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Investigation of microstructure and magnetic properties of FePt–X films grown on MgO and STO substrates
Journal of Magnetism and Magnetic Materials 402 (2016) 124–130
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Current Perspectives
Investigation of microstructure and magnetic properties of FePt–X films grown on MgO and STO substrates K.F. Dong a,n, F. Jin a, W.Q. Mo a, J.L. Song a, W.M. Cheng b a b
School of Automation, China University of Geosciences, Wuhan 430074, China School of Optical and Electronic Information, Huazhong University of Science & Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 September 2015 Received in revised form 8 November 2015 Accepted 21 November 2015 Available online 22 November 2015
Different thickness (001) textured FePt–X films were fabricated on MgO and SrTiO3 substrates, and the microstructure and magnetic properties of FePt–X films was systematically investigated. The study showed that the substrates with different lattice mismatch resulted in different crystallographic texture and microstructure. The larger mismatch strain with using MgO substrates would cause FePt films to follow the island growth mode, and the negligible mismatch strain with using STO substrate would form continuous film. Doping SiNx and C into FePt films caused the increase of lattice mismatch strain between FePt and MgO/STO, which would induce FePt films to forming island growth. The perpendicular anisotropy of FePt–SiNx–C films grown on MgO substrate was larger than that of using SrTiO3 substrate, which was attributed to the larger strain induced by larger lattice mismatch. & 2015 Elsevier B.V. All rights reserved.
Keywords: FePt films Lattice mismatch Different substrates
1. Introduction Epitaxial growth of thin films in the early stages is observed to follow primarily three modes [1,2]: (a) planar growth or layer-bylayer growth (Frank-van der Merwe growth); (b) three-dimensional or island growth (Volmer–Weber growth); and (c) planar growth followed by island growth (Stranski–Krastonov growth). In all three types of growth, exploring how the lattice-mismatch affects the epitaxial growth mode is an important issue, as the epitaxial growth mode can dramatically influence the microstructure and magnetic properties of magnetic thin films. Generally, it is accepted that depending on the magnitude of misfit strain ε and the interfacial and surface energies, growth may proceed by layer-by-island [Stranski–Krastonov (SK)] mode, or at a higher ε via direct islanding (Volmer–Weber) growth. Moreover, lattice matched and closely lattice-mismatch systems would induce in a layer-by-layer (Frank-van der Merwe) growth mode [3,4]. L10 ordered FePt alloy with high magnetocrystalline anisotropy (Ku 7 107 erg/cc) and chemical stability is the most promising material to realize the potential of heat assisted magnetic recording (HAMR) [5,6]. For the exploration of practical applications, L10-FePt films with (001) texture and high magnetocrystalline anisotropy have been fabricated by epitaxial growth on singlecrystalline MgO (001) substrates [7,8] or on polycrystalline under n
Corresponding author. E-mail address: [email protected] (K.F. Dong).
http://dx.doi.org/10.1016/j.jmmm.2015.11.060 0304-8853/& 2015 Elsevier B.V. All rights reserved.
layers such as Pt (001), MgO (200), RuAl (200) and TiN (200) due to the small lattice mismatch between FePt films and underlayer/ substrate [9–15]. However, the systematic analyses of mismatch effects on the magnetic properties and microstructure of FePt films have rarely been reported. Therefore, it would be worthwhile to clarify the microstructure and magnetic properties of FePt films with different mismatch. In the present study, FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films were fabricated on MgO and SrTiO3 substrates, and the systematic investigation of the mismatch effect on the microstructure and magnetic properties of FePt films were carried out.
2. Experiments FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates with (100) orientation were deposited using an AJA sputtering system with a base pressure better than 2 10 8 Torr. FePt–SiNx–C films were fabricated by co-sputtering FePt, SiNx and C targets. The Ar working pressure was 10 mTorr for FePt–SiNx–C, and the sputtering temperature was fixed at 380 °C. The crystallographic texture was examined with X-ray diffraction (XRD) using Cu Kα radiation. The microstructure of the films was characterized by transmission electron microscopy (TEM). The morphologies of the samples were examined by scanning electron microscopy (SEM). The magnetic properties were measured by the vibrating sample magnetometry (VSM) at a maximum applied field of 20 kOe at room temperature and the superconduction quantum inference device (SQUID) at a
K.F. Dong et al. / Journal of Magnetism and Magnetic Materials 402 (2016) 124–130
Fig. 1. XRD 2θ spectra of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates.
maximum applied field of 60 kOe at room temperature.
3. Results and discussion 3.1. Effects of SiNx and C doping on FePt films Fig. 1 shows the crystallographic texture of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates. FePt (001), (002) and (003) peaks were observed and no peaks from any other FePt planes were present with MgO and STO substrates, indicating that all the FePt films exhibited good L10 (001) texture. This was due to the epitaxial growth of FePt on the (100) textured MgO and STO single crystal substrates. With SiNx
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and C doping into FePt films, FePt (001), (002) and (003) peaks became weaker, especially for FePt–SiNx–C film with using STO substrate. Moreover, the FePt (001), (002) and (003) peaks shifted to higher angle, indicating the c-lattice constant decreased with SiNx and C doping. This would be caused by the reduce of the strain acting on FePt layer due to the change of microstructure of FePt layer from continuous structure to granular structure or existence of some C atoms in the interface between FePt and the substrates. The out-of-plane and in-plane hysteresis loops of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/ FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C are shown in Fig. 2. The magnetic anisotropy (Ku1) can be approximately calculated from Ku1 ¼Ku1eff þ2π Ms2, where Ku1eff is estimated by integrating the area between the easy-axis loop and the hard-axis loop [16,17]. It can be seen in Fig. 2(a) and (b) that FePt 10 nm film with using MgO and STO substrates exhibited high perpendicular anisotropy (2.08 107 ergs/cc for MgO and 1.82 107 ergs/cc for STO, respectively), with a high out-of-plane coercivity and a low in-plane coercivity. FePt films with using MgO substrate exhibited better perpendicular magnetic anisotropy than that of using STO substrate, which was attributed to the larger strain induced by larger lattice mismatch. With introduced SiNx and C into FePt films, the out-of-plane coercivity of FePt–SiNx–C film with using MgO substrate decreased from 19.29 kOe to 16.72 kOe, and the out-of-plane coercivity of FePt–SiNx–C film with using STO substrate increased dramatically from 2.93 kOe to 23.36 kOe. The Ku1 decreased to 1.91 107 ergs/cc for MgO, and increased to 1.88 107 ergs/cc for STO. This was due to the change of the
Fig. 2. M–H loops of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
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Fig. 3. SEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
microstructure with doping SiNx and C into FePt films. Furthermore, very clear kinks appeared in the out-of-plane hysteresis loop, and the in-plane hysteresis loop showed very large open-up (Fig. 2c and d), indicating the formation of some fcc FePt phase and the increase in contents of in-plane variance. This will be confirmed later by TEM results. It also can be observed the slope at coercivity of FePt films with using MgO substrate decreased slightly from 4.97 to 4.34, and a dramatic reduction from 13.08 to 2.29 could be obtained with using STO substrate. The decrease of the slope of the hysteresis loop implied the decrease in the exchange coupling. It is well-established that the coercivity increased with the decrease of the exchange coupling [18]. Therefore, the increase of the out-of-plane coercivity of FePt–SiNx–C film with using STO substrate would be attributed to the decrease of the exchange coupling. The change of microstructure caused the increase of the in-plane coercivity and a slight decrease of the outof-plane coercivity of FePt–SiNx–C film with using MgO substrate. The evolution of film growth mechanism of FePt films with different substrates were investigated by scanning electron microscopy (SEM). Fig. 3 shows the surface morphologies of of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. Distinct grain boundaries are observed in Fig. 3(a) for FePt 10 nm film with using MgO substrate, which indicated island growth of FePt grains was formed. For FePt 10 nm films with using STO substrate,
continuous FePt film was exhibited (Fig. 3b). Therefore, it can be said that by reducing the lattice constant of the substrate from MgO to STO, the evolution of the growth mechanism was changing from island to continuous film. With introduced SiNx and C into FePt films, FePt grains became smaller and island growth were formed. To further investigate the relationship between the microstructure and the substrate the low magnification and a high resolution cross-sectional TEM were carried out. Fig. 4 shows the low magnification cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. It can be seen that FePt grains were grown on the top of MgO substrates with island growth mode (Fig. 4a), and continuous FePt film was formed on the STO substrate (Fig. 4b). Doping SiNx and C into FePt films resulted in the formation of two layers of well-isolated FePt grains due to excess C diffusing to the surface and causing FePt renucleation. These TEM results are consistent with the results of the SEM results. Fig. 5 shows high resolution cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. For all the four samples, the FePt grains in
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Fig. 4. The low magnification cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
layer 1 with (001) orientation were epitaxially grown on the (200) textured MgO and STO layer and the atomic planes across the MgO/STO and FePt interface matched well with each other. With introduced SiNx and C into FePt films, despite obtaining small FePt grains, the FePt grains began to form double or multiple layers. Except for the (002) orientation FePt grains, some (111) orientation FePt grains were formed. It was reported in our former studies that the L10 FePt particle nucleated on the L10 FePt particles without C coverage, while the fcc FePt particle in layer 2 nucleated on the C coverage, which would then cause the chemical ordering decrease [19]. Moreover, the co-existence of the FePt (111) and (001) phases would produce two variances in the same direction when measured the M–H loops. Adding these two variances together would form kinks in the M–H loops. This is the origin of the kinks in the Fig. 2(c) and (d). In the IFFT image, the (200) lattice planes of FePt and substrate (MgO and STO) were connected together, implying a good epitaxial growth of FePt on (MgO and STO) substrate. In addition, it was found that the dislocations at the interface were formed to release the strain energy. Furthermore, with using MgO substrate more lattice dislocations were formed than that of using STO substrate, which were induced by larger lattice mismatch between FePt and MgO. 3.2. Effects of SiNx and C doping on mismatch strain of FePt films Although (001) textured FePt–SiNx–C films with high coercivity were obtained at the deposition temperature of 380 °C, the two-
layer structure of these films was not desirable for investigating the evolution of the mismatch strain with doping SiNx and C, which was attributed to the mismatch strain would be released during the formation of two-layer structure. In order to investigate the evolution of the mismatch strain, [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates with (100) orientation were fabricated. Fig. 6 shows the crystallographic texture of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates. As compared to the FePt (001) and (002) peaks in Fig. 1, the FePt (001) and (002) peaks shown in Fig. 6 were much weaker, especially for FePt 4 nm-SiNx–C film grown on STO substrate. This was mainly due to the thinner thickness of FePt films and the smaller grain size of FePt grains. The corresponding M–H loops are shown in Fig. 7. As shown in Fig. 7(a), the FePt 4 nm-SiNx–C film grown on MgO substrate exhibited high perpendicular anisotropy Ku1 of 0.62 107 ergs/cc, with a high out-of-plane coercivity of 7.63 kOe and a low in-plane coercivity of 0.32 kOe. The poor (001) texture of FePt 4 nm-SiNx–C film grown on STO substrate caused the deviation of perpendicular anisotropy Ku1 to 0.23 107 ergs/cc (Fig. 7b). This was consistence with the XRD results. Fig. 8 shows SEM and low magnification cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) and (c) MgO/FePt 4 nm-SiNx–C, (b) and (d) STO/FePt 4 nm-SiNx–C. It can be seen that when FePt thickness was 4 nm, very smaller FePt grains could be observed in Fig. 8 (a) and (b). Moreover, well-isolated FePt grains were grown on the
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Fig. 5. High resolution cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images.
Fig. 6. XRD 2θ spectra of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates.
top of MgO and STO substrates with one-layer structure (Fig. 8c and d). To further investigate the evolution of mismatch strain, high resolution cross-sectional TEM was carried out. Fig. 9 reveals high resolution cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/
FePt 4 nm-SiNx–C and (b) STO/FePt 4 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. It showed the matching of the atomic planes across the FePt and substrate interface from the HR-TEM images. The interface was sharp and clear. In the IFFT image, the (200) lattice planes of FePt and substrate (MgO and STO) were connected together, implying a good epitaxial growth of FePt on (MgO and STO) substrates. Furthermore, compared to the IFFT image of FePt films in Fig. 5(a) and (b), it was found that with SiNx and C doping more lattice dislocations was observed, which was due to the formation of more grain boundaries when the microstructure of FePt layer changed from continuous structure to granular structure. This implied that lattice mismatch strain between FePt and MgO/STO increased with doping SiNx and C into FePt films. The lattice constant a and c, as well as in-plane mismatch ε are summarized in Table 1. With SiNx and C doping, lattice constant a and c slightly decreased, and in-plane mismatch ε increased. The different growth mode of FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates were due to the different mismatch strain between FePt and substrate. Chen et al. [20] had proposed that the mismatched film preferred island growth when the film size exceeded the critical size, which is inversely proportional to the mismatch strain ϵ6xx . As mentioned above, the mismatch strain εxx between FePt and MgO, between FePt and STO were 5.8% and 0.1%, respectively. As SiNx and C doping into FePt films, the mismatch strain εxx between
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Fig. 7. M–H loops of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 4 nm-SiNx–C and (b) STO/FePt 4 nm-SiNx–C.
Fig. 8. SEM and low magnification cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) and (c) MgO/FePt 4 nm-SiN x–C, (b) and (d) STO/FePt 4 nm-SiNx–C.
FePt and MgO, and between FePt and STO increased to 6.5% and 3.2%, respectively. Based on Chen's study, the mismatch strain with using MgO substrates would cause FePt films to follow the island growth mode, and the negligible mismatch strain with using STO substrate would form continuous film. Doping SiNx and C into FePt films caused the increase of lattice mismatch strain, which thus induced the formation of island growth. These inferences were well in agreement with the SEM and TEM results.
4. Conclusion The systematic investigation of strain evolution of FePt–SiNx–C films was carried out. The results showed that with increasing the substrate lattice constant from STO (3.905 Å) to MgO (4.216 Å), the microstructure of FePt films changed gradually from continuous film to islands. Doping SiNx and C could effectively tune the lattice mismatch, thus affected the growth mode of FePt films. Moreover,
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Fig. 9. High resolution cross-sectional TEM images of [FePt 4 nm- 40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 4 nm-SiNx–C and (b) STO/ FePt 4 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. Table 1 Summaries of structure parameters and magnetic properties of FePt films grown on different substrates: lattice constant a and c, in-plane mismatch ε¼ (asub aFePt)/asub, as well as perpendicular and in-plane coercivity Hc⊥ and Hc⫽. Films
a (Å) ( 7 5%)
c (Å) ( 7 5%)
ε ( 7 5%) Hc⊥ (kOe) Hc⫽ (kOe)
MgO/FePt 10 nm STO/FePt 10 nm MgO/FePt 4 nmSiNx–C STO/FePt 4 nm-SiN x–C
3.971 3.912 3.941
3.716 3.717 3.698
5.8% 0.18% 6.5%
19.26 2.93 7.67
0.31 0.41 0.32
3.781
3.701
3.2%
3.45
2.17
The perpendicular anisotropy of FePt–SiNx–C films films grown on MgO was larger than that of grown on SrTiO3, which was attributed to the larger strain induced by larger lattice mismatch.
Acknowledgment This work is partially supported by the National Natural Science Foundation of China (Grant no. 51501168, 41574175, 41204083, 61432007 and 51001051), the Key Science and Technology Support Program of Hubei Province (Grant no. 2015BCE054) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG 150632).
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Current Perspectives
Investigation of microstructure and magnetic properties of FePt–X films grown on MgO and STO substrates K.F. Dong a,n, F. Jin a, W.Q. Mo a, J.L. Song a, W.M. Cheng b a b
School of Automation, China University of Geosciences, Wuhan 430074, China School of Optical and Electronic Information, Huazhong University of Science & Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 September 2015 Received in revised form 8 November 2015 Accepted 21 November 2015 Available online 22 November 2015
Different thickness (001) textured FePt–X films were fabricated on MgO and SrTiO3 substrates, and the microstructure and magnetic properties of FePt–X films was systematically investigated. The study showed that the substrates with different lattice mismatch resulted in different crystallographic texture and microstructure. The larger mismatch strain with using MgO substrates would cause FePt films to follow the island growth mode, and the negligible mismatch strain with using STO substrate would form continuous film. Doping SiNx and C into FePt films caused the increase of lattice mismatch strain between FePt and MgO/STO, which would induce FePt films to forming island growth. The perpendicular anisotropy of FePt–SiNx–C films grown on MgO substrate was larger than that of using SrTiO3 substrate, which was attributed to the larger strain induced by larger lattice mismatch. & 2015 Elsevier B.V. All rights reserved.
Keywords: FePt films Lattice mismatch Different substrates
1. Introduction Epitaxial growth of thin films in the early stages is observed to follow primarily three modes [1,2]: (a) planar growth or layer-bylayer growth (Frank-van der Merwe growth); (b) three-dimensional or island growth (Volmer–Weber growth); and (c) planar growth followed by island growth (Stranski–Krastonov growth). In all three types of growth, exploring how the lattice-mismatch affects the epitaxial growth mode is an important issue, as the epitaxial growth mode can dramatically influence the microstructure and magnetic properties of magnetic thin films. Generally, it is accepted that depending on the magnitude of misfit strain ε and the interfacial and surface energies, growth may proceed by layer-by-island [Stranski–Krastonov (SK)] mode, or at a higher ε via direct islanding (Volmer–Weber) growth. Moreover, lattice matched and closely lattice-mismatch systems would induce in a layer-by-layer (Frank-van der Merwe) growth mode [3,4]. L10 ordered FePt alloy with high magnetocrystalline anisotropy (Ku 7 107 erg/cc) and chemical stability is the most promising material to realize the potential of heat assisted magnetic recording (HAMR) [5,6]. For the exploration of practical applications, L10-FePt films with (001) texture and high magnetocrystalline anisotropy have been fabricated by epitaxial growth on singlecrystalline MgO (001) substrates [7,8] or on polycrystalline under n
Corresponding author. E-mail address: [email protected] (K.F. Dong).
http://dx.doi.org/10.1016/j.jmmm.2015.11.060 0304-8853/& 2015 Elsevier B.V. All rights reserved.
layers such as Pt (001), MgO (200), RuAl (200) and TiN (200) due to the small lattice mismatch between FePt films and underlayer/ substrate [9–15]. However, the systematic analyses of mismatch effects on the magnetic properties and microstructure of FePt films have rarely been reported. Therefore, it would be worthwhile to clarify the microstructure and magnetic properties of FePt films with different mismatch. In the present study, FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films were fabricated on MgO and SrTiO3 substrates, and the systematic investigation of the mismatch effect on the microstructure and magnetic properties of FePt films were carried out.
2. Experiments FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates with (100) orientation were deposited using an AJA sputtering system with a base pressure better than 2 10 8 Torr. FePt–SiNx–C films were fabricated by co-sputtering FePt, SiNx and C targets. The Ar working pressure was 10 mTorr for FePt–SiNx–C, and the sputtering temperature was fixed at 380 °C. The crystallographic texture was examined with X-ray diffraction (XRD) using Cu Kα radiation. The microstructure of the films was characterized by transmission electron microscopy (TEM). The morphologies of the samples were examined by scanning electron microscopy (SEM). The magnetic properties were measured by the vibrating sample magnetometry (VSM) at a maximum applied field of 20 kOe at room temperature and the superconduction quantum inference device (SQUID) at a
K.F. Dong et al. / Journal of Magnetism and Magnetic Materials 402 (2016) 124–130
Fig. 1. XRD 2θ spectra of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates.
maximum applied field of 60 kOe at room temperature.
3. Results and discussion 3.1. Effects of SiNx and C doping on FePt films Fig. 1 shows the crystallographic texture of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates. FePt (001), (002) and (003) peaks were observed and no peaks from any other FePt planes were present with MgO and STO substrates, indicating that all the FePt films exhibited good L10 (001) texture. This was due to the epitaxial growth of FePt on the (100) textured MgO and STO single crystal substrates. With SiNx
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and C doping into FePt films, FePt (001), (002) and (003) peaks became weaker, especially for FePt–SiNx–C film with using STO substrate. Moreover, the FePt (001), (002) and (003) peaks shifted to higher angle, indicating the c-lattice constant decreased with SiNx and C doping. This would be caused by the reduce of the strain acting on FePt layer due to the change of microstructure of FePt layer from continuous structure to granular structure or existence of some C atoms in the interface between FePt and the substrates. The out-of-plane and in-plane hysteresis loops of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/ FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C are shown in Fig. 2. The magnetic anisotropy (Ku1) can be approximately calculated from Ku1 ¼Ku1eff þ2π Ms2, where Ku1eff is estimated by integrating the area between the easy-axis loop and the hard-axis loop [16,17]. It can be seen in Fig. 2(a) and (b) that FePt 10 nm film with using MgO and STO substrates exhibited high perpendicular anisotropy (2.08 107 ergs/cc for MgO and 1.82 107 ergs/cc for STO, respectively), with a high out-of-plane coercivity and a low in-plane coercivity. FePt films with using MgO substrate exhibited better perpendicular magnetic anisotropy than that of using STO substrate, which was attributed to the larger strain induced by larger lattice mismatch. With introduced SiNx and C into FePt films, the out-of-plane coercivity of FePt–SiNx–C film with using MgO substrate decreased from 19.29 kOe to 16.72 kOe, and the out-of-plane coercivity of FePt–SiNx–C film with using STO substrate increased dramatically from 2.93 kOe to 23.36 kOe. The Ku1 decreased to 1.91 107 ergs/cc for MgO, and increased to 1.88 107 ergs/cc for STO. This was due to the change of the
Fig. 2. M–H loops of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
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Fig. 3. SEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
microstructure with doping SiNx and C into FePt films. Furthermore, very clear kinks appeared in the out-of-plane hysteresis loop, and the in-plane hysteresis loop showed very large open-up (Fig. 2c and d), indicating the formation of some fcc FePt phase and the increase in contents of in-plane variance. This will be confirmed later by TEM results. It also can be observed the slope at coercivity of FePt films with using MgO substrate decreased slightly from 4.97 to 4.34, and a dramatic reduction from 13.08 to 2.29 could be obtained with using STO substrate. The decrease of the slope of the hysteresis loop implied the decrease in the exchange coupling. It is well-established that the coercivity increased with the decrease of the exchange coupling [18]. Therefore, the increase of the out-of-plane coercivity of FePt–SiNx–C film with using STO substrate would be attributed to the decrease of the exchange coupling. The change of microstructure caused the increase of the in-plane coercivity and a slight decrease of the outof-plane coercivity of FePt–SiNx–C film with using MgO substrate. The evolution of film growth mechanism of FePt films with different substrates were investigated by scanning electron microscopy (SEM). Fig. 3 shows the surface morphologies of of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. Distinct grain boundaries are observed in Fig. 3(a) for FePt 10 nm film with using MgO substrate, which indicated island growth of FePt grains was formed. For FePt 10 nm films with using STO substrate,
continuous FePt film was exhibited (Fig. 3b). Therefore, it can be said that by reducing the lattice constant of the substrate from MgO to STO, the evolution of the growth mechanism was changing from island to continuous film. With introduced SiNx and C into FePt films, FePt grains became smaller and island growth were formed. To further investigate the relationship between the microstructure and the substrate the low magnification and a high resolution cross-sectional TEM were carried out. Fig. 4 shows the low magnification cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. It can be seen that FePt grains were grown on the top of MgO substrates with island growth mode (Fig. 4a), and continuous FePt film was formed on the STO substrate (Fig. 4b). Doping SiNx and C into FePt films resulted in the formation of two layers of well-isolated FePt grains due to excess C diffusing to the surface and causing FePt renucleation. These TEM results are consistent with the results of the SEM results. Fig. 5 shows high resolution cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. For all the four samples, the FePt grains in
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Fig. 4. The low magnification cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C.
layer 1 with (001) orientation were epitaxially grown on the (200) textured MgO and STO layer and the atomic planes across the MgO/STO and FePt interface matched well with each other. With introduced SiNx and C into FePt films, despite obtaining small FePt grains, the FePt grains began to form double or multiple layers. Except for the (002) orientation FePt grains, some (111) orientation FePt grains were formed. It was reported in our former studies that the L10 FePt particle nucleated on the L10 FePt particles without C coverage, while the fcc FePt particle in layer 2 nucleated on the C coverage, which would then cause the chemical ordering decrease [19]. Moreover, the co-existence of the FePt (111) and (001) phases would produce two variances in the same direction when measured the M–H loops. Adding these two variances together would form kinks in the M–H loops. This is the origin of the kinks in the Fig. 2(c) and (d). In the IFFT image, the (200) lattice planes of FePt and substrate (MgO and STO) were connected together, implying a good epitaxial growth of FePt on (MgO and STO) substrate. In addition, it was found that the dislocations at the interface were formed to release the strain energy. Furthermore, with using MgO substrate more lattice dislocations were formed than that of using STO substrate, which were induced by larger lattice mismatch between FePt and MgO. 3.2. Effects of SiNx and C doping on mismatch strain of FePt films Although (001) textured FePt–SiNx–C films with high coercivity were obtained at the deposition temperature of 380 °C, the two-
layer structure of these films was not desirable for investigating the evolution of the mismatch strain with doping SiNx and C, which was attributed to the mismatch strain would be released during the formation of two-layer structure. In order to investigate the evolution of the mismatch strain, [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates with (100) orientation were fabricated. Fig. 6 shows the crystallographic texture of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates. As compared to the FePt (001) and (002) peaks in Fig. 1, the FePt (001) and (002) peaks shown in Fig. 6 were much weaker, especially for FePt 4 nm-SiNx–C film grown on STO substrate. This was mainly due to the thinner thickness of FePt films and the smaller grain size of FePt grains. The corresponding M–H loops are shown in Fig. 7. As shown in Fig. 7(a), the FePt 4 nm-SiNx–C film grown on MgO substrate exhibited high perpendicular anisotropy Ku1 of 0.62 107 ergs/cc, with a high out-of-plane coercivity of 7.63 kOe and a low in-plane coercivity of 0.32 kOe. The poor (001) texture of FePt 4 nm-SiNx–C film grown on STO substrate caused the deviation of perpendicular anisotropy Ku1 to 0.23 107 ergs/cc (Fig. 7b). This was consistence with the XRD results. Fig. 8 shows SEM and low magnification cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) and (c) MgO/FePt 4 nm-SiNx–C, (b) and (d) STO/FePt 4 nm-SiNx–C. It can be seen that when FePt thickness was 4 nm, very smaller FePt grains could be observed in Fig. 8 (a) and (b). Moreover, well-isolated FePt grains were grown on the
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Fig. 5. High resolution cross-sectional TEM images of FePt 10 nm and [FePt 10 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 10 nm, (b) STO/FePt 10 nm, (c) MgO/FePt 10 nm-SiNx–C and (d) STO/FePt 10 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images.
Fig. 6. XRD 2θ spectra of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates.
top of MgO and STO substrates with one-layer structure (Fig. 8c and d). To further investigate the evolution of mismatch strain, high resolution cross-sectional TEM was carried out. Fig. 9 reveals high resolution cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/
FePt 4 nm-SiNx–C and (b) STO/FePt 4 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. It showed the matching of the atomic planes across the FePt and substrate interface from the HR-TEM images. The interface was sharp and clear. In the IFFT image, the (200) lattice planes of FePt and substrate (MgO and STO) were connected together, implying a good epitaxial growth of FePt on (MgO and STO) substrates. Furthermore, compared to the IFFT image of FePt films in Fig. 5(a) and (b), it was found that with SiNx and C doping more lattice dislocations was observed, which was due to the formation of more grain boundaries when the microstructure of FePt layer changed from continuous structure to granular structure. This implied that lattice mismatch strain between FePt and MgO/STO increased with doping SiNx and C into FePt films. The lattice constant a and c, as well as in-plane mismatch ε are summarized in Table 1. With SiNx and C doping, lattice constant a and c slightly decreased, and in-plane mismatch ε increased. The different growth mode of FePt 10 nm and [FePt (10 and 4 nm)-40 vol% SiNx]-20 vol% C films grown on MgO and SrTiO3 substrates were due to the different mismatch strain between FePt and substrate. Chen et al. [20] had proposed that the mismatched film preferred island growth when the film size exceeded the critical size, which is inversely proportional to the mismatch strain ϵ6xx . As mentioned above, the mismatch strain εxx between FePt and MgO, between FePt and STO were 5.8% and 0.1%, respectively. As SiNx and C doping into FePt films, the mismatch strain εxx between
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Fig. 7. M–H loops of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 4 nm-SiNx–C and (b) STO/FePt 4 nm-SiNx–C.
Fig. 8. SEM and low magnification cross-sectional TEM images of [FePt 4 nm-40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) and (c) MgO/FePt 4 nm-SiN x–C, (b) and (d) STO/FePt 4 nm-SiNx–C.
FePt and MgO, and between FePt and STO increased to 6.5% and 3.2%, respectively. Based on Chen's study, the mismatch strain with using MgO substrates would cause FePt films to follow the island growth mode, and the negligible mismatch strain with using STO substrate would form continuous film. Doping SiNx and C into FePt films caused the increase of lattice mismatch strain, which thus induced the formation of island growth. These inferences were well in agreement with the SEM and TEM results.
4. Conclusion The systematic investigation of strain evolution of FePt–SiNx–C films was carried out. The results showed that with increasing the substrate lattice constant from STO (3.905 Å) to MgO (4.216 Å), the microstructure of FePt films changed gradually from continuous film to islands. Doping SiNx and C could effectively tune the lattice mismatch, thus affected the growth mode of FePt films. Moreover,
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Fig. 9. High resolution cross-sectional TEM images of [FePt 4 nm- 40 vol% SiNx]-20 vol% C films grown on MgO and STO substrates, (a) MgO/FePt 4 nm-SiNx–C and (b) STO/ FePt 4 nm-SiNx–C. The insets in (a) and (b) are corresponding inverse fast Fourier transform (IFFT) images. Table 1 Summaries of structure parameters and magnetic properties of FePt films grown on different substrates: lattice constant a and c, in-plane mismatch ε¼ (asub aFePt)/asub, as well as perpendicular and in-plane coercivity Hc⊥ and Hc⫽. Films
a (Å) ( 7 5%)
c (Å) ( 7 5%)
ε ( 7 5%) Hc⊥ (kOe) Hc⫽ (kOe)
MgO/FePt 10 nm STO/FePt 10 nm MgO/FePt 4 nmSiNx–C STO/FePt 4 nm-SiN x–C
3.971 3.912 3.941
3.716 3.717 3.698
5.8% 0.18% 6.5%
19.26 2.93 7.67
0.31 0.41 0.32
3.781
3.701
3.2%
3.45
2.17
The perpendicular anisotropy of FePt–SiNx–C films films grown on MgO was larger than that of grown on SrTiO3, which was attributed to the larger strain induced by larger lattice mismatch.
Acknowledgment This work is partially supported by the National Natural Science Foundation of China (Grant no. 51501168, 41574175, 41204083, 61432007 and 51001051), the Key Science and Technology Support Program of Hubei Province (Grant no. 2015BCE054) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG 150632).
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