] C. Gómez-Polo, J.I. Pérez-Landazabal, V. Recarte, Phys. Rev. B 66 (2002) 012401. [23] A. Hernando, A. González, J. Non-Cryst. Solids 287 (2001) 256–257.