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Nd60Fe30 − xNixAl10 bulk metallic glasses with high hard magnetic properties
Materials Letters 61 (2007) 219 – 222 www.elsevier.com/locate/matlet
Nd60Fe30 − xNixAl10 bulk metallic glasses with high hard magnetic properties Ming-xu Xia
a,⁎
, Qing-ge Meng a , Shu-guang Zhang a , Jian Liu a , Chao-li Ma b , Jian-guo Li
a
a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China b School of Materials Science and Engineering, Beihang University, Beijing 100083, China Received 1 December 2005; accepted 6 April 2006 Available online 5 May 2006
Abstract Bulk metallic glasses (BMGs) Nd60Fe30 − xNixAl10 were prepared by suction cast method. The glass forming abilities (GFAs) and the hard magnetic properties of the BMGs were examined by differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM), respectively. The results show that the largest GFA (Tx / Tm = 0.61) of the alloys was obtained when Fe was substituted by 10% Ni and the rods of Nd60Fe20Ni10Al10 have the coercivity up to 323 kA/m and the remanence up to 9.41 Am2/kg as high as Nd–Fe–Co–Al, the Nd–Fe-based BMGs with highest hard magnetic properties up to date. The homogeneous distribution of Fe-rich nano-clusters, Nd(FeNiAl)2, in amorphous matrix is responsible for the enhancement. © 2006 Elsevier B.V. All rights reserved. Keywords: Suction casting method; Bulk metallic glass; Magnetic properties
1. Introduction Several years ago, a new class of BMGs with magic hard magnetic properties at room temperature were prepared by Inoue et al. [1,2] and Ding et al. [3]. Further studies indicated that the attracting properties depend on the fine ferromagnetic phases (metastable or non-equilibrium magnetic phases) in amorphous matrix (non-magnetic phases) [4]. Among these phases, the eutectic ferromagnetic phase consists of Nd phase and rich-Fe phase and the amorphous matrix consists of rich-Nd phase. The exchange–coupling interaction between Nd phase and rich-Fe phase in eutectic phase leads to the properties mentioned above [5,6]. However, the hard magnetic properties of Nd–Fe-based BMGs were very low until Co was introduced in the alloys. The as-cast Nd–Fe–Co–Al BMG samples show large coercivities up to 0.4 T [7–10] at room temperature and large reduced crystalline temperature Tx / Tm, the ratio of crystalline temperature (Tx) and melting temperature(Tm), up to 0.96, which has been proposed to assess the GFA of Nd–Febased amorphous alloys [1,2]. More alloying elements have
⁎ Corresponding author. Tel.: +86 21 62932569; fax: +86 21 62933074. E-mail address: [email protected] (M. Xia). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.035
been attempted [11–13] to enhance the GFA of alloys, e.g. the transition metals Ni and Cu, but the results are disappointing. Although Ni is ferromagnetic like Co and both Ni and Cu have similar atom radius with Fe as well as Co, the Nd–Fe–Ni–Cu– Al amorphous alloys still exhibited low coercivity (70 kA/m) or even paramagnetic properties at room temperature [11–13]. So, it is very important to study the variation properties of Nd–Fe– M–Al BMGs with different alloying elements for science interest and potential technical application. In fact, little reports have been presented in these aspects. In our experiments, Ni was considered individually to be added into Nd60Fe30Al10. The microstructure mechanism for high hard magnetic properties was involved in this article. 2. Experiments The Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ingots were prepared by arc melting from elemental Nd, Fe, Ni, Al with a purity of 99.9% in a titanium-gettered argon atmosphere. Ribbons with a cross section of about 30 μm thickness and 2 mm width were prepared from the ingots in argon atmosphere by melting spinning. A Copper roller had a diameter of 200 mm and its rotation speed was fixed at 4000 rpm. Cylindrical specimens of 3 mm in diameter and 50 mm in length
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M. Xia et al. / Materials Letters 61 (2007) 219–222
Fig. 1. DSC curves of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons.
Fig. 2. XRD patterns of the as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods.
were prepared by suction casting method under argon atmosphere. X-ray diffractometry (XRD) in a D8 Discover X-ray diffractometer with Cu Kα radiation was used to confirm the amorphous structures of the ribbons and the as-cast rods. Thermal analysis was performed by a differential scanning calorimeter Netzsch DSC C404 and a differential thermal analyzer DTA1600 under argon atmosphere with heating rates of 0.33 and 0.17 K/s, respectively. The metallographic study on the sections of as-cast alloys was performed by an optical microscopy (OM) and a JEOL 6460 scanning electron microscopy (SEM) equipped with an in situ energy dispersive X-ray spectroscopy (EDX). JEOL 2100 transmission electron microscope (TEM) was also applied to study the microstructure of the samples. The samples were polished and etched for 20 s at room temperature in a solution of 2% hydrofluoric acid and 98% distilled water in volume ratio before metallographic study. Magnetic measurements were performed using a vibrating sample magnetometer with a maximum applied field of 1.8 T at room temperature.
decreased when the concentration increases sequentially. Although the mixing heat of Ni with Nd, − 31 kJ/mol, is more negative than Co, −22 kJ/mol and Cu, −22 kJ/mol, respectively, which should have suggested higher GFA according to Inoue's experimental rules [14], the DSC results of Nd–Fe–Ni–Al show lower GFA than Nd–Fe–Al [1] (Tx = 784 K, Tx / Tm = 0.85) and Nd–Fe–Co–Al [10] (Tx = 764 K, Tx / Tm = 0.94). The lower GFA of Nd–Fe–Ni–Al might be caused by some devitrification of easy crystallized phases, which is proved by the low first crystalline temperature in DSC. Fig. 2 shows the XRD patterns of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods measured with Cu Kα radiation. Crystalline peaks, presented together with the amorphous humps in the sample with x = 5, 15, 20, indicated the coexisting of amorphous and crystalline phases. Some crystalline peaks in XRD patterns could be identified as hexagonal neodymium phase as shown in Fig. 2. Others are identified as the coexisting of the ferromagnetic Fe-rich phase with a structure of NdFe2 and the Nd-rich phase such as Nd3Al. The further composition analysis shows that the Fe-rich phase is Nd34Fe42Ni13Al11 and Nd-rich phase is Nd67Fe11Ni13Al9. Combined with the XRD result, the component of Fe-rich phase can be inferred as Nd(Fe,Ni,Al)2. The image of scanning electron microscopy (SEM) revealed that the ferromagnetic phase is black cross-like and imbeds homogenously in the amorphous matrix. The size of the cross-like
3. Results and discussions In the XRD patterns of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons, two bumps were found at about 30° and 55°, which revealed that the ribbons are fully amorphous. DSC curves of the ribbons are shown in Fig. 1. No obvious glass transition was observed in the curves. Several exothermic crystalline peaks in the DSC curves suggest that there are multi-crystallization in the heating process. Tx, Tm, Tx / Tm are listed in Table 1. The thermoanalysis results show that GFA increased with the increase of Ni concentration at first and then
Table 1 The Tx, Tm and Tx / Tm of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons Nd60Fe30 − xNixAl10
Tx, K
Tm, K
Tx / Tm
x=5 x = 10 x = 15 x = 20
460 482 470 4556
789 794 801 780
0.58 0.61 0.59 0.58
Fig. 3. Back scattered SEM micrographs of as-cast Nd60Fe10Ni20Al10 alloys.
M. Xia et al. / Materials Letters 61 (2007) 219–222
221
Fig. 6. The remanence and coercivity of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods with different Ni component. Fig. 4. Bright field TEM image of the as-cast Nd60Fe10Ni20Al10 alloys. The inset shows a typical selected area diffraction pattern.
structure is about 5–10 μm. A fine eutectic structure was found in the back scattering electron (BSE) image of these phases (Fig. 3). However, the XRD pattern of Nd60Fe20Ni10Al10 rod didn't show any crystalline peak except typically broad halos (Fig. 2). Further observed by TEM, unexpected large amounts of white particles with an average size smaller than 5 nm were found in the black matrix although the selected area diffraction pattern of TEM (insert of Fig. 4) suggests a fully amorphous structure. It means that the amorphous structure consists of large amount of nano-clusters. For the limited resolution, the structure of nano-crystalline can not be identified by XRD and selected area diffraction. The M–H hysteresis loops are shown in Fig. 5, which were measured at a maximum applied field of 1.8 T at room temperature. The results show that the substitution of Ni for Fe in Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) caused the increase of remanence from 3.80 to 9.41 Am2/kg with x = 10 and then sharply decreases to 4.28 Am2/kg with x = 20 (Fig. 6), which is different with the magnetization of Nd60Fe30 − xCoxAl10 (5 ≤ x ≤ 30) decreasing continuously with the Co concentration. The values of coercivity are fairly constant with the change of Ni concentration (Fig. 6), which manifested the formation of the same magnetic phase in these alloys because the coercivity is an inherent property directly related to the phase constituent of alloys. For
the same reason, the ferromagnetic nano-clusters of Nd60Fe20Ni10Al10 were inferred as Nd(Fe,Ni,Al)2. The magnetization mechanism of Nd–Fe-based BMG is attributed to the cluster model that the exchange–coupling interaction among magnetic clusters with random anisotropy could cause the high coercivity of the magnetic system [15–18]. Inoue [19] has proved the presence of these short-range ordered clusters in Pr–Fe–Al and Nd– Fe–Al BMGs by HRTEM. In our experiments, the nano-clusters, Nd (Fe,Ni,Al)2, were also found embedding in the amorphous matrix, which would play the same roles for the similar phase constituent and cause the increase of the hard magnetic properties. Furthermore, the presence of strong exchange couple is also confirmed by the significant remanence enhancement of our BMGs. The Mr/Ms values of Nd60 Fe30 − xNixAl10 (x = 5, 10, 15, 20) are 0.666, 0.672, 0.637 and 0.622, respectively, which is obviously greater than 0.5, the value predicted for randomly oriented and non-interacting particles by the Stoner– Wohlfarth theory [20]. However, because of the different GFAs, partial crystallization of the Nd60Fe30 − xNixAl10 (x = 5, 15, 20), which has been proved by XRD and OM images (Fig. 7), abated the contents of the clusters. Thus, although the coercivity of these alloys has little fluctuation for the same magnetic phases, the remanence will decrease with crystallization for the decreasing GFA, which have also been verified by H. Chiriac [21] and at the same time, the crystallization decreases the effect of exchange couple interaction shown in Mr/Ms as well. That is to say, the appropriate concentration of Ni can suppress the growth of ferromagnetic Nd(Fe,Ni,Al)2 phases and at last leads to high hard magnetic properties.
4. Summary
Fig. 5. M–H hysteresis loops of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods.
The substitution of Ni for Fe decreased the GFA of Nd–Fe– Ni–Al. The largest GFA (Tx / Tm = 0.61) was obtained when 10% Ni was introduced into Nd60Fe30Al10. The as-cast Nd60Fe20Ni10Al10 rod reveals hard magnetic properties with coercivity of 323 kA/m and remanence of 9.41 Am 2 /kg almost as high as the Nd–Fe–Co–Al alloys. The latter was the Nd– Fe-based BMGs with highest hard magnetic properties up to now. The high hard magnetic properties were caused by the distribution of ferromagnetic nano-clusters embedded in amorphous matrix. The nano-clusters, Nd(Fe,Ni,Al)2, are responsible for the high hard magnetic properties. The results of study suggest that the hard magnetic properties of Nd–Fe–
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Fig. 7. The OM images of (a) Nd60Fe25Ni5Al10, (b) Nd60Fe20Ni10Al10, (b) Nd60Fe15Ni15Al10 and (b) Nd60Fe10Ni20Al10 BMGs. Among them, (a), (c) and (d) are partial crystalline which will be responsible for the decrease of remanence, while (b) is fully amorphous with the highest hard magnetic properties.
Al can be enhanced by introducing an appropriate alloying element. Acknowledgements The authors express their appreciation for the financial support of the National Natural Science Foundation of China under Grant No. 50274051 and the National Natural Science Foundation for Distinguished Young Scholars of China under Grant No. 50125101. References [1] A. Inoue, T. Zhang, W. Zhang, A. Takeuchi, Mater. Trans., JIM 37 (1996) 99. [2] A. Inoue, T. Zhang, A. Takeuchi, W. Zhang, Mater. Trans., JIM 37 (1996) 636. [3] J. Ding, Y. Li, X.Z. Wang, J. Phys., D, Appl. Phys. 32 (1999) 713. [4] G. Ausanio, H. Chiriac, V. Iannottii, C. Hison, L. Lanotte, N. Lupu, J. Magn. Magn. Mater. 265 (2003) 138. [5] Z.G. Sun, W. Löser, J. Echert, K.-H. Müller, L. Schultz, J. Magn. Magn. Mater. 261 (2003) 122. [6] C.E. Rodríguez Torres, A.F. Cabrera, F.H. Sánchez, O.V. Billoni, S.E. Urreta, L.M. Fabietti, J. Magn. Magn. Mater. 267 (2003) 92. [7] B.C. Wei, Y. Zhang, Y.X. Zhuang, D.Q. Zhao, M.X. Pan, W.H. Wang, W. R. Hu, J. Appl. Phys. 89 (2001) 3529.
[8] R. Sato Turtelli, D. Triyono, R. Grössinger, H. Michor, J.H. Espina, J.P. Sinnecker, H. Sassik, J. Eckert, G. Kumar, Z.G. Sun, G.J. Fan, Phys. Rev., B 66 (2002) 054441. [9] G. Kumar, J. Echert, S. Roth, K.-H. Müller, L. Schultz, J. Alloys Compd. 348 (2003) 309. [10] G. Kumar, J. Echert, S. Roth, W. Löser, L. Schultz, S. Ram, Acta Mater. 51 (2003) 229. [11] Z. Zhang, W.H. Wang, Y. Hirotsu, Mater. Sci. Eng., A 385 (2004) 38. [12] M.B. Tang, D.Q. Zhang, M.X. Pan, B.C. Wei, W.H. Wang, J. Phys., D, Appl. Phys. 37 (2004) 973. [13] W.H. Li, B.C. Wei, Y.F. Sun, L. Xia, Y.R. Wang, Y.D. Dong, Mater. Sci. Eng., A 385 (2004) 397. [14] A. Inoue, Mater. Sci. Forum 179/181 (1995) 691. [15] L. Wang, J. Ding, H.Z. Kong, Y. Li, T.P. Feng, Phys. Rev., B 64 (2001) 214410. [16] L. Wang, J. Ding, Y. Li, Y.P. Feng, N.X. Phuc, N.H. Dan, J. Appl. Phys. 89 (12) (2001) 8046. [17] K. Siratori, K. Nagayama, H. Ino, N. Saito, Y. Nakagawa, J. Phys. Soc. Jpn. 59 (1990) 2483. [18] R.C. Taylor, T.R. McGuire, J.M.D. Coey, A. Gangulee, J. Appl. Phys. 49 (1978) 2885. [19] A. Inoue, A. Takeuchi, T. Zhang, Metall. Mater. Trans., A 29 (1998) 1779. [20] M.A. Al-Khafaji, W.M. Raiforth, M.R.J. Gibbs, H.A. Davies, J.E.L. Bishop, J. Magn. Magn. Mater. 182 (1998) 111. [21] H. Chiriac, N. Lupu, J.M. Greneche, J. Magn. Magn. Mater. 242–245 (2002) 1310.
Nd60Fe30 − xNixAl10 bulk metallic glasses with high hard magnetic properties Ming-xu Xia
a,⁎
, Qing-ge Meng a , Shu-guang Zhang a , Jian Liu a , Chao-li Ma b , Jian-guo Li
a
a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China b School of Materials Science and Engineering, Beihang University, Beijing 100083, China Received 1 December 2005; accepted 6 April 2006 Available online 5 May 2006
Abstract Bulk metallic glasses (BMGs) Nd60Fe30 − xNixAl10 were prepared by suction cast method. The glass forming abilities (GFAs) and the hard magnetic properties of the BMGs were examined by differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM), respectively. The results show that the largest GFA (Tx / Tm = 0.61) of the alloys was obtained when Fe was substituted by 10% Ni and the rods of Nd60Fe20Ni10Al10 have the coercivity up to 323 kA/m and the remanence up to 9.41 Am2/kg as high as Nd–Fe–Co–Al, the Nd–Fe-based BMGs with highest hard magnetic properties up to date. The homogeneous distribution of Fe-rich nano-clusters, Nd(FeNiAl)2, in amorphous matrix is responsible for the enhancement. © 2006 Elsevier B.V. All rights reserved. Keywords: Suction casting method; Bulk metallic glass; Magnetic properties
1. Introduction Several years ago, a new class of BMGs with magic hard magnetic properties at room temperature were prepared by Inoue et al. [1,2] and Ding et al. [3]. Further studies indicated that the attracting properties depend on the fine ferromagnetic phases (metastable or non-equilibrium magnetic phases) in amorphous matrix (non-magnetic phases) [4]. Among these phases, the eutectic ferromagnetic phase consists of Nd phase and rich-Fe phase and the amorphous matrix consists of rich-Nd phase. The exchange–coupling interaction between Nd phase and rich-Fe phase in eutectic phase leads to the properties mentioned above [5,6]. However, the hard magnetic properties of Nd–Fe-based BMGs were very low until Co was introduced in the alloys. The as-cast Nd–Fe–Co–Al BMG samples show large coercivities up to 0.4 T [7–10] at room temperature and large reduced crystalline temperature Tx / Tm, the ratio of crystalline temperature (Tx) and melting temperature(Tm), up to 0.96, which has been proposed to assess the GFA of Nd–Febased amorphous alloys [1,2]. More alloying elements have
⁎ Corresponding author. Tel.: +86 21 62932569; fax: +86 21 62933074. E-mail address: [email protected] (M. Xia). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.035
been attempted [11–13] to enhance the GFA of alloys, e.g. the transition metals Ni and Cu, but the results are disappointing. Although Ni is ferromagnetic like Co and both Ni and Cu have similar atom radius with Fe as well as Co, the Nd–Fe–Ni–Cu– Al amorphous alloys still exhibited low coercivity (70 kA/m) or even paramagnetic properties at room temperature [11–13]. So, it is very important to study the variation properties of Nd–Fe– M–Al BMGs with different alloying elements for science interest and potential technical application. In fact, little reports have been presented in these aspects. In our experiments, Ni was considered individually to be added into Nd60Fe30Al10. The microstructure mechanism for high hard magnetic properties was involved in this article. 2. Experiments The Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ingots were prepared by arc melting from elemental Nd, Fe, Ni, Al with a purity of 99.9% in a titanium-gettered argon atmosphere. Ribbons with a cross section of about 30 μm thickness and 2 mm width were prepared from the ingots in argon atmosphere by melting spinning. A Copper roller had a diameter of 200 mm and its rotation speed was fixed at 4000 rpm. Cylindrical specimens of 3 mm in diameter and 50 mm in length
220
M. Xia et al. / Materials Letters 61 (2007) 219–222
Fig. 1. DSC curves of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons.
Fig. 2. XRD patterns of the as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods.
were prepared by suction casting method under argon atmosphere. X-ray diffractometry (XRD) in a D8 Discover X-ray diffractometer with Cu Kα radiation was used to confirm the amorphous structures of the ribbons and the as-cast rods. Thermal analysis was performed by a differential scanning calorimeter Netzsch DSC C404 and a differential thermal analyzer DTA1600 under argon atmosphere with heating rates of 0.33 and 0.17 K/s, respectively. The metallographic study on the sections of as-cast alloys was performed by an optical microscopy (OM) and a JEOL 6460 scanning electron microscopy (SEM) equipped with an in situ energy dispersive X-ray spectroscopy (EDX). JEOL 2100 transmission electron microscope (TEM) was also applied to study the microstructure of the samples. The samples were polished and etched for 20 s at room temperature in a solution of 2% hydrofluoric acid and 98% distilled water in volume ratio before metallographic study. Magnetic measurements were performed using a vibrating sample magnetometer with a maximum applied field of 1.8 T at room temperature.
decreased when the concentration increases sequentially. Although the mixing heat of Ni with Nd, − 31 kJ/mol, is more negative than Co, −22 kJ/mol and Cu, −22 kJ/mol, respectively, which should have suggested higher GFA according to Inoue's experimental rules [14], the DSC results of Nd–Fe–Ni–Al show lower GFA than Nd–Fe–Al [1] (Tx = 784 K, Tx / Tm = 0.85) and Nd–Fe–Co–Al [10] (Tx = 764 K, Tx / Tm = 0.94). The lower GFA of Nd–Fe–Ni–Al might be caused by some devitrification of easy crystallized phases, which is proved by the low first crystalline temperature in DSC. Fig. 2 shows the XRD patterns of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods measured with Cu Kα radiation. Crystalline peaks, presented together with the amorphous humps in the sample with x = 5, 15, 20, indicated the coexisting of amorphous and crystalline phases. Some crystalline peaks in XRD patterns could be identified as hexagonal neodymium phase as shown in Fig. 2. Others are identified as the coexisting of the ferromagnetic Fe-rich phase with a structure of NdFe2 and the Nd-rich phase such as Nd3Al. The further composition analysis shows that the Fe-rich phase is Nd34Fe42Ni13Al11 and Nd-rich phase is Nd67Fe11Ni13Al9. Combined with the XRD result, the component of Fe-rich phase can be inferred as Nd(Fe,Ni,Al)2. The image of scanning electron microscopy (SEM) revealed that the ferromagnetic phase is black cross-like and imbeds homogenously in the amorphous matrix. The size of the cross-like
3. Results and discussions In the XRD patterns of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons, two bumps were found at about 30° and 55°, which revealed that the ribbons are fully amorphous. DSC curves of the ribbons are shown in Fig. 1. No obvious glass transition was observed in the curves. Several exothermic crystalline peaks in the DSC curves suggest that there are multi-crystallization in the heating process. Tx, Tm, Tx / Tm are listed in Table 1. The thermoanalysis results show that GFA increased with the increase of Ni concentration at first and then
Table 1 The Tx, Tm and Tx / Tm of the Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) ribbons Nd60Fe30 − xNixAl10
Tx, K
Tm, K
Tx / Tm
x=5 x = 10 x = 15 x = 20
460 482 470 4556
789 794 801 780
0.58 0.61 0.59 0.58
Fig. 3. Back scattered SEM micrographs of as-cast Nd60Fe10Ni20Al10 alloys.
M. Xia et al. / Materials Letters 61 (2007) 219–222
221
Fig. 6. The remanence and coercivity of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods with different Ni component. Fig. 4. Bright field TEM image of the as-cast Nd60Fe10Ni20Al10 alloys. The inset shows a typical selected area diffraction pattern.
structure is about 5–10 μm. A fine eutectic structure was found in the back scattering electron (BSE) image of these phases (Fig. 3). However, the XRD pattern of Nd60Fe20Ni10Al10 rod didn't show any crystalline peak except typically broad halos (Fig. 2). Further observed by TEM, unexpected large amounts of white particles with an average size smaller than 5 nm were found in the black matrix although the selected area diffraction pattern of TEM (insert of Fig. 4) suggests a fully amorphous structure. It means that the amorphous structure consists of large amount of nano-clusters. For the limited resolution, the structure of nano-crystalline can not be identified by XRD and selected area diffraction. The M–H hysteresis loops are shown in Fig. 5, which were measured at a maximum applied field of 1.8 T at room temperature. The results show that the substitution of Ni for Fe in Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) caused the increase of remanence from 3.80 to 9.41 Am2/kg with x = 10 and then sharply decreases to 4.28 Am2/kg with x = 20 (Fig. 6), which is different with the magnetization of Nd60Fe30 − xCoxAl10 (5 ≤ x ≤ 30) decreasing continuously with the Co concentration. The values of coercivity are fairly constant with the change of Ni concentration (Fig. 6), which manifested the formation of the same magnetic phase in these alloys because the coercivity is an inherent property directly related to the phase constituent of alloys. For
the same reason, the ferromagnetic nano-clusters of Nd60Fe20Ni10Al10 were inferred as Nd(Fe,Ni,Al)2. The magnetization mechanism of Nd–Fe-based BMG is attributed to the cluster model that the exchange–coupling interaction among magnetic clusters with random anisotropy could cause the high coercivity of the magnetic system [15–18]. Inoue [19] has proved the presence of these short-range ordered clusters in Pr–Fe–Al and Nd– Fe–Al BMGs by HRTEM. In our experiments, the nano-clusters, Nd (Fe,Ni,Al)2, were also found embedding in the amorphous matrix, which would play the same roles for the similar phase constituent and cause the increase of the hard magnetic properties. Furthermore, the presence of strong exchange couple is also confirmed by the significant remanence enhancement of our BMGs. The Mr/Ms values of Nd60 Fe30 − xNixAl10 (x = 5, 10, 15, 20) are 0.666, 0.672, 0.637 and 0.622, respectively, which is obviously greater than 0.5, the value predicted for randomly oriented and non-interacting particles by the Stoner– Wohlfarth theory [20]. However, because of the different GFAs, partial crystallization of the Nd60Fe30 − xNixAl10 (x = 5, 15, 20), which has been proved by XRD and OM images (Fig. 7), abated the contents of the clusters. Thus, although the coercivity of these alloys has little fluctuation for the same magnetic phases, the remanence will decrease with crystallization for the decreasing GFA, which have also been verified by H. Chiriac [21] and at the same time, the crystallization decreases the effect of exchange couple interaction shown in Mr/Ms as well. That is to say, the appropriate concentration of Ni can suppress the growth of ferromagnetic Nd(Fe,Ni,Al)2 phases and at last leads to high hard magnetic properties.
4. Summary
Fig. 5. M–H hysteresis loops of as-cast Nd60Fe30 − xNixAl10 (x = 5, 10, 15, 20) rods.
The substitution of Ni for Fe decreased the GFA of Nd–Fe– Ni–Al. The largest GFA (Tx / Tm = 0.61) was obtained when 10% Ni was introduced into Nd60Fe30Al10. The as-cast Nd60Fe20Ni10Al10 rod reveals hard magnetic properties with coercivity of 323 kA/m and remanence of 9.41 Am 2 /kg almost as high as the Nd–Fe–Co–Al alloys. The latter was the Nd– Fe-based BMGs with highest hard magnetic properties up to now. The high hard magnetic properties were caused by the distribution of ferromagnetic nano-clusters embedded in amorphous matrix. The nano-clusters, Nd(Fe,Ni,Al)2, are responsible for the high hard magnetic properties. The results of study suggest that the hard magnetic properties of Nd–Fe–
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M. Xia et al. / Materials Letters 61 (2007) 219–222
Fig. 7. The OM images of (a) Nd60Fe25Ni5Al10, (b) Nd60Fe20Ni10Al10, (b) Nd60Fe15Ni15Al10 and (b) Nd60Fe10Ni20Al10 BMGs. Among them, (a), (c) and (d) are partial crystalline which will be responsible for the decrease of remanence, while (b) is fully amorphous with the highest hard magnetic properties.
Al can be enhanced by introducing an appropriate alloying element. Acknowledgements The authors express their appreciation for the financial support of the National Natural Science Foundation of China under Grant No. 50274051 and the National Natural Science Foundation for Distinguished Young Scholars of China under Grant No. 50125101. References [1] A. Inoue, T. Zhang, W. Zhang, A. Takeuchi, Mater. Trans., JIM 37 (1996) 99. [2] A. Inoue, T. Zhang, A. Takeuchi, W. Zhang, Mater. Trans., JIM 37 (1996) 636. [3] J. Ding, Y. Li, X.Z. Wang, J. Phys., D, Appl. Phys. 32 (1999) 713. [4] G. Ausanio, H. Chiriac, V. Iannottii, C. Hison, L. Lanotte, N. Lupu, J. Magn. Magn. Mater. 265 (2003) 138. [5] Z.G. Sun, W. Löser, J. Echert, K.-H. Müller, L. Schultz, J. Magn. Magn. Mater. 261 (2003) 122. [6] C.E. Rodríguez Torres, A.F. Cabrera, F.H. Sánchez, O.V. Billoni, S.E. Urreta, L.M. Fabietti, J. Magn. Magn. Mater. 267 (2003) 92. [7] B.C. Wei, Y. Zhang, Y.X. Zhuang, D.Q. Zhao, M.X. Pan, W.H. Wang, W. R. Hu, J. Appl. Phys. 89 (2001) 3529.
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