- Email: [email protected]
Study of band structure of iron nitrides by angle resolved photoemission spectroscopy
Journal of Magnetism and Magnetic Materials 475 (2019) 759–762
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Study of band structure of iron nitrides by angle resolved photoemission spectroscopy
T
Henan Fang , Junlin Chen, Ying Li, Xiang Peng, Zhikuo Tao ⁎
College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
ARTICLE INFO
ABSTRACT
Keyword: Iron nitrides Electronic structure Angle resolved photoemission spectroscopy Magnetic materials
The band structure of Fe-N film is studied by angle resolved photoemission spectroscopy. It is found that the 3d band of Fe-N film disappears due to the hybridization of Fe 3d states and N 2p states. The result can account for the less ferromagnetism and conductivity of Fe-N film when compared with those of Fe film. This finding indicates that the high content of nitrogen can significantly change the band structure of iron nitrides, which exceeds the conventional theoretical prediction. The present work may evoke the re-recognization to the effect of coupling of Fe and N, and promote the further research of band structure of iron nitrides.
1. Introduction
remove the surface oxide and impurities.
Iron nitrides have been paid much attention due to their chemically inert, abrasive resistance, and magnetic properties [1–4]. Since 1980s, extensive works have been carried out to study the electronic band structure of iron nitrides [5–16]. However, these works focused on the theoretical investigation, and the experimental results were barely reported. In this paper, we try to experimentally investigate the band structure of iron nitrides prepared on GaN film using angle resolved photoemission spectroscopy (ARPES). Here, the choice of GaN as the substrate is due to the fact that several kinds of iron nitrides, i.e. -Fe4N, -Fe3N and -FeN, have been fabricated on it [4,17–20].
3. Results and discussion
2. Experimental The semi-insulated GaN (0001) substrate used was grown by HVPE on c-plane sapphire, and its thickness is more than 30 μm. First, the Fe film was prepared on the GaN substrates by direct current magnetron sputtering. The base pressure of the sputtering system was 10 5 Pa. High-purity four-nines Fe target was used. The sputtering is under room temperature. The processing pressure is 0.5 Pa, the argon flow rate is 100 sccm, and the sputtering power is 100 W. Correspondingly, the growth rate is 4.5 nm/min. The sputtering time is 20 min, with the resultant Fe film being 90 nm. And then, the Fe film was nitrated in a tube furnace under the optimal conditions of using the nitriding temperature 950 °C, the nitriding time 4 h, and the NH3 flow rate 200 sccm. As can be seen in the following, the Fe film was wholly nitrated at such conditions. Finally, the sample was further etched by ICP for 120 s to
⁎
The crystal structures of the Fe and Fe-N films were analyzed using X-ray diffraction (XRD). The results are shown in Fig. 1. For the pattern of the Fe film, the scattering peak with large width located at 44.7° suggests the existence of Fe (1 1 0) phase [21]. On the contrary, the Fe (1 1 0) peak completely disappears in the pattern of Fe-N film, which suggests that the Fe film should be wholly nitrated. Instead, the major phase is Fe2N, and the two strongest peaks located at 40.74° and 42.8° correspond to Fe2N (0 0 2) [22] and Fe2N (0 1 1) [23] respectively. In addition, there are other four peaks at diffraction angles of 36.32°, 37.36°, 41.4°, and 44.32°, and they can respectively be indexed to FeN (1 1 1) [19], Fe3N (1 1 0) [21], Fe3N (0 0 2) [24], and Fe3N (2 0 0) [25]. The chemical state analyses of the N and Fe atoms in the Fe and FeN films were investigated by X-ray photoelectron spectroscopy (XPS). According to routine procedures, argon ion sputtering was performed before the XPS measurement. The binding energies have been calibrated using the C 1s peak (284.6 eV) as a reference. Fig. 2(a) and (c) exhibit the N 1s spectra of the Fe film and Fe-N film respectively. As can be seen from Fig. 2(a), there is not any peaks, which is a natural result for the Fe film. With regard to Fe-N film, it can be found that there are two peaks which are located at 396 eV and 397.1 eV. The peak of 397.1 eV should be ascribed to N-Fe which can show the existence of iron nitrides on the Fe-N film [25,26], while the other corresponds to NO [27]. Fig. 2(b) and (d) depict the Fe 2p spectra of the Fe film and Fe-N film respectively. In order to eliminate the noise, the signal of Fe-N film
Corresponding author. E-mail address: [email protected] (H. Fang).
https://doi.org/10.1016/j.jmmm.2018.12.034 Received 25 September 2018; Received in revised form 6 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 475 (2019) 759–762
H. Fang et al.
This can be explained by the XRD results: There exist several phases of iron nitrides in the film, and they are unable to be fused with each other. Fig. 4 displays the magnetic hysteresis loops of the Fe and Fe-N films at 5 K and room temperature (300 K) measured by superconducting quantum interference device (SQUID) magnetometer. The external magnetic fields were applied parallel to the sample plane. The influence of diamagnetism of the substrates has been removed. As shown in Fig. 4, both the Fe film and Fe-N film exhibit ferromagnetism, no matter at 5 K or 300 K. It also can be observed that the saturation magnetic moment of the Fe-N film is much less than that of the Fe film. Since the Fe film was wholly nitrated as stated above, there is no residual single iron in the Fe-N film. Therefore, the ferromagnetism of Fe-N film must originate from iron nitrides. Among the several phases existing in the Fe-N film, only the Fe3N phase can provide the ferromagnetism because both the FeN and Fe2N phases are nonferromagnetic at room temperature [3,29]. In addition, one can see that the saturation magnetic moment of the Fe-N film at 300 K is much weaker than that at 5 K, while the difference is little for the Fe film. As well known, the closer to Curie temperature the temperature is, the more quickly the spontaneous magnetization will decrease. Therefore, the Curie temperature of the Fe-N film is closer to 300 K and lower than the Fe film. As to the coercivities, they are 248Oe at 5 K and 105Oe at 300 K for the Fe film, while 338Oe and 336Oe for Fe-N film. This means that the coercivity of Fe film changes much more significantly with temperature than Fe-N film. Since the effect of temperature on coercivities mainly originates from the non-uniform size distribution of the magnetic domains [30], it can be concluded that the magnetic uniformity of the Fe-N film is better than the Fe film. The significant differences of coercivities between the
Fig. 1. XRD patterns of Fe and Fe-N films.
has been smoothed by Savitzky-Golay algorithm. It can be deduced from Fig. 2(b) that the peaks of Fe film should correspond to Fe-O since there exist satellite peaks. It is because that the XPS can only detect the surface information of the sample, and the surface of the Fe film was readily oxidized. For the Fe-N film, two major peaks are located around 709.2 eV and 723.3 eV. Since there is no broad satellite peaks, they should correspond to Fe-N 2p3/2 and Fe-N 2p1/2, respectively [28]. Fig. 3(a) presents the SEM image of the Fe film under 30000 magnifications. It can be observed that the surface is smooth and flat. The SEM image of the Fe-N film is displayed in Fig. 3(b). One can see that the surface of the Fe-N film is composed of irregular crystalline regions.
Fig. 2. XPS spectra of (a) N 1s of Fe film, (b) Fe 2p of Fe film, (c) N 1s of Fe-N film, and (d) Fe 2p of Fe-N film. 760
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Fig. 3. SEM photographs of (a) Fe film and (b) Fe-N film after ICP etching.
Fig. 4. Magnetic hysteresis loops of (a) Fe film at 5 K, (b) Fe film at 300 K, (c) Fe-N film at 5 K, and (d) Fe-N film at 300 K.
Fe film and Fe-N film also indicate that the ferromagnetism of Fe-N film can not originate from single iron. The I-V curves were measured at room temperature for both the Fe and Fe-N films by the system consists of Keithley 4200 semiconductor parameter analyzer and Chroma probe. Before measuring the I-V curves, Al electrodes are fabricated on the films using a BAK640 electron beam deposition system. The results are plotted in Fig. 5. Here, the resistivity of GaN template is more than 106 ·cm, and thus the influence of the substrate can be ignored. As depicted in Fig. 5, the linear I-V curve of the Fe-N film indicates good conductivity. Compared with the Fe Film, its resistivity is only 3 times higher. The results are in accordance with the previous literature [1]. Finally, the ARPES measurements were performed on laboratorybased ARPES system comprising a SPECS PHOI-BOS 150 electron analyzer and an UVS-300 lamp. Here, since the samples are polycrystalline, the angle-resolved information is isotropic. And thus, only the densities of states (DOS) are shown in Fig. 6 where the Fermi level is
at 0 eV. For the Fe film, it can be observed that there exists the so-called 3d band of Fe around Fermi level. As well known, the 3d band mainly consists of 3d states of Fe, which is account for the itinerant electron magnetism. For the Fe-N film, it is interesting that the 3d band disappears. This may be due to that the Fe 3d states are hybridized with the N 2p states, and the energies of the states are decreased. Accordingly, the results of SQUID and I-V curves can be explained as following: (1) The disappearance of 3d band decreases the ferromagnetism, which causes that the saturation magnetic moment of the Fe-N film is much less than that of the Fe film. (2) The disappearance of 3d band also decreases the conductivity, which leads to the higher resistivity of Fe-N film. It is worth noting that ARPES mainly detects the surface information of the sample. Among the phases of Fe-N film, FeN and Fe2N should exist on the surface and Fe3N in the inner because the surface will be more fully nitrated. And thus, the ARPES only gives the electronic structure of superficial FeN or Fe2N that provides less ferromagnetism and conductivity. On the contrary, the results of SQUID and 761
Journal of Magnetism and Magnetic Materials 475 (2019) 759–762
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Research Foundation of Nanjing University Communications (NY215083, NY217046).
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Posts
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References [1] M. Naito, K. Uehara, R. Takeda, Y. Taniyasu, H. Yamamoto, Growth of iron nitride thin films by molecular beam epitaxy, J. Cryst. Growth 415 (2015) 36–40. [2] H.N. Fang, R. Zhang, B. Liu, Z.K. Tao, M.W. Xiao, X.F. Wang, Z.L. Xie, X.Q. Xiu, Y.D. Zheng, Analysis of magnetic structures of iron nitrides by Landau’s theory of second-order phase transitions, AIP Adv. 3 (2013) 072136 . [3] C. Navio, J. Alvarez, M.J. Capitan, F. Yndurain, R. Miranda, Nonmagnetic -FeN thin films epitaxially grown on Cu(001): Electronic structure and thermal stability, Phys. Rev. B 78 (2008) 155417 . [4] H.N. Fang, R. Zhang, B. Liu, Z.K. Tao, X.F. Wang, Z.L. Xie, X.Q. Xiu, Y.D. Zheng, Magnetic and electrical properties of ε-Fe3N on c-plane GaN, J. Phys. D: Appl. Phys. 45 (2012) 315002 . [5] W. Zhou, L.J. Qu, Q.M. Zhang, D.S. Wang, Interaction and charge transfer in the iron nitride Fe4N, Phys. Rev. B 40 (1989) 6393–6397. [6] W.Y. Lai, Q.Q. Zheng, W.Y. Hu, The giant magnetic moment and electronic correlation effect in ferromagnetic nitride Fe16N2, J. Phys.: Condens. Matter 6 (1994) L259–L264. [7] J.S. He, Y.M. Zhou, W.Y. Lai, C.Y. Pan, Electronic structure and enhancement of magnetic moments in the ferromagnetic nitride Fe16N2, Phys. Rev. B 52 (1995) 6193–6196. [8] A. Sakuma, Electronic and magnetic structure of iron nitride, Fe16N2, J. Appl. Phys. 79 (1996) 5570–5575. [9] M. Sifkovits, H. Smolinski, S. Hellwig, W. Weber, Interplay of chemical bonding and magnetism in Fe4N, Fe3N and ζ-Fe2N, J. Magn. Magn. Mater. 204 (1999) 191–198. [10] S. Kokado, N. Fujima, K. Harigaya, H. Shimizu, A. Sakuma, Spin polarized transport of iron nitride Fe4N: Analysis using a combination of first principles calculation and model calculation, Phys. Stat. Sol. (C) 3 (2006) 3303–3306. [11] A. Houari, S.F. Matar, M.A. Belkhir, M. Nakhl, Structural stability and magnetism of FeN from first principles, Phys. Rev. B 75 (2007) 064420 . [12] D. Li, J.W. Roh, K.J. Jeon, Y.S. Gu, W. Lee, First principles calculations of the magnetic properties of FeN systems, Phys. Stat. Sol. (B) 245 (2008) 2581–2585. [13] A.V.G. Rebaza, J. Desimoni, E.L. Peltzer y Blancá, Study of the magnetic and electronic properties of the Fe4N with pressure, Physica B 404 (2009) 2872–2875. [14] S.D. Gupta, S.K. Gupta, P.K. Jha, Structural, electronic and dynamical stability of heavy metal iron pernitride: a spin polarized first-principles study, Eur. Phys. J. B 86 (2013) 8. [15] Z.L. Zhao, K. Bao, D.F. Duan, F.B. Tian, B.B. Liu, T. Cui, Effects of magnetic ordering and electron correlations on the stability of FeN, RSC Adv. 5 (2015) 31270–31274. [16] T. Chihi, M. Fatmi, M.A. Ghebouli, B. Ghebouli, H. Djemai, Experimental and theoretical study of the structural, mechanical and electronic properties of the FeyN (y = 1, 2, 3, 4) phases, Chinese. J. Phys. 55 (2017) 674–691. [17] M. Kimura, S. Hasegawa, Growth evolution of γprime-Fe4N films on GaN(0001) and their interfacial structure, Jpn. J. Appl. Phys. 55 (2016) 05FD02. [18] Z.K. Tao, X.G. Cui, R. Zhang, X.Q. Xiu, Z.L. Xie, Y.D. Zheng, Ferromagnetic Fe3N films grown on GaN(0002) substrates by MOCVD, J. Cryst. Growth 312 (2010) 1525–1528. [19] W.Z. Lin, J. Pak, D.C. Ingram, A.R. Smith, Molecular beam epitaxial growth of zincblende FeN(1 1 1) on wurtzite GaN(0 0 0 1), J. Alloys Compd. 463 (2008) 257–262. [20] H.N. Fang, J.L. Chen, X. Peng, Y. Li, Z.K. Tao, Iron nitrides on semi-insulated GaN film, Mater. Lett. 233 (2018) 191–194. [21] Vinod K. Rohith, P. Saravanan, M. Sakar, V.T.P. Vinod, S. Miroslav Cernik, Large Balakumar, scale synthesis and formation mechanism of highly magnetic and stable iron nitride (ε-Fe3N) nanoparticles, RSC Adv. 5 (2015) 56045–56048. [22] Y. Zhang, Y. Xie, Y.T. Zhou, X.W. Wang, K. Pan, Well dispersed Fe2N nanoparticles on surface of nitrogen-doped reduced graphite oxide for highly efficient electrochemical hydrogen evolution, J. Mater. Res. 32 (2017) 1770–1776. [23] L. Cao, Z.H. Li, Y. Gu, D.H. Li, K.M. Su, D.J. Yang, B.W. Cheng, Rational design of Ndoped carbon nanobox-supported Fe/Fe2N/Fe3C nanoparticles as efficient oxygen reduction catalysts for Znair batteries, J. Mater. Chem. A 5 (2017) 11340–11347. [24] N. Zhao, W. Wang, X. Lei, Z.T. Ye, X.D. Chen, H. Ding, H. Yang, Synthesis structure and magnetic properties of Fe3N nanoparticles, J. Mater. Sci.: Mater. Electron. 28 (2017) 15701–15707. [25] W.W. Yin, L. Li, X.D. Jiang, P.P. Liu, F.M. Liu, Y. Li, F. Peng, D.W. He, High pressure synthesis and properties studies on spherical bulk ε-Fe3N, High Pressure Res. 34 (2014) 317–326. [26] K. Rohith Vinod, P. Saravanan, M. Sakara, S. Balakumar, Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles, RSC Adv. 6 (2016) 45850–45857. [27] I. Milošev, H.H. Strehblow, B. Navinšek, M. Metikoshukovic, Electrochemical and thermal oxidation of TiN coatings studied by XPS, Surf. Interface Anal. 23 (1995) 529–539. [28] X.X. Huang, Z.Y. Yang, B. Dong, Y.Z. Wang, T.Y. Tang, Y.L. Hou, In situ Fe2N@Ndoped porous carbon hybrids as superior catalysts for oxygen reduction reaction, Nanoscale 9 (2017) 8102–8106. [29] H. Naganuma, Y. Endo, R. Nakatani, Y. Kawamura, M. Yamamoto, Magnetic properties of weak itinerant ferromagnetic ζ-Fe2N film, Sci. Technol. Adv. Mat. 5 (2004) 83–87. [30] M.G. Chen, Z.W. Jiao, S.J. Yu, M.Z. Yu, F.B. Bao, K. Nakamura, Temperature dependence of coercivity behavior in Fe films on fractal rough ceramics surfaces, Jpn. J. Appl. Phys. 52 (2013) 01AC13.
Fig. 5. I-V curves of Fe and Fe-N films.
Fig. 6. Densities of states of Fe and Fe-N films.
I-V curves detect the bulk properties of the Fe-N film, and hence show the ferromagnetism and conductivity which basically come from the internal Fe3N. The present result indicates that, when the N content is close to Fe, the coupling of Fe and N can significantly change the band structure of iron nitrides, and further affect the ferromagnetism and conductivity. 4. Conclusion In conclusion, Fe-N film was fabricated by means of nitridation of Fe film. A series of measurements characterize the structure, chemical environment, surface morphology, magnetic and electrical properties of the Fe-N film. Most importantly, the band structures of the Fe and Fe-N film are investigated by ARPES. Through the analysis of the DOS, it is found that the 3d band of Fe-N film disappears which can account for the less ferromagnetism and conductivity. This indicates that the high content of nitrogen can significantly change the band structure of iron nitrides. The interesting result exceeds the conventional theoretical prediction, and will promote the further research of the iron nitrides. Acknowledgements This work is supported by the National Natural Science Foundation of China (11704197, 61574079), the Nature Science of Foundation of Jiangsu Province (BK20130866), the University Nature Science Research Project of Jiangsu province (14KJB510020), the Scientific 762
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Study of band structure of iron nitrides by angle resolved photoemission spectroscopy
T
Henan Fang , Junlin Chen, Ying Li, Xiang Peng, Zhikuo Tao ⁎
College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
ARTICLE INFO
ABSTRACT
Keyword: Iron nitrides Electronic structure Angle resolved photoemission spectroscopy Magnetic materials
The band structure of Fe-N film is studied by angle resolved photoemission spectroscopy. It is found that the 3d band of Fe-N film disappears due to the hybridization of Fe 3d states and N 2p states. The result can account for the less ferromagnetism and conductivity of Fe-N film when compared with those of Fe film. This finding indicates that the high content of nitrogen can significantly change the band structure of iron nitrides, which exceeds the conventional theoretical prediction. The present work may evoke the re-recognization to the effect of coupling of Fe and N, and promote the further research of band structure of iron nitrides.
1. Introduction
remove the surface oxide and impurities.
Iron nitrides have been paid much attention due to their chemically inert, abrasive resistance, and magnetic properties [1–4]. Since 1980s, extensive works have been carried out to study the electronic band structure of iron nitrides [5–16]. However, these works focused on the theoretical investigation, and the experimental results were barely reported. In this paper, we try to experimentally investigate the band structure of iron nitrides prepared on GaN film using angle resolved photoemission spectroscopy (ARPES). Here, the choice of GaN as the substrate is due to the fact that several kinds of iron nitrides, i.e. -Fe4N, -Fe3N and -FeN, have been fabricated on it [4,17–20].
3. Results and discussion
2. Experimental The semi-insulated GaN (0001) substrate used was grown by HVPE on c-plane sapphire, and its thickness is more than 30 μm. First, the Fe film was prepared on the GaN substrates by direct current magnetron sputtering. The base pressure of the sputtering system was 10 5 Pa. High-purity four-nines Fe target was used. The sputtering is under room temperature. The processing pressure is 0.5 Pa, the argon flow rate is 100 sccm, and the sputtering power is 100 W. Correspondingly, the growth rate is 4.5 nm/min. The sputtering time is 20 min, with the resultant Fe film being 90 nm. And then, the Fe film was nitrated in a tube furnace under the optimal conditions of using the nitriding temperature 950 °C, the nitriding time 4 h, and the NH3 flow rate 200 sccm. As can be seen in the following, the Fe film was wholly nitrated at such conditions. Finally, the sample was further etched by ICP for 120 s to
⁎
The crystal structures of the Fe and Fe-N films were analyzed using X-ray diffraction (XRD). The results are shown in Fig. 1. For the pattern of the Fe film, the scattering peak with large width located at 44.7° suggests the existence of Fe (1 1 0) phase [21]. On the contrary, the Fe (1 1 0) peak completely disappears in the pattern of Fe-N film, which suggests that the Fe film should be wholly nitrated. Instead, the major phase is Fe2N, and the two strongest peaks located at 40.74° and 42.8° correspond to Fe2N (0 0 2) [22] and Fe2N (0 1 1) [23] respectively. In addition, there are other four peaks at diffraction angles of 36.32°, 37.36°, 41.4°, and 44.32°, and they can respectively be indexed to FeN (1 1 1) [19], Fe3N (1 1 0) [21], Fe3N (0 0 2) [24], and Fe3N (2 0 0) [25]. The chemical state analyses of the N and Fe atoms in the Fe and FeN films were investigated by X-ray photoelectron spectroscopy (XPS). According to routine procedures, argon ion sputtering was performed before the XPS measurement. The binding energies have been calibrated using the C 1s peak (284.6 eV) as a reference. Fig. 2(a) and (c) exhibit the N 1s spectra of the Fe film and Fe-N film respectively. As can be seen from Fig. 2(a), there is not any peaks, which is a natural result for the Fe film. With regard to Fe-N film, it can be found that there are two peaks which are located at 396 eV and 397.1 eV. The peak of 397.1 eV should be ascribed to N-Fe which can show the existence of iron nitrides on the Fe-N film [25,26], while the other corresponds to NO [27]. Fig. 2(b) and (d) depict the Fe 2p spectra of the Fe film and Fe-N film respectively. In order to eliminate the noise, the signal of Fe-N film
Corresponding author. E-mail address: [email protected] (H. Fang).
https://doi.org/10.1016/j.jmmm.2018.12.034 Received 25 September 2018; Received in revised form 6 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 475 (2019) 759–762
H. Fang et al.
This can be explained by the XRD results: There exist several phases of iron nitrides in the film, and they are unable to be fused with each other. Fig. 4 displays the magnetic hysteresis loops of the Fe and Fe-N films at 5 K and room temperature (300 K) measured by superconducting quantum interference device (SQUID) magnetometer. The external magnetic fields were applied parallel to the sample plane. The influence of diamagnetism of the substrates has been removed. As shown in Fig. 4, both the Fe film and Fe-N film exhibit ferromagnetism, no matter at 5 K or 300 K. It also can be observed that the saturation magnetic moment of the Fe-N film is much less than that of the Fe film. Since the Fe film was wholly nitrated as stated above, there is no residual single iron in the Fe-N film. Therefore, the ferromagnetism of Fe-N film must originate from iron nitrides. Among the several phases existing in the Fe-N film, only the Fe3N phase can provide the ferromagnetism because both the FeN and Fe2N phases are nonferromagnetic at room temperature [3,29]. In addition, one can see that the saturation magnetic moment of the Fe-N film at 300 K is much weaker than that at 5 K, while the difference is little for the Fe film. As well known, the closer to Curie temperature the temperature is, the more quickly the spontaneous magnetization will decrease. Therefore, the Curie temperature of the Fe-N film is closer to 300 K and lower than the Fe film. As to the coercivities, they are 248Oe at 5 K and 105Oe at 300 K for the Fe film, while 338Oe and 336Oe for Fe-N film. This means that the coercivity of Fe film changes much more significantly with temperature than Fe-N film. Since the effect of temperature on coercivities mainly originates from the non-uniform size distribution of the magnetic domains [30], it can be concluded that the magnetic uniformity of the Fe-N film is better than the Fe film. The significant differences of coercivities between the
Fig. 1. XRD patterns of Fe and Fe-N films.
has been smoothed by Savitzky-Golay algorithm. It can be deduced from Fig. 2(b) that the peaks of Fe film should correspond to Fe-O since there exist satellite peaks. It is because that the XPS can only detect the surface information of the sample, and the surface of the Fe film was readily oxidized. For the Fe-N film, two major peaks are located around 709.2 eV and 723.3 eV. Since there is no broad satellite peaks, they should correspond to Fe-N 2p3/2 and Fe-N 2p1/2, respectively [28]. Fig. 3(a) presents the SEM image of the Fe film under 30000 magnifications. It can be observed that the surface is smooth and flat. The SEM image of the Fe-N film is displayed in Fig. 3(b). One can see that the surface of the Fe-N film is composed of irregular crystalline regions.
Fig. 2. XPS spectra of (a) N 1s of Fe film, (b) Fe 2p of Fe film, (c) N 1s of Fe-N film, and (d) Fe 2p of Fe-N film. 760
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Fig. 3. SEM photographs of (a) Fe film and (b) Fe-N film after ICP etching.
Fig. 4. Magnetic hysteresis loops of (a) Fe film at 5 K, (b) Fe film at 300 K, (c) Fe-N film at 5 K, and (d) Fe-N film at 300 K.
Fe film and Fe-N film also indicate that the ferromagnetism of Fe-N film can not originate from single iron. The I-V curves were measured at room temperature for both the Fe and Fe-N films by the system consists of Keithley 4200 semiconductor parameter analyzer and Chroma probe. Before measuring the I-V curves, Al electrodes are fabricated on the films using a BAK640 electron beam deposition system. The results are plotted in Fig. 5. Here, the resistivity of GaN template is more than 106 ·cm, and thus the influence of the substrate can be ignored. As depicted in Fig. 5, the linear I-V curve of the Fe-N film indicates good conductivity. Compared with the Fe Film, its resistivity is only 3 times higher. The results are in accordance with the previous literature [1]. Finally, the ARPES measurements were performed on laboratorybased ARPES system comprising a SPECS PHOI-BOS 150 electron analyzer and an UVS-300 lamp. Here, since the samples are polycrystalline, the angle-resolved information is isotropic. And thus, only the densities of states (DOS) are shown in Fig. 6 where the Fermi level is
at 0 eV. For the Fe film, it can be observed that there exists the so-called 3d band of Fe around Fermi level. As well known, the 3d band mainly consists of 3d states of Fe, which is account for the itinerant electron magnetism. For the Fe-N film, it is interesting that the 3d band disappears. This may be due to that the Fe 3d states are hybridized with the N 2p states, and the energies of the states are decreased. Accordingly, the results of SQUID and I-V curves can be explained as following: (1) The disappearance of 3d band decreases the ferromagnetism, which causes that the saturation magnetic moment of the Fe-N film is much less than that of the Fe film. (2) The disappearance of 3d band also decreases the conductivity, which leads to the higher resistivity of Fe-N film. It is worth noting that ARPES mainly detects the surface information of the sample. Among the phases of Fe-N film, FeN and Fe2N should exist on the surface and Fe3N in the inner because the surface will be more fully nitrated. And thus, the ARPES only gives the electronic structure of superficial FeN or Fe2N that provides less ferromagnetism and conductivity. On the contrary, the results of SQUID and 761
Journal of Magnetism and Magnetic Materials 475 (2019) 759–762
H. Fang et al.
Research Foundation of Nanjing University Communications (NY215083, NY217046).
of
Posts
and
References [1] M. Naito, K. Uehara, R. Takeda, Y. Taniyasu, H. Yamamoto, Growth of iron nitride thin films by molecular beam epitaxy, J. Cryst. Growth 415 (2015) 36–40. [2] H.N. Fang, R. Zhang, B. Liu, Z.K. Tao, M.W. Xiao, X.F. Wang, Z.L. Xie, X.Q. Xiu, Y.D. Zheng, Analysis of magnetic structures of iron nitrides by Landau’s theory of second-order phase transitions, AIP Adv. 3 (2013) 072136 . [3] C. Navio, J. Alvarez, M.J. Capitan, F. Yndurain, R. Miranda, Nonmagnetic -FeN thin films epitaxially grown on Cu(001): Electronic structure and thermal stability, Phys. Rev. B 78 (2008) 155417 . [4] H.N. Fang, R. Zhang, B. Liu, Z.K. Tao, X.F. Wang, Z.L. Xie, X.Q. Xiu, Y.D. Zheng, Magnetic and electrical properties of ε-Fe3N on c-plane GaN, J. Phys. D: Appl. Phys. 45 (2012) 315002 . [5] W. Zhou, L.J. Qu, Q.M. Zhang, D.S. Wang, Interaction and charge transfer in the iron nitride Fe4N, Phys. Rev. B 40 (1989) 6393–6397. [6] W.Y. Lai, Q.Q. Zheng, W.Y. 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Fig. 5. I-V curves of Fe and Fe-N films.
Fig. 6. Densities of states of Fe and Fe-N films.
I-V curves detect the bulk properties of the Fe-N film, and hence show the ferromagnetism and conductivity which basically come from the internal Fe3N. The present result indicates that, when the N content is close to Fe, the coupling of Fe and N can significantly change the band structure of iron nitrides, and further affect the ferromagnetism and conductivity. 4. Conclusion In conclusion, Fe-N film was fabricated by means of nitridation of Fe film. A series of measurements characterize the structure, chemical environment, surface morphology, magnetic and electrical properties of the Fe-N film. Most importantly, the band structures of the Fe and Fe-N film are investigated by ARPES. Through the analysis of the DOS, it is found that the 3d band of Fe-N film disappears which can account for the less ferromagnetism and conductivity. This indicates that the high content of nitrogen can significantly change the band structure of iron nitrides. The interesting result exceeds the conventional theoretical prediction, and will promote the further research of the iron nitrides. Acknowledgements This work is supported by the National Natural Science Foundation of China (11704197, 61574079), the Nature Science of Foundation of Jiangsu Province (BK20130866), the University Nature Science Research Project of Jiangsu province (14KJB510020), the Scientific 762