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The characterization of phase separations for FeCo–Al2O3 nanogranular films
Materials Letters 57 (2003) 2168 – 2173 www.elsevier.com/locate/matlet
The characterization of phase separations for FeCo–Al2O3 nanogranular films Ning Zhou, Changzheng Wang, Zhenghong Guo, Yonghua Rong*, T.Y. Hsu1 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 29 July 2002; accepted 29 August 2002
Abstract The phase separations of FeCo(41 vol.%) – Al2O3 nanogranular films sputtered at both room temperature (RT) and 823 K were characterized by analytical electron microscopy. The results indicate that RT sputtered film consists of FeCo amorphous phase accompanying a little amount of particles as bcc a-Fe(Co) super-saturation solid solution in Al2O3 matrix; meanwhile, the retained Fe and Co atoms almost homogeneously distribute in Al3O2 matrix. Whereas for 823 K sputtered film, the a-Fe particles with bcc structure and a-Co particles with hcp structure individually appear in Al2O3 matrix. According to the above experimental results, the evolution sequence of two-stage phase separations is suggested if the RT sputtered FeCo – Al2O3 film is gradually heated up below 973 K. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase separation; FeCo – Al2O3 nanogranular film; Analytical electron microscopy
1. Introduction Since Berkowitz et al. [1] and Xiao et al. [2] independently discovered the giant magnetoresistance (GMR) effect of Co – Cu granular film in 1992, various kinds of magnetic heterogeneous alloys with ferromagnetic granules (Fe, Co, Ni) embedded in nonmagnetic metals (such as Cu, Ag) [3] or in insulator (such as SiO2, Al2O3) [4,5] have been investigated. These investigations indicate that the prerequisite condition for exhibiting GMR effect is mutual insolubility between the ferromagnetic metals * Corresponding author. Fax: +86-21-6293-2435. 1 Xu Zuyao.
and nonferromegnetic metals or insulators, namely, the phase separation is required [6]. In the present paper, the feature of the phase separations in FeCo – Al2O3 granular film with GMR effect, as a new system, at different sputtering conditions, will be studied by analytical electron microscopy.
2. Experimental procedure The FeCo(41 vol.%)– Al2O3 granular films were sputtered respectively on a KCl substrate at room temperature (RT) and at 823 K with an spc350 multi-target magnetron controlled sputtering system made by ANELVA. FeCo target (the weight ratio of
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01168-0
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
Fe and Co is 1:1) and Al2O3 target (99.9% purity) were separately installed on two independently controlled RF cathodes, and were alternatively employed to sputter these films for about 7 min with 20 circles of substrate per minute. The thickness of films is about 50 nm, measured by a-step meter. The volume fraction of FeCo in the FeCo – Al2O3 films was controlled by changing the sputtering power of FeCo target and was determined by means of energy dispersive spectrum (EDS) in JEM-2010 transmission electron microscope (TEM). The sputtering gas (Ar) pressure was 4 10 1 Pa. The film specimens for TEM observation were obtained by putting a sputtered substrate into de-ionized water with a little acetone, then by using a copper grid to hold the film floating on the water after the dissolution of the KCl substrate. TEM observations were carried out on JEM-100CX, JEM-2010 and
2169
Philips Tecnai F20 with electron energy-loss spectrometer (EELS).
3. Results and discussions 3.1. As-room temperature sputtered FeCo – Al2O3 granular film Fig. 1(a) and (b) shows the TEM bright field image and dark field image of FeCo(41 vol.%) – Al2O3 granular film sputtered at RT, respectively. It can be found that a little amount of a-Fe(Co) particles with bcc structure (see Fig. 1(c)) disperse in Al2O3 matrix. EDS analysis as shown in Fig. 1(d) reveals that all particles contain both Fe and Co atoms. Fig. 2 is a high-resolution transmission electron microscopy (HRTEM) image for a local area in as-RT
Fig. 1. Analysis of particles in FeCo(41 vol.%) – Al2O3 film sputtered at RT. (a) Bright field image, (b) dark field image, (c) selected area diffraction pattern, (d) EDS of a particle (Cu peak from copper grid).
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Fe(Co) with bcc structure and FeCo amorphous phase in Al2O3 matrix. Fig. 3(a) and (b) shows the EELS iron map and cobalt map, respectively, revealing the almost homogeneous distribution of Fe and Co in the granular film. In Co – Fe binary phase diagram, there is a miscibility gap in wide composition range [7]; accordingly, Fe– Co alloy with the composition ratio of 1: 1 will result in the phase separation from bcc a-Fe(Co) under the cooling from about 973 K. However, this phenomenon was not observed in RT sputtered FeCo – Al2O3 film. The reason may be the difficulty of Fe and Co diffusion at RT. For the sake, the FeCo – Al2O3 film with same composition sputtered at 823 K will be investigated. 3.2. 823 K sputtered FeCo –Al2O3 granular film
Fig. 2. HRTEM image of as-room temperature sputtered FeCo – Al2O3 film and its corresponding Fourier transformed pattern.
sputtered FeCo – Al2O3 film examined by Philips Tecnai F20 and its corresponding Fourier transformed pattern inserted in the right lower corner in Fig. 2. No lattice fringes in HRTEM image and its corresponding spots in Fourier transformed pattern can be observed in Fig. 2; as a consequence, these particles less than about 3 nm in size are amorphous phase or cluster. The above experiments indicate that as-RT sputtered FeCo(41 vol.%) –Al2O3 film consists of a little amount of a-
Fig. 4 is a set of EELS maps of 823 K sputtered FeCo – Al2O3 granular film, including the (a) iron map, (b) cobalt map, (c) oxygen map, and (d) aluminum map. It can be found from the comparison of these maps that Co and Fe particles (bright contrast) form individually in Al2O3 matrix, exhibiting the phase separation from a-Fe(Co) particle, which is attributed to the increase of diffusion coefficient of Fe and Co atoms at 823 K. It is worthy to note that the partial Fe atoms form the ferrous oxide during 823 K sputtering according to the overlapping of bright contrast spots (area) of iron and oxygen comparing with Fig. 4(a) and (c), but this phenomenon was not found for cobalt,
Fig. 3. EELS maps of as-room temperature sputtered FeCo – Al2O3 film. (a) Iron map, (b) cobalt map.
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
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Fig. 4. EELS maps of 823 K sputtered FeCo – Al2O3 granular film. (a) Iron map, (b) cobalt map, (c) oxygen map, (d) aluminum map.
which indicates that the degree of phase separation between Fe and Al2O3 is worse than that between Co and Al2O3. Therefore, it can be predicted that the GMR effect of Fe – Al2O3 (SiO2) system will be worse than that of Co –Al2O3 (SiO2) system, although the magnetic moment of Fe is slightly larger than that of Co, if they are heated up to a proper high temperature to anneal from as-RT sputtered state, which is consistent with the reported experimental result [8,9]. Among the above four elements, aluminum is the most homogeneous distribution in Al2O3 matrix. HRTEM imaging of Co (Fig. 5(a)) and Fe (Fig. 5(b)) particles was carried out after they were identified by their EELS maps, and shows the lattice fringes or two-
dimensional structures of Co particles and Fe as the crystalline feature; meanwhile, it also shows the crystalline and amorphous feature of Al2O3 or ferrous oxide. Comparing with Fig. 5(a) and (b), it can be found that the interface between Co and Al2O3 is sharper than that between Fe and Al2O3, which may be explained as the existence of oxygen in Fe particles. The sharper diffraction rings accompanying brighter diffraction spots in SADP for 823 K sputtered film than those for as-RT sputtered film exhibit the existence of the crystalline bcc Fe (a-Fe) and hcp Co (aCo) particles as indexed in Fig. 5(c), as well as the oxides (not indexed in Fig. 5(c)).
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Fig. 5. HRTEM images and their selected area diffraction pattern (SADP) of Fe and Co particles in 823 K sputtered FeCo – Al2O3 granular film. (a) Fe particles, (b) Co particles, (c) SADP.
By the comprehensive analysis of RT sputtered and 823 K sputtered films, the evolution sequence of the two-stage phase separations for the FeCo – Al2O3 granular film is suggested as follows. If the RT sputtered FeCo – Al2O3 film is gradually heated up below 993 K, the amorphous phase or cluster containing both Fe and Co atoms will firstly transform to the super-saturation bcc a-Fe(Co) particles by amorphous crystallization, promoting the phase separation between a-Fe(Co) and Al2O3, then the super-saturation a-Fe(Co) gradually precipitates a-Co particles to further produce the phase separation between Fe and Co until to the equilibrium state. It is conceivable that the proper annealing technology resulting in the phase
separation is favorable for the improvement of GMR effect for RT sputtered FeCo – Al2O3 granular films.
4. Conclusions (a) The RT sputtered FeCo(41 vol.%) –Al2O3 (the weight ratio of Fe and Co as 1:1) granular film consists of FeCo amorphous phase and a little amount of super-saturation bcc a-Fe(Co) particles in Al2O3 matrix; at the same time, the retained Fe and Co atoms distribute in Al2O3 matrix homogeneously. (b) The a-Fe particles with bcc structure and a-Co particles with hcp structure appear in 823 K sputtered
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
FeCo(41 vol.%) – Al2O3 granular film through the phase separation from the super-saturation a-Fe(Co) particles. (c) The evolution sequence of two-stage phase separations of FeCo(41 vol.%)– Al2O3 film is suggested, namely, the super-saturation a-Fe(Co) particles firstly form through amorphous crystallization, promoting the first phase separation between aFe(Co) and Al2O3, then the hcp a-Co particles appear through the precipitation of Co atoms from the supersaturation a-Fe(Co), which leads to the second phase separation between Fe and Co.
Acknowledgements Y.R. would like to thank Dr. Bert H. Freitag and Timon F. Fliervoet at FEI Company in Netherlands for their kind help with Tacnai F20 analytical electron microscopy. This work is financially supported by the
2173
National Nature Science Foundation of China under Grant No. 50071033.
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, et al., Phys. Rev. Lett. 68 (1992) 3745. [2] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [3] S.H. Ge, Y.Y. Lu, Z.Z. Zhang, et al., J. Magn. Magn. Mater. 168 (1999) 35. [4] S. Honda, T. Okada, M. Nawate, Phys. Rev., B 56 (1997) 14566. [5] S. Mitani, H. Fujimori, K. Takanashi, et al., J. Magn. Magn. Mater. 198 (1999) 179. [6] J.Q. Wang, G. Xiao, Phys. Rev., B 49 (1994) 3983. [7] T. Lyman, Metals Handbook, vol. 8, 8th Ed., American Society for Metals, Metals Park, OH, 1973. [8] S.R. Teixeira, M.A.S. Boff, W.H. Flores, et al., J. Magn. Magn. Mater. 233 (2001) 96. [9] J.G. Lin, S.Y. Tang, W.D. Hsu, P.C. Kuo, J. Magn. Magn. Mater. 239 (2001) 154.
The characterization of phase separations for FeCo–Al2O3 nanogranular films Ning Zhou, Changzheng Wang, Zhenghong Guo, Yonghua Rong*, T.Y. Hsu1 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 29 July 2002; accepted 29 August 2002
Abstract The phase separations of FeCo(41 vol.%) – Al2O3 nanogranular films sputtered at both room temperature (RT) and 823 K were characterized by analytical electron microscopy. The results indicate that RT sputtered film consists of FeCo amorphous phase accompanying a little amount of particles as bcc a-Fe(Co) super-saturation solid solution in Al2O3 matrix; meanwhile, the retained Fe and Co atoms almost homogeneously distribute in Al3O2 matrix. Whereas for 823 K sputtered film, the a-Fe particles with bcc structure and a-Co particles with hcp structure individually appear in Al2O3 matrix. According to the above experimental results, the evolution sequence of two-stage phase separations is suggested if the RT sputtered FeCo – Al2O3 film is gradually heated up below 973 K. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Phase separation; FeCo – Al2O3 nanogranular film; Analytical electron microscopy
1. Introduction Since Berkowitz et al. [1] and Xiao et al. [2] independently discovered the giant magnetoresistance (GMR) effect of Co – Cu granular film in 1992, various kinds of magnetic heterogeneous alloys with ferromagnetic granules (Fe, Co, Ni) embedded in nonmagnetic metals (such as Cu, Ag) [3] or in insulator (such as SiO2, Al2O3) [4,5] have been investigated. These investigations indicate that the prerequisite condition for exhibiting GMR effect is mutual insolubility between the ferromagnetic metals * Corresponding author. Fax: +86-21-6293-2435. 1 Xu Zuyao.
and nonferromegnetic metals or insulators, namely, the phase separation is required [6]. In the present paper, the feature of the phase separations in FeCo – Al2O3 granular film with GMR effect, as a new system, at different sputtering conditions, will be studied by analytical electron microscopy.
2. Experimental procedure The FeCo(41 vol.%)– Al2O3 granular films were sputtered respectively on a KCl substrate at room temperature (RT) and at 823 K with an spc350 multi-target magnetron controlled sputtering system made by ANELVA. FeCo target (the weight ratio of
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01168-0
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
Fe and Co is 1:1) and Al2O3 target (99.9% purity) were separately installed on two independently controlled RF cathodes, and were alternatively employed to sputter these films for about 7 min with 20 circles of substrate per minute. The thickness of films is about 50 nm, measured by a-step meter. The volume fraction of FeCo in the FeCo – Al2O3 films was controlled by changing the sputtering power of FeCo target and was determined by means of energy dispersive spectrum (EDS) in JEM-2010 transmission electron microscope (TEM). The sputtering gas (Ar) pressure was 4 10 1 Pa. The film specimens for TEM observation were obtained by putting a sputtered substrate into de-ionized water with a little acetone, then by using a copper grid to hold the film floating on the water after the dissolution of the KCl substrate. TEM observations were carried out on JEM-100CX, JEM-2010 and
2169
Philips Tecnai F20 with electron energy-loss spectrometer (EELS).
3. Results and discussions 3.1. As-room temperature sputtered FeCo – Al2O3 granular film Fig. 1(a) and (b) shows the TEM bright field image and dark field image of FeCo(41 vol.%) – Al2O3 granular film sputtered at RT, respectively. It can be found that a little amount of a-Fe(Co) particles with bcc structure (see Fig. 1(c)) disperse in Al2O3 matrix. EDS analysis as shown in Fig. 1(d) reveals that all particles contain both Fe and Co atoms. Fig. 2 is a high-resolution transmission electron microscopy (HRTEM) image for a local area in as-RT
Fig. 1. Analysis of particles in FeCo(41 vol.%) – Al2O3 film sputtered at RT. (a) Bright field image, (b) dark field image, (c) selected area diffraction pattern, (d) EDS of a particle (Cu peak from copper grid).
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Fe(Co) with bcc structure and FeCo amorphous phase in Al2O3 matrix. Fig. 3(a) and (b) shows the EELS iron map and cobalt map, respectively, revealing the almost homogeneous distribution of Fe and Co in the granular film. In Co – Fe binary phase diagram, there is a miscibility gap in wide composition range [7]; accordingly, Fe– Co alloy with the composition ratio of 1: 1 will result in the phase separation from bcc a-Fe(Co) under the cooling from about 973 K. However, this phenomenon was not observed in RT sputtered FeCo – Al2O3 film. The reason may be the difficulty of Fe and Co diffusion at RT. For the sake, the FeCo – Al2O3 film with same composition sputtered at 823 K will be investigated. 3.2. 823 K sputtered FeCo –Al2O3 granular film
Fig. 2. HRTEM image of as-room temperature sputtered FeCo – Al2O3 film and its corresponding Fourier transformed pattern.
sputtered FeCo – Al2O3 film examined by Philips Tecnai F20 and its corresponding Fourier transformed pattern inserted in the right lower corner in Fig. 2. No lattice fringes in HRTEM image and its corresponding spots in Fourier transformed pattern can be observed in Fig. 2; as a consequence, these particles less than about 3 nm in size are amorphous phase or cluster. The above experiments indicate that as-RT sputtered FeCo(41 vol.%) –Al2O3 film consists of a little amount of a-
Fig. 4 is a set of EELS maps of 823 K sputtered FeCo – Al2O3 granular film, including the (a) iron map, (b) cobalt map, (c) oxygen map, and (d) aluminum map. It can be found from the comparison of these maps that Co and Fe particles (bright contrast) form individually in Al2O3 matrix, exhibiting the phase separation from a-Fe(Co) particle, which is attributed to the increase of diffusion coefficient of Fe and Co atoms at 823 K. It is worthy to note that the partial Fe atoms form the ferrous oxide during 823 K sputtering according to the overlapping of bright contrast spots (area) of iron and oxygen comparing with Fig. 4(a) and (c), but this phenomenon was not found for cobalt,
Fig. 3. EELS maps of as-room temperature sputtered FeCo – Al2O3 film. (a) Iron map, (b) cobalt map.
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
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Fig. 4. EELS maps of 823 K sputtered FeCo – Al2O3 granular film. (a) Iron map, (b) cobalt map, (c) oxygen map, (d) aluminum map.
which indicates that the degree of phase separation between Fe and Al2O3 is worse than that between Co and Al2O3. Therefore, it can be predicted that the GMR effect of Fe – Al2O3 (SiO2) system will be worse than that of Co –Al2O3 (SiO2) system, although the magnetic moment of Fe is slightly larger than that of Co, if they are heated up to a proper high temperature to anneal from as-RT sputtered state, which is consistent with the reported experimental result [8,9]. Among the above four elements, aluminum is the most homogeneous distribution in Al2O3 matrix. HRTEM imaging of Co (Fig. 5(a)) and Fe (Fig. 5(b)) particles was carried out after they were identified by their EELS maps, and shows the lattice fringes or two-
dimensional structures of Co particles and Fe as the crystalline feature; meanwhile, it also shows the crystalline and amorphous feature of Al2O3 or ferrous oxide. Comparing with Fig. 5(a) and (b), it can be found that the interface between Co and Al2O3 is sharper than that between Fe and Al2O3, which may be explained as the existence of oxygen in Fe particles. The sharper diffraction rings accompanying brighter diffraction spots in SADP for 823 K sputtered film than those for as-RT sputtered film exhibit the existence of the crystalline bcc Fe (a-Fe) and hcp Co (aCo) particles as indexed in Fig. 5(c), as well as the oxides (not indexed in Fig. 5(c)).
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Fig. 5. HRTEM images and their selected area diffraction pattern (SADP) of Fe and Co particles in 823 K sputtered FeCo – Al2O3 granular film. (a) Fe particles, (b) Co particles, (c) SADP.
By the comprehensive analysis of RT sputtered and 823 K sputtered films, the evolution sequence of the two-stage phase separations for the FeCo – Al2O3 granular film is suggested as follows. If the RT sputtered FeCo – Al2O3 film is gradually heated up below 993 K, the amorphous phase or cluster containing both Fe and Co atoms will firstly transform to the super-saturation bcc a-Fe(Co) particles by amorphous crystallization, promoting the phase separation between a-Fe(Co) and Al2O3, then the super-saturation a-Fe(Co) gradually precipitates a-Co particles to further produce the phase separation between Fe and Co until to the equilibrium state. It is conceivable that the proper annealing technology resulting in the phase
separation is favorable for the improvement of GMR effect for RT sputtered FeCo – Al2O3 granular films.
4. Conclusions (a) The RT sputtered FeCo(41 vol.%) –Al2O3 (the weight ratio of Fe and Co as 1:1) granular film consists of FeCo amorphous phase and a little amount of super-saturation bcc a-Fe(Co) particles in Al2O3 matrix; at the same time, the retained Fe and Co atoms distribute in Al2O3 matrix homogeneously. (b) The a-Fe particles with bcc structure and a-Co particles with hcp structure appear in 823 K sputtered
N. Zhou et al. / Materials Letters 57 (2003) 2168–2173
FeCo(41 vol.%) – Al2O3 granular film through the phase separation from the super-saturation a-Fe(Co) particles. (c) The evolution sequence of two-stage phase separations of FeCo(41 vol.%)– Al2O3 film is suggested, namely, the super-saturation a-Fe(Co) particles firstly form through amorphous crystallization, promoting the first phase separation between aFe(Co) and Al2O3, then the hcp a-Co particles appear through the precipitation of Co atoms from the supersaturation a-Fe(Co), which leads to the second phase separation between Fe and Co.
Acknowledgements Y.R. would like to thank Dr. Bert H. Freitag and Timon F. Fliervoet at FEI Company in Netherlands for their kind help with Tacnai F20 analytical electron microscopy. This work is financially supported by the
2173
National Nature Science Foundation of China under Grant No. 50071033.
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, et al., Phys. Rev. Lett. 68 (1992) 3745. [2] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [3] S.H. Ge, Y.Y. Lu, Z.Z. Zhang, et al., J. Magn. Magn. Mater. 168 (1999) 35. [4] S. Honda, T. Okada, M. Nawate, Phys. Rev., B 56 (1997) 14566. [5] S. Mitani, H. Fujimori, K. Takanashi, et al., J. Magn. Magn. Mater. 198 (1999) 179. [6] J.Q. Wang, G. Xiao, Phys. Rev., B 49 (1994) 3983. [7] T. Lyman, Metals Handbook, vol. 8, 8th Ed., American Society for Metals, Metals Park, OH, 1973. [8] S.R. Teixeira, M.A.S. Boff, W.H. Flores, et al., J. Magn. Magn. Mater. 233 (2001) 96. [9] J.G. Lin, S.Y. Tang, W.D. Hsu, P.C. Kuo, J. Magn. Magn. Mater. 239 (2001) 154.