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Preparation and microstructures of FeSiBNbCu thin films
ARTICLE IN PRESS
Physica B 355 (2005) 182–187 www.elsevier.com/locate/physb
Preparation and microstructures of FeSiBNbCu thin films Fangting Lin, Wangzhou Shi, Xueming Ma Nano-Center, Department of Physics, East China Normal University, Shanghai 200062, PR China Received 11 September 2004; received in revised form 25 October 2004; accepted 25 October 2004
Abstract We have prepared FeSiBNbCu thin films by radio frequency (RF) magnetron sputtering and investigated the influence of the sputtering power on the microstructures of FeSiBNbCu thin films through analyzing their microstructural configurations by X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy. The results showed that FeSiBNbCu thin films were amorphous when deposited with low sputtering power density and exhibited the mixed structure of crystalline and amorphous components with increasing the sputtering power density, without any heat treatment. The crystallites contained nanosized a-Fe(Si) and a-Fe(B) solid solutions of which the volume fractions and the microstructural configurations changed with the sputtering power. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 81.15.Cd; 68.55.a Keywords: FeSiBNbCu thin films; RF magnetron sputtering; Microstructure; Sputtering power; Mo¨ssbauer spectroscopy
1. Introduction Nanocrystalline FeSiBNbCu alloys have been the focus of many studies due to their extensive technological application. In particular, the perspective of novel application of giant magnetoimpedance (GMI) effect in the field of magnetic sensors and magnetic recording in the highfrequency range has been extensively supported in the last years’ research in soft magnetic Corresponding
author. Tel.: +86 21 6223 2001; +86 21 6223 3219. E-mail address: [email protected] (F. Lin).
fax:
materials [1–3]. Up to now a large number of investigations have been carried out on the structures and properties of nanocrystalline FeSiBNbCu materials in the shape of ribbons, wires and films [2–7]. The results indicate that the key to obtaining excellent soft magnetic properties is to form the structure of nanocrystalline a-Fe(Si) solid solution embedded in the residual amorphous matrix. In order to achieve the aim, the existing preparative methods always take two steps. The first step is to prepare the amorphous sample; then through appropriate heat treatment nanocrystalline a-Fe(Si) phase will separate out from the amorphous matrix [4,5,7]. Although these methods
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.098
ARTICLE IN PRESS F. Lin et al. / Physica B 355 (2005) 182–187
have been mature, the interface effect will be produced during the post-annealing process. For thin films the interface effect will change the states of their surfaces and boundaries, which has an especially distinct effect on the structures of multilayer films. Therefore, it is of great importance for the improvement in properties of film devices to develop a new method to deposit nanocrystalline FeSiBNbCu films without heat treatment. At first, we increased the substrate temperature but found that a high substrate temperature would favor the growth of grains and inhibit the formation of the structure of nanocrystallites embedded in the amorphous matrix. Then we changed the RF sputtering power at a fixed substrate temperature to increase the energy of the deposited particles and successfully realized the transformation from amorphous state to the embedded structure. Here we report our investigations of the influence of the sputtering power on the microstructures of FeSiBNbCu thin films by XRD and Mo¨ssbauer spectroscopy.
2. Experimental details FeSiBNbCu thin films were prepared using an RF magnetron sputtering system with an RF signal generator (Model RFX600) produced by Advanced Energy Industries. The target was Fe73.5Si13.5B9Nb3Cu1 alloy with a size of F 51 3 mm2. Cover-glass substrates of 18 18 mm2 size and 180 mm thickness were used. The distance between target and substrate was fixed at 90 mm. The target and substrate were cooled with circulating water of which the temperature was kept at 5 1C by refrigeration. The base pressure was 2.0 104 Pa. High purity (99.99%) argon was introduced as working gas with the pressure of 1 Pa during the sputtering process. According to the different sputtering power of 50, 100, 150 and 200 W, the deposition time was set to 200, 150, 120 and 90 min, respectively, to obtain the typical film thickness of 2000 nm. An X-ray diffractometer (Model D/ Max-2500 V) using CuKa radiation was employed to study the phase composition of the films. Relative contents and microstructures of different
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phases were analyzed by means of transmission Mo¨ssbauer spectroscopy.
3. Results and discussion 3.1. XRD results Fig. 1 presents the X-ray diffraction patterns for FeSiBNbCu samples prepared at different sputtering power. As can be seen from Fig. 1(a) only in the range of 2y from 401 to 501 existed a broad diffraction peak, which was in accord with the case of ribbon-shaped amorphous materials, indicating the amorphous phase was the only product obtained at the sputtering power of 50 W. When the sputtering power was increased to 100 W, a-Fe diffraction peak is observed from Fig. 1(b), by which we judged that a-Fe-like nanocrystallites had begun to separate out from the amorphous matrix. With increasing the sputtering power up to 150 W, the further sharpening of crystalline diffraction peak against the background of broad amorphous diffraction was observed, which indicated that the sample further crystallized and the grains had the tendency to grow, however, there still existed the amorphous phase. When the sputtering power was increased to 200 W, the information about amorphous diffraction weakened. At the same time further sharpening of crystalline diffraction peak was noted accompanied by the appearance of asymmetry. This fact
Fig. 1. XRD patterns for samples deposited at the different sputtering power of (a) 50 W, (b) 100 W, (c) 150 W and (d) 200 W.
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suggested that with increasing the sputtering power to 200 W, the amorphous phase relatively decreased and nanocrystal grains further grew up. The asymmetry of the diffraction peak indicated that there probably separated out two kinds of solid solutions that slightly differed in lattice constant. Further analysis showed that the diffraction peak was the superposition of two peaks corresponding to the a-Fe(Si) and a-Fe(B) solid solutions, respectively, which indicated that not only a-Fe(Si) nanocrystallite but also a-Fe(B) nanocrystallite separated out from the amorphous matrix. In the meantime we also found that no other diffraction peaks were detected except Fe(1 1 0) diffraction peak, implying nanocrystallites that separated out from the amorphous matrix were highly oriented along Fe(1 1 0) plane. 3.2. Mo¨ssbauer results In order to further study the differences in the microstructures of samples prepared at different sputtering power, we recorded the Mo¨ssbauer spectra of samples in transmission geometry and fitted them using the Hesse method. According to the Mo¨ssbauer results, we analyzed the local configurations of the Fe sites for different samples. The Mo¨ssbauer spectra are shown in Fig. 2 where (a), (b), (c) and (d) correspond to the sputtering power of 50, 100, 150 and 200 W, respectively. From the Mo¨ssbauer spectra, the evident differences in the absorption spectra of four samples reflected the differences in their microstructural characteristics. In Fig. 2(a), for the sample deposited at the sputtering power of 50 W, its absorption spectrum was composed of two components and was fitted using five subspectra, four of which were sextets in accordance with the a-Fe(Si) and a-Fe(B) solid solutions and the last one represented the amorphous component. In the BCC structure of the a-Fe(Si) solid solution there were three different 57Fe sites which were well distinguished by Mo¨ssbauer spectroscopy. These configurations had the hyperfine-field values well differentiated, namely, Hhf ¼ 280, 242 and 188 kOe for the A6, A5 and A4 configurations, respectively. A6 represents there are six Fe and two Si atoms in the nearest-neighborhood of the
Fig. 2. Mo¨ssbauer spectra for samples deposited at the different sputtering power of (a) 50 W, (b) 100 W, (c) 150 W and (d) 200 W.
body-centered 57Fe atom. A5 corresponds to five Fe and three Si nearest neighbors around the body-centered 57Fe atom. The rest may be deduced by analogy. For the a-Fe(B) solid solution and the amorphous component, the average hyperfine fields were 235 and 239 kOe, respectively. It can be concluded from the Mo¨ssbauer results that the ordering of interatomic configurations has initiated although the sample was determined to be amorphous structure by XRD. In other words, besides the amorphous phase, the sample also contained the embryo of crystalline A6, A5, A4 and a-Fe(B) structures. As the sputtering power was increased to 100 W, the XRD results have indicated the existence of nanocrystallites besides the disordered amorphous phase and the Mo¨ssbauer results further proved that the sample contained A6, A5, A4 and a-Fe(B) structures. At the same time, for the four sextets, the intensity increased significantly and the relative intensity (I2,5/I1,6) of the second (fifth) peak to the first (sixth) peak changed. The hyperfine field Hhf of the a-Fe(B) phase declined from 235 to 229 kOe. With increasing the sputtering power to 150 W, the disordered amorphous phase further decreased whereas the crystalline phase further increased, compared with the sample at the sputtering power of 100 W. And the hyperfine field Hhf of the
ARTICLE IN PRESS F. Lin et al. / Physica B 355 (2005) 182–187
a-Fe(B) phase reduced to 223 kOe. When the sputtering power reached 200 W, the sample was mainly composed of nanocrystalline phases and the amorphous phase relatively decreased. The hyperfine field Hhf of the a-Fe(B) phase further reduced from 223 to 218 kOe. From the results above, it can be seen that the compositional proportions of the a-Fe(Si), a-Fe(B) solid solution phases and the amorphous phase changed with the sputtering power (see Fig. 3). At the sputtering power of 50 W, the film consisted mostly of the amorphous phase, containing 51% amorphous component, 40% a-Fe(Si) solid solution (the sum of A6, A5 and A4 structures) and 9% a-Fe(B) solid solution by the volume fraction. When the sputtering power was 100 W, the volume fraction of the amorphous phase decreased to 34% whereas the a-Fe(Si) and a-Fe(B) solid solutions increased to 55% and 11%, respectively, which implied that the embryo of the ordered a-Fe(Si) and a-Fe(B) phases had begun to be transformed into nanocrystallites. With increasing the sputtering power to 150 W, the amorphous phase, the aFe(Si) and a-Fe(B) solid solution phases accounted for 29, 58 and 13 vol%, respectively. The changes of the volume fractions for the three phases were relatively small compared with the case at the sputtering power of 100 W, which was attributed to the dominance of the growth of nanocrystal-
Fig. 3. Volume fractions of the amorphous component, aFe(Si) and a-Fe(B) nanocrystallites in samples deposited at different sputtering power.
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lites. When the sputtering power increased up to 200 W, the normalized volume fraction of the amorphous phase fell to 17% whereas the a-Fe(Si) and a-Fe(B) solid solution phases increased to 69% and 14%, respectively. Here, probably the film was mainly composed of a-Fe(Si) and a-Fe(B) nanocrystallites and the amorphous component probably originated from the contribution of grain boundaries. As we can see from the analysis above, with the increase in the sputtering power, the amorphous component in the film decreased whereas the a-Fe(Si) and a-Fe(B) solid solutions increased. Moreover, the increase in the a-Fe(B) phase was approximately linear. In order to further investigate the variation of the a-Fe(B) configuration with the sputtering power, the dependence of the hyperfine field Hhf of the a-Fe(B) phase on the sputtering power was analyzed as shown in Fig. 4. It can be seen that within the range of our experimental parameters the hyperfine field Hhf of the a-Fe(B) phase showed an approximately linear diminution with the sputtering power. At the sputtering power of 50 W, the hyperfine field Hhf of the a-Fe(B) phase was 235 kOe. With increasing the sputtering power, the corresponding hyperfine field Hhf decreased from 229 kOe for 100 W through 223 kOe for 150 W to 218 kOe for 200 W. It is known that the hyperfine field will decrease when the number of the B atoms in the nearest-neighborhood of the Fe
Fig. 4. Hyperfine fields of the a-Fe(B) phase for different sputtering power.
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atom increases. According to this influence of the occupation of the B atoms around the Fe atom on the hyperfine filed, it can be concluded that the occupation probability of the B atoms was small at low sputtering power and increased with increasing the sputtering power, i.e. the number of the B atoms among the eight nearest neighbors of the Fe atom increased. At the same time, we also found that for the four sextets corresponding to nanocrystalline phases the relative intensity I2,5/I1,6, where I1,6 and I2,5 were the intensity of the peaks 1,6 and 2,5, respectively, markedly varied with the sputtering power. It can be seen in Fig. 5 that the values of I2,5/I1,6 were 1.3, 0.75, 0.65 and 0.3, corresponding to the sputtering power of 50, 100, 150 and 200 W, respectively. This result was due to the variation in the angle b included between the magnetic moments of the nanocrystalline component and the normal to the film plane. Using the variation of I2,5/I1,6 with b; namely, 3 ð14 sin2 bÞ=½16 ð1 þ cos2 bÞ ¼ I 2;5 =I 1;6 ; we found 6I 2;5 =I 1;6 2 sin b ¼ 3I 2;5 =I 1;6 þ4 ; in terms of which the values of b were calculated to be 841, 581, 541 and 371, corresponding to the sputtering power of 50, 100, 150 and 200 W, respectively. This result indicated that the sputtering power had an obvious effect on the orientation of magnetic moments in the deposited thin films. When the sputtering power was 50 W, there existed the ordered nucleus in the
thin film where the orientation of magnetic moments was mostly perpendicular to the normal to the film plane, i.e. it was mostly parallel to the film plane. With increasing the sputtering power, the ordered nucleus gradually grew into nanocrystallites and the angle b reduced little by little, from 841 for the sputtering power of 50 W to 371 for 200 W. This result may be due to the variation in the stress of the films induced by the variation in the particle energy. However, the relatively small variation in the orientation of magnetic moments was observed in the range of the sputtering power from 100 to 150 W where we suggested that the growth of nanocrystallites dominated during the variation in the microstructure and the orientation of magnetic moments remained almost unchanged. It can be concluded from the discussion above that nanocrystalline embedded structure could be formed by increasing the sputtering power and the increase in the sputtering power not only influenced nanocrystallites to separate out but also influenced the relative contents of the a-Fe(Si) and a-Fe(B) phases. The increase in the sputtering power would lead to the increase in the particle energy. When the sputtering power was low, the particles had low energy and the deposited thin film was mainly composed of the amorphous phase. With increasing the sputtering power, nanocrystallites separated out from the amorphous matrix. Meanwhile the volume fractions of a-Fe(Si) and a-Fe(B) nanocrystallites increased whereas the amorphous component decreased. This fact was similar to the case of the annealing of amorphous samples [5,8]. When annealed at a low temperature (o470 1C), the amorphous samples would not crystallize. If an appropriate annealing temperature (510–570 1C) was given, there would separate out nanocrystallites from the amorphous matrix. At a high annealing temperature (4610 1C), nanocrystal grains would rapidly grow up and it would be easier for the aFe(B) phase to separate out.
4. Conclusion Fig. 5. Variation of the angle b included between the magnetic moments of the nanocrystalline component and the normal to the film plane with sputtering power.
We have prepared FeSiBNbCu thin films at different sputtering power by RF magnetron
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sputtering, realizing the transformation of thin films from amorphous state to nanocrystallites and the formation of nanocrystalline embedded structure. The sputtering power had a distinct effect on the microstructures of thin films. When the sputtering power was low, the film consisted mostly of the amorphous component. At high sputtering power, a-Fe(Si) and a-Fe(B) nanocrystallites would separate out and the embedded structure was formed. With increasing the sputtering power, the volume fractions of nanocrystalline phases increased whereas the amorphous phase decreased. At the same time, the occupation probability of the B atoms around the Fe atom in the a-Fe(B) phase increased with the sputtering power, consequently leading to the reduction of the hyperfine field of the a-Fe(B) phase. In addition, we also found that the angle b included between the orientation of magnetic moments in the ordered structure of the film and the normal to the film plane decreased with increasing the sputtering power. From the analysis of the experimental results, we found that in the range
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of the sputtering power from 100 to 150 W the growth of nanocrystallites dominated during the variation in the microstructure of the film whereas the volume fractions of nanocrystallites and the orientation of magnetic moments remained almost unchanged. References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] J.A. Moya, B. Arcondo, H. Sirkin, M.L. Sartorelli, M. Knobel, M. Va´zquez, J. Magn. Magn. Mater. 203 (1999) 117. [3] J. Petzold, Scripta Mater. 48 (2003) 895. [4] M. Knobel, R. Sato Turtelli, H.R. Rechenberg, J. Appl. Phys. 71 (1992) 6008. [5] H.S. Yang, G.C. Tu, X.T. Xiong, Z.X. Xu, R.Z. Ma, J. Magn. Magn. Mater. 138 (1994) 94. [6] L. Brunetti, M. Coisson, P. Tiberto, F. Vinai, H. Chiriac, F. Borza, Sensors Actuators A 91 (2001) 203. [7] S.Q. Xiao, Y.H. Liu, S.S. Yan, Y.Y. Dai, L. Zhang, L.M. Mei, Phys. Rev. B 61 (2000) 5734. [8] W.Z. Chen, Z.H. Li, G.X. Zhang, J. Magn. Magn. Mater. 146 (1995) 354.
Physica B 355 (2005) 182–187 www.elsevier.com/locate/physb
Preparation and microstructures of FeSiBNbCu thin films Fangting Lin, Wangzhou Shi, Xueming Ma Nano-Center, Department of Physics, East China Normal University, Shanghai 200062, PR China Received 11 September 2004; received in revised form 25 October 2004; accepted 25 October 2004
Abstract We have prepared FeSiBNbCu thin films by radio frequency (RF) magnetron sputtering and investigated the influence of the sputtering power on the microstructures of FeSiBNbCu thin films through analyzing their microstructural configurations by X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy. The results showed that FeSiBNbCu thin films were amorphous when deposited with low sputtering power density and exhibited the mixed structure of crystalline and amorphous components with increasing the sputtering power density, without any heat treatment. The crystallites contained nanosized a-Fe(Si) and a-Fe(B) solid solutions of which the volume fractions and the microstructural configurations changed with the sputtering power. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 81.15.Cd; 68.55.a Keywords: FeSiBNbCu thin films; RF magnetron sputtering; Microstructure; Sputtering power; Mo¨ssbauer spectroscopy
1. Introduction Nanocrystalline FeSiBNbCu alloys have been the focus of many studies due to their extensive technological application. In particular, the perspective of novel application of giant magnetoimpedance (GMI) effect in the field of magnetic sensors and magnetic recording in the highfrequency range has been extensively supported in the last years’ research in soft magnetic Corresponding
author. Tel.: +86 21 6223 2001; +86 21 6223 3219. E-mail address: [email protected] (F. Lin).
fax:
materials [1–3]. Up to now a large number of investigations have been carried out on the structures and properties of nanocrystalline FeSiBNbCu materials in the shape of ribbons, wires and films [2–7]. The results indicate that the key to obtaining excellent soft magnetic properties is to form the structure of nanocrystalline a-Fe(Si) solid solution embedded in the residual amorphous matrix. In order to achieve the aim, the existing preparative methods always take two steps. The first step is to prepare the amorphous sample; then through appropriate heat treatment nanocrystalline a-Fe(Si) phase will separate out from the amorphous matrix [4,5,7]. Although these methods
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.098
ARTICLE IN PRESS F. Lin et al. / Physica B 355 (2005) 182–187
have been mature, the interface effect will be produced during the post-annealing process. For thin films the interface effect will change the states of their surfaces and boundaries, which has an especially distinct effect on the structures of multilayer films. Therefore, it is of great importance for the improvement in properties of film devices to develop a new method to deposit nanocrystalline FeSiBNbCu films without heat treatment. At first, we increased the substrate temperature but found that a high substrate temperature would favor the growth of grains and inhibit the formation of the structure of nanocrystallites embedded in the amorphous matrix. Then we changed the RF sputtering power at a fixed substrate temperature to increase the energy of the deposited particles and successfully realized the transformation from amorphous state to the embedded structure. Here we report our investigations of the influence of the sputtering power on the microstructures of FeSiBNbCu thin films by XRD and Mo¨ssbauer spectroscopy.
2. Experimental details FeSiBNbCu thin films were prepared using an RF magnetron sputtering system with an RF signal generator (Model RFX600) produced by Advanced Energy Industries. The target was Fe73.5Si13.5B9Nb3Cu1 alloy with a size of F 51 3 mm2. Cover-glass substrates of 18 18 mm2 size and 180 mm thickness were used. The distance between target and substrate was fixed at 90 mm. The target and substrate were cooled with circulating water of which the temperature was kept at 5 1C by refrigeration. The base pressure was 2.0 104 Pa. High purity (99.99%) argon was introduced as working gas with the pressure of 1 Pa during the sputtering process. According to the different sputtering power of 50, 100, 150 and 200 W, the deposition time was set to 200, 150, 120 and 90 min, respectively, to obtain the typical film thickness of 2000 nm. An X-ray diffractometer (Model D/ Max-2500 V) using CuKa radiation was employed to study the phase composition of the films. Relative contents and microstructures of different
183
phases were analyzed by means of transmission Mo¨ssbauer spectroscopy.
3. Results and discussion 3.1. XRD results Fig. 1 presents the X-ray diffraction patterns for FeSiBNbCu samples prepared at different sputtering power. As can be seen from Fig. 1(a) only in the range of 2y from 401 to 501 existed a broad diffraction peak, which was in accord with the case of ribbon-shaped amorphous materials, indicating the amorphous phase was the only product obtained at the sputtering power of 50 W. When the sputtering power was increased to 100 W, a-Fe diffraction peak is observed from Fig. 1(b), by which we judged that a-Fe-like nanocrystallites had begun to separate out from the amorphous matrix. With increasing the sputtering power up to 150 W, the further sharpening of crystalline diffraction peak against the background of broad amorphous diffraction was observed, which indicated that the sample further crystallized and the grains had the tendency to grow, however, there still existed the amorphous phase. When the sputtering power was increased to 200 W, the information about amorphous diffraction weakened. At the same time further sharpening of crystalline diffraction peak was noted accompanied by the appearance of asymmetry. This fact
Fig. 1. XRD patterns for samples deposited at the different sputtering power of (a) 50 W, (b) 100 W, (c) 150 W and (d) 200 W.
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suggested that with increasing the sputtering power to 200 W, the amorphous phase relatively decreased and nanocrystal grains further grew up. The asymmetry of the diffraction peak indicated that there probably separated out two kinds of solid solutions that slightly differed in lattice constant. Further analysis showed that the diffraction peak was the superposition of two peaks corresponding to the a-Fe(Si) and a-Fe(B) solid solutions, respectively, which indicated that not only a-Fe(Si) nanocrystallite but also a-Fe(B) nanocrystallite separated out from the amorphous matrix. In the meantime we also found that no other diffraction peaks were detected except Fe(1 1 0) diffraction peak, implying nanocrystallites that separated out from the amorphous matrix were highly oriented along Fe(1 1 0) plane. 3.2. Mo¨ssbauer results In order to further study the differences in the microstructures of samples prepared at different sputtering power, we recorded the Mo¨ssbauer spectra of samples in transmission geometry and fitted them using the Hesse method. According to the Mo¨ssbauer results, we analyzed the local configurations of the Fe sites for different samples. The Mo¨ssbauer spectra are shown in Fig. 2 where (a), (b), (c) and (d) correspond to the sputtering power of 50, 100, 150 and 200 W, respectively. From the Mo¨ssbauer spectra, the evident differences in the absorption spectra of four samples reflected the differences in their microstructural characteristics. In Fig. 2(a), for the sample deposited at the sputtering power of 50 W, its absorption spectrum was composed of two components and was fitted using five subspectra, four of which were sextets in accordance with the a-Fe(Si) and a-Fe(B) solid solutions and the last one represented the amorphous component. In the BCC structure of the a-Fe(Si) solid solution there were three different 57Fe sites which were well distinguished by Mo¨ssbauer spectroscopy. These configurations had the hyperfine-field values well differentiated, namely, Hhf ¼ 280, 242 and 188 kOe for the A6, A5 and A4 configurations, respectively. A6 represents there are six Fe and two Si atoms in the nearest-neighborhood of the
Fig. 2. Mo¨ssbauer spectra for samples deposited at the different sputtering power of (a) 50 W, (b) 100 W, (c) 150 W and (d) 200 W.
body-centered 57Fe atom. A5 corresponds to five Fe and three Si nearest neighbors around the body-centered 57Fe atom. The rest may be deduced by analogy. For the a-Fe(B) solid solution and the amorphous component, the average hyperfine fields were 235 and 239 kOe, respectively. It can be concluded from the Mo¨ssbauer results that the ordering of interatomic configurations has initiated although the sample was determined to be amorphous structure by XRD. In other words, besides the amorphous phase, the sample also contained the embryo of crystalline A6, A5, A4 and a-Fe(B) structures. As the sputtering power was increased to 100 W, the XRD results have indicated the existence of nanocrystallites besides the disordered amorphous phase and the Mo¨ssbauer results further proved that the sample contained A6, A5, A4 and a-Fe(B) structures. At the same time, for the four sextets, the intensity increased significantly and the relative intensity (I2,5/I1,6) of the second (fifth) peak to the first (sixth) peak changed. The hyperfine field Hhf of the a-Fe(B) phase declined from 235 to 229 kOe. With increasing the sputtering power to 150 W, the disordered amorphous phase further decreased whereas the crystalline phase further increased, compared with the sample at the sputtering power of 100 W. And the hyperfine field Hhf of the
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a-Fe(B) phase reduced to 223 kOe. When the sputtering power reached 200 W, the sample was mainly composed of nanocrystalline phases and the amorphous phase relatively decreased. The hyperfine field Hhf of the a-Fe(B) phase further reduced from 223 to 218 kOe. From the results above, it can be seen that the compositional proportions of the a-Fe(Si), a-Fe(B) solid solution phases and the amorphous phase changed with the sputtering power (see Fig. 3). At the sputtering power of 50 W, the film consisted mostly of the amorphous phase, containing 51% amorphous component, 40% a-Fe(Si) solid solution (the sum of A6, A5 and A4 structures) and 9% a-Fe(B) solid solution by the volume fraction. When the sputtering power was 100 W, the volume fraction of the amorphous phase decreased to 34% whereas the a-Fe(Si) and a-Fe(B) solid solutions increased to 55% and 11%, respectively, which implied that the embryo of the ordered a-Fe(Si) and a-Fe(B) phases had begun to be transformed into nanocrystallites. With increasing the sputtering power to 150 W, the amorphous phase, the aFe(Si) and a-Fe(B) solid solution phases accounted for 29, 58 and 13 vol%, respectively. The changes of the volume fractions for the three phases were relatively small compared with the case at the sputtering power of 100 W, which was attributed to the dominance of the growth of nanocrystal-
Fig. 3. Volume fractions of the amorphous component, aFe(Si) and a-Fe(B) nanocrystallites in samples deposited at different sputtering power.
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lites. When the sputtering power increased up to 200 W, the normalized volume fraction of the amorphous phase fell to 17% whereas the a-Fe(Si) and a-Fe(B) solid solution phases increased to 69% and 14%, respectively. Here, probably the film was mainly composed of a-Fe(Si) and a-Fe(B) nanocrystallites and the amorphous component probably originated from the contribution of grain boundaries. As we can see from the analysis above, with the increase in the sputtering power, the amorphous component in the film decreased whereas the a-Fe(Si) and a-Fe(B) solid solutions increased. Moreover, the increase in the a-Fe(B) phase was approximately linear. In order to further investigate the variation of the a-Fe(B) configuration with the sputtering power, the dependence of the hyperfine field Hhf of the a-Fe(B) phase on the sputtering power was analyzed as shown in Fig. 4. It can be seen that within the range of our experimental parameters the hyperfine field Hhf of the a-Fe(B) phase showed an approximately linear diminution with the sputtering power. At the sputtering power of 50 W, the hyperfine field Hhf of the a-Fe(B) phase was 235 kOe. With increasing the sputtering power, the corresponding hyperfine field Hhf decreased from 229 kOe for 100 W through 223 kOe for 150 W to 218 kOe for 200 W. It is known that the hyperfine field will decrease when the number of the B atoms in the nearest-neighborhood of the Fe
Fig. 4. Hyperfine fields of the a-Fe(B) phase for different sputtering power.
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atom increases. According to this influence of the occupation of the B atoms around the Fe atom on the hyperfine filed, it can be concluded that the occupation probability of the B atoms was small at low sputtering power and increased with increasing the sputtering power, i.e. the number of the B atoms among the eight nearest neighbors of the Fe atom increased. At the same time, we also found that for the four sextets corresponding to nanocrystalline phases the relative intensity I2,5/I1,6, where I1,6 and I2,5 were the intensity of the peaks 1,6 and 2,5, respectively, markedly varied with the sputtering power. It can be seen in Fig. 5 that the values of I2,5/I1,6 were 1.3, 0.75, 0.65 and 0.3, corresponding to the sputtering power of 50, 100, 150 and 200 W, respectively. This result was due to the variation in the angle b included between the magnetic moments of the nanocrystalline component and the normal to the film plane. Using the variation of I2,5/I1,6 with b; namely, 3 ð14 sin2 bÞ=½16 ð1 þ cos2 bÞ ¼ I 2;5 =I 1;6 ; we found 6I 2;5 =I 1;6 2 sin b ¼ 3I 2;5 =I 1;6 þ4 ; in terms of which the values of b were calculated to be 841, 581, 541 and 371, corresponding to the sputtering power of 50, 100, 150 and 200 W, respectively. This result indicated that the sputtering power had an obvious effect on the orientation of magnetic moments in the deposited thin films. When the sputtering power was 50 W, there existed the ordered nucleus in the
thin film where the orientation of magnetic moments was mostly perpendicular to the normal to the film plane, i.e. it was mostly parallel to the film plane. With increasing the sputtering power, the ordered nucleus gradually grew into nanocrystallites and the angle b reduced little by little, from 841 for the sputtering power of 50 W to 371 for 200 W. This result may be due to the variation in the stress of the films induced by the variation in the particle energy. However, the relatively small variation in the orientation of magnetic moments was observed in the range of the sputtering power from 100 to 150 W where we suggested that the growth of nanocrystallites dominated during the variation in the microstructure and the orientation of magnetic moments remained almost unchanged. It can be concluded from the discussion above that nanocrystalline embedded structure could be formed by increasing the sputtering power and the increase in the sputtering power not only influenced nanocrystallites to separate out but also influenced the relative contents of the a-Fe(Si) and a-Fe(B) phases. The increase in the sputtering power would lead to the increase in the particle energy. When the sputtering power was low, the particles had low energy and the deposited thin film was mainly composed of the amorphous phase. With increasing the sputtering power, nanocrystallites separated out from the amorphous matrix. Meanwhile the volume fractions of a-Fe(Si) and a-Fe(B) nanocrystallites increased whereas the amorphous component decreased. This fact was similar to the case of the annealing of amorphous samples [5,8]. When annealed at a low temperature (o470 1C), the amorphous samples would not crystallize. If an appropriate annealing temperature (510–570 1C) was given, there would separate out nanocrystallites from the amorphous matrix. At a high annealing temperature (4610 1C), nanocrystal grains would rapidly grow up and it would be easier for the aFe(B) phase to separate out.
4. Conclusion Fig. 5. Variation of the angle b included between the magnetic moments of the nanocrystalline component and the normal to the film plane with sputtering power.
We have prepared FeSiBNbCu thin films at different sputtering power by RF magnetron
ARTICLE IN PRESS F. Lin et al. / Physica B 355 (2005) 182–187
sputtering, realizing the transformation of thin films from amorphous state to nanocrystallites and the formation of nanocrystalline embedded structure. The sputtering power had a distinct effect on the microstructures of thin films. When the sputtering power was low, the film consisted mostly of the amorphous component. At high sputtering power, a-Fe(Si) and a-Fe(B) nanocrystallites would separate out and the embedded structure was formed. With increasing the sputtering power, the volume fractions of nanocrystalline phases increased whereas the amorphous phase decreased. At the same time, the occupation probability of the B atoms around the Fe atom in the a-Fe(B) phase increased with the sputtering power, consequently leading to the reduction of the hyperfine field of the a-Fe(B) phase. In addition, we also found that the angle b included between the orientation of magnetic moments in the ordered structure of the film and the normal to the film plane decreased with increasing the sputtering power. From the analysis of the experimental results, we found that in the range
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of the sputtering power from 100 to 150 W the growth of nanocrystallites dominated during the variation in the microstructure of the film whereas the volume fractions of nanocrystallites and the orientation of magnetic moments remained almost unchanged. References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] J.A. Moya, B. Arcondo, H. Sirkin, M.L. Sartorelli, M. Knobel, M. Va´zquez, J. Magn. Magn. Mater. 203 (1999) 117. [3] J. Petzold, Scripta Mater. 48 (2003) 895. [4] M. Knobel, R. Sato Turtelli, H.R. Rechenberg, J. Appl. Phys. 71 (1992) 6008. [5] H.S. Yang, G.C. Tu, X.T. Xiong, Z.X. Xu, R.Z. Ma, J. Magn. Magn. Mater. 138 (1994) 94. [6] L. Brunetti, M. Coisson, P. Tiberto, F. Vinai, H. Chiriac, F. Borza, Sensors Actuators A 91 (2001) 203. [7] S.Q. Xiao, Y.H. Liu, S.S. Yan, Y.Y. Dai, L. Zhang, L.M. Mei, Phys. Rev. B 61 (2000) 5734. [8] W.Z. Chen, Z.H. Li, G.X. Zhang, J. Magn. Magn. Mater. 146 (1995) 354.