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Effects of Fe(Pt) single layer thickness and carbon doping on (001) orientation and magnetic properties of FePt thin films
Superlattices and Microstructures 85 (2015) 198–205
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effects of Fe(Pt) single layer thickness and carbon doping on (001) orientation and magnetic properties of FePt thin films Yumei Zhang a, Haibo Cheng a, Yongsheng Yu b, Mei Liu a, Haibo Li a,c,⇑ a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 17 February 2015 Received in revised form 15 May 2015 Accepted 18 May 2015 Available online 23 May 2015 Keywords: L10 FePt film Single layer thickness Carbon doping Texture Magnetic properties
a b s t r a c t The effects of the Fe(Pt) single layer thickness and carbon doping on the chemical ordering, (001) orientation, and magnetic properties of the [Fe/Pt]n thin films have been studied. The [Fe/Pt]n thin films with various Fe(Pt) single layer thicknesses and carbon contents were prepared on thermally oxidized Si(100) substrates by using magnetron sputtering at room temperature and annealed in hydrogen atmosphere at 600 °C. It was found that the annealed [Fe/Pt]n film with a single layer thickness of 0.5 nm exhibited high chemical ordering and (001) preferred orientation. Suitable carbon doping induced high (001) texture and enhanced soft and hard magnetic exchange couple. The hard magnetic property of the films deteriorated with increasing carbon content. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction L10 ordered FePt films have attracted much attention for magnetic recording applications because of their high magnetocrystalline anisotropy constant, small superparamagnetic critical size, moderate saturation magnetization, and good corrosion resistance [1]. Usually, FePt films deposited on amorphous or oxidized Si substrates tend to exhibit (111) preferred or random orientations and their easy axis is tilted 37 ° away from the film plane [2]. As perpendicular magnetic recording media, it is desirable to achieve a perfect (001) texture. In many cases, some single crystals such as MgO (001) [3], Pt (001) substrates [4], and CrX (200) (X = Ru, Mo, W, V) [5–7] underlayers are used to enhance the (001) orientation. In contrast, for the nonepitaxial method, rapid thermal annealing [8] and [Fe/Pt]n multilayer films prepared by monatomic layer deposition [9] have been devoted to promotion of the L10 FePt (001) preferred orientation as a perpendicular magnetic recording media. However, it is always a challenge to obtain a perfect (001) texture with nonepitaxial techniques and understand the mechanism of nonepitaxial growth. In this paper, we prepared [Fe(C)/Pt]n films annealed in hydrogen atmosphere in order to obtain the high (001) preferred orientation and clarify the mechanism of crystalline orientation for nonepitaxial FePt films. The thickness of Fe(Pt) single layers can play an important role in the nucleation of ordering phase and growth of preferred orientation in the films. Carbon as a nonmagnetic material can also reduce the magnetic interaction and expand the relaxation length of spinning in the media. Additionally it is expected to remove the internal oxygen using hydrogen atmosphere annealing. The effects
⇑ Corresponding author at: Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.spmi.2015.05.022 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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of Fe(Pt) single layer thickness and carbon doping on the microstructure and magnetic properties of the FePt films annealed in hydrogen atmosphere were investigated in detail.
2. Experiments [Fe(t nm)C/Pt(t nm)]n (t = 0.33, 0.5, 1.0 and 5.0 nm) films with various carbon sputtering powers (0, 60, 120, and 210 W) were prepared on thermal oxidized Si(100) substrates with an ATC 1800-F sputtering system by dc- and rf-magnetron sputtering. The nominal thickness of [Fe/Pt]n films was 10.0 nm. The thickness tFe of Fe single layer was equal to that (tPt) of Pt single layer. The sputtering power of C target was changed to vary the carbon content in [FeC/Pt]n multilayer films. The base pressure of the deposition chamber was about 7.5 107 Torr and Ar gas was kept at a pressure of 5.0 mTorr during sputtering. The substrates were rotated at a speed of 20 rpm during deposition to obtain uniform films. The as-deposited films were annealed at 600 °C for 1 h in H2 atmosphere. The thicknesses of [Fe(C)/Pt]n films were determined by X-ray reflectivity (XRR) measurement. The structure of the as-deposited and annealed films were characterized by X-ray diffraction (XRD) on Rigaku D/max-2500 diffractometer with Cu Ka radiation using a current of 300 mA and voltage of 40 kV. The microstructure and thickness of the film were investigated by JEM-2100HR transmission electron microscopy (TEM). The magnetic properties of the films were measured at room temperature using Lake Shore 7407 vibrating sample magnetometer (VSM) with maximum applied field of 20 kOe.
3. Results and discussions 3.1. The effect of Fe(Pt) single layer thickness on structure and magnetic properties The XRD patterns of the annealed [Fe/Pt]n films are shown in Fig. 1. The diffraction peaks around 24.0°, 49.3°, and 78.0° originate from the (001), (002), and (003) superlattice reflections of the L10 FePt phase, respectively. The (00l) diffraction peaks are predominant while the (111), (200) diffraction peaks are weak, indicating that all the films exhibit (001) texture. It is noteworthy that two visible symmetric satellite peaks locate at two sides of the (001) and (002) peaks in Fig. 1((a)–(c)), suggesting a well-aligned (001)-oriented film [10]. We estimated the ‘‘perpendicular’’ grain sizes from the half-peak widths of (00l) diffraction peaks, according to the Hall equation [11] expressed as b cos h=k ¼ 2gðsin h=kÞ þ ð1=eÞ, where b is the full width at half maximum of the diffraction peak, 2h is the Bragg angle, k is X-ray wavelength of 0.15406 nm, e is the grain size and g is the lattice strain. The grain sizes along (001) direction are about 12.1, 12.0, 12.1 and 11.6 nm for [Fe/Pt]n films with t = 0.33, 0.5, 1.0 and 5.0 nm, respectively, which are slightly larger than the nominal thickness of 10.0 nm. Fig. 2(a) shows the integrated intensity ratio of the (001) peak to the (002) one as a function of Fe(Pt) single layer thickness t. The chemical ordering parameter S is proportional to (I001/I002)1/2 [12]. I001/I002 increases firstly and then decreases. When t is 0.5 nm, I001/I002 reaches the maximum value of 2.15, which indicates that the chemical ordering improves as well. It can be observed that the change in the degree of perpendicular orientation via the integrated intensity ratio of the (001) peak of the FePt film to (111) one, I001/I111, as shown in Fig. 2(b). In the case of [Fe/Pt]n film with t = 0.5 nm, I001/I111 exhibits the maximum value. The [Fe/Pt]n film with t = 0.5 nm has high chemical ordering and (001) preferred orientation. These results indicate that the thickness of Fe(Pt) single layer can affect the nucleation of ordering phase and growth of preferred orientation in the films [13].
Fig. 1. XRD patterns of the annealed [Fe(t nm)/Pt(t nm)]n films with various single layer thicknesses of t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
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Fig. 2. Variations of I001/I002 (a) and I001/I111 (b) with the single layer thickness t.
Fig. 3. XRD rocking curves of the (0 0 1) diffraction peaks in the [Fe(t nm)/Pt(t nm)]n films with various single layer thicknesses of t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
To evaluate the degree of orientation of [Fe/Pt]n films with various single layer thicknesses, XRD rocking curves of the (001) diffraction peaks were measured. The XRD rocking curves can be fitted by a Gaussian function. The full width at half maximum FWHM and peak position xc of FePt (001) diffraction peak are shown in Fig. 3. The minimum Gaussian FWHM of the (001) diffraction peak is 5.8° in the [Fe/Pt]n film with t = 0.5 nm, suggesting that the film has the highest (001) texture.
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Fig. 4. The out-of-plane and in-plane hysteresis loops of the annealed [Fe/Pt]n films with various single layer thicknesses: t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
Table 1 The coercivity Hc and remanence ratio Mr/Ms (\: out-of-plane, ‚: in-plane) of the [Fe/Pt]n films with various single layer thicknesses t. t (nm)
Hc\(kOe)
Hc‚ (kOe)
Mr\/Ms\
0.33 0.5 1.0 5.0
9.46 9.65 10.18 7.32
3.18 2.06 1.49 1.88
0.67 0.76 0.73 0.59
Fig. 4 shows the out-of-plane and in-plane hysteresis loops of annealed [Fe/Pt]n films with various single layer thicknesses. The coercivity Hc and remanence ratio Mr/Ms of the films are given in Table 1. All of the films show the hard ferromagnetic characteristic with out-of-plane coercivities of more than 7.3 kOe after annealing at 600 °C, which indicates that the L10 ordered structures are formed in the films. The easy magnetization axis is oriented perpendicular to the plane of L10 FePt films. From Table 1, we can see the film with t = 0.5 nm has the largest remanence ratio of 0.76 in the out-of-plane hysteresis loop, revealing that it has a high perpendicular magnetic anisotropy. Apparent shoulders appear in the hysteresis loops due to the coexistence of soft and hard magnetic phases in the corresponding films. This indicates that the part of the soft magnetic phase is not full exchange coupling with the hard magnetic phase.
3.2. The effect of carbon doping on structure and magnetic properties Fig. 5 presents the TEM cross-sectional image (a) and corresponding HRTEM cross-sectional image (b) of annealed film with carbon sputtering power of 210 W. From Fig. 5, the measured thickness of the film is about 9.60 nm, consistent well with the evaluation from the controlled experiments. The (001) lattice planes are parallel to the film plane. The measured interplanar distance of the fringe is about 0.37 nm corresponding to the (001) crystallographic plane of L10 FePt. Fig. 6 shows XRD patterns of [Fe(0.5 nm)C/Pt(0.5 nm)]n films with various carbon sputtering powers. As shown in Fig. 6, only the (00l) diffraction peaks of the L10 FePt phase are seen obviously, which means that FePt films have (001) texture. In
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Fig. 5. TEM cross-sectional image (a) and corresponding HRTEM cross-sectional image (b) of annealed film with carbon sputtering power of 210 W.
Fig. 6. XRD patterns of the annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), 210 W (d) and the enlarged patterns of (1 1 1) diffraction peaks (e).
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the XRD patterns of partial enlargement, the weaker (111) diffraction peaks can be observed in the films without carbon doping and with carbon doping when carbon sputtering power is 60 W. As carbon sputtering power is 120 W and above, the (111) diffraction peak disappears. These results mean that increasing carbon content could improve the (001) orientation of L10 phase. The grain sizes along (001) direction are about 12.0, 11.4, 11.7 and 11.7 nm for [FeC/Pt]n films when the carbon sputtering power is 0, 60, 120 and 210 W, respectively, indicating that the grain sizes do not change much with increasing the carbon content. Züttel et al. [14] have reported that hydrogen atoms can dissolve in the octahedral interstitial sites of Pd. The disordered FePt has the same structure as Pd, so hydrogen atoms may dissolve in the interstitial sites of disordered face-centered-cubic FePt. The radius of carbon atom (0.086 nm) is slightly larger than that of hydrogen atom (0.078 nm). Therefore, it is likely that carbon atom substitutes for hydrogen atom when hydrogen atom escapes during high-temperature annealing and interstitial carbon makes d-spacing of FePt (00l) enlarge. In Table 2, (001), (002) and (003) diffraction peak positions of the [FeC/Pt]n films are listed, (00l) diffraction peaks of FePt shift to small angle with increasing carbon sputtering power, confirming that carbon atoms dissolve in the interstitial sites of FePt phase [15]. We also evaluated the degree of orientation of [Fe(0.5 nm)C/Pt(0.5 nm)]n films with various carbon sputtering powers by XRD rocking curves for (001) diffraction peaks, as shown in Fig. 7. The FWHM fitted with Gaussian functions decreases from 5.8° to 2.3° in the films with increasing carbon content, suggesting that the films with carbon doping can also induce high (001) texture.
Table 2 (0 0 1), (0 0 2) and (0 0 3) diffraction peak positions of the annealed [FeC/Pt]n films with various carbon sputtering powers. Carbon sputtering power (W)
0
60
120
210
(0 0 1) peak position (0 0 2) peak position (0 0 3) peak position
24.131 ± 0.001° 49.389 ± 0.001° 77.648 ± 0.005°
24.009 ± 0.001° 49.160 ± 0.002° 77.378 ± 0.012°
23.998 ± 0.001° 49.159 ± 0.002° 77.282 ± 0.004°
23.991 ± 0.001° 49.120 ± 0.004° 77.263 ± 0.003°
Fig. 7. XRD rocking curves of annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), and 210 W (d).
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Fig. 8. The out-of-plane and in-plane hysteresis loops of the annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), and 210 W (d).
Table 3 Coercivities and remanence ratios (\: out-of-plane, ‚: in-plane) of the films with various carbon sputtering powers. Carbon sputtering power (W)
Hc\ (kOe)
Hc‚ (kOe)
Mr\/Ms\
0 60 120 210
9.65 8.68 5.76 5.63
2.06 7.58 2.54 0.99
0.76 0.79 0.89 0.73
Fig. 8 shows the out-of-plane and in-plane hysteresis loops of the annealed [Fe(0.5 nm)C/Pt (0.5 nm)]n films with various carbon sputtering powers. All the films have obvious perpendicular anisotropy. Apparent shoulders appear in the hysteresis loops of the films prepared with carbon sputtering power of 0 and 210 W, whereas those of the films prepared with carbon sputtering power 60 and 120 W significantly decrease. Yu et al. have reported that shoulders in the corresponding hysteresis loops indicate there is a duplex phase structure due to the coexistence of soft and hard phases in the films [16]. On the basis of the above results and analysis, we can conclude that carbon content has an effect on strength of soft/hard magnetic exchange coupling. The suitable carbon doping can enhance exchange coupling, but excess carbon doping induces a weak soft and hard magnetic exchange couple. The coercivity Hc and remanence ratio Mr/Ms of the films are listed in Table 3. From Table 3, we find that increasing the carbon content causes a decrease in coercivity, which is similar to those reported by Chen and co-workers [17]. It has been demonstrated that d-spacing of FePt (00l) is expanded by carbon doping and then ordering degree of L10 FePt film decreases. The film prepared with carbon sputtering power of 120 W has the largest out-of-plane remanence ratio of 0.89, revealing that it has a high perpendicular magnetic anisotropy.
4. Conclusions We have studied the structure and magnetic properties of [Fe/Pt]n films with various thicknesses of Fe(Pt) single layer and carbon contents. It was found that the single layer thickness could affect the nucleation of ordering phase and growth of
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preferred orientation in the films. [Fe/Pt]n film with single layer thickness of 0.5 nm exhibited high chemical ordering and (001) preferred orientation. Apparent shoulders appeared in magnetic hysteresis loops of the annealed films due to the coexistence of soft and hard magnetic phases in the films. Suitable carbon doping induced high (001) texture and enhanced soft and hard magnetic exchange couple. But the out-of-plane coercivity of the films decreased with increasing the carbon content. Acknowledgment The work was supported by the National Natural Science Foundation of China (Nos. 20971055 and 21371071). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effects of Fe(Pt) single layer thickness and carbon doping on (001) orientation and magnetic properties of FePt thin films Yumei Zhang a, Haibo Cheng a, Yongsheng Yu b, Mei Liu a, Haibo Li a,c,⇑ a
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 17 February 2015 Received in revised form 15 May 2015 Accepted 18 May 2015 Available online 23 May 2015 Keywords: L10 FePt film Single layer thickness Carbon doping Texture Magnetic properties
a b s t r a c t The effects of the Fe(Pt) single layer thickness and carbon doping on the chemical ordering, (001) orientation, and magnetic properties of the [Fe/Pt]n thin films have been studied. The [Fe/Pt]n thin films with various Fe(Pt) single layer thicknesses and carbon contents were prepared on thermally oxidized Si(100) substrates by using magnetron sputtering at room temperature and annealed in hydrogen atmosphere at 600 °C. It was found that the annealed [Fe/Pt]n film with a single layer thickness of 0.5 nm exhibited high chemical ordering and (001) preferred orientation. Suitable carbon doping induced high (001) texture and enhanced soft and hard magnetic exchange couple. The hard magnetic property of the films deteriorated with increasing carbon content. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction L10 ordered FePt films have attracted much attention for magnetic recording applications because of their high magnetocrystalline anisotropy constant, small superparamagnetic critical size, moderate saturation magnetization, and good corrosion resistance [1]. Usually, FePt films deposited on amorphous or oxidized Si substrates tend to exhibit (111) preferred or random orientations and their easy axis is tilted 37 ° away from the film plane [2]. As perpendicular magnetic recording media, it is desirable to achieve a perfect (001) texture. In many cases, some single crystals such as MgO (001) [3], Pt (001) substrates [4], and CrX (200) (X = Ru, Mo, W, V) [5–7] underlayers are used to enhance the (001) orientation. In contrast, for the nonepitaxial method, rapid thermal annealing [8] and [Fe/Pt]n multilayer films prepared by monatomic layer deposition [9] have been devoted to promotion of the L10 FePt (001) preferred orientation as a perpendicular magnetic recording media. However, it is always a challenge to obtain a perfect (001) texture with nonepitaxial techniques and understand the mechanism of nonepitaxial growth. In this paper, we prepared [Fe(C)/Pt]n films annealed in hydrogen atmosphere in order to obtain the high (001) preferred orientation and clarify the mechanism of crystalline orientation for nonepitaxial FePt films. The thickness of Fe(Pt) single layers can play an important role in the nucleation of ordering phase and growth of preferred orientation in the films. Carbon as a nonmagnetic material can also reduce the magnetic interaction and expand the relaxation length of spinning in the media. Additionally it is expected to remove the internal oxygen using hydrogen atmosphere annealing. The effects
⇑ Corresponding author at: Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.spmi.2015.05.022 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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of Fe(Pt) single layer thickness and carbon doping on the microstructure and magnetic properties of the FePt films annealed in hydrogen atmosphere were investigated in detail.
2. Experiments [Fe(t nm)C/Pt(t nm)]n (t = 0.33, 0.5, 1.0 and 5.0 nm) films with various carbon sputtering powers (0, 60, 120, and 210 W) were prepared on thermal oxidized Si(100) substrates with an ATC 1800-F sputtering system by dc- and rf-magnetron sputtering. The nominal thickness of [Fe/Pt]n films was 10.0 nm. The thickness tFe of Fe single layer was equal to that (tPt) of Pt single layer. The sputtering power of C target was changed to vary the carbon content in [FeC/Pt]n multilayer films. The base pressure of the deposition chamber was about 7.5 107 Torr and Ar gas was kept at a pressure of 5.0 mTorr during sputtering. The substrates were rotated at a speed of 20 rpm during deposition to obtain uniform films. The as-deposited films were annealed at 600 °C for 1 h in H2 atmosphere. The thicknesses of [Fe(C)/Pt]n films were determined by X-ray reflectivity (XRR) measurement. The structure of the as-deposited and annealed films were characterized by X-ray diffraction (XRD) on Rigaku D/max-2500 diffractometer with Cu Ka radiation using a current of 300 mA and voltage of 40 kV. The microstructure and thickness of the film were investigated by JEM-2100HR transmission electron microscopy (TEM). The magnetic properties of the films were measured at room temperature using Lake Shore 7407 vibrating sample magnetometer (VSM) with maximum applied field of 20 kOe.
3. Results and discussions 3.1. The effect of Fe(Pt) single layer thickness on structure and magnetic properties The XRD patterns of the annealed [Fe/Pt]n films are shown in Fig. 1. The diffraction peaks around 24.0°, 49.3°, and 78.0° originate from the (001), (002), and (003) superlattice reflections of the L10 FePt phase, respectively. The (00l) diffraction peaks are predominant while the (111), (200) diffraction peaks are weak, indicating that all the films exhibit (001) texture. It is noteworthy that two visible symmetric satellite peaks locate at two sides of the (001) and (002) peaks in Fig. 1((a)–(c)), suggesting a well-aligned (001)-oriented film [10]. We estimated the ‘‘perpendicular’’ grain sizes from the half-peak widths of (00l) diffraction peaks, according to the Hall equation [11] expressed as b cos h=k ¼ 2gðsin h=kÞ þ ð1=eÞ, where b is the full width at half maximum of the diffraction peak, 2h is the Bragg angle, k is X-ray wavelength of 0.15406 nm, e is the grain size and g is the lattice strain. The grain sizes along (001) direction are about 12.1, 12.0, 12.1 and 11.6 nm for [Fe/Pt]n films with t = 0.33, 0.5, 1.0 and 5.0 nm, respectively, which are slightly larger than the nominal thickness of 10.0 nm. Fig. 2(a) shows the integrated intensity ratio of the (001) peak to the (002) one as a function of Fe(Pt) single layer thickness t. The chemical ordering parameter S is proportional to (I001/I002)1/2 [12]. I001/I002 increases firstly and then decreases. When t is 0.5 nm, I001/I002 reaches the maximum value of 2.15, which indicates that the chemical ordering improves as well. It can be observed that the change in the degree of perpendicular orientation via the integrated intensity ratio of the (001) peak of the FePt film to (111) one, I001/I111, as shown in Fig. 2(b). In the case of [Fe/Pt]n film with t = 0.5 nm, I001/I111 exhibits the maximum value. The [Fe/Pt]n film with t = 0.5 nm has high chemical ordering and (001) preferred orientation. These results indicate that the thickness of Fe(Pt) single layer can affect the nucleation of ordering phase and growth of preferred orientation in the films [13].
Fig. 1. XRD patterns of the annealed [Fe(t nm)/Pt(t nm)]n films with various single layer thicknesses of t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
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Fig. 2. Variations of I001/I002 (a) and I001/I111 (b) with the single layer thickness t.
Fig. 3. XRD rocking curves of the (0 0 1) diffraction peaks in the [Fe(t nm)/Pt(t nm)]n films with various single layer thicknesses of t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
To evaluate the degree of orientation of [Fe/Pt]n films with various single layer thicknesses, XRD rocking curves of the (001) diffraction peaks were measured. The XRD rocking curves can be fitted by a Gaussian function. The full width at half maximum FWHM and peak position xc of FePt (001) diffraction peak are shown in Fig. 3. The minimum Gaussian FWHM of the (001) diffraction peak is 5.8° in the [Fe/Pt]n film with t = 0.5 nm, suggesting that the film has the highest (001) texture.
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Fig. 4. The out-of-plane and in-plane hysteresis loops of the annealed [Fe/Pt]n films with various single layer thicknesses: t = 0.33 nm (a), 0.5 nm (b), 1.0 nm (c) and 5.0 nm (d).
Table 1 The coercivity Hc and remanence ratio Mr/Ms (\: out-of-plane, ‚: in-plane) of the [Fe/Pt]n films with various single layer thicknesses t. t (nm)
Hc\(kOe)
Hc‚ (kOe)
Mr\/Ms\
0.33 0.5 1.0 5.0
9.46 9.65 10.18 7.32
3.18 2.06 1.49 1.88
0.67 0.76 0.73 0.59
Fig. 4 shows the out-of-plane and in-plane hysteresis loops of annealed [Fe/Pt]n films with various single layer thicknesses. The coercivity Hc and remanence ratio Mr/Ms of the films are given in Table 1. All of the films show the hard ferromagnetic characteristic with out-of-plane coercivities of more than 7.3 kOe after annealing at 600 °C, which indicates that the L10 ordered structures are formed in the films. The easy magnetization axis is oriented perpendicular to the plane of L10 FePt films. From Table 1, we can see the film with t = 0.5 nm has the largest remanence ratio of 0.76 in the out-of-plane hysteresis loop, revealing that it has a high perpendicular magnetic anisotropy. Apparent shoulders appear in the hysteresis loops due to the coexistence of soft and hard magnetic phases in the corresponding films. This indicates that the part of the soft magnetic phase is not full exchange coupling with the hard magnetic phase.
3.2. The effect of carbon doping on structure and magnetic properties Fig. 5 presents the TEM cross-sectional image (a) and corresponding HRTEM cross-sectional image (b) of annealed film with carbon sputtering power of 210 W. From Fig. 5, the measured thickness of the film is about 9.60 nm, consistent well with the evaluation from the controlled experiments. The (001) lattice planes are parallel to the film plane. The measured interplanar distance of the fringe is about 0.37 nm corresponding to the (001) crystallographic plane of L10 FePt. Fig. 6 shows XRD patterns of [Fe(0.5 nm)C/Pt(0.5 nm)]n films with various carbon sputtering powers. As shown in Fig. 6, only the (00l) diffraction peaks of the L10 FePt phase are seen obviously, which means that FePt films have (001) texture. In
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Fig. 5. TEM cross-sectional image (a) and corresponding HRTEM cross-sectional image (b) of annealed film with carbon sputtering power of 210 W.
Fig. 6. XRD patterns of the annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), 210 W (d) and the enlarged patterns of (1 1 1) diffraction peaks (e).
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the XRD patterns of partial enlargement, the weaker (111) diffraction peaks can be observed in the films without carbon doping and with carbon doping when carbon sputtering power is 60 W. As carbon sputtering power is 120 W and above, the (111) diffraction peak disappears. These results mean that increasing carbon content could improve the (001) orientation of L10 phase. The grain sizes along (001) direction are about 12.0, 11.4, 11.7 and 11.7 nm for [FeC/Pt]n films when the carbon sputtering power is 0, 60, 120 and 210 W, respectively, indicating that the grain sizes do not change much with increasing the carbon content. Züttel et al. [14] have reported that hydrogen atoms can dissolve in the octahedral interstitial sites of Pd. The disordered FePt has the same structure as Pd, so hydrogen atoms may dissolve in the interstitial sites of disordered face-centered-cubic FePt. The radius of carbon atom (0.086 nm) is slightly larger than that of hydrogen atom (0.078 nm). Therefore, it is likely that carbon atom substitutes for hydrogen atom when hydrogen atom escapes during high-temperature annealing and interstitial carbon makes d-spacing of FePt (00l) enlarge. In Table 2, (001), (002) and (003) diffraction peak positions of the [FeC/Pt]n films are listed, (00l) diffraction peaks of FePt shift to small angle with increasing carbon sputtering power, confirming that carbon atoms dissolve in the interstitial sites of FePt phase [15]. We also evaluated the degree of orientation of [Fe(0.5 nm)C/Pt(0.5 nm)]n films with various carbon sputtering powers by XRD rocking curves for (001) diffraction peaks, as shown in Fig. 7. The FWHM fitted with Gaussian functions decreases from 5.8° to 2.3° in the films with increasing carbon content, suggesting that the films with carbon doping can also induce high (001) texture.
Table 2 (0 0 1), (0 0 2) and (0 0 3) diffraction peak positions of the annealed [FeC/Pt]n films with various carbon sputtering powers. Carbon sputtering power (W)
0
60
120
210
(0 0 1) peak position (0 0 2) peak position (0 0 3) peak position
24.131 ± 0.001° 49.389 ± 0.001° 77.648 ± 0.005°
24.009 ± 0.001° 49.160 ± 0.002° 77.378 ± 0.012°
23.998 ± 0.001° 49.159 ± 0.002° 77.282 ± 0.004°
23.991 ± 0.001° 49.120 ± 0.004° 77.263 ± 0.003°
Fig. 7. XRD rocking curves of annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), and 210 W (d).
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Fig. 8. The out-of-plane and in-plane hysteresis loops of the annealed [FeC/Pt]n films with various carbon sputtering powers: 0 W (a), 60 W (b), 120 W (c), and 210 W (d).
Table 3 Coercivities and remanence ratios (\: out-of-plane, ‚: in-plane) of the films with various carbon sputtering powers. Carbon sputtering power (W)
Hc\ (kOe)
Hc‚ (kOe)
Mr\/Ms\
0 60 120 210
9.65 8.68 5.76 5.63
2.06 7.58 2.54 0.99
0.76 0.79 0.89 0.73
Fig. 8 shows the out-of-plane and in-plane hysteresis loops of the annealed [Fe(0.5 nm)C/Pt (0.5 nm)]n films with various carbon sputtering powers. All the films have obvious perpendicular anisotropy. Apparent shoulders appear in the hysteresis loops of the films prepared with carbon sputtering power of 0 and 210 W, whereas those of the films prepared with carbon sputtering power 60 and 120 W significantly decrease. Yu et al. have reported that shoulders in the corresponding hysteresis loops indicate there is a duplex phase structure due to the coexistence of soft and hard phases in the films [16]. On the basis of the above results and analysis, we can conclude that carbon content has an effect on strength of soft/hard magnetic exchange coupling. The suitable carbon doping can enhance exchange coupling, but excess carbon doping induces a weak soft and hard magnetic exchange couple. The coercivity Hc and remanence ratio Mr/Ms of the films are listed in Table 3. From Table 3, we find that increasing the carbon content causes a decrease in coercivity, which is similar to those reported by Chen and co-workers [17]. It has been demonstrated that d-spacing of FePt (00l) is expanded by carbon doping and then ordering degree of L10 FePt film decreases. The film prepared with carbon sputtering power of 120 W has the largest out-of-plane remanence ratio of 0.89, revealing that it has a high perpendicular magnetic anisotropy.
4. Conclusions We have studied the structure and magnetic properties of [Fe/Pt]n films with various thicknesses of Fe(Pt) single layer and carbon contents. It was found that the single layer thickness could affect the nucleation of ordering phase and growth of
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preferred orientation in the films. [Fe/Pt]n film with single layer thickness of 0.5 nm exhibited high chemical ordering and (001) preferred orientation. Apparent shoulders appeared in magnetic hysteresis loops of the annealed films due to the coexistence of soft and hard magnetic phases in the films. Suitable carbon doping induced high (001) texture and enhanced soft and hard magnetic exchange couple. But the out-of-plane coercivity of the films decreased with increasing the carbon content. Acknowledgment The work was supported by the National Natural Science Foundation of China (Nos. 20971055 and 21371071). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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