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Study of FePt films prepared by reactive sputtering
ARTICLE IN PRESS
Vacuum 81 (2007) 1077–1081 www.elsevier.com/locate/vacuum
Study of FePt films prepared by reactive sputtering V. Raghavendra Reddya,, Shikha Puranikb, Ajay Guptaa, Wolfram Leitenbergerc a
UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India b School of Physics, DAV University, Khandwa Road, Indore 452 017, India c Universita¨t Potsdam, Institut fu¨r Physik, Postfach 601553, 14415 Potsdam, Germany Received 4 August 2006; received in revised form 13 January 2007; accepted 2 February 2007
Abstract In the present study, 57FePt films are prepared with reactive ion beam sputtering using mixture of argon and nitrogen gases. Energydispersive X-ray reflectivity is used to estimate the thickness of the as-deposited films. Structural and magnetic properties of the asdeposited and annealed films are studied using grazing incidence X-ray diffraction (GIXRD), magneto-optical Kerr effect (MOKE) and conversion electron Mossbauer spectroscopy (CEMS). Significant difference in structural and magnetic properties i.e., formation of ordered L10 phase and perpendicular magnetic anisotropy are observed for the films prepared with mixture of nitrogen and argon as compared to the film prepared with argon only. From the GIXRD, peaks corresponding to the ordered face-centred tetragonal FePt phase are observed for the films prepared with mixture gas. The results of CEMS clearly show the perpendicular magnetic anisotropy (PMA) for the films prepared with mixture of nitrogen and argon. The observed enhanced chemical ordering and the development of PMA in the films prepared with mixture gas is due to the role played by the defects created as a consequence of nitrogen escape in the films with high temperature annealing. r 2007 Elsevier Ltd. All rights reserved. Keywords: L10 ordering in FePt; Mossbauer spectroscopy; Perpendicular magnetic anisotropy; Reactive ion beam sputtering
1. Introduction The intermetallic FePt alloys are the focus of current research due to their huge magneto-crystalline anisotropy constant (Ku) values [1]. The transformation of the disordered face-centered cubic (fcc) structure to the ordered face-centered tetragonal (fct) structure (known as L10) is attributed to be the reason for the huge Ku. There have been many efforts in reducing the ordering temperature and improving the hard magnetic properties of these materials [2]. The concept of intergrain exchange coupling is studied theoretically in nano-composite magnets consisting hard magnetic phases such as Nd2Fe14B, SmCo5 and soft phases such as a-Fe and are shown experimentally to have improved energy product values [3]. To improve the hard magnetic properties of FePt, it would be Corresponding author. Tel.: +91 0731 5049232; fax: +91 0731 2462294. E-mail addresses: [email protected], [email protected] (V. Raghavendra Reddy).
0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.02.001
interesting to look at the possibility of intergrain exchange coupling mechanism. Fe4N and Fe16N2 are soft magnetic materials with high saturation magnetization [4] and can be introduced conveniently into FePt films by sputtering in nitrogen–argon mixture [5]. The presence of nitrogen in the sample is expected to improve the hard magnetic properties of FePt system in two ways. First, with the intergrain exchange coupling mechanism between soft magnetic nitride and hard magnetic FePt phase. Secondly, voids created with the nitrogen escape with high-temperature annealing are expected to play a role in increasing the chemical ordering. Recently, it was shown that it is possible to increase the coercivity of FePt hard magnets by nitrogen addition [6]. The aim of the present study is (i) to study the formation of ordered fct FePt phase, (ii) and also to understand the mechanism responsible for the improvement of hard magnetic properties of the FePt films prepared with reactive ion beam sputtering using argon and nitrogen gases using microscopic technique such as Mossbauer spectroscopy. Mossbauer effect is expected to distinguish
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FeN and FePt phases unambiguously whereas the structural peaks in XRD corresponding to some FeN phases and FePt overlap appreciably. 2. Experimental In the present study, FePt compound target is used to prepare FePt thin films. The compound target is obtained by placing Pt and 57Fe-enriched Fe target side by side. Silicon was used as the substrate. The substrates are thoroughly cleaned prior to the deposition. Ion beam sputtering using 3 cm Kaufman type hot cathode source is used for the preparation of thin films. A base vacuum of 1 10 7 Torr was achieved before deposition. Three films are prepared using a mixture of Ar and nitrogen with 0%, 20% and 40% nitrogen (designated as N0, N20 and N40 henceforth) as the sputtering gas. The samples are prepared by sputtering FePt target for the same amount of time (20 min) and prepared in the similar conditions, i.e., without breaking the vacuum. The annealing of the samples was carried out in a vacuum better than 1 10 6 Torr simultaneously for the three samples at a given temperature. Energy-dispersive X-ray reflectivity (EDXRR), grazing incidence X-ray diffraction (GIXRD) are used for the structural characterisation and conversion electron Mossbauer spectroscopy (CEMS), and magnetooptical Kerr effect in longitudinal mode (L-MOKE) are used for the study of magnetic properties of the asdeposited and annealed films.
3. Results and discussion Fig. 1 shows the X-ray measurements of the as-deposited films. The EDXRR [7] pattern is used to estimate the thickness of the films. The obtained thickness values are 55.5, 50.4 and 40.770.1 nm for N0, N20 and N40 films, respectively. As the sputtering yield with nitrogen is less as compared to Ar, the thickness is observed to decrease for increasing N2 concentration. The GIXRD patterns of the as-deposited samples are shown in Fig. 1(d–f). Peaks at 41.271, 47.791, 69.851 and 84.111 in N0 are due to the disordered fcc phase of FePt (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections. In N20, sample peak at 40.811 matches closely with either fcc FePt (1 1 1) or Fe4N (1 1 1) and peaks at 45.381, 69.151 might be due to Fe2N(0 1 2), Fe3N(3 0 0), respectively. However, from the Mossbauer measurements as discussed below, the N20 sample consists only iron nitride phases. The diffraction pattern of N40 sample showed the presence of Pt peaks in addition to the broad iron nitride peak at 45.751. No peak due to the FePt is seen in N40 sample. Peaks at 39.951, 67.791 and 81.911 in N40 are due to Pt (1 1 1), (2 2 0) and (3 1 1), respectively. Fig. 2(a–c) shows the CEMS spectra of the asdeposited samples. The experimental data is fitted with NORMOS program for the evaluation of the hyperfine parameters relative to natural iron. A nearly zero quadrupole splitting (QS) is observed for N0 sample indicating the fcc structure of the as-deposited film, which is due to the fact that as-deposited FePt exists in disordered fcc structure. The obtained isomer shift (IS) and internal field
N0
N0
FePt(111) FePt(200)
FePt(311)
FePt(220)
N20
N20
FePt/Fe4N(111) Fe2N (012)
Fe3N (300)
N40
N40
Pt(111) Fe2N (012) Pt(220) Pt(311)
0.16
0.18 q(A-1)
0.20
0.22
30
40
50
60
70
80
90
2θ (deg)
Fig. 1. Energy-dispersive X-ray reflectivity patterns ((a), (b) and (c) for N0, N20 and N40, respectively, the solid represents the fit to data) and the GIXRD patterns ((d), (e) and (f) for N0, N20 and N40, respectively) of the as-deposited films.
ARTICLE IN PRESS V. Raghavendra Reddy et al. / Vacuum 81 (2007) 1077–1081
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1.02 N0 1.01
Relative Transmission
1.00
1.02
N20
1.01
1.00
1.03 N40 1.02 1.01 1.00 -9
-6
-3
0 Velocity (mm/s)
3
6
9
Fig. 2. Mossbauer spectra of the as-deposited films.
(BHF) are 0.224 mm/s and 31.18 T, respectively for N0 film matches with that of literature [8]. The A23 parameter, the area ratio of the second and third lines of the Mossbauer spectra, gives information about the orientation of the magnetic moment. The A23 value of four, two and zero indicates the in-plane, random and out-of plane spin orientation, respectively, in the sample [9]. The observed A23 value close to four indicates the in-plane magnetization for the as-deposited film of N0. The spectrum of the N20 film prepared with argon and 20% nitrogen as sputtering gases is fitted with one doublet and one sextet. The obtained IS, QS and BHF for the sextet are 0.294 mm/ s, 0.01 mm/s and 22.71 T, respectively. The obtained IS and QS for the doublet are 0.34 and 1.2 mm/s, respectively. The Mossbauer parameters of N20 matches with that of magnetic iron nitride and the central doublet are due to the non-magnetic iron nitride Fe3N phase [5,10]. In the Mossbauer spectrum of N20, no signature of FePt phase is seen. The Mossbauer spectrum of the N40 sample indicates that the film is non-magnetic in consistent with the MOKE measurements and the obtained IS and QS are 0.36 and 0.98 mm/s, respectively. It may be noted here that as the phase diagram of FeN is very complex, the critical analysis of the Mossbauer data to assign different phases of FeN system is not discussed here, as the aim of the present experiment is to study the formation of FePt phase. The GIXRD and CEMS results, therefore, demonstrate the
N20 at 580 celsius N20 at 680 celsius N20 at 500 celsius N20 (pristine)
N0 at 680 celsius N0 at 580 celsius N0 at 500 celsius N0 (pristine)
-1000 -800 -600 -400 -200 0 200 Field(oe)
400
600
800 1000
Fig. 3. Magnetization data of N0 and N20 samples at different temperatures obtained from L-MOKE.
formation of iron nitride in the films prepared with mixture of argon and nitrogen as the sputtering gases. Fig. 3 shows the magnetic measurements of the asdeposited and annealed samples obtained from L-MOKE. From the magnetization loops, both as-deposited N0 and N20 samples are observed to be soft magnetic. This is expected as the as-deposited disordered FePt alloy is soft
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magnetic and the Fe4N is also a soft magnetic material. As shown in Fig. 3, it is observed that the coercivity increases with the annealing at higher temperatures. Significant changes in the structural and magnetic properties after 680 1C annealing are observed. For the sample annealed at 680 1C, the coercivity (Hc) increased from 28 to 91 Oe and from 24 to 217 Oe for N0 and N20, respectively. The increase of the coercivity in the longitudinal geometry is considered due to the rotation of magnetization from inplane to out-of plane. We could not record the hysteresis curve of the as-deposited N40 (due to the non-magnetic nature) and also for the film annealed at 680 1C (due to the insufficient field, 1.8 kOe, of our magnet). Fig. 4(d–f) shows the GIXRD pattern of the films annealed at 680 1C for 1 h. The pattern of N40 film clearly shows the ordered fct FePt (0 0 1), (1 1 0), (1 1 1) and (2 0 0) at 24.21, 32.91, 41.11 and 47.41, respectively. FePt(0 0 1) peak with very small intensity at 241 is observed for N20 film after annealing at 680 1C. But, in the case of N0 film, no such peaks corresponding to the ordered phase are observed. Therefore, GIXRD suggests that after annealing at 680 1C films prepared with nitrogen as the sputtering gas in addition to the argon are having more chemical ordering as compared to N0 film prepared with argon only. This observation is further evidenced from the Mossbauer data as discussed below. Fig. 4(a–c) shows the Mossbauer patterns of the films annealed at 680 1C for 1 h. The observed QS values are 0.036, 0.027 and 0.27 mm/s for N0, N20 and N40, respectively. The QS value of 0.27 mm/s clearly indicates the deviation from cubic symmetry i.e., the formation of fct FePt phase in N40 sample. The observed A23 values are 3.83, 3.69 and 1.53 for N0, N20 and N40, N0
respectively. The A23 value of less than 2 demonstrates the out-of plane magnetization i.e., development of perpendicular magnetic anisotropy (PMA) in N40. Because of the strong /1 1 1S texture, the A23 value is not zero for N40. The N20 sample also shows an A23 value less as compared to N0 film. The Mossbauer results clearly demonstrate the development of PMA in the films prepared with nitrogen as compared to the N0 film. The results also demonstrate that more the nitrogen in the film, more ordering is observed with the annealing. These observations can be explained as the following. The nitrogen incorporated in the film is known to escape with high temperature annealing. At annealing temperatures above 673 K, the nitrogen escape is mainly due to degassing kinetics [11]. Hence, with the high-temperature annealing the N escape rapidly from the FePt phase yielding lot of vacancies inside the film. The ordering of L10 alloys such as FePt, FePd and CoPt usually proceeds by a nucleation and growth process [12]. The formation of vacancies because of nitrogen escape increases the mobility of Fe and Pt atoms and hence diffusion. Therefore, the ordered grain grows faster and the kinetics of L10 ordering is enhanced for the FePt films prepared with argon and nitrogen gas mixture. The observed BHF from the Mossbauer spectra did not indicate the presence of any nitride phase [5,10] indicating that the nitrogen has escaped from the system with annealing. The dissolution of nitrogen inside the FePt below the detection limit of Mossbauer measurement cannot be ruled out. The observation of no significant ordering in N0 film even at 680 1C in the present study might be due to the fact that the film may not be N0 FePt(111) FePt(200)
N20
N20 fct FePt(001)
N40
FePt(111)
FePt(200)
N40 FePt(111) fct FePt(001)
-6
-4
-2 0 2 Velocity (mm/s)
4
6
20
25
fct FePt(200) fct FePt(110)
30 35 2θ (deg)
40
45
50
Fig. 4. Mossbauer spectra ((a), (b) and (c) for N0, N20 and N40, respectively) and GIXRD patterns ((d), (e) and (f) for N0, N20 and N40, respectively) of the films annealed at 680 1C for 1 h.
ARTICLE IN PRESS V. Raghavendra Reddy et al. / Vacuum 81 (2007) 1077–1081
equi-atomic as the target was obtained by placing the Fe and Pt side by side [13]. But, it is to be noted that all the three films are prepared with the same target in the same conditions and hence the comparison is meaningful in terms of structural and magnetic properties. In conclusion, from the present studies it is observed that it is possible to improve the ordering in FePt films prepared with reactive ion beam sputtering using nitrogen gas in addition to the argon. The voids created with the escape of nitrogen might be responsible for the enhanced mobility and the improvement in the ordering. References [1] Ivanov OA, Solina LV, Demishina VA. Phys Met Metallogr 1973;35:81. [2] Maeda T, Kai T, Nagase T, Akiyama J. Appl Phys Lett 2002;80:2147; Cellollada A, Weller D, Sticht J, Harp GR, Farrow RFC, Marks R, et al. Phys Rev B 1994;50:3419;
[3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13]
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Chen SK, Yuan FT, Chen GS, Chang WC. Physica B 2003;327:366; Raghavendra Reddy V, Kavita S, Ajay Gupta A. J Appl Phys 2006;99:113906. Skomski R, Coey JMD. Phys Rev B 1993;48:15812. Sugita Y, Takahashi H, Komuro M, Mitsuoka K, Sakuma A. J Appl Phys 1994;76:6637. Dubey R, Gupta A. J Appl Phys 2005;98:83903. Wang HY, Mao WH, Ma XK, Zhang HY, Chen YB, He YJ, et al. J Appl Phys 2004;95:2564; You CY, Takahashi YK, Hono K. J Appl Phys 2005;98:013902. Pietsch U, Grenzer J, Geue Th, Neissendorfer F, Brezsesinski G, Symietz Ch, et al. Nucl Inst Methods Phys Res A 2001;467:1077. Spada FE, Parkar FT, Platt CL, Howard JK. J Appl Phys 2003;94:5123. Shinjo T, Keune W. J Magn Magn Mater 1999;200:598. Schaaf P. Hyperfine Interact 1998;111:113. Han M, Carpene E, Landry F, Lieb KP, Schaaf P. J Appl Phys 2001;89:4619. Klemmer TJ, Liu C, Shukla N, Wu XW, Weller D, Tanase M, et al. J Magn Magn Mater 2003;266:79. Seki T, Shima T, Takanashi K, Takahashi Y, Matsubara E, Hono K. Appl Phys Lett 2003;82:2461.
Vacuum 81 (2007) 1077–1081 www.elsevier.com/locate/vacuum
Study of FePt films prepared by reactive sputtering V. Raghavendra Reddya,, Shikha Puranikb, Ajay Guptaa, Wolfram Leitenbergerc a
UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India b School of Physics, DAV University, Khandwa Road, Indore 452 017, India c Universita¨t Potsdam, Institut fu¨r Physik, Postfach 601553, 14415 Potsdam, Germany Received 4 August 2006; received in revised form 13 January 2007; accepted 2 February 2007
Abstract In the present study, 57FePt films are prepared with reactive ion beam sputtering using mixture of argon and nitrogen gases. Energydispersive X-ray reflectivity is used to estimate the thickness of the as-deposited films. Structural and magnetic properties of the asdeposited and annealed films are studied using grazing incidence X-ray diffraction (GIXRD), magneto-optical Kerr effect (MOKE) and conversion electron Mossbauer spectroscopy (CEMS). Significant difference in structural and magnetic properties i.e., formation of ordered L10 phase and perpendicular magnetic anisotropy are observed for the films prepared with mixture of nitrogen and argon as compared to the film prepared with argon only. From the GIXRD, peaks corresponding to the ordered face-centred tetragonal FePt phase are observed for the films prepared with mixture gas. The results of CEMS clearly show the perpendicular magnetic anisotropy (PMA) for the films prepared with mixture of nitrogen and argon. The observed enhanced chemical ordering and the development of PMA in the films prepared with mixture gas is due to the role played by the defects created as a consequence of nitrogen escape in the films with high temperature annealing. r 2007 Elsevier Ltd. All rights reserved. Keywords: L10 ordering in FePt; Mossbauer spectroscopy; Perpendicular magnetic anisotropy; Reactive ion beam sputtering
1. Introduction The intermetallic FePt alloys are the focus of current research due to their huge magneto-crystalline anisotropy constant (Ku) values [1]. The transformation of the disordered face-centered cubic (fcc) structure to the ordered face-centered tetragonal (fct) structure (known as L10) is attributed to be the reason for the huge Ku. There have been many efforts in reducing the ordering temperature and improving the hard magnetic properties of these materials [2]. The concept of intergrain exchange coupling is studied theoretically in nano-composite magnets consisting hard magnetic phases such as Nd2Fe14B, SmCo5 and soft phases such as a-Fe and are shown experimentally to have improved energy product values [3]. To improve the hard magnetic properties of FePt, it would be Corresponding author. Tel.: +91 0731 5049232; fax: +91 0731 2462294. E-mail addresses: [email protected], [email protected] (V. Raghavendra Reddy).
0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.02.001
interesting to look at the possibility of intergrain exchange coupling mechanism. Fe4N and Fe16N2 are soft magnetic materials with high saturation magnetization [4] and can be introduced conveniently into FePt films by sputtering in nitrogen–argon mixture [5]. The presence of nitrogen in the sample is expected to improve the hard magnetic properties of FePt system in two ways. First, with the intergrain exchange coupling mechanism between soft magnetic nitride and hard magnetic FePt phase. Secondly, voids created with the nitrogen escape with high-temperature annealing are expected to play a role in increasing the chemical ordering. Recently, it was shown that it is possible to increase the coercivity of FePt hard magnets by nitrogen addition [6]. The aim of the present study is (i) to study the formation of ordered fct FePt phase, (ii) and also to understand the mechanism responsible for the improvement of hard magnetic properties of the FePt films prepared with reactive ion beam sputtering using argon and nitrogen gases using microscopic technique such as Mossbauer spectroscopy. Mossbauer effect is expected to distinguish
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FeN and FePt phases unambiguously whereas the structural peaks in XRD corresponding to some FeN phases and FePt overlap appreciably. 2. Experimental In the present study, FePt compound target is used to prepare FePt thin films. The compound target is obtained by placing Pt and 57Fe-enriched Fe target side by side. Silicon was used as the substrate. The substrates are thoroughly cleaned prior to the deposition. Ion beam sputtering using 3 cm Kaufman type hot cathode source is used for the preparation of thin films. A base vacuum of 1 10 7 Torr was achieved before deposition. Three films are prepared using a mixture of Ar and nitrogen with 0%, 20% and 40% nitrogen (designated as N0, N20 and N40 henceforth) as the sputtering gas. The samples are prepared by sputtering FePt target for the same amount of time (20 min) and prepared in the similar conditions, i.e., without breaking the vacuum. The annealing of the samples was carried out in a vacuum better than 1 10 6 Torr simultaneously for the three samples at a given temperature. Energy-dispersive X-ray reflectivity (EDXRR), grazing incidence X-ray diffraction (GIXRD) are used for the structural characterisation and conversion electron Mossbauer spectroscopy (CEMS), and magnetooptical Kerr effect in longitudinal mode (L-MOKE) are used for the study of magnetic properties of the asdeposited and annealed films.
3. Results and discussion Fig. 1 shows the X-ray measurements of the as-deposited films. The EDXRR [7] pattern is used to estimate the thickness of the films. The obtained thickness values are 55.5, 50.4 and 40.770.1 nm for N0, N20 and N40 films, respectively. As the sputtering yield with nitrogen is less as compared to Ar, the thickness is observed to decrease for increasing N2 concentration. The GIXRD patterns of the as-deposited samples are shown in Fig. 1(d–f). Peaks at 41.271, 47.791, 69.851 and 84.111 in N0 are due to the disordered fcc phase of FePt (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections. In N20, sample peak at 40.811 matches closely with either fcc FePt (1 1 1) or Fe4N (1 1 1) and peaks at 45.381, 69.151 might be due to Fe2N(0 1 2), Fe3N(3 0 0), respectively. However, from the Mossbauer measurements as discussed below, the N20 sample consists only iron nitride phases. The diffraction pattern of N40 sample showed the presence of Pt peaks in addition to the broad iron nitride peak at 45.751. No peak due to the FePt is seen in N40 sample. Peaks at 39.951, 67.791 and 81.911 in N40 are due to Pt (1 1 1), (2 2 0) and (3 1 1), respectively. Fig. 2(a–c) shows the CEMS spectra of the asdeposited samples. The experimental data is fitted with NORMOS program for the evaluation of the hyperfine parameters relative to natural iron. A nearly zero quadrupole splitting (QS) is observed for N0 sample indicating the fcc structure of the as-deposited film, which is due to the fact that as-deposited FePt exists in disordered fcc structure. The obtained isomer shift (IS) and internal field
N0
N0
FePt(111) FePt(200)
FePt(311)
FePt(220)
N20
N20
FePt/Fe4N(111) Fe2N (012)
Fe3N (300)
N40
N40
Pt(111) Fe2N (012) Pt(220) Pt(311)
0.16
0.18 q(A-1)
0.20
0.22
30
40
50
60
70
80
90
2θ (deg)
Fig. 1. Energy-dispersive X-ray reflectivity patterns ((a), (b) and (c) for N0, N20 and N40, respectively, the solid represents the fit to data) and the GIXRD patterns ((d), (e) and (f) for N0, N20 and N40, respectively) of the as-deposited films.
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1.02 N0 1.01
Relative Transmission
1.00
1.02
N20
1.01
1.00
1.03 N40 1.02 1.01 1.00 -9
-6
-3
0 Velocity (mm/s)
3
6
9
Fig. 2. Mossbauer spectra of the as-deposited films.
(BHF) are 0.224 mm/s and 31.18 T, respectively for N0 film matches with that of literature [8]. The A23 parameter, the area ratio of the second and third lines of the Mossbauer spectra, gives information about the orientation of the magnetic moment. The A23 value of four, two and zero indicates the in-plane, random and out-of plane spin orientation, respectively, in the sample [9]. The observed A23 value close to four indicates the in-plane magnetization for the as-deposited film of N0. The spectrum of the N20 film prepared with argon and 20% nitrogen as sputtering gases is fitted with one doublet and one sextet. The obtained IS, QS and BHF for the sextet are 0.294 mm/ s, 0.01 mm/s and 22.71 T, respectively. The obtained IS and QS for the doublet are 0.34 and 1.2 mm/s, respectively. The Mossbauer parameters of N20 matches with that of magnetic iron nitride and the central doublet are due to the non-magnetic iron nitride Fe3N phase [5,10]. In the Mossbauer spectrum of N20, no signature of FePt phase is seen. The Mossbauer spectrum of the N40 sample indicates that the film is non-magnetic in consistent with the MOKE measurements and the obtained IS and QS are 0.36 and 0.98 mm/s, respectively. It may be noted here that as the phase diagram of FeN is very complex, the critical analysis of the Mossbauer data to assign different phases of FeN system is not discussed here, as the aim of the present experiment is to study the formation of FePt phase. The GIXRD and CEMS results, therefore, demonstrate the
N20 at 580 celsius N20 at 680 celsius N20 at 500 celsius N20 (pristine)
N0 at 680 celsius N0 at 580 celsius N0 at 500 celsius N0 (pristine)
-1000 -800 -600 -400 -200 0 200 Field(oe)
400
600
800 1000
Fig. 3. Magnetization data of N0 and N20 samples at different temperatures obtained from L-MOKE.
formation of iron nitride in the films prepared with mixture of argon and nitrogen as the sputtering gases. Fig. 3 shows the magnetic measurements of the asdeposited and annealed samples obtained from L-MOKE. From the magnetization loops, both as-deposited N0 and N20 samples are observed to be soft magnetic. This is expected as the as-deposited disordered FePt alloy is soft
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magnetic and the Fe4N is also a soft magnetic material. As shown in Fig. 3, it is observed that the coercivity increases with the annealing at higher temperatures. Significant changes in the structural and magnetic properties after 680 1C annealing are observed. For the sample annealed at 680 1C, the coercivity (Hc) increased from 28 to 91 Oe and from 24 to 217 Oe for N0 and N20, respectively. The increase of the coercivity in the longitudinal geometry is considered due to the rotation of magnetization from inplane to out-of plane. We could not record the hysteresis curve of the as-deposited N40 (due to the non-magnetic nature) and also for the film annealed at 680 1C (due to the insufficient field, 1.8 kOe, of our magnet). Fig. 4(d–f) shows the GIXRD pattern of the films annealed at 680 1C for 1 h. The pattern of N40 film clearly shows the ordered fct FePt (0 0 1), (1 1 0), (1 1 1) and (2 0 0) at 24.21, 32.91, 41.11 and 47.41, respectively. FePt(0 0 1) peak with very small intensity at 241 is observed for N20 film after annealing at 680 1C. But, in the case of N0 film, no such peaks corresponding to the ordered phase are observed. Therefore, GIXRD suggests that after annealing at 680 1C films prepared with nitrogen as the sputtering gas in addition to the argon are having more chemical ordering as compared to N0 film prepared with argon only. This observation is further evidenced from the Mossbauer data as discussed below. Fig. 4(a–c) shows the Mossbauer patterns of the films annealed at 680 1C for 1 h. The observed QS values are 0.036, 0.027 and 0.27 mm/s for N0, N20 and N40, respectively. The QS value of 0.27 mm/s clearly indicates the deviation from cubic symmetry i.e., the formation of fct FePt phase in N40 sample. The observed A23 values are 3.83, 3.69 and 1.53 for N0, N20 and N40, N0
respectively. The A23 value of less than 2 demonstrates the out-of plane magnetization i.e., development of perpendicular magnetic anisotropy (PMA) in N40. Because of the strong /1 1 1S texture, the A23 value is not zero for N40. The N20 sample also shows an A23 value less as compared to N0 film. The Mossbauer results clearly demonstrate the development of PMA in the films prepared with nitrogen as compared to the N0 film. The results also demonstrate that more the nitrogen in the film, more ordering is observed with the annealing. These observations can be explained as the following. The nitrogen incorporated in the film is known to escape with high temperature annealing. At annealing temperatures above 673 K, the nitrogen escape is mainly due to degassing kinetics [11]. Hence, with the high-temperature annealing the N escape rapidly from the FePt phase yielding lot of vacancies inside the film. The ordering of L10 alloys such as FePt, FePd and CoPt usually proceeds by a nucleation and growth process [12]. The formation of vacancies because of nitrogen escape increases the mobility of Fe and Pt atoms and hence diffusion. Therefore, the ordered grain grows faster and the kinetics of L10 ordering is enhanced for the FePt films prepared with argon and nitrogen gas mixture. The observed BHF from the Mossbauer spectra did not indicate the presence of any nitride phase [5,10] indicating that the nitrogen has escaped from the system with annealing. The dissolution of nitrogen inside the FePt below the detection limit of Mossbauer measurement cannot be ruled out. The observation of no significant ordering in N0 film even at 680 1C in the present study might be due to the fact that the film may not be N0 FePt(111) FePt(200)
N20
N20 fct FePt(001)
N40
FePt(111)
FePt(200)
N40 FePt(111) fct FePt(001)
-6
-4
-2 0 2 Velocity (mm/s)
4
6
20
25
fct FePt(200) fct FePt(110)
30 35 2θ (deg)
40
45
50
Fig. 4. Mossbauer spectra ((a), (b) and (c) for N0, N20 and N40, respectively) and GIXRD patterns ((d), (e) and (f) for N0, N20 and N40, respectively) of the films annealed at 680 1C for 1 h.
ARTICLE IN PRESS V. Raghavendra Reddy et al. / Vacuum 81 (2007) 1077–1081
equi-atomic as the target was obtained by placing the Fe and Pt side by side [13]. But, it is to be noted that all the three films are prepared with the same target in the same conditions and hence the comparison is meaningful in terms of structural and magnetic properties. In conclusion, from the present studies it is observed that it is possible to improve the ordering in FePt films prepared with reactive ion beam sputtering using nitrogen gas in addition to the argon. The voids created with the escape of nitrogen might be responsible for the enhanced mobility and the improvement in the ordering. References [1] Ivanov OA, Solina LV, Demishina VA. Phys Met Metallogr 1973;35:81. [2] Maeda T, Kai T, Nagase T, Akiyama J. Appl Phys Lett 2002;80:2147; Cellollada A, Weller D, Sticht J, Harp GR, Farrow RFC, Marks R, et al. Phys Rev B 1994;50:3419;
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