- Email: [email protected]
Influence of Si-N interlayer on the microstructure and magnetic properties of γ′-Fe4N films
Available online at www.sciencedirect.com
Applied Surface Science 254 (2008) 4786–4792 www.elsevier.com/locate/apsusc
Influence of Si-N interlayer on the microstructure and magnetic properties of g0-Fe4N films N. Ma a, X. Wang a, W.T. Zheng a,*, L.L. Wang a, M.W. Wang a, P.J. Cao b, X.C. Ma b a
Department of Materials Science, Key Laboratory of Automobile Materials of MOE, and State Key Laboratory of Integrated Optoelectronics, Jilin University, Changchun 130012, China b Key Laboratory of Special Functional Materials of Shenzhen City, Shenzhen 518060, China
Received 5 September 2007; received in revised form 12 December 2007; accepted 21 January 2008 Available online 2 February 2008
Abstract The (g0 -Fe4N/Si-N)n (n: number of layers) multilayer films and g0 -Fe4N single layer film synthesized on Si (1 0 0) substrates by direct current magnetron sputtering were annealed at different temperatures. The structures and magnetic properties of as-deposited films and films annealed at different temperatures were characterized using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The results showed that the insertion of Si-N layer had a significant influence on the structures and magnetic properties of g0 -Fe4N film. Without the addition of Si-N lamination, the iron nitride g0 -Fe4N tended to transform to a-Fe when annealed at the temperatures over 300 8C. However, the phase transition from g0 -Fe4N to e-Fe3N occurred at annealing temperature of 300 8C for the multilayer films. Furthermore, with increasing annealing temperature up to 400 8C or above, e-Fe3N transformed back into g0 -Fe4N. The magnetic investigations indicated that coercivity of magnetic phase g0 -Fe4N for as-deposited films decreased from 152 Oe (for single layer) to 57.23 Oe with increasing n up to 30. For the annealed multilayer films, the coercivity values decreased with increasing annealing temperature, except that the film annealed at 300 8C due to the appearance of phase e-Fe3N. # 2008 Elsevier B.V. All rights reserved. Keywords: (g0 -Fe4N/Si-N)n multilayer film; Phase transition; Magnetic properties
1. Introduction Iron nitride thin films have attracted much attention due to their excellent magnetic properties, corrosion and wear resistance. It is well known that iron nitrides show a variety of phases such as g0 -Fe4N (fcc), e-Fe2–3N (hcp), and a00 -Fe16N2 (bct) [1–10]. Among different phases of iron nitrides, the g0 Fe4N phase has the face-centered cubic iron lattice with a nitrogen atom positioned at the body-center site. Compared to the phase a00 -Fe16N2, the Fe4N phase has a better thermal stability and chemical stability. Hence, Fe4N has been considered to be a potential candidate for a high-density recording material [11–13]. In the past few decades, several studies have reported on the growth and properties of g0 -Fe4N using different fabrication and characterization techniques [14– 20]. However, up to now, lamination of g0 -Fe4N with
* Corresponding author. E-mail address: [email protected] (W.T. Zheng). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.112
nonmagnetic ceramic interlayer has received little attention [21,22]. In this work, we present our investigations on the structures and magnetic properties for g0 -Fe4N single layer and (g0 -Fe4N/Si-N)n multilayer films, deposited by reactive direct current (DC) magnetron sputtering. The influence of Si-N insertions on the microstructure and magnetic properties of iron nitride g0 -Fe4N film has been explored. 2. Experimental (g0 -Fe4N/Si-N)n multilayer films on Si(1 0 0) were grown by DC magnetron sputtering Fe (99.9%) and radio-frequency (RF) magnetron sputtering Si (99.9%), respectively. Both Fe and Si targets had a thickness of 3 mm and 60 mm of diameter. The distance between the substrate holder and the target was 6.5 cm, and the base pressure in chamber was 2 104 Pa. Prior to deposition, the substrates were cleaned ultrasonically in acetone and alcohol consecutively. During deposition, DC and RF power were kept at 110 and 100 W, respectively. The pure argon (99.999%) and nitrogen (99.999%) gases was inlet
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
4787
mass of the samples was obtained using electronic balance (R200) for evaluating the saturation magnetization. 3. Results and discussion 3.1. Film structures
Fig. 1. XRD patterns for a single layer g0 -Fe4N films as-deposited and annealed at different temperatures.
into the chamber, controlled by two independent mass-flow controllers. The ratio of gas flow rate N2/(N2 + Ar) and total pressure were fixed at 10% and 1.5 Pa, respectively, while substrate bias voltage and temperature were kept at 80 V and 250 8C, respectively. Annealing (g0 -Fe4N/Si-N)n multilayer and g0 -Fe4N single layer was carried out in a vacuum furnace. The vacuum chamber was first pumped down to 5.0 104 Pa, and then temperature was increased to the setting temperature at a rate of 10 8C/min. The final temperature was maintained for 2 h. The structures of the films were analyzed by X-ray diffraction (XRD) with Cu Ka radiation (Bruker D8-tools), and scanning electron microscope (SEM) (JOEL JSM-6700F). Magnetic properties of the films were measured by vibrating sample magnetometer (VSM) (Lake Shore M-7407), and the
Fig. 1 shows the XRD patterns for g0 -Fe4N single layer film and films annealed at different temperatures. Only one peak from g0 -Fe4N (1 1 1) is observed and no other nitride phases are found for all the films in XRD patterns (plotted in a linear scale). A weak peak from a-Fe can be distinguished if X-ray spectra are plotted in a log scale. As the sample is annealed at 500 8C, another peak indexed from a-Fe (1 1 0) appears, which means that g0 -Fe4N decomposes into a-Fe and releases N2 during annealing. Fig. 2(a) exhibits the structures of the (g0 -Fe4N/Si-N)n multilayer films deposited on Si(1 0 0) with the different Si-N layers (the number of layers is represented by n, and n is chosen as 10, 20, and 30, respectively). For these films, a total thickness of 2000 nm for g0 -Fe4N is fixed and each Si-N interlayer has a thickness of 50 nm. It can be seen that only two peaks from g0 Fe4N (1 1 1) and g0 -Fe4N (2 0 0) can be observed, which indicates that g0 -Fe4N can be synthesized when Si-N layers are inserted. No peaks from Si-N phase were found in XRD patterns, which may suggest that Si-N layers are amorphous. Fig. 2(b–d) display the XRD patterns for (g0 -Fe4N/Si-N)n multilayer films annealed at different temperatures (300, 400, and 500 8C, respectively). For multilayer films with different n annealed at 300 8C, one peak at 2u = 43.78 from e-Fe3N (1 1 1) appears in Fig. 2(b), which reveals that g0 -Fe4N transforms into e-Fe3N during thermal annealing due to the insertion of Si-N
Fig. 2. XRD patterns for (g0 -Fe4N/Si-N)n multilayer films as-deposited (a) and annealed at different temperature of (b) 300 8C, (c) 400 8C, (d) 500 8C, respectively (total thickness is 2000 nm for g0 -Fe4N, while the thickness of each layer is 50 nm for Si-N).
4788
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
laminations. Based on the Gibbs free energy calculated by Vredenberg and Perez-Martin [23], g0 phase becomes unstable below 310 8C, and should decompose into a + e phase. However, there is few experimental support for this prediction of g0 ! a + e, because any phase transition at temperatures below 350 8C is strongly limited by kinetic factors for most binary systems. de Wit et al. [24] have used ion implantation technique to overcome these kinetic limitations and found that the g0 phase converts to e-Fe2N1x (x = 0.5) at room temperature. In our work, the phase transition g0 ! e appears only for the film annealed at temperature of 300 8C, no other phases such as a phase are observed. We believe that this phase transition is driven by the insertion of Si-N layer. During annealing at temperatures of 300 8C, the nitrogen atoms may more easily escape from Si-N layer than g0 -Fe4N layer and diffuse into g0 -Fe4N layer, forming the e-Fe3N phase. When the multilayer films are annealed at the temperature of 400 8C, the e-Fe3N phase transforms back into g0 -Fe4N, as shown in Fig. 2(c), due to the release of nitrogen from e-Fe3N phase during high temperature annealing. From Fig. 2(c) for multilayer film with n = 30, an unclear peak appears at 2u = 44.18 beside the peak from g0 -Fe4N. According to Zhou and Li [25], this unclear peak may come from a0 -Fe8N, which means that nitrogen may more easily escape from thinner g0 Fe4N layer in the (g0 -Fe4N/Si-N)n multilayer film, resulting in a poor nitrogen content in nitrides. As the annealing temperature is raised up to 500 8C, the peak from a-Fe (1 1 0) appears for all samples, as shown in Fig. 2(d), which implies that the g0 -Fe4N phase partially decomposes to the more stable phase of a-Fe after N atoms are released. 3.2. Film magnetic properties Fig. 3 shows the magnetic hysteresis loops for a single layer g0 -Fe4N film. The hysteresis loops are collected with the magnetic fields parallel and perpendicular to the film surface. One notable feature is that the hysteresis loops are quite sensitive to the applied field direction. The saturation field parallel to the film surface is much smaller than that perpendicular to the film surface. As shown in Fig. 3, the
Fig. 3. Magnetic hysteresis loops of g0 -Fe4N a single layer film with external fields parallel and perpendicular to the film surface.
Fig. 4. Cross-sectional SEM image of a single layer g0 -Fe4N film.
single layer film exhibits zero remanent magnetization along the direction perpendicular to the film surface, while a considerable value of remanent magnetization is found along the direction parallel to the film surface. This result implies that the in-plane uniaxial anisotropy exists in the single layer g0 Fe4N film. Fig. 4 shows a cross-sectional SEM image for asdeposited g0 -Fe4N a single layer film, which reveals a structure with a high degree of alignment of columns. However, as Si-N layers are inserted into g0 -Fe4N single layer, the columns are cut short and the thickness of individual g0 -Fe4N layer decreases, as shown in Fig. 5. With decreasing the thickness of individual g0 Fe4N layer, the domain walls may transform from Bloch walls to Neel walls (or cross-tie walls) [26,27]. Consequently, the coercivity of the multilayer film decreases monotonously, as shown in Fig. 6. The coercivity reduction in multilayer films may be attributed to (1) the wall interaction in multilayer films [28] and (2) a domain-wall energy reduction due to
Fig. 5. Cross-sectional SEM image of (g0 -Fe4N/Si-N)n multilayer (n = 10) film.
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
Fig. 6. Coercivity for a single layer g0 -Fe4N and (g0 -Fe4N/Si-N)n multilayer films as-deposited vs. number of Si-N layers.
magnetostatic coupling of the magnetic layers across the nonmagnetic interlayer [29–31], As the number of Si-N layers increases and each Fe4N layer becomes thinner, both (1) and (2) should play an important role. Fig. 7 exhibits the evolution of coercivity for a single layer and multilayer films annealed at different temperatures. It is well known that coercivity is influenced by a lot of factors including stress s and grain size D, which may play a prominent role in our experiment, since s and D can be changed by annealing treatment. The evolution of average grains size, evaluated by Scherrer formula, for a single layer and multilayer films annealed at different temperatures is shown in Fig. 8, where it can be seen that as annealing temperature increases, grains tend to grow up for all samples. According to the law of Hc / D6, when the grain size D is of nm order of magnitude [32], an increase in grain size will favor an increase in coercivity.
4789
Intrinsic stress in the film can be quantitatively evaluated by the peak position in XRD. The evolution of peak position from g0 -Fe4N (1 1 1) for a single layer and multilayer films annealed at different temperatures is displayed in Fig. 9. Compare to the standard peak position from g0 -Fe4N (1 1 1) in the PDF cards (dash line in Fig. 9), a shift towards a high angle for all samples is observed, which means a significant compressive stress exist in a single layer and multilayer films. As annealing temperature increases, the peak shifts towards the standard position for a single layer g0 -Fe4N films and multilayer films except one annealed at 300 8C. The peak shift toward a low angle means that the stress is significantly released after annealing treatment. According to the law of Hc / s [33], a release of stress favors a decrease in coercivity. However, the peak position for the multilayer film annealed at 300 8C has an unusual large shift, compared to the standard peak position, which can be ascribed to the presence of e-Fe3N phase in multilayer films. If only the change of coercivity with grain size is considered, it seems that coercivity should increase with increasing annealing temperature. However, this is against our results (Fig. 7). In Fig. 7, the coercivity for multilayer films decreases with increasing the annealing temperature. Above 300 8C, a significant compressive stress is released from multilayer films. Furthermore, when annealing temperature increases up to 500 8C, the appearance of low coercivity a-Fe phase will also lead to a decrease in coercivity. Hence we believe that the intrinsic stress in films should play a predominant role in governing coercivity. Fig. 10 shows hysteresis loops for multilayer samples (n = 30) as-deposited and annealed at different temperatures as the magnetic field is applied along the direction parallel to the film surface, and the change of the saturation magnetization
Fig. 7. Evolution of coercivity for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
4790
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
Fig. 8. Evolution of grains size e for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
Fig. 9. Evolution of peak (g0 -Fe4N (1 1 1)) position for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
(Ms) annealed at different temperatures is exhibited in Fig. 11. The value of Ms for as-deposited multilayer film is 119 emu/g, which is lower than 186 emu/g for bulk Fe4N, reported by Coey and Smith [2]. This difference can be ascribed to the presence of nonmagnetic Si-N interlayer in multilayer film and the interfaces mixture between g0 -Fe4N and Si-N layer. As annealing temperature reaches 300 8C, the value of Ms for multilayer film
decreases to 93 emu/g, which is due to that a mixture of e-Fe3N and g0 -Fe4N phase appears in film. According to Jacobs [34], Ms for bulk e-Fe3N is 153 emu/g, lower than that for g0 -Fe4N. At annealing temperature of 400 8C, the value of Ms increases to 118 emu/g since the phase transition from e phase to g0 occurs. In addition, there maybe exists a0 -Fe8N phase in multilayer film. At annealing temperature of 500 8C, the highest value of 201 emu/g
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
4791
Fig. 10. Magnetic hysteresis loops for (g0 -Fe4N/Si-N)n multilayer (n = 30) film at different temperature of (a) as-deposited, (b) 300 8C, (c) 400 8C, (d) 500 8C, respectively.
will also decrease the value of coercivity for multilayer films due to the release of compressive stress. Acknowledgement The authors gratefully appreciate the financial support by the Key Laboratory of Special Functional Materials of Shenzhen City under Grant No. 0605. References
Fig. 11. Saturation magnetization for (g0 -Fe4N/Si-N)n multilayer (n = 30) film as-deposited and at different annealed temperatures.
is obtained due to the presence of a-Fe phase in the multilayer film. 4. Conclusions The g0 -Fe4N films, deposited by magnetron sputtering, can be obtained by insertion of Si-N interlayer, and the phase transition in the following
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
g0 ! g0 þ e ! g0 ða0 Þ ! g0 þ a-Fe
[15] [16]
can be realized if (g0 -Fe4N/Si-N)n multilayer films are annealed at different temperatures. It is found that the values of coercivity for as-deposited g0 -Fe4N decrease to 57.23 Oe with increasing the numbers of multilayer films up to 30. Annealing treatment
[17] [18] [19] [20] [21]
300 C
400 C
500 C
K.H. Jack, Proc. R. Soc. Lond. Ser. A 208 (1951) 200. J.M.D. Coey, P.A.I. Smith, J. Magn. Magn. Mater. 200 (1999) 405. W.T. Zheng, C.Q. Sun, J. Prog. Solid State Chem. 34 (2006) 1. C. Russo, R. Kaplow, Surf. Sci. 69 (1977) 453. T. Yamaguchi, M. Sakita, M. Nakamura, J. Magn. Magn. Mater. 215/216 (2000) 529. T. Takahashi, N. Takahashi, N. Tamura, J. Mater. Chem. 11 (2001) 3154. M.A. Abdellateef, C. Heiden, H. Lemke, J. Magn. Magn. Mater. 256 (2003) 214. Q. Zhan, R. Yu, L.L. He, Thin Solid Films 411 (2002) 225. S. Iwatsubo, M. Naoe, Vacuum 66 (2002) 251. Y.F. Chen, E.Y. Jiang, Z.Q. Li, J. Phys. D: Appl. Phys. 37 (2004) 1429. J.Q. Xiao, C.L. Chien, Appl. Phys. Lett. 64 (1994) 384. J.L. Costa-Kra¨mer, D.M. Borsa, Phys. Rev. B 69 (2004) 144402. J.C. Wood, A.J. Nozik, Phys. Rev. B 4 (1971) 2224. L.L. Wang, X. Wang, W.T. Zheng, Mater. Chem. Phys. 100 (2006) 304. H. Chatbi, M. Vergnat, Ph. Bauer, Appl. Phys. Lett. 67 (1995) 430. D.M. Borsa, S. Grachev, D.O. Boerma, Appl. Phys. Lett. 79 (2001) 994. C. Navio, J. Alvarez, Phys. Rev. B 75 (2007) 125422. J.M. Gallego, S.Y. Grachev, D.M. Borsa, Phys. Rev. B 70 (2004) 115417. L.L. Wang, W. Wang, W.T. Zheng, Surf. Coat. Tech. 201 (2006) 786. X. Wang, W.T. Zheng, J. Magn. Magn. Mater. 283 (2004) 282. R. Loloee, K.R. Nikolaev, W.P. Pratt, Appl. Phys. Lett. 82 (2003) 3281.
4792 [22] [23] [24] [25] [26] [27] [28]
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792 D.M. Borsa, S. Grachev, C. Presura, Appl. Phys. Lett. 80 (2002) 1823. A.M. Vredenberg, C.M. Perez-Martin, J. Mater. Res. 7 (1992) 2689. L. de Wit, T. Weber, J.S. Custer, Phys. Rev. Lett. 72 (1994) 3835. J.P. Zhou, D. Li, J. Magn. Magn. Mater. 238 (2002) 1. T. Trunk, M. Redjdal, J. Appl. Phys. 89 (2001) 7606. J. McCord, J. Westwood, J. Appl. Phys. 87 (2000) 6502. H. Clow, Nature 194 (1962) 1035.
[29] [30] [31] [32] [33] [34]
J.C. Slonczewsky, S. Middelhoek, Appl. Phys. Lett. 6 (1965) 139. L.V. Kirensky, R.C. Barker, J. Appl. Phys. 39 (1968) 1181. M.H. Kryder, S. Wang, J. Appl. Phys. 69 (8) (1991) 5626. G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. J.P. Zhou, D. Li, IEEE Trans. Magn. 37 (2001) 3844. H. Jacobs, D. Rechenbach, U. Zachwieja, J. Alloys Compd. 227 (1995) 10.
Applied Surface Science 254 (2008) 4786–4792 www.elsevier.com/locate/apsusc
Influence of Si-N interlayer on the microstructure and magnetic properties of g0-Fe4N films N. Ma a, X. Wang a, W.T. Zheng a,*, L.L. Wang a, M.W. Wang a, P.J. Cao b, X.C. Ma b a
Department of Materials Science, Key Laboratory of Automobile Materials of MOE, and State Key Laboratory of Integrated Optoelectronics, Jilin University, Changchun 130012, China b Key Laboratory of Special Functional Materials of Shenzhen City, Shenzhen 518060, China
Received 5 September 2007; received in revised form 12 December 2007; accepted 21 January 2008 Available online 2 February 2008
Abstract The (g0 -Fe4N/Si-N)n (n: number of layers) multilayer films and g0 -Fe4N single layer film synthesized on Si (1 0 0) substrates by direct current magnetron sputtering were annealed at different temperatures. The structures and magnetic properties of as-deposited films and films annealed at different temperatures were characterized using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The results showed that the insertion of Si-N layer had a significant influence on the structures and magnetic properties of g0 -Fe4N film. Without the addition of Si-N lamination, the iron nitride g0 -Fe4N tended to transform to a-Fe when annealed at the temperatures over 300 8C. However, the phase transition from g0 -Fe4N to e-Fe3N occurred at annealing temperature of 300 8C for the multilayer films. Furthermore, with increasing annealing temperature up to 400 8C or above, e-Fe3N transformed back into g0 -Fe4N. The magnetic investigations indicated that coercivity of magnetic phase g0 -Fe4N for as-deposited films decreased from 152 Oe (for single layer) to 57.23 Oe with increasing n up to 30. For the annealed multilayer films, the coercivity values decreased with increasing annealing temperature, except that the film annealed at 300 8C due to the appearance of phase e-Fe3N. # 2008 Elsevier B.V. All rights reserved. Keywords: (g0 -Fe4N/Si-N)n multilayer film; Phase transition; Magnetic properties
1. Introduction Iron nitride thin films have attracted much attention due to their excellent magnetic properties, corrosion and wear resistance. It is well known that iron nitrides show a variety of phases such as g0 -Fe4N (fcc), e-Fe2–3N (hcp), and a00 -Fe16N2 (bct) [1–10]. Among different phases of iron nitrides, the g0 Fe4N phase has the face-centered cubic iron lattice with a nitrogen atom positioned at the body-center site. Compared to the phase a00 -Fe16N2, the Fe4N phase has a better thermal stability and chemical stability. Hence, Fe4N has been considered to be a potential candidate for a high-density recording material [11–13]. In the past few decades, several studies have reported on the growth and properties of g0 -Fe4N using different fabrication and characterization techniques [14– 20]. However, up to now, lamination of g0 -Fe4N with
* Corresponding author. E-mail address: [email protected] (W.T. Zheng). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.112
nonmagnetic ceramic interlayer has received little attention [21,22]. In this work, we present our investigations on the structures and magnetic properties for g0 -Fe4N single layer and (g0 -Fe4N/Si-N)n multilayer films, deposited by reactive direct current (DC) magnetron sputtering. The influence of Si-N insertions on the microstructure and magnetic properties of iron nitride g0 -Fe4N film has been explored. 2. Experimental (g0 -Fe4N/Si-N)n multilayer films on Si(1 0 0) were grown by DC magnetron sputtering Fe (99.9%) and radio-frequency (RF) magnetron sputtering Si (99.9%), respectively. Both Fe and Si targets had a thickness of 3 mm and 60 mm of diameter. The distance between the substrate holder and the target was 6.5 cm, and the base pressure in chamber was 2 104 Pa. Prior to deposition, the substrates were cleaned ultrasonically in acetone and alcohol consecutively. During deposition, DC and RF power were kept at 110 and 100 W, respectively. The pure argon (99.999%) and nitrogen (99.999%) gases was inlet
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
4787
mass of the samples was obtained using electronic balance (R200) for evaluating the saturation magnetization. 3. Results and discussion 3.1. Film structures
Fig. 1. XRD patterns for a single layer g0 -Fe4N films as-deposited and annealed at different temperatures.
into the chamber, controlled by two independent mass-flow controllers. The ratio of gas flow rate N2/(N2 + Ar) and total pressure were fixed at 10% and 1.5 Pa, respectively, while substrate bias voltage and temperature were kept at 80 V and 250 8C, respectively. Annealing (g0 -Fe4N/Si-N)n multilayer and g0 -Fe4N single layer was carried out in a vacuum furnace. The vacuum chamber was first pumped down to 5.0 104 Pa, and then temperature was increased to the setting temperature at a rate of 10 8C/min. The final temperature was maintained for 2 h. The structures of the films were analyzed by X-ray diffraction (XRD) with Cu Ka radiation (Bruker D8-tools), and scanning electron microscope (SEM) (JOEL JSM-6700F). Magnetic properties of the films were measured by vibrating sample magnetometer (VSM) (Lake Shore M-7407), and the
Fig. 1 shows the XRD patterns for g0 -Fe4N single layer film and films annealed at different temperatures. Only one peak from g0 -Fe4N (1 1 1) is observed and no other nitride phases are found for all the films in XRD patterns (plotted in a linear scale). A weak peak from a-Fe can be distinguished if X-ray spectra are plotted in a log scale. As the sample is annealed at 500 8C, another peak indexed from a-Fe (1 1 0) appears, which means that g0 -Fe4N decomposes into a-Fe and releases N2 during annealing. Fig. 2(a) exhibits the structures of the (g0 -Fe4N/Si-N)n multilayer films deposited on Si(1 0 0) with the different Si-N layers (the number of layers is represented by n, and n is chosen as 10, 20, and 30, respectively). For these films, a total thickness of 2000 nm for g0 -Fe4N is fixed and each Si-N interlayer has a thickness of 50 nm. It can be seen that only two peaks from g0 Fe4N (1 1 1) and g0 -Fe4N (2 0 0) can be observed, which indicates that g0 -Fe4N can be synthesized when Si-N layers are inserted. No peaks from Si-N phase were found in XRD patterns, which may suggest that Si-N layers are amorphous. Fig. 2(b–d) display the XRD patterns for (g0 -Fe4N/Si-N)n multilayer films annealed at different temperatures (300, 400, and 500 8C, respectively). For multilayer films with different n annealed at 300 8C, one peak at 2u = 43.78 from e-Fe3N (1 1 1) appears in Fig. 2(b), which reveals that g0 -Fe4N transforms into e-Fe3N during thermal annealing due to the insertion of Si-N
Fig. 2. XRD patterns for (g0 -Fe4N/Si-N)n multilayer films as-deposited (a) and annealed at different temperature of (b) 300 8C, (c) 400 8C, (d) 500 8C, respectively (total thickness is 2000 nm for g0 -Fe4N, while the thickness of each layer is 50 nm for Si-N).
4788
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
laminations. Based on the Gibbs free energy calculated by Vredenberg and Perez-Martin [23], g0 phase becomes unstable below 310 8C, and should decompose into a + e phase. However, there is few experimental support for this prediction of g0 ! a + e, because any phase transition at temperatures below 350 8C is strongly limited by kinetic factors for most binary systems. de Wit et al. [24] have used ion implantation technique to overcome these kinetic limitations and found that the g0 phase converts to e-Fe2N1x (x = 0.5) at room temperature. In our work, the phase transition g0 ! e appears only for the film annealed at temperature of 300 8C, no other phases such as a phase are observed. We believe that this phase transition is driven by the insertion of Si-N layer. During annealing at temperatures of 300 8C, the nitrogen atoms may more easily escape from Si-N layer than g0 -Fe4N layer and diffuse into g0 -Fe4N layer, forming the e-Fe3N phase. When the multilayer films are annealed at the temperature of 400 8C, the e-Fe3N phase transforms back into g0 -Fe4N, as shown in Fig. 2(c), due to the release of nitrogen from e-Fe3N phase during high temperature annealing. From Fig. 2(c) for multilayer film with n = 30, an unclear peak appears at 2u = 44.18 beside the peak from g0 -Fe4N. According to Zhou and Li [25], this unclear peak may come from a0 -Fe8N, which means that nitrogen may more easily escape from thinner g0 Fe4N layer in the (g0 -Fe4N/Si-N)n multilayer film, resulting in a poor nitrogen content in nitrides. As the annealing temperature is raised up to 500 8C, the peak from a-Fe (1 1 0) appears for all samples, as shown in Fig. 2(d), which implies that the g0 -Fe4N phase partially decomposes to the more stable phase of a-Fe after N atoms are released. 3.2. Film magnetic properties Fig. 3 shows the magnetic hysteresis loops for a single layer g0 -Fe4N film. The hysteresis loops are collected with the magnetic fields parallel and perpendicular to the film surface. One notable feature is that the hysteresis loops are quite sensitive to the applied field direction. The saturation field parallel to the film surface is much smaller than that perpendicular to the film surface. As shown in Fig. 3, the
Fig. 3. Magnetic hysteresis loops of g0 -Fe4N a single layer film with external fields parallel and perpendicular to the film surface.
Fig. 4. Cross-sectional SEM image of a single layer g0 -Fe4N film.
single layer film exhibits zero remanent magnetization along the direction perpendicular to the film surface, while a considerable value of remanent magnetization is found along the direction parallel to the film surface. This result implies that the in-plane uniaxial anisotropy exists in the single layer g0 Fe4N film. Fig. 4 shows a cross-sectional SEM image for asdeposited g0 -Fe4N a single layer film, which reveals a structure with a high degree of alignment of columns. However, as Si-N layers are inserted into g0 -Fe4N single layer, the columns are cut short and the thickness of individual g0 -Fe4N layer decreases, as shown in Fig. 5. With decreasing the thickness of individual g0 Fe4N layer, the domain walls may transform from Bloch walls to Neel walls (or cross-tie walls) [26,27]. Consequently, the coercivity of the multilayer film decreases monotonously, as shown in Fig. 6. The coercivity reduction in multilayer films may be attributed to (1) the wall interaction in multilayer films [28] and (2) a domain-wall energy reduction due to
Fig. 5. Cross-sectional SEM image of (g0 -Fe4N/Si-N)n multilayer (n = 10) film.
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
Fig. 6. Coercivity for a single layer g0 -Fe4N and (g0 -Fe4N/Si-N)n multilayer films as-deposited vs. number of Si-N layers.
magnetostatic coupling of the magnetic layers across the nonmagnetic interlayer [29–31], As the number of Si-N layers increases and each Fe4N layer becomes thinner, both (1) and (2) should play an important role. Fig. 7 exhibits the evolution of coercivity for a single layer and multilayer films annealed at different temperatures. It is well known that coercivity is influenced by a lot of factors including stress s and grain size D, which may play a prominent role in our experiment, since s and D can be changed by annealing treatment. The evolution of average grains size, evaluated by Scherrer formula, for a single layer and multilayer films annealed at different temperatures is shown in Fig. 8, where it can be seen that as annealing temperature increases, grains tend to grow up for all samples. According to the law of Hc / D6, when the grain size D is of nm order of magnitude [32], an increase in grain size will favor an increase in coercivity.
4789
Intrinsic stress in the film can be quantitatively evaluated by the peak position in XRD. The evolution of peak position from g0 -Fe4N (1 1 1) for a single layer and multilayer films annealed at different temperatures is displayed in Fig. 9. Compare to the standard peak position from g0 -Fe4N (1 1 1) in the PDF cards (dash line in Fig. 9), a shift towards a high angle for all samples is observed, which means a significant compressive stress exist in a single layer and multilayer films. As annealing temperature increases, the peak shifts towards the standard position for a single layer g0 -Fe4N films and multilayer films except one annealed at 300 8C. The peak shift toward a low angle means that the stress is significantly released after annealing treatment. According to the law of Hc / s [33], a release of stress favors a decrease in coercivity. However, the peak position for the multilayer film annealed at 300 8C has an unusual large shift, compared to the standard peak position, which can be ascribed to the presence of e-Fe3N phase in multilayer films. If only the change of coercivity with grain size is considered, it seems that coercivity should increase with increasing annealing temperature. However, this is against our results (Fig. 7). In Fig. 7, the coercivity for multilayer films decreases with increasing the annealing temperature. Above 300 8C, a significant compressive stress is released from multilayer films. Furthermore, when annealing temperature increases up to 500 8C, the appearance of low coercivity a-Fe phase will also lead to a decrease in coercivity. Hence we believe that the intrinsic stress in films should play a predominant role in governing coercivity. Fig. 10 shows hysteresis loops for multilayer samples (n = 30) as-deposited and annealed at different temperatures as the magnetic field is applied along the direction parallel to the film surface, and the change of the saturation magnetization
Fig. 7. Evolution of coercivity for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
4790
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
Fig. 8. Evolution of grains size e for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
Fig. 9. Evolution of peak (g0 -Fe4N (1 1 1)) position for a single layer (a) and multilayer film (b: n = 10; c: n = 20; d: n = 30) as-deposited and at different annealing temperatures.
(Ms) annealed at different temperatures is exhibited in Fig. 11. The value of Ms for as-deposited multilayer film is 119 emu/g, which is lower than 186 emu/g for bulk Fe4N, reported by Coey and Smith [2]. This difference can be ascribed to the presence of nonmagnetic Si-N interlayer in multilayer film and the interfaces mixture between g0 -Fe4N and Si-N layer. As annealing temperature reaches 300 8C, the value of Ms for multilayer film
decreases to 93 emu/g, which is due to that a mixture of e-Fe3N and g0 -Fe4N phase appears in film. According to Jacobs [34], Ms for bulk e-Fe3N is 153 emu/g, lower than that for g0 -Fe4N. At annealing temperature of 400 8C, the value of Ms increases to 118 emu/g since the phase transition from e phase to g0 occurs. In addition, there maybe exists a0 -Fe8N phase in multilayer film. At annealing temperature of 500 8C, the highest value of 201 emu/g
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792
4791
Fig. 10. Magnetic hysteresis loops for (g0 -Fe4N/Si-N)n multilayer (n = 30) film at different temperature of (a) as-deposited, (b) 300 8C, (c) 400 8C, (d) 500 8C, respectively.
will also decrease the value of coercivity for multilayer films due to the release of compressive stress. Acknowledgement The authors gratefully appreciate the financial support by the Key Laboratory of Special Functional Materials of Shenzhen City under Grant No. 0605. References
Fig. 11. Saturation magnetization for (g0 -Fe4N/Si-N)n multilayer (n = 30) film as-deposited and at different annealed temperatures.
is obtained due to the presence of a-Fe phase in the multilayer film. 4. Conclusions The g0 -Fe4N films, deposited by magnetron sputtering, can be obtained by insertion of Si-N interlayer, and the phase transition in the following
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
g0 ! g0 þ e ! g0 ða0 Þ ! g0 þ a-Fe
[15] [16]
can be realized if (g0 -Fe4N/Si-N)n multilayer films are annealed at different temperatures. It is found that the values of coercivity for as-deposited g0 -Fe4N decrease to 57.23 Oe with increasing the numbers of multilayer films up to 30. Annealing treatment
[17] [18] [19] [20] [21]
300 C
400 C
500 C
K.H. Jack, Proc. R. Soc. Lond. Ser. A 208 (1951) 200. J.M.D. Coey, P.A.I. Smith, J. Magn. Magn. Mater. 200 (1999) 405. W.T. Zheng, C.Q. Sun, J. Prog. Solid State Chem. 34 (2006) 1. C. Russo, R. Kaplow, Surf. Sci. 69 (1977) 453. T. Yamaguchi, M. Sakita, M. Nakamura, J. Magn. Magn. Mater. 215/216 (2000) 529. T. Takahashi, N. Takahashi, N. Tamura, J. Mater. Chem. 11 (2001) 3154. M.A. Abdellateef, C. Heiden, H. Lemke, J. Magn. Magn. Mater. 256 (2003) 214. Q. Zhan, R. Yu, L.L. He, Thin Solid Films 411 (2002) 225. S. Iwatsubo, M. Naoe, Vacuum 66 (2002) 251. Y.F. Chen, E.Y. Jiang, Z.Q. Li, J. Phys. D: Appl. Phys. 37 (2004) 1429. J.Q. Xiao, C.L. Chien, Appl. Phys. Lett. 64 (1994) 384. J.L. Costa-Kra¨mer, D.M. Borsa, Phys. Rev. B 69 (2004) 144402. J.C. Wood, A.J. Nozik, Phys. Rev. B 4 (1971) 2224. L.L. Wang, X. Wang, W.T. Zheng, Mater. Chem. Phys. 100 (2006) 304. H. Chatbi, M. Vergnat, Ph. Bauer, Appl. Phys. Lett. 67 (1995) 430. D.M. Borsa, S. Grachev, D.O. Boerma, Appl. Phys. Lett. 79 (2001) 994. C. Navio, J. Alvarez, Phys. Rev. B 75 (2007) 125422. J.M. Gallego, S.Y. Grachev, D.M. Borsa, Phys. Rev. B 70 (2004) 115417. L.L. Wang, W. Wang, W.T. Zheng, Surf. Coat. Tech. 201 (2006) 786. X. Wang, W.T. Zheng, J. Magn. Magn. Mater. 283 (2004) 282. R. Loloee, K.R. Nikolaev, W.P. Pratt, Appl. Phys. Lett. 82 (2003) 3281.
4792 [22] [23] [24] [25] [26] [27] [28]
N. Ma et al. / Applied Surface Science 254 (2008) 4786–4792 D.M. Borsa, S. Grachev, C. Presura, Appl. Phys. Lett. 80 (2002) 1823. A.M. Vredenberg, C.M. Perez-Martin, J. Mater. Res. 7 (1992) 2689. L. de Wit, T. Weber, J.S. Custer, Phys. Rev. Lett. 72 (1994) 3835. J.P. Zhou, D. Li, J. Magn. Magn. Mater. 238 (2002) 1. T. Trunk, M. Redjdal, J. Appl. Phys. 89 (2001) 7606. J. McCord, J. Westwood, J. Appl. Phys. 87 (2000) 6502. H. Clow, Nature 194 (1962) 1035.
[29] [30] [31] [32] [33] [34]
J.C. Slonczewsky, S. Middelhoek, Appl. Phys. Lett. 6 (1965) 139. L.V. Kirensky, R.C. Barker, J. Appl. Phys. 39 (1968) 1181. M.H. Kryder, S. Wang, J. Appl. Phys. 69 (8) (1991) 5626. G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. J.P. Zhou, D. Li, IEEE Trans. Magn. 37 (2001) 3844. H. Jacobs, D. Rechenbach, U. Zachwieja, J. Alloys Compd. 227 (1995) 10.