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Effect of nitrogen and cobalt additions on surface morphology and magnetic properties of Fe thin films
Journal of Alloys and Compounds 662 (2016) 541e545
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of nitrogen and cobalt additions on surface morphology and magnetic properties of Fe thin films Jiuping Fan, Juan Sun, Yang Yang, Riuyan Liang, Yannan Jiang, Jun Zhang, Xiaohong Xu* School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and Magnetic Information Materials, Ministry of Education, Shanxi Normal University, Linfen 041004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 2 December 2015 Accepted 7 December 2015 Available online 11 December 2015
Fe, FeN, and FeCoN films were fabricated by direct-current (DC) magnetron sputtering. The dependence of surface morphology, magnetic properties, and resistivity on phase structure was systematically investigated. a00 -Fe16N2 (202) phase occurred with the incorporation of nitrogen (N) and cobalt (Co), and the surface morphology evolved from random-leafs-like shapes to triangular-pyramid-like islands. Clear reproducible striped domains appeared at the FeCoN film surfaces, because of high perpendicular anisotropy in the film. The additions of Co enhanced the formation of the a00 -Fe16N2 phase, which caused the saturation magnetization (Ms) value of the FeCoN film to increase to 2088 emu/cm3, considerably higher than that of pure Fe. Moreover, the significant amount of a00 phase leads to higher resistivity and therefore improved high-frequency permeability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Thin films Anisotropy Transmission electron microscopy Magnetic measurements
1. Introduction Over the past decades, Fe-based films have evolved because of their high saturation magnetization and high resistivity [1,2]. The magnetization of Fe was enhanced by introducing a small amount of N, which happened because of the incorporation of N atoms at the interstitial sites, thereby resulting in expansion and distortion of body-centered cubic (bcc)-Fe units cells. A significant increase occurs in the volume fraction of grain boundaries or interfaces, resulting in reasonably high values of saturation magnetization [3]. One of the most studied iron (Fe) nitride compounds is a00 -Fe16N2, which possesses a giant magnetic moment and large magnetocrystalline anisotropy, and is considered as a potential candidate for rare-earth-free magnets because of its promising magnetic-energy product performance [4]. However, pure a00 -Fe16N2 is difficult to prepare because of the limitations from the FeN-phase diagrams [5,6]. Co doping into Fe nitrides has been reported to change their magnetic properties of these materials [7e13]. According to several studies, the addition of Co is in favor of enhancing the amount of a00 phase [7e10], and the noticeably increased electrical resistivity is very useful for a variety of technological applications; thus studying the Co-doped FeN system is meaningful [14]. The following two
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.12.042 0925-8388/© 2015 Elsevier B.V. All rights reserved.
points are considered as the reasons to choose Co as the additive element: (i) bcc structure is stably formed up to a composition of 75 at.% at room temperature (RT), which is essential because a00 Fe16N2 has an analogous bct structure [15]; and (ii) the chemical affinity of Co for N is weaker than that of Fe, which prevents Co from undergoing preferential nitridation [16]. As reported, the FeCoN film has large resistivity, but the Ms of this sample is lower [14], and we observed a larger Ms and a higher resistivity in one FeCoN film. In addition, to the best of our knowledge, few studies have systemically compared the effect of N and Co additions on Fe thin films. In this paper, we describe the surface morphology, structure, magnetic properties and resistivity of N and Co additions in pure Fe film, to determine the relationship between the structure and magnetic properties of the Fe system, as well as those of the FeN and FeCoN systems. 2. Experimental Fe, FeN, and FeCoN thin films were deposited on glass substrates at RT by DC magnetron sputtering and the FeCoN film for transmission electron microscopy (TEM) was grown on NaCl substrate. The sputtering targets were pure Fe (99.99%) and composite materials consisting of Co chips placed on Fe target. Co atomic concentration percentage (at%) was 13 at% in FeCoN films. The sputtering gas was Ar (99.999%) and reactive gas was N2 (99.999%).
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The sputtering chamber pressure was reduced to 8 105 Pa before deposition. During sputtering, the partial pressure in the chamber was maintained at 0.8 Pa; the argon gas flow was kept at 60 standard cubic centimeters per minute (sccm); and the nitrogen gas flow was varied at 0, 3, and 3 sccm for the Fe, FeN, and FeCoN films, respectively. The films were deposited at RT and subsequently annealed at 200 C for 3 h. All annealing processes were conducted under a base vacuum higher than 2 104 Pa. The thickness of the films was measured by a surface profiler. The structures were determined by X-ray diffraction (XRD) and TEM. The morphological analyses of the three samples were conducted by scanning electron microscopy (SEM). The Co concentration in the FeCoN film was measured using energy dispersive spectroscopy. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) measurements were also performed to determine the surface topography, grain structure and domain structures on the surface of the three films, respectively. The magnetic properties were measured by a vibrating sample magnetometer (VSM) with magnetic field of up to 10 kOe. 3. Results and discussion 3.1. Scanning electron microscopy Fig. 1 shows the SEM images of the Fe, FeN and FeCoN films. In Fig. 1(a), the islands on the pure Fe film surface resemble leaves. The surface morphology is changed dramatically for the FeN film shown in Fig. 1(b), where the island shape does not look like leaves, but like a triangular pyramid. In Fig. 1(c), the island shape still looks like triangular pyramid, but the number of triangular pyramids is increased. 3.2. X-ray diffraction The variation of the surface morphology may be due to the change of the structures [17]; thus the XRD measurements were performed. Fig. 2 shows the XRD patterns of the three films. For the pure Fe film, the main diffraction peaks from Fe (110), (200), and (211) can be observed, which suggests that the pure Fe film prefers to grow along the bcc a-Fe (110) orientation. As N was introduced, the diffraction peaks from a00 -Fe16N2 (202) and (220) appear, and the intensity of Fe (211) weakened, which could be the reason for the triangular-pyramid-like surface morphology, but the peak of Fe (211) was still observed in the FeN sample. As Co additions, the peak of Fe (211) disappeared, and only a00 -(Fe,Co)16N2 (202) could be observed. No evidence existed for related compounds of Co because of the larger affinity of Fe for N than that of Co for N [16], which would suggest that the Co atoms replaced the Fe atoms in the Fe
Fig. 2. XRD patterns of three films (a) Fe, (b) FeN, and (c) FeCoN.
lattice. The intensity of a00 (202) in the FeCoN film was stronger than that in the FeN film, which indicated that the a00 phase was the dominant phase and the amount of the a00 phase in the FeCoN film was larger than that in the FeN film. According to the XRD results, proper Co addition helps to enhance the a00 phase. This condition is discussed as follows: owing to Co substitution, the lattice parameter of the bcc FeeCo phase increased relative to that of the bcc pure Fe [18]. Based on the assumption that the atomic volume of Co is the same as that of Fe, the interstitial sites are presumably wider in the FeeCo lattice than in the Fe lattice. This idea is supported by the fact that the solubility of N is increased by Co addition [19]. The
Fig. 1. SEM images for (a) Fe, (b) FeN, and (c) FeCoN films.
J. Fan et al. / Journal of Alloys and Compounds 662 (2016) 541e545
a00 phase is considered to be an N ordered form of the tetragonal FeeN solid solution, which is derived without changing the basic arrangement of Fe atoms in the bcc structure. Therefore, we can be speculate that the Co-containing 16:2 nitride also forms with less strain energy, leading to higher stability. The observed enhancement of formation of the 16:2 nitride by Co addition can be related to the improved stability of the 16:2 structure [7]. 3.3. Transmission electron microscopy For reliable phase identification in the FeCoN film, TEM was used to characterize the crystal structure and morphology. The grain sizes estimated from the bright-field images (shown in Fig. 3(a)) of the FeCoN film are approximately 12e15 nm. As the grain size decreases below the ferromagnetic exchange length (~20 nm for Fe), the averaging is conducted on very fine grains and magnetization follows the easy direction of each individual grain. This condition results in very low coercivity and high values of saturation magnetization [3]. The selected area diffraction (SAD) pattern in Fig. 3(b) exhibits some discontinuous diffraction rings with bright spots, showing that the FeCoN film is polycrystalline and some texture and specific orientation relationships exist in the film. The superlattice reflections from (201), (220), (301), (400), (422) of the a00 phase and from (300) of the g0 phase are clearly observed. These findings indicate that the major Co containing a00 alloy phase has the same structure as the a00 -Fe16N2 phase. The existence of the g0 phase in our film, caused this film to have lower Ms than those reported by other researchers [20]. No diffraction spots or rings of Co compounds were observed in the SAD patterns. The high-resolution transmission electron microscopy (HRTEM) image in Fig. 3(c) shows the crystallized a00 phase grains with (202) lattice d-spacing of ~2.1 Å, (220) lattice d-spacing of ~2.0 Å, and (301) lattice d-spacing of ~1.8 Å. 3.4. AFMeFMF studies Fig. 4 shows the AFM (left) and MFM (right) images of the three films. All AFM and MFM images were obtained through a consecutive scan. AFM observation demonstrates that the films have smooth surfaces. The shape of the Fe film looks like leaves. When N and Co were added, the leafs shape was changed significantly changed and the column nodules were detached from one another. The different surface morphology may be due to the differences of the lattice orientations of the grains on the film surface. The leaflike surface morphology should be the result from the coexistence of the a-Fe [110], [200], and [211] oriented grains. The aforementioned relation between surface morphology and lattice orientation have been confirmed by the following XRD results. In general, the average surface roughness Ra is defined as the arithmetic average deviation from the mean line within the assessment
543
length L [21]; the average surface roughness is 2.568, 2.630, and 3.136 nm for the Fe, FeN, and FeCoN films, respectively; and the varied roughness values with different samples may be due to the varying energy of sputtered particles and growth mechanisms [22]. These images confirm that the surface morphology of the three films agreed with the SEM results. The MFM measurement detects the magnetic force gradient arising from magnetic charge on the surface and shows the domain structures of the samples. The difference in texture of the Fe, FeN, and FeCoN films seems to have a marked influence on these domains. The magnetic image in Fig. 4 (d) is blurred as only very weak stray fields were produced by the Fe film; the domain structure was finer for the FeN film (in Fig. 4 (e)); they look connected across the adjacent patches but become more orderly for Co doping. The reproducible image contrast appears in the FeCoN film (in Fig. 4 (f)), which suggests that the film has sufficiently high perpendicular anisotropy (shown in the insets of Fig. 5), which can self-organize the domain structure in regular patterns [23,24]. Theoretically, the competition between the longrange dipolar energy and the short range magnetic interaction can result in striped domains [25,26]. 3.5. Magnetic measurements Fig. 5 shows the RT MeH curves of the three samples. Clearly, the Ms increased as a result of N addition and Co doping. The increase in MS of the FeN film shows that the N atoms alter the atomic states of Fe [27]. The exchange energy determines the spin order and the enhancement of the exchange energy results in a good degree of spin order and therefore larger saturation magnetization. The implication is that the small amount of N atoms in the interstices of Fe increases the exchange force. As generally known, the compound Fe16N2 exhibits giant saturation magnetization [28,29]. However, only a few research groups have succeeded in fabricating single-phase Fe16N2, and the reported Ms of Fe16N2 is in the 1710e2310 emu/cm3 range [30e33], the reduced magnetization of our FeN film may have resulted from the Fe phases that can be detected by XRD. In the present study, the magnetization in FeN film improved with the addition of Co, and the value of Ms in the sample FeCoN was about 2088 emu/cm3, higher than that of FeN (1795 emu/cm3) and pure Fe (1445 emu/cm3). Larger Ms of the FeCoN film could be attributed to a decrease in the Fe phase and an increase in the a00 phase formation. These findings are in good agreement with the XRD data, as shown in Fig. 2(c). In terms of Hc, the values for the Fe, FeN, and FeCoN films were 123, 329, and 123 Oe, respectively. Incorporation of N in Fe led to magnetic anisotropy and an increase in coercivity [3]. Hc of the FeCoN thin film was lower than that of the corresponding FeN film because of the random distribution of the particles [34]. To verify the perpendicular anisotropy, the typical MeH curves of the FeCoN film with external fields parallel and perpendicular to the film plane are
Fig. 3. (a) Bright-field TEM image, (b) SAD pattern, and (c) HRTEM image of the FeCoN thin film.
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Fig. 4. AFM images: (a) Fe, (b) FeN, (c) FeCoN, and MFM measurements: (d) Fe, (e) FeN, (f) FeCoN.
presented in the inset of Fig. 5. Notably, the in-plane (IP) hysteresis loop is saturated at low field, but the out-of-plane (OP) hysteresis loop is difficult to saturate, which exhibits very good magnetic anisotropy with the easy axis on the film plane.
3.6. Resistivity properties The corresponding resistivities are 52.1, 187.9, and 455.5 mU cm for the Fe, FeN and FeCoN films. These results indicate that N and Co
J. Fan et al. / Journal of Alloys and Compounds 662 (2016) 541e545
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51301099, 11274214), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20121404130001) and the Natural Science Foundation of Shanxi Province (Grant No. 2013011014-4).
References
Fig. 5. RT in-plane MeH curves of the polycrystalline three samples. The insets show IP and OP hysteresis loops measured on the FeCoN film.
can effectively adjust the resistivity of Fe films. Such an increase in resistivity may be due to two types of contributions to the scattering of the conduction electrons: one is the non-metallic N content in the film, which causes the number of free electrons per unit volume to decrease and creates more defects (such as dislocations, porus, impurities, and strain) and the other is the disorderly atom arrangement in crystallites [35]. The resistivity of our FeCoN film is much higher than that of the widely studied FeCoN thin films that have a resistivity of approximately 211 mU cm [14]. High resistivity is an important parameter for high frequency application because higher resistivity can suppress eddy current loss more effectively. In this respect, using FeCoN thin films in magnetic recording heads is a good choice. 4. Conclusions We conducted a comparative study on the morphology, structure, magnetism properties, and resistivity of Fe, FeN, and FeCoN thin films. The phases formed in these films were strongly dependent on the N and Co addition. The addition of Co was favorable to the formation of an a00 phase and suppressed the formation of Fe phase. Owing to the sufficiently high perpendicular anisotropy, the FeCoN film had smoother surfaces and clear reproducible striped domains. According to the VSM results, the Ms of 2088 emu/cm3 was obtained by Co doping. The resistivity of the Fe film increased from 52.1 mU cm to 455.5 mU cm with the introduction of N and Co. The variations in the morphology, structure, magnetic properties, and resistivity were attributed to the change in the phase structure of the films. These properties indicated that the FeCoN film had good potential for use in rare-earth-free permanent magnets, magnetic writers, and spintronic devices in the future.
[1] D. Muraca, V. Cremaschi, J. Moya, H. Sirkin, J. Magn. Magn. Mater. 320 (2008) 1639e1644. [2] M.E. McHenry, M.A. Willard, D.E. Laughlin, Prog. Mater. Sci. 44 (1999) 291e433. [3] R. Gupta, A. Tayal, S.M. Amir, M. Gupta, A. Gupta, M. Horisberger, J. Stahn, J. Appl. Phys. 111 (2012) 103520. [4] J.P. Wang, N. Ji, X.Q. Liu, Y.H. Xu, C. S anchez-Hanke, Y.M. Wu, F.M.F.D. Groot, L.F. Allard, E. Lara-Curzio, IEEE Trans. Magn. 48 (2012) 1710e1717. [5] K.H. Jack, Proc. R. Soc. A 208 (1951) 200e215. [6] E.Z. Fratczak, J.E. Prieo, M.E. Moneta, J. Alloys Compd. 586 (2014) 375e379. [7] Y. Inoue, S. Takebayashi, T. Mukai, J. Appl. Phys. 76 (1994) 6653e6655. [8] H.Y. Wang, E.Y. Jiang, Z.W. Ma, Y.J. He, H.S. Huang, J. Phys. Condens. Matter 11 (1999) 989e995. [9] H.Y. Wang, E.Y. Jiang, Appl. Phys. A 65 (1997) 203e206. [10] E.Y. Jiang, H.Y. Wang, Z.W. Ma, J. Appl. Phys. 85 (1999) 4488e4490. [11] Y.P. Wu, Y. Yang, Z.H. Yang, B.Y. Zong, J. Ding, IEEE. Magn. Lett. 6 (2015) 5700204. [12] Y.P. Wu, Y. Yang, Z.H. Yang, B.Y. Zong, IEEE Magn. Lett. 6 (2015) 5000204. [13] Y.P. Wu, Y. Yang, Z.H. Yang, N.N. Phouc, F. Ma, B.Y. Zong, J. Ding, J. Appl. Phys. 118 (2015) 013902. [14] Y. Liu, Z.W. Liu, C.Y. Tan, C.K. Ong, J. Appl. Phys. 100 (2006) 093912. [15] S. Atiq, H.S. Ko, S.A. Siddiqi, S.C. Shin, J. Alloys Compd. 479 (2009) 755e758. [16] H.Y. Wang, E.Y. Jiang, J. Phys. Condens. Matter 9 (1997) 8547e8552. [17] X.C. Wang, W.B. Mi, G.F. Chen, X.M. Chen, B.H. Yang, Appl. Surf. Sci. 258 (2012) 4764e4769. [18] W.B. Pearson, A Handbook of Lattice Spacing and Structures of Metals and Alloys, 1958. [19] V. Raghavan, Phase Diagrams of Ternary Iron Alloys, Part 1, 1987, p. 167. [20] Y. Sugita, H. Takahashi, M. Komuro, J. Appl. Phys. 76 (1994) 6637e6641. [21] W.B. Mi, L. Guo, E.Y. Jiang, Z.Q. Li, P. Wu, H.L. Bai, J. Phys. D Appl. Phys. 36 (2003) 2393e2399. [22] X. Wang, W.T. Zheng, H.W. Tian, S.S. Yu, W. Xu, S.H. Meng, X.D. He, J.C. Han, C.Q. Sun, B.K. Tay, Appl. Surf. Sci. 220 (2003) 30e39. [23] C.B. Craus, A.R. Chezan, M.H. Siekman, J.C. Lodder, D.O. Boerma, L. Niesen, J. Magn. Mater. Magn. 240 (2002) 423e426. [24] Y.Z. Wu, C. Won, A. Scholl, A. Doran, H.W. Zhao, X.F. Jin, Z.Q. Qiu, Phys. Rev. Lett. 93 (2004) 117205. [25] A. Vaterlaus, C. Stamm, U. Maier, M.G. Pini, P. Politi, D. Pescia, Phys. Rev. Lett. 84 (2000) 2247e2250. [26] T. Garel, S. Doniach, Phys. Rev. B 26 (1982) 325e329. [27] M. Nagakubo, T. Yamamoto, M. Naoe, J. Appl. Phys. 63 (1988) 4309e4311. [28] T.K. Kim, M. Takahashi, Appl. Phys. Lett. 20 (1972) 492e494. [29] N. Ji, M.S. Osofsky, V. Lauter, L.F. Allard, X. Li, K.L. Jensen, H. Ambaye, E. LaraCurzio, J.P. Wang, Phys. Rev. B 84 (2011) 245310. [30] D.C. Sun, E.Y. Jiang, M.B. Tian, C. Lin, X.X. Zhang, J. Appl. Phys. 79 (1996) 5440e5442. [31] M. Takahashi, H. Shoji, H. Takahashi, H. Nashi, T. Wakiyama, M. Doi, M. Matsui, J. Appl. Phys. 76 (1994) 6642e6647. [32] H. Takahashi, K. Mitsuoka, M. Komuro, Y. Sugita, J. Appl. Phys. 73 (1993) 6060e6062. [33] Y. Utsushikawa, K. Niizuma, J. Alloys Compd. 222 (1995) 188e192. [34] R. Gupta, M. Gupta, Phys. Rev. B 72 (2005) 024202. [35] H.B. Nie, S.Y. Xu, C.K. Ong, Q. Zhan, D.X. Li, J.P. Wang, Thin Solid Films 440 (2003) 35e40.
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Effect of nitrogen and cobalt additions on surface morphology and magnetic properties of Fe thin films Jiuping Fan, Juan Sun, Yang Yang, Riuyan Liang, Yannan Jiang, Jun Zhang, Xiaohong Xu* School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and Magnetic Information Materials, Ministry of Education, Shanxi Normal University, Linfen 041004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 2 December 2015 Accepted 7 December 2015 Available online 11 December 2015
Fe, FeN, and FeCoN films were fabricated by direct-current (DC) magnetron sputtering. The dependence of surface morphology, magnetic properties, and resistivity on phase structure was systematically investigated. a00 -Fe16N2 (202) phase occurred with the incorporation of nitrogen (N) and cobalt (Co), and the surface morphology evolved from random-leafs-like shapes to triangular-pyramid-like islands. Clear reproducible striped domains appeared at the FeCoN film surfaces, because of high perpendicular anisotropy in the film. The additions of Co enhanced the formation of the a00 -Fe16N2 phase, which caused the saturation magnetization (Ms) value of the FeCoN film to increase to 2088 emu/cm3, considerably higher than that of pure Fe. Moreover, the significant amount of a00 phase leads to higher resistivity and therefore improved high-frequency permeability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Thin films Anisotropy Transmission electron microscopy Magnetic measurements
1. Introduction Over the past decades, Fe-based films have evolved because of their high saturation magnetization and high resistivity [1,2]. The magnetization of Fe was enhanced by introducing a small amount of N, which happened because of the incorporation of N atoms at the interstitial sites, thereby resulting in expansion and distortion of body-centered cubic (bcc)-Fe units cells. A significant increase occurs in the volume fraction of grain boundaries or interfaces, resulting in reasonably high values of saturation magnetization [3]. One of the most studied iron (Fe) nitride compounds is a00 -Fe16N2, which possesses a giant magnetic moment and large magnetocrystalline anisotropy, and is considered as a potential candidate for rare-earth-free magnets because of its promising magnetic-energy product performance [4]. However, pure a00 -Fe16N2 is difficult to prepare because of the limitations from the FeN-phase diagrams [5,6]. Co doping into Fe nitrides has been reported to change their magnetic properties of these materials [7e13]. According to several studies, the addition of Co is in favor of enhancing the amount of a00 phase [7e10], and the noticeably increased electrical resistivity is very useful for a variety of technological applications; thus studying the Co-doped FeN system is meaningful [14]. The following two
* Corresponding author. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.12.042 0925-8388/© 2015 Elsevier B.V. All rights reserved.
points are considered as the reasons to choose Co as the additive element: (i) bcc structure is stably formed up to a composition of 75 at.% at room temperature (RT), which is essential because a00 Fe16N2 has an analogous bct structure [15]; and (ii) the chemical affinity of Co for N is weaker than that of Fe, which prevents Co from undergoing preferential nitridation [16]. As reported, the FeCoN film has large resistivity, but the Ms of this sample is lower [14], and we observed a larger Ms and a higher resistivity in one FeCoN film. In addition, to the best of our knowledge, few studies have systemically compared the effect of N and Co additions on Fe thin films. In this paper, we describe the surface morphology, structure, magnetic properties and resistivity of N and Co additions in pure Fe film, to determine the relationship between the structure and magnetic properties of the Fe system, as well as those of the FeN and FeCoN systems. 2. Experimental Fe, FeN, and FeCoN thin films were deposited on glass substrates at RT by DC magnetron sputtering and the FeCoN film for transmission electron microscopy (TEM) was grown on NaCl substrate. The sputtering targets were pure Fe (99.99%) and composite materials consisting of Co chips placed on Fe target. Co atomic concentration percentage (at%) was 13 at% in FeCoN films. The sputtering gas was Ar (99.999%) and reactive gas was N2 (99.999%).
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The sputtering chamber pressure was reduced to 8 105 Pa before deposition. During sputtering, the partial pressure in the chamber was maintained at 0.8 Pa; the argon gas flow was kept at 60 standard cubic centimeters per minute (sccm); and the nitrogen gas flow was varied at 0, 3, and 3 sccm for the Fe, FeN, and FeCoN films, respectively. The films were deposited at RT and subsequently annealed at 200 C for 3 h. All annealing processes were conducted under a base vacuum higher than 2 104 Pa. The thickness of the films was measured by a surface profiler. The structures were determined by X-ray diffraction (XRD) and TEM. The morphological analyses of the three samples were conducted by scanning electron microscopy (SEM). The Co concentration in the FeCoN film was measured using energy dispersive spectroscopy. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) measurements were also performed to determine the surface topography, grain structure and domain structures on the surface of the three films, respectively. The magnetic properties were measured by a vibrating sample magnetometer (VSM) with magnetic field of up to 10 kOe. 3. Results and discussion 3.1. Scanning electron microscopy Fig. 1 shows the SEM images of the Fe, FeN and FeCoN films. In Fig. 1(a), the islands on the pure Fe film surface resemble leaves. The surface morphology is changed dramatically for the FeN film shown in Fig. 1(b), where the island shape does not look like leaves, but like a triangular pyramid. In Fig. 1(c), the island shape still looks like triangular pyramid, but the number of triangular pyramids is increased. 3.2. X-ray diffraction The variation of the surface morphology may be due to the change of the structures [17]; thus the XRD measurements were performed. Fig. 2 shows the XRD patterns of the three films. For the pure Fe film, the main diffraction peaks from Fe (110), (200), and (211) can be observed, which suggests that the pure Fe film prefers to grow along the bcc a-Fe (110) orientation. As N was introduced, the diffraction peaks from a00 -Fe16N2 (202) and (220) appear, and the intensity of Fe (211) weakened, which could be the reason for the triangular-pyramid-like surface morphology, but the peak of Fe (211) was still observed in the FeN sample. As Co additions, the peak of Fe (211) disappeared, and only a00 -(Fe,Co)16N2 (202) could be observed. No evidence existed for related compounds of Co because of the larger affinity of Fe for N than that of Co for N [16], which would suggest that the Co atoms replaced the Fe atoms in the Fe
Fig. 2. XRD patterns of three films (a) Fe, (b) FeN, and (c) FeCoN.
lattice. The intensity of a00 (202) in the FeCoN film was stronger than that in the FeN film, which indicated that the a00 phase was the dominant phase and the amount of the a00 phase in the FeCoN film was larger than that in the FeN film. According to the XRD results, proper Co addition helps to enhance the a00 phase. This condition is discussed as follows: owing to Co substitution, the lattice parameter of the bcc FeeCo phase increased relative to that of the bcc pure Fe [18]. Based on the assumption that the atomic volume of Co is the same as that of Fe, the interstitial sites are presumably wider in the FeeCo lattice than in the Fe lattice. This idea is supported by the fact that the solubility of N is increased by Co addition [19]. The
Fig. 1. SEM images for (a) Fe, (b) FeN, and (c) FeCoN films.
J. Fan et al. / Journal of Alloys and Compounds 662 (2016) 541e545
a00 phase is considered to be an N ordered form of the tetragonal FeeN solid solution, which is derived without changing the basic arrangement of Fe atoms in the bcc structure. Therefore, we can be speculate that the Co-containing 16:2 nitride also forms with less strain energy, leading to higher stability. The observed enhancement of formation of the 16:2 nitride by Co addition can be related to the improved stability of the 16:2 structure [7]. 3.3. Transmission electron microscopy For reliable phase identification in the FeCoN film, TEM was used to characterize the crystal structure and morphology. The grain sizes estimated from the bright-field images (shown in Fig. 3(a)) of the FeCoN film are approximately 12e15 nm. As the grain size decreases below the ferromagnetic exchange length (~20 nm for Fe), the averaging is conducted on very fine grains and magnetization follows the easy direction of each individual grain. This condition results in very low coercivity and high values of saturation magnetization [3]. The selected area diffraction (SAD) pattern in Fig. 3(b) exhibits some discontinuous diffraction rings with bright spots, showing that the FeCoN film is polycrystalline and some texture and specific orientation relationships exist in the film. The superlattice reflections from (201), (220), (301), (400), (422) of the a00 phase and from (300) of the g0 phase are clearly observed. These findings indicate that the major Co containing a00 alloy phase has the same structure as the a00 -Fe16N2 phase. The existence of the g0 phase in our film, caused this film to have lower Ms than those reported by other researchers [20]. No diffraction spots or rings of Co compounds were observed in the SAD patterns. The high-resolution transmission electron microscopy (HRTEM) image in Fig. 3(c) shows the crystallized a00 phase grains with (202) lattice d-spacing of ~2.1 Å, (220) lattice d-spacing of ~2.0 Å, and (301) lattice d-spacing of ~1.8 Å. 3.4. AFMeFMF studies Fig. 4 shows the AFM (left) and MFM (right) images of the three films. All AFM and MFM images were obtained through a consecutive scan. AFM observation demonstrates that the films have smooth surfaces. The shape of the Fe film looks like leaves. When N and Co were added, the leafs shape was changed significantly changed and the column nodules were detached from one another. The different surface morphology may be due to the differences of the lattice orientations of the grains on the film surface. The leaflike surface morphology should be the result from the coexistence of the a-Fe [110], [200], and [211] oriented grains. The aforementioned relation between surface morphology and lattice orientation have been confirmed by the following XRD results. In general, the average surface roughness Ra is defined as the arithmetic average deviation from the mean line within the assessment
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length L [21]; the average surface roughness is 2.568, 2.630, and 3.136 nm for the Fe, FeN, and FeCoN films, respectively; and the varied roughness values with different samples may be due to the varying energy of sputtered particles and growth mechanisms [22]. These images confirm that the surface morphology of the three films agreed with the SEM results. The MFM measurement detects the magnetic force gradient arising from magnetic charge on the surface and shows the domain structures of the samples. The difference in texture of the Fe, FeN, and FeCoN films seems to have a marked influence on these domains. The magnetic image in Fig. 4 (d) is blurred as only very weak stray fields were produced by the Fe film; the domain structure was finer for the FeN film (in Fig. 4 (e)); they look connected across the adjacent patches but become more orderly for Co doping. The reproducible image contrast appears in the FeCoN film (in Fig. 4 (f)), which suggests that the film has sufficiently high perpendicular anisotropy (shown in the insets of Fig. 5), which can self-organize the domain structure in regular patterns [23,24]. Theoretically, the competition between the longrange dipolar energy and the short range magnetic interaction can result in striped domains [25,26]. 3.5. Magnetic measurements Fig. 5 shows the RT MeH curves of the three samples. Clearly, the Ms increased as a result of N addition and Co doping. The increase in MS of the FeN film shows that the N atoms alter the atomic states of Fe [27]. The exchange energy determines the spin order and the enhancement of the exchange energy results in a good degree of spin order and therefore larger saturation magnetization. The implication is that the small amount of N atoms in the interstices of Fe increases the exchange force. As generally known, the compound Fe16N2 exhibits giant saturation magnetization [28,29]. However, only a few research groups have succeeded in fabricating single-phase Fe16N2, and the reported Ms of Fe16N2 is in the 1710e2310 emu/cm3 range [30e33], the reduced magnetization of our FeN film may have resulted from the Fe phases that can be detected by XRD. In the present study, the magnetization in FeN film improved with the addition of Co, and the value of Ms in the sample FeCoN was about 2088 emu/cm3, higher than that of FeN (1795 emu/cm3) and pure Fe (1445 emu/cm3). Larger Ms of the FeCoN film could be attributed to a decrease in the Fe phase and an increase in the a00 phase formation. These findings are in good agreement with the XRD data, as shown in Fig. 2(c). In terms of Hc, the values for the Fe, FeN, and FeCoN films were 123, 329, and 123 Oe, respectively. Incorporation of N in Fe led to magnetic anisotropy and an increase in coercivity [3]. Hc of the FeCoN thin film was lower than that of the corresponding FeN film because of the random distribution of the particles [34]. To verify the perpendicular anisotropy, the typical MeH curves of the FeCoN film with external fields parallel and perpendicular to the film plane are
Fig. 3. (a) Bright-field TEM image, (b) SAD pattern, and (c) HRTEM image of the FeCoN thin film.
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Fig. 4. AFM images: (a) Fe, (b) FeN, (c) FeCoN, and MFM measurements: (d) Fe, (e) FeN, (f) FeCoN.
presented in the inset of Fig. 5. Notably, the in-plane (IP) hysteresis loop is saturated at low field, but the out-of-plane (OP) hysteresis loop is difficult to saturate, which exhibits very good magnetic anisotropy with the easy axis on the film plane.
3.6. Resistivity properties The corresponding resistivities are 52.1, 187.9, and 455.5 mU cm for the Fe, FeN and FeCoN films. These results indicate that N and Co
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51301099, 11274214), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20121404130001) and the Natural Science Foundation of Shanxi Province (Grant No. 2013011014-4).
References
Fig. 5. RT in-plane MeH curves of the polycrystalline three samples. The insets show IP and OP hysteresis loops measured on the FeCoN film.
can effectively adjust the resistivity of Fe films. Such an increase in resistivity may be due to two types of contributions to the scattering of the conduction electrons: one is the non-metallic N content in the film, which causes the number of free electrons per unit volume to decrease and creates more defects (such as dislocations, porus, impurities, and strain) and the other is the disorderly atom arrangement in crystallites [35]. The resistivity of our FeCoN film is much higher than that of the widely studied FeCoN thin films that have a resistivity of approximately 211 mU cm [14]. High resistivity is an important parameter for high frequency application because higher resistivity can suppress eddy current loss more effectively. In this respect, using FeCoN thin films in magnetic recording heads is a good choice. 4. Conclusions We conducted a comparative study on the morphology, structure, magnetism properties, and resistivity of Fe, FeN, and FeCoN thin films. The phases formed in these films were strongly dependent on the N and Co addition. The addition of Co was favorable to the formation of an a00 phase and suppressed the formation of Fe phase. Owing to the sufficiently high perpendicular anisotropy, the FeCoN film had smoother surfaces and clear reproducible striped domains. According to the VSM results, the Ms of 2088 emu/cm3 was obtained by Co doping. The resistivity of the Fe film increased from 52.1 mU cm to 455.5 mU cm with the introduction of N and Co. The variations in the morphology, structure, magnetic properties, and resistivity were attributed to the change in the phase structure of the films. These properties indicated that the FeCoN film had good potential for use in rare-earth-free permanent magnets, magnetic writers, and spintronic devices in the future.
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