- Email: [email protected]
Mössbauer study of RF-sputtered FexN(x≈2) film
Nuclear Instruments and Methods in Physics Research B 168 (2000) 422±426
www.elsevier.nl/locate/nimb
ssbauer study of RF-sputtered Fex N x 2 ®lm Mo Yingwen Cai a
a,* ,
Aiguo Li a, Jianqing Cao a, Xinbo Ni a, Guilin Zhang a, Ganghui Yu b, Weixin Xu b
Key Lab of Nuclear Analysis Techniques, Chinese Academy of Sciences, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800, People's Republic of China b Key Lab of Shanghai Iron and Steel Research Institute 200940, Shanghai, People's Republic of China Received 20 September 1999; received in revised form 3 December 1999
Abstract Fex N 1:6 < x < 2:2 is produced by RF-sputtering of pure iron under high purity argon and nitrogen atmosphere. Abundances of Fe, N, O and C are analyzed by proton deep elastic scattering, while X-ray photoelectron spectroscopy (XPS) is used to detect the oxidation of iron. M ossbauer spectroscopy was used to characterize the samples as-sputtered and after annealing at 200°C, 250°C and 300°C for 30 minutes, respectively. X-ray diraction and M ossbauer results show that for the chemical ratio of iron in Fex N approaching 2, orthorhombic f-Fe2 N will transform into hcp e-Fex N or quasi-ZnS type structure with an increase of nitrogen content in the sputtering atmosphere or after annealing. The ®rst neighbor shell of iron is either rich or depleted of nitrogen in e-Fe2 N. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.18.Fs; 68.55.Nq Keywords: Iron nitride; M ossbauer eect; Proton deep elastic scattering
1. Introduction Iron nitride has attracted extensive research interests for a long time due to its superior magnetic properties as well as improvement in the corrosion and abrasion resistance of materials surface [1,2]. Especially with the recent discovery of as high as 3:5 lB magnetic moment of iron in metastable a00 -Fe16 N2 [3,4], even more interests have been excited. Through X-ray diraction
*
Corresponding author. Tel.: +86-21-5955-3998; fax: +8621-5955-3021. E-mail address: [email protected] (Y. Cai).
(XRD), electron diraction, M ossbauer spectroscopy, and various magnetic measurements, the microstructure, magnetic properties, and phase transformation of iron nitride are investigated. The characteristics of many phases are investigated intensively, and their structure and magnetic properties are relatively clear, such as c-Fe4 N [5], f-Fe2 N [6,7], etc. For hcp and ZnS-type Fex N, many investigations have been done in the range 2:0 < x < 3:0 and 1:0 < x < 1:6 [8±11]. Oueldennaoua et al. [11] found NaCl-type iron nitride at x 1:03, while Nakagawa et al. [10] found a novel phase with ZnS-type structure when 1:1 < x < 1:6. It is well known that the stoichiometric region around x 2 is sensitive to phase
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 2 0 2 - 1
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
transformation and magnetic properties variation, in this range there still exists a lot of controversy. Therefore, it is very important to study the relationship between chemical composition and phase structure around Fe2 N. In this paper, RF-sputtering is adopted with mixed argon and nitrogen atmosphere to prepare iron nitride thin ®lm with 1:6 < x < 2:2. M ossbauer eect combined with proton deep elastic scattering (PDES), XRD and X-ray photoelectron spectroscopy (XPS) measurements were performed to study the hyper®ne interactions, phase structure and composition of Fex N, as well as the transformation of iron nitride ®lm after isochronous annealing. 2. Experiments Iron nitride ®lms about 500 nm thick are prepared on aluminum substrate by RF-sputtering of pure iron under atmosphere with dierent composition ratio of high purity argon and nitrogen. The vacuum before sputtering is pumped to 2 10ÿ3 Pa, while the atmosphere during sputtering is kept at 5 Pa. The nitrogen pressure to the total atmosphere is 0%, 5%, 10% and 22% (which is represented by the subscript N in PN afterwards), respectively, resulting in four groups of iron nitride ®lms with dierent nitrogen contents. The relative content of nitrogen in the ®lm was measured by PDES with a condition as follows: 2.8 MeV incident proton energy, gold surface silicon barrier detector with an energy resolution 18 keV, the angle between incident and scattering beams is 170°. XPS (MicroLab MKII) was used to detect the oxidation of iron during sputtering, the result manifests a very sharp P3=2 peak of Fe at binding energy of 707.2 eV, while no peak of oxidized iron appears. This indicates that iron is not oxidized within detection sensitivity. Phase identi®cation of the sample mainly relies on conversion electron M ossbauer spectroscopy (CEMS), assisted by XRD. The penetration depth of 7.3 keV conversion electron is about 300 nm. The detector used in our experiment is a resonance counter with working gas mixed by 90% He plus ossbauer spectrometer was operated 10% CH4 . M in a mirror-like triangular mode at constant
423
acceleration. M ossbauer source was 57 Co Pd of 8 9:25 10 Bq activity. In this work P10 sample was isochronally annealed for 30 min under H2 atmosphere (6:6 103 Pa) at 200°C; 250°C and 300°C, respectively. 3. Results and discussion Fig. 1 is a typical PDES spectrum, which shows Fe, N, O and C peaks of P10 sample after subtracting A1 substrate spectrum. RBS using a particles cannot distinguish these light elements, but now they can be distinguished using relatively high-energy protons instead of a-particles. The energy of protons was chosen such that the protons can penetrate the Coulomb barrier of N, O and C, but not A1. Thus the collision cross-section is much increased due to the nuclear force interaction between protons and N, O, C atoms, compared with that between protons and A1 atoms. Fe/N ratios by PDES of P5 , P10 and P22 samples were 1.4(2), 2.0(2) and 2.5(2), respectively. Although the computation error was relatively large, it is clear that oxygen and carbon contents were rather low. M ossbauer spectra of as-sputtered P5 ; P10 and P22 samples are shown in Fig. 2, while the M ossbauer spectra of annealed P10 sample are shown in Fig. 3. M ossbauer parameters of isomer shifts (IS), quadrupole splittings (QS), line widths (W) and the percentage of each component are
Fig. 1. PDES spectra of as-sputtered P10 with A1 background subtracted.
424
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
Fig. 2. M ossbauer spectra of as-sputtered P0 ; P5 ; P10 and P22 samples.
Fig. 3. M ossbauer spectra of P10 after isochronous annealing at dierent temperatures.
listed in Table 1. IS provided in this paper refer to a-Fe. From Fig. 2, it can be seen that M ossbauer spectrum obtained under the atmosphere without nitrogen appears to be of pure a-Fe, while under other three atmospheres paramagnetic iron nitride forms. Among the latter, the two samples prepared under low nitrogen content have a M ossbauer spectrum composed of two doublets Q1 and Q2, while the one prepared under high nitrogen content has a spectrum composed of a doublet Q1 and a single line S1. From Table 1, it can be seen that IS and QS of Q2 are 0.48 and 0.28 mm/s, respectively. Thus we can assign Q2 to Fe2 N based on [8,12,13]. XRD results (Fig. 4) also show that Q2 is mainly f-Fe2 N. A single line S1 appears at P22 , its IS is 0.23 mm/s, which is far greater than c-Fe and c-austenite containing nitrogen. This component appears only when N% P 22%, showing that its nitrogen content is rather high. It has also been found in the research of Nakagawa et al., which is a satellite line of ZnS-type Fex N, compound formed in the range 1:1 < x < 1:6. Each Fe atom in ZnS-type iron nitride should have four neighboring N atoms. If one of the ®rst neighbors of some Fe atom is lacking, aforementioned satellite line will appear [10]. Therefore, we deem that S1 is a non-stoichiometric ZnS-type nitride as N approaches 22, it is seriously lacking nitrogen when compared with standard ZnS phase, since no apparent line with four ®rst neighboring N (IS 0.08 mm/s) was observed in M ossbauer spectrum. From XRD (Fig. 4), 1 1 1 and 2 2 0 peaks of ZnS-type structure appear in P22 , but the lattice constant is about 0.445 nm, not the standard value of 0.433 nm of ZnS-type structure. This might be mainly the result of interstitial N atoms during phase separation to form a disordered ZnS structure. Since N does not occupy the normal lattice position, they can enlarge the layer distance between Fe atoms and the lattice constant. Another component Q1 has an IS of 0.37 mm/s and QS of 0.62 mm/s in P5 . With the increase of PN , QS decreases from 0.62 to 0.53 mm/s, correspondingly the line width becomes narrow and the relative intensity increases. In P22 , Q2 disappears, in which some transforms into single line (S1), and most into Q1. This has been observed by Mosca
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
425
Table 1 M ossbauer parameters and relative area of each componenta Q1
a
Q2
S1
PN (%)
T °C
IS
QS
W
f (%)
IS
QS
W
f (%)
IS
QS
W
f (%)
5 10 22 10 10
RT RT RT 200 250
0.37 0.40 0.38 0.34 0.34
0.62 0.56 0.53 0.53 0.54
0.53 0.48 0.40 0.33 0.33
0.45 0.55 0.86 0.66 0.64
0.48 0.48 ± ± ±
0.28 0.29 ± ± ±
0.26 0.26 ± ± ±
0.55 0.45 ± ± ±
± ± 0.23 0.25 0.26
± ± 0.00 0.00 0.00
± ± 0.29 0.44 0.47
± ± 0.14 0.34 0.36
IS, QS, W in mm/s, all refer to a-Fe.
Fig. 4. XRD of as-sputtered P22 ; P5 and P10 samples (top to bottom).
et al. [12], they considered it as Fe2 N phase in which the ®rst neighboring of Fe atoms lack N. Here we found its IS is pretty close to that of Fe2 N obtained by Mosca, therefore we tend to regard Q1 as Fe2 N phase, but the number of ®rst neighboring N atoms of Fe is not consistent. Obviously, the nitrogen content of Q1 increases with PN , so the neighboring N atoms around Fe might vary from less than 3 to greater than 3. Since there is a certain distribution of ®rst neighboring nitrogen atoms, the line-width of Q2 is relatively large. XRD indicates that very strong f-Fe2 N and e-Fe2 N±Fe3 N
peaks show up in P5 sample, while in P10 , these two peak groups become weak, but two new peaks appear at 2h 35:3° and 60°, which correspond to hcp e-Fex N (x is around 2). This indicates a transition from orthorhombic f-Fe2 N and nonstoichiometric hcp e-Fe2 N±Fe3 N lacking N atoms to stoichiometric hcp e-Fe2 N. M ossbauer spectra of annealed P10 sample are shown in Fig. 3 (in the ®gure only the small velocity range is shown). After 200°C and 250°C annealings, the spectrum changed from two QS into one QS doublet and one single line. On the
426
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
other hand, after 300°C annealing, it changed into a spectrum composed of multiple sextet. It can be seen from M ossbauer parameters listed in Table 1, IS of Q1 doublet in P10 after 200°C and 250°C annealing are smaller than that in as-sputtered P10 . The single line with IS close to S1 in P22 also appears. This means that some stoichiometric f-Fe2 N transforms into ZnS-type iron nitride, but it is seriously lacking N compared with stoichimetric ZnS-type iron nitride. We found that the ratios of peak area to background remain close to that of as-sputtered after 200°C and 250°C annealing suggesting that N atoms do not escape out of the sample. This indicates that all the structure and transformation of iron nitride is only the result of short-range interaction, most probably vacancy migration and ordering. Due to mass conservation, Q1 after annealing must be lacking N compared with that of as-sputtered. Refer to the analysis of Zhou for c0 -Fe4 N [13] by means of LAPW, if Fe4s is the prevailing bonding in Fex N and isomer shift values depend linearly on the number of N atoms, n, at nearest neighbor interstitial sites, the isomer shift will depend linearly on n. Our results seem to support this analysis though Fe2 N here is not a dilute system. After 300°C annealing for P10 , it can be seen from M ossbauer spectrum that all paramagnetic iron nitride transforms into ferromagnetic phases. The two sextets are a-Fe and c0 -Fe4 N, respectively. It is well known that Fex N compound shows ferromagnetic at room temperature as x > 2:2 [8], from this it can be concluded that after 300°C annealing, part of N atoms diuse out of the thin ®lm, forming N2 and escaping, resulting in ferromagnetic iron nitride. These results are quite similar to the annealing behavior of N implanted into a-Fe [13]. In [13] it is found that after 300°C annealing, Fe2 N phase disappears, while after 350°C annealing there exists only 25% nitrogen in the sample. In conclusion, Fex N compound ®lms obtained in the above PN atmosphere have an x < 2:2. Nakagawa et al. [10] found that ZnS-type structure occurs at x < 1:6, and we found ZnS-type phase lacking nitrogen for P22 , hence we think x 1:6 is at least the lower limit of Fex N in P22 . This range of x 1:6 < x < 2:2 is consistent with PDES results within experimental errors.
4. Conclusion Using properly mixing ratio PN of Ar and N, iron nitride with stoichiometric composition around Fe2 N can be prepared by RF-sputtering. In Fe2 N phase, N can either be stoichiometric or not, i.e., the ®rst neighboring N atoms of Fe can be 3, more or less. Under dierent nitrogen atmosphere, Fe2 N phase appears with dierent structure. With the increase of nitrogen content, or after annealing, orthorhombic f-Fe2 N will transform into hcp e-Fe2 N or ZnS-type iron nitride. Besides, PDES is a simple and rapid way for nondestructive detection of Fe, N, O and C components.
Acknowledgements The authors are indebted to the ®nancial support of Natural Science Foundation of China (under contract number #19975067).
References [1] S.F. Matar, G. Demazeau, B. Siberchicot, IEEE Trans. Magn. MAG 26 (1990) 60. [2] M. Komuro, Y. Kozono, M. Hanazono, Y. Sugita, J. Appl. Phys. 67 (1990) 5126. [3] Y. Sugita, K. Mitsuoka, M. Komuro, H. Hoshiya, Y. Kozono, M. Hanazono, J. Appl. Phys. 70 (1991) 5977. [4] J.Q. Xiao, C.L. Chien, Appl. Phys. Lett. 64 (1994) 384. [5] A.J. Nozik, J.C. Wood, G. Haacke, Solid State Commun. 7 (1969) 1677. [6] J. Baibridge, D.A. Channing, W.H. White, R.E. Pendolburge, J. Phys. Chem. Solid 34 (1973) 1579. [7] M. Chabanel, C. Janot, J. Motte, C.R. Held, Seances Acad. Sci. Ser B B266 (1968) 419. [8] G.M. Chen, N.K. Jaggl, J.B. Butt, E.B. Yeh, L.H. Schwartz, J. Phys. Chem. 87 (1983) 5326. [9] Kim Kyu-Jin, Kebji Sumiyama, Hideya Onodera, Kenji Suzuki, Jpn. J. Appl. Phys. 33 (1994) 6539. [10] H. Nakagawa, S. Nasu, H. Fuji, M. Takahashi, F. Kanamaru, Hyper®ne Interactions 69 (1991) 455. [11] A. Oueldennaoua, E. Bauer-Grosse, M. Foos, C. Frantz, Scr. Metall. 19 (1985) 1503. [12] D.H. Mosca, P.H. Dionisio, W.H. Schreiner et al., J. Appl. Phys. 67 (1990) 7514. [13] W. Zhou, L. Qu, Q. Zhang, Phys. Rev. B B40 (1989) 6393.
www.elsevier.nl/locate/nimb
ssbauer study of RF-sputtered Fex N x 2 ®lm Mo Yingwen Cai a
a,* ,
Aiguo Li a, Jianqing Cao a, Xinbo Ni a, Guilin Zhang a, Ganghui Yu b, Weixin Xu b
Key Lab of Nuclear Analysis Techniques, Chinese Academy of Sciences, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800, People's Republic of China b Key Lab of Shanghai Iron and Steel Research Institute 200940, Shanghai, People's Republic of China Received 20 September 1999; received in revised form 3 December 1999
Abstract Fex N 1:6 < x < 2:2 is produced by RF-sputtering of pure iron under high purity argon and nitrogen atmosphere. Abundances of Fe, N, O and C are analyzed by proton deep elastic scattering, while X-ray photoelectron spectroscopy (XPS) is used to detect the oxidation of iron. M ossbauer spectroscopy was used to characterize the samples as-sputtered and after annealing at 200°C, 250°C and 300°C for 30 minutes, respectively. X-ray diraction and M ossbauer results show that for the chemical ratio of iron in Fex N approaching 2, orthorhombic f-Fe2 N will transform into hcp e-Fex N or quasi-ZnS type structure with an increase of nitrogen content in the sputtering atmosphere or after annealing. The ®rst neighbor shell of iron is either rich or depleted of nitrogen in e-Fe2 N. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.18.Fs; 68.55.Nq Keywords: Iron nitride; M ossbauer eect; Proton deep elastic scattering
1. Introduction Iron nitride has attracted extensive research interests for a long time due to its superior magnetic properties as well as improvement in the corrosion and abrasion resistance of materials surface [1,2]. Especially with the recent discovery of as high as 3:5 lB magnetic moment of iron in metastable a00 -Fe16 N2 [3,4], even more interests have been excited. Through X-ray diraction
*
Corresponding author. Tel.: +86-21-5955-3998; fax: +8621-5955-3021. E-mail address: [email protected] (Y. Cai).
(XRD), electron diraction, M ossbauer spectroscopy, and various magnetic measurements, the microstructure, magnetic properties, and phase transformation of iron nitride are investigated. The characteristics of many phases are investigated intensively, and their structure and magnetic properties are relatively clear, such as c-Fe4 N [5], f-Fe2 N [6,7], etc. For hcp and ZnS-type Fex N, many investigations have been done in the range 2:0 < x < 3:0 and 1:0 < x < 1:6 [8±11]. Oueldennaoua et al. [11] found NaCl-type iron nitride at x 1:03, while Nakagawa et al. [10] found a novel phase with ZnS-type structure when 1:1 < x < 1:6. It is well known that the stoichiometric region around x 2 is sensitive to phase
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 1 2 0 2 - 1
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
transformation and magnetic properties variation, in this range there still exists a lot of controversy. Therefore, it is very important to study the relationship between chemical composition and phase structure around Fe2 N. In this paper, RF-sputtering is adopted with mixed argon and nitrogen atmosphere to prepare iron nitride thin ®lm with 1:6 < x < 2:2. M ossbauer eect combined with proton deep elastic scattering (PDES), XRD and X-ray photoelectron spectroscopy (XPS) measurements were performed to study the hyper®ne interactions, phase structure and composition of Fex N, as well as the transformation of iron nitride ®lm after isochronous annealing. 2. Experiments Iron nitride ®lms about 500 nm thick are prepared on aluminum substrate by RF-sputtering of pure iron under atmosphere with dierent composition ratio of high purity argon and nitrogen. The vacuum before sputtering is pumped to 2 10ÿ3 Pa, while the atmosphere during sputtering is kept at 5 Pa. The nitrogen pressure to the total atmosphere is 0%, 5%, 10% and 22% (which is represented by the subscript N in PN afterwards), respectively, resulting in four groups of iron nitride ®lms with dierent nitrogen contents. The relative content of nitrogen in the ®lm was measured by PDES with a condition as follows: 2.8 MeV incident proton energy, gold surface silicon barrier detector with an energy resolution 18 keV, the angle between incident and scattering beams is 170°. XPS (MicroLab MKII) was used to detect the oxidation of iron during sputtering, the result manifests a very sharp P3=2 peak of Fe at binding energy of 707.2 eV, while no peak of oxidized iron appears. This indicates that iron is not oxidized within detection sensitivity. Phase identi®cation of the sample mainly relies on conversion electron M ossbauer spectroscopy (CEMS), assisted by XRD. The penetration depth of 7.3 keV conversion electron is about 300 nm. The detector used in our experiment is a resonance counter with working gas mixed by 90% He plus ossbauer spectrometer was operated 10% CH4 . M in a mirror-like triangular mode at constant
423
acceleration. M ossbauer source was 57 Co Pd of 8 9:25 10 Bq activity. In this work P10 sample was isochronally annealed for 30 min under H2 atmosphere (6:6 103 Pa) at 200°C; 250°C and 300°C, respectively. 3. Results and discussion Fig. 1 is a typical PDES spectrum, which shows Fe, N, O and C peaks of P10 sample after subtracting A1 substrate spectrum. RBS using a particles cannot distinguish these light elements, but now they can be distinguished using relatively high-energy protons instead of a-particles. The energy of protons was chosen such that the protons can penetrate the Coulomb barrier of N, O and C, but not A1. Thus the collision cross-section is much increased due to the nuclear force interaction between protons and N, O, C atoms, compared with that between protons and A1 atoms. Fe/N ratios by PDES of P5 , P10 and P22 samples were 1.4(2), 2.0(2) and 2.5(2), respectively. Although the computation error was relatively large, it is clear that oxygen and carbon contents were rather low. M ossbauer spectra of as-sputtered P5 ; P10 and P22 samples are shown in Fig. 2, while the M ossbauer spectra of annealed P10 sample are shown in Fig. 3. M ossbauer parameters of isomer shifts (IS), quadrupole splittings (QS), line widths (W) and the percentage of each component are
Fig. 1. PDES spectra of as-sputtered P10 with A1 background subtracted.
424
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
Fig. 2. M ossbauer spectra of as-sputtered P0 ; P5 ; P10 and P22 samples.
Fig. 3. M ossbauer spectra of P10 after isochronous annealing at dierent temperatures.
listed in Table 1. IS provided in this paper refer to a-Fe. From Fig. 2, it can be seen that M ossbauer spectrum obtained under the atmosphere without nitrogen appears to be of pure a-Fe, while under other three atmospheres paramagnetic iron nitride forms. Among the latter, the two samples prepared under low nitrogen content have a M ossbauer spectrum composed of two doublets Q1 and Q2, while the one prepared under high nitrogen content has a spectrum composed of a doublet Q1 and a single line S1. From Table 1, it can be seen that IS and QS of Q2 are 0.48 and 0.28 mm/s, respectively. Thus we can assign Q2 to Fe2 N based on [8,12,13]. XRD results (Fig. 4) also show that Q2 is mainly f-Fe2 N. A single line S1 appears at P22 , its IS is 0.23 mm/s, which is far greater than c-Fe and c-austenite containing nitrogen. This component appears only when N% P 22%, showing that its nitrogen content is rather high. It has also been found in the research of Nakagawa et al., which is a satellite line of ZnS-type Fex N, compound formed in the range 1:1 < x < 1:6. Each Fe atom in ZnS-type iron nitride should have four neighboring N atoms. If one of the ®rst neighbors of some Fe atom is lacking, aforementioned satellite line will appear [10]. Therefore, we deem that S1 is a non-stoichiometric ZnS-type nitride as N approaches 22, it is seriously lacking nitrogen when compared with standard ZnS phase, since no apparent line with four ®rst neighboring N (IS 0.08 mm/s) was observed in M ossbauer spectrum. From XRD (Fig. 4), 1 1 1 and 2 2 0 peaks of ZnS-type structure appear in P22 , but the lattice constant is about 0.445 nm, not the standard value of 0.433 nm of ZnS-type structure. This might be mainly the result of interstitial N atoms during phase separation to form a disordered ZnS structure. Since N does not occupy the normal lattice position, they can enlarge the layer distance between Fe atoms and the lattice constant. Another component Q1 has an IS of 0.37 mm/s and QS of 0.62 mm/s in P5 . With the increase of PN , QS decreases from 0.62 to 0.53 mm/s, correspondingly the line width becomes narrow and the relative intensity increases. In P22 , Q2 disappears, in which some transforms into single line (S1), and most into Q1. This has been observed by Mosca
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
425
Table 1 M ossbauer parameters and relative area of each componenta Q1
a
Q2
S1
PN (%)
T °C
IS
QS
W
f (%)
IS
QS
W
f (%)
IS
QS
W
f (%)
5 10 22 10 10
RT RT RT 200 250
0.37 0.40 0.38 0.34 0.34
0.62 0.56 0.53 0.53 0.54
0.53 0.48 0.40 0.33 0.33
0.45 0.55 0.86 0.66 0.64
0.48 0.48 ± ± ±
0.28 0.29 ± ± ±
0.26 0.26 ± ± ±
0.55 0.45 ± ± ±
± ± 0.23 0.25 0.26
± ± 0.00 0.00 0.00
± ± 0.29 0.44 0.47
± ± 0.14 0.34 0.36
IS, QS, W in mm/s, all refer to a-Fe.
Fig. 4. XRD of as-sputtered P22 ; P5 and P10 samples (top to bottom).
et al. [12], they considered it as Fe2 N phase in which the ®rst neighboring of Fe atoms lack N. Here we found its IS is pretty close to that of Fe2 N obtained by Mosca, therefore we tend to regard Q1 as Fe2 N phase, but the number of ®rst neighboring N atoms of Fe is not consistent. Obviously, the nitrogen content of Q1 increases with PN , so the neighboring N atoms around Fe might vary from less than 3 to greater than 3. Since there is a certain distribution of ®rst neighboring nitrogen atoms, the line-width of Q2 is relatively large. XRD indicates that very strong f-Fe2 N and e-Fe2 N±Fe3 N
peaks show up in P5 sample, while in P10 , these two peak groups become weak, but two new peaks appear at 2h 35:3° and 60°, which correspond to hcp e-Fex N (x is around 2). This indicates a transition from orthorhombic f-Fe2 N and nonstoichiometric hcp e-Fe2 N±Fe3 N lacking N atoms to stoichiometric hcp e-Fe2 N. M ossbauer spectra of annealed P10 sample are shown in Fig. 3 (in the ®gure only the small velocity range is shown). After 200°C and 250°C annealings, the spectrum changed from two QS into one QS doublet and one single line. On the
426
Y. Cai et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 422±426
other hand, after 300°C annealing, it changed into a spectrum composed of multiple sextet. It can be seen from M ossbauer parameters listed in Table 1, IS of Q1 doublet in P10 after 200°C and 250°C annealing are smaller than that in as-sputtered P10 . The single line with IS close to S1 in P22 also appears. This means that some stoichiometric f-Fe2 N transforms into ZnS-type iron nitride, but it is seriously lacking N compared with stoichimetric ZnS-type iron nitride. We found that the ratios of peak area to background remain close to that of as-sputtered after 200°C and 250°C annealing suggesting that N atoms do not escape out of the sample. This indicates that all the structure and transformation of iron nitride is only the result of short-range interaction, most probably vacancy migration and ordering. Due to mass conservation, Q1 after annealing must be lacking N compared with that of as-sputtered. Refer to the analysis of Zhou for c0 -Fe4 N [13] by means of LAPW, if Fe4s is the prevailing bonding in Fex N and isomer shift values depend linearly on the number of N atoms, n, at nearest neighbor interstitial sites, the isomer shift will depend linearly on n. Our results seem to support this analysis though Fe2 N here is not a dilute system. After 300°C annealing for P10 , it can be seen from M ossbauer spectrum that all paramagnetic iron nitride transforms into ferromagnetic phases. The two sextets are a-Fe and c0 -Fe4 N, respectively. It is well known that Fex N compound shows ferromagnetic at room temperature as x > 2:2 [8], from this it can be concluded that after 300°C annealing, part of N atoms diuse out of the thin ®lm, forming N2 and escaping, resulting in ferromagnetic iron nitride. These results are quite similar to the annealing behavior of N implanted into a-Fe [13]. In [13] it is found that after 300°C annealing, Fe2 N phase disappears, while after 350°C annealing there exists only 25% nitrogen in the sample. In conclusion, Fex N compound ®lms obtained in the above PN atmosphere have an x < 2:2. Nakagawa et al. [10] found that ZnS-type structure occurs at x < 1:6, and we found ZnS-type phase lacking nitrogen for P22 , hence we think x 1:6 is at least the lower limit of Fex N in P22 . This range of x 1:6 < x < 2:2 is consistent with PDES results within experimental errors.
4. Conclusion Using properly mixing ratio PN of Ar and N, iron nitride with stoichiometric composition around Fe2 N can be prepared by RF-sputtering. In Fe2 N phase, N can either be stoichiometric or not, i.e., the ®rst neighboring N atoms of Fe can be 3, more or less. Under dierent nitrogen atmosphere, Fe2 N phase appears with dierent structure. With the increase of nitrogen content, or after annealing, orthorhombic f-Fe2 N will transform into hcp e-Fe2 N or ZnS-type iron nitride. Besides, PDES is a simple and rapid way for nondestructive detection of Fe, N, O and C components.
Acknowledgements The authors are indebted to the ®nancial support of Natural Science Foundation of China (under contract number #19975067).
References [1] S.F. Matar, G. Demazeau, B. Siberchicot, IEEE Trans. Magn. MAG 26 (1990) 60. [2] M. Komuro, Y. Kozono, M. Hanazono, Y. Sugita, J. Appl. Phys. 67 (1990) 5126. [3] Y. Sugita, K. Mitsuoka, M. Komuro, H. Hoshiya, Y. Kozono, M. Hanazono, J. Appl. Phys. 70 (1991) 5977. [4] J.Q. Xiao, C.L. Chien, Appl. Phys. Lett. 64 (1994) 384. [5] A.J. Nozik, J.C. Wood, G. Haacke, Solid State Commun. 7 (1969) 1677. [6] J. Baibridge, D.A. Channing, W.H. White, R.E. Pendolburge, J. Phys. Chem. Solid 34 (1973) 1579. [7] M. Chabanel, C. Janot, J. Motte, C.R. Held, Seances Acad. Sci. Ser B B266 (1968) 419. [8] G.M. Chen, N.K. Jaggl, J.B. Butt, E.B. Yeh, L.H. Schwartz, J. Phys. Chem. 87 (1983) 5326. [9] Kim Kyu-Jin, Kebji Sumiyama, Hideya Onodera, Kenji Suzuki, Jpn. J. Appl. Phys. 33 (1994) 6539. [10] H. Nakagawa, S. Nasu, H. Fuji, M. Takahashi, F. Kanamaru, Hyper®ne Interactions 69 (1991) 455. [11] A. Oueldennaoua, E. Bauer-Grosse, M. Foos, C. Frantz, Scr. Metall. 19 (1985) 1503. [12] D.H. Mosca, P.H. Dionisio, W.H. Schreiner et al., J. Appl. Phys. 67 (1990) 7514. [13] W. Zhou, L. Qu, Q. Zhang, Phys. Rev. B B40 (1989) 6393.