- Email: [email protected]
Controlling the crystallographic orientation and easy axis of magnetic anisotropy in L10 FePt films with Cu additive
Surface & Coatings Technology 198 (2005) 270 – 273 www.elsevier.com/locate/surfcoat
Controlling the crystallographic orientation and easy axis of magnetic anisotropy in L10 FePt films with Cu additive Y.F. Dinga, J.S. Chenb, E. Liua,* a
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Data Storage Institute, DSI building, 5 Engineering Drive 1, Singapore 11760, Singapore Available online 2 December 2004
Abstract By co-sputtering Cu and FePt onto Cr90Mo10 underlayers, the preferred orientation and easy axis of magnetic anisotropy of FePt films were successfully changed from the perpendicular to the in-plane direction. The pure FePt film grown on the Cr90Mo10 underlayer showed a (001) preferred orientation with out-of-plane magnetic anisotropy. As 20 and 40 vol.% Cu were doped, the FePt films showed a (200) preferred orientation with in-plane magnetic anisotropy. The pure FePt film showed a continuous microstructure, while the Cu-doped FePt films showed a mixture of particle-like and continuous microstructures. The change of the preferred orientation in the FePt films by Cu doping might be due to the competition of grain-boundary energy, surface free energy and epitaxial-strain energy. The experimental results suggest that the Cu doping be a promising method for the fabrication of the in-plane oriented FePt films. The Cu-doped FePt films had a lower Ms than the pure FePt film. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 75.50.Ss Keywords: Co-sputtering; FePt alloy; Preferred orientation; Magnetic anisotropy
1. Introduction FePt thin films have drawn considerable attention as a potential high-density magnetic recording material, due to a large magnetocrystalline anisotropy of ordered tetragonal L10 FePt phase. In-plane magnetized FePt films are also needed for novel applications such as bias magnets in monolithic microwave-integrated circuits and exchange spring magnets with a large energy product. However, as-deposited FePt films tend to have a (111) preferred orientation. In view of the technological interest in L10 FePt films for above applications, there is a need to control the orientation of the c-axis. Various underlayers or substrates, such as MgO [1–4], Ag [5], CrRu [6], and oxidized Si [7] have been successfully used to promote the L10 FePt (001) texture in the case of perpendicular
* Corresponding author. Tel.: +65 6790 5504; fax: +65 6791 1859. E-mail address: [email protected] (E. Liu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.080
magnetic recording. Substrates such as MgO (110) [8], Cu (100) [9] and SrTiO3 (110) [10] single-crystal substrates have been used to promote the L10 FePt (220) or FePt (200) texture. From a practical point of view, single-crystal substrates are expensive which limit their applications. In a previous work, we reported that the preferred orientation and easy axis of magnetic anisotropy of FePt films could be controlled with Cr90Mo10 underlayer [11] at relatively low temperatures. In this study, we show that the crystallographic orientation and easy axis of magnetic anisotropy in L10 FePt films can be controlled with Cu additive.
2. Experimental Films with a structure of (FePt)1 x Cux (20 nm)/Pt (4 nm)/Cr90Mo10 (30 nm)/Glass substrate were prepared by DC magnetron sputtering. The base pressure was lower than 110 8 Torr. The deposition temperature for all the
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
271
layers was fixed at 350 8C. The targets used were Cr90Mo10 and Fe50Pt50 alloys, and pure Pt and Cu. The substrate holder was rotated at approximately 40 rpm to ensure the film uniformity during deposition. The working gas pressures were 3 mTorr for the Cr90Mo10 underlayer, and 10 mTorr for the Pt and (FePt)1 x Cux layers, respectively. The Cu contents in the films were 0, 20 and 40 vol.% achieved by varying the sputtering power applied on the Cu target, whereas keeping that for FePt fixed, which corresponded to the Cu atomic ratios of 0%, 8% and 20%, respectively, in the films, as determined by X-ray photoelectron spectroscopy (XPS) (ULTRA, Kratos). The thicknesses of (FePt)1 x Cux layers (x=vol.%) and Cr90Mo10 layers were fixed at 20 and 30 nm, respectively. A 4-nm thick Pt buffer layer between Cr90Mo10 underlayers and FePt magnetic layers was used to suppress the diffusion of Cr into the FePt layer [12]. The magnetic properties were measured using vibrating sample magnetometer (VSM). The film crystallographic texture was measured with high-resolution X-ray diffraction (HRXRD) (Philips X’Pert) using a Cu K a radiation. The microstructural properties of the films were analyzed by transmission electron microscopy (TEM) (JEOL-2010).
3. Results and discussion Fig. 1 shows the XRD 2h scans of the FePt thin films deposited on the Cr90Mo10 underlayers with various Cu contents. The pure FePt film depicts a (001) preferred orientation (Spectrum a). With 20 vol.% Cu, the FePt (001) peak is weakened drastically, and the peak between 458 and 508 becomes dominant (Spectrum b) and shifts to a lower angle. It is well known that the three diffraction peaks of L10 FePt (002), (200) and fcc FePt (200) locate at 48.978, 47.358 and 47.668, respectively. The weak FePt (001) peak (near 23.98), the peak near
Fig. 1. XRD spectra of the FePt films grown on Cr90Mo10 underlayers (a) without Cu doping, (b) with 20 vol.% Cu, and (c) with 40 vol.% Cu.
Fig. 2. Out-of-plane and in-plane hysteresis loops of FePt films grown on Cr90Mo10 underlayers with various Cu contents.
47.358 of Spectrum b and the high in-plane coercivity value of the film (N3300 Oe) (as shown in Fig. 4) suggest that the FePt (200) peak dominate. When 40 vol.% Cu is doped, the FePt (001) peak almost disappears. In addition, the peak between 458 and 508 shifts to a higher angle at 47.68 (Spectrum c), which depicts a smaller lattice constant of the FePt film. The absence of the FePt (001) peak and the smaller lattice constant illustrate the formation of the FePtCu alloy [13]. The XRD spectra show that the preferred orientations of the FePt films have been changed from FePt (001) to FePt (200) as 20 and 40 vol.% Cu are doped. The changes of the preferred orientation in the FePt films are also reflected by their hysteresis loops. Fig. 2 shows both the in-plane and the out-of-plane hysteresis loops of all samples. The pure FePt film shows an out-ofplane magnetic anisotropy. As Cu is doped, the films show an in-plane magnetic anisotropy. The hysteresis loops illustrate that the magnetic easy axis of the FePt films has been successfully changed from out-of-plane to
272
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
in-plane when Cu is added. The variation of magnetic easy axis with Cu content is consistent with the evolution of the crystallographic orientation for the FePt films as reflected by the XRD spectra in Fig. 1. In order to study the microstructural properties of the Cu-doped FePt films and the mechanism of the change of preferred orientation in the FePt films with Cu doping, the TEM bright-field images and corresponding selected area electron diffraction (SAED) patterns of the films are taken (Fig. 3). The image of the pure FePt film does not reveal the clear grain boundaries, and the grains are found to have formed a continuous microstructure. As 20 and 40 vol.% Cu are doped, the films show a mixture of particle-like and continuous microstructures. Combining the XRD results as discussed above with the TEM results, we hypothesize that the continuous microstructure helps keep the epitaxial strain energy along the film normal direction and induce the growth of the FePt films with a (001) preferred orientation, while the particle-like microstructure releases the strain energy. Hence, the FePt film is not contracted and does not
Fig. 3. TEM bright field images and corresponding selected area diffraction patterns of FePt films grown on Cr90Mo10 underlayers with various Cu contents.
Fig. 4. In-plane and out-of-plane coercivities and saturation magnetization (Ms) as a function of Cu content.
form the (001)-oriented L10 grains when Cu is doped. The films with Cu doped show a (200) preferred orientation rather than any other orientation such as (111) which is the most close-packed plane of both the fcc and the fct phases. It might be explained by the competition of the grain-boundary energy, surface free energy and epitaxial-strain energy [12]. In this case, the grain boundary energy may be most important with Cu doping. However, the mechanism needs to be further understood. From the SAED pattern of pure FePt film, a sharp superlattice (110) reflection which results from the (001)oriented grains is observed, indicating that the film is well ordered. The appearance of the FePt (110) reflection and the absence of the FePt (001) reflection which results from (100)- or (010)-oriented FePt grains indicate that the c-axis (magnetic easy axis) is perpendicular to the film plane. As 20 vol.% Cu is doped, the weak (110) and (100) superlattice reflections indicate a lower ordering degree. When 40 vol.% Cu is doped, only the fundamental FePt reflections are observed, indicating a further decrease of the ordering degree of the film. The in-plane and out-of-plane coercivities (Hc) and saturation magnetization (Ms) as a function of Cu content are shown in Fig. 4. The saturation magnetization of the FePt films decreases with increased Cu content. It is well known that the Ms in FePt film is from the Fe atoms. Thus, the reduction of the Ms should be due to the reduction of the Fe volume fraction in the Cu-doped FePt films. The in-plane coercivity increases whereas the out-of-plane one decreases with increasing Cu content from 0 to 20 vol.%, which confirms the switch of magnetic easy axis from the perpendicular to the longitudinal direction. Both the in-plane and the outof-plane coercivities decrease with increasing Cu content from 20 to 40 vol.%, which may be due to the lowered ordering degree of the FePt film containing 40 vol.% Cu.
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
273
4. Conclusions
Acknowledgement
The microstructure and magnetic properties of Cudoped FePt films were studied with the following conclusions:
One of the authors, Y.F. Ding, would like to thank Nanyang Technological University (Singapore) for providing the scholarship for this research.
(1) The pure FePt film grown on the Cr90Mo10 underlayer showed a (001) preferred orientation with out-of-plane magnetic anisotropy. When 20 and 40 vol.% Cu were doped, the FePt films illustrated a (200) preferred orientation with inplane magnetic anisotropy. (2) The pure FePt film had a continuous microstructure, while the Cu-doped FePt films got a mixture of particle-like and continuous microstructures. The change of the preferred orientation in the FePt films caused by Cu doping might be due to the competition between grain-boundary energy, surface free energy, and epitaxial-strain energy. (3) The Cu-doped FePt films had a lower Ms value than that of the pure FePt films. The experimental results suggest that Cu additive be a promising method for fabrication of in-plane oriented FePt films.
References [1] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [2] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [3] M.G. Kim, S.C. Shin, K. Kang, Appl. Phys. Lett. 80 (2002) 3802. [4] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [5] Y.-N. Hsu, S. Jeong, D. Laughlin, D.N. Lambeth, J. Appl. Phys. 89 (2001) 7068. [6] Y. Xu, J.S. Chen, J.P. Wang, Appl. Phys. Lett. 80 (2002) 3325. [7] H. Zeng, M.L. Yan, N. Powers, D.J. Sellmyer, Appl. Phys. Lett. 80 (2002) 2350. [8] T. Shima, T. Seki, K. Takanashi, Y.K. Takahashi, K. Hono, J. Magn. Magn. Mater. 272–276 (2004) 557. [9] C.J. Sun, G.M. Chow, J.P. Wang, Appl. Phys. Lett. 82 (2003) 1902. [10] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (8) (1996) 5967. [11] Y.F. Ding, J.S. Chen, E. Liu, J. Magn. Magn. Mater. (in press). [12] J.S. Chen, Y.F. Xu, J.P. Wang, J. Appl. Phys. 93 (2003) 1661. [13] T. Maeda, T. Kai, A. Kikitsu, T. Nagasa, J. Akiyama, Appl. Phys. Lett. 80 (2002) 2147.
Controlling the crystallographic orientation and easy axis of magnetic anisotropy in L10 FePt films with Cu additive Y.F. Dinga, J.S. Chenb, E. Liua,* a
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b Data Storage Institute, DSI building, 5 Engineering Drive 1, Singapore 11760, Singapore Available online 2 December 2004
Abstract By co-sputtering Cu and FePt onto Cr90Mo10 underlayers, the preferred orientation and easy axis of magnetic anisotropy of FePt films were successfully changed from the perpendicular to the in-plane direction. The pure FePt film grown on the Cr90Mo10 underlayer showed a (001) preferred orientation with out-of-plane magnetic anisotropy. As 20 and 40 vol.% Cu were doped, the FePt films showed a (200) preferred orientation with in-plane magnetic anisotropy. The pure FePt film showed a continuous microstructure, while the Cu-doped FePt films showed a mixture of particle-like and continuous microstructures. The change of the preferred orientation in the FePt films by Cu doping might be due to the competition of grain-boundary energy, surface free energy and epitaxial-strain energy. The experimental results suggest that the Cu doping be a promising method for the fabrication of the in-plane oriented FePt films. The Cu-doped FePt films had a lower Ms than the pure FePt film. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 75.50.Ss Keywords: Co-sputtering; FePt alloy; Preferred orientation; Magnetic anisotropy
1. Introduction FePt thin films have drawn considerable attention as a potential high-density magnetic recording material, due to a large magnetocrystalline anisotropy of ordered tetragonal L10 FePt phase. In-plane magnetized FePt films are also needed for novel applications such as bias magnets in monolithic microwave-integrated circuits and exchange spring magnets with a large energy product. However, as-deposited FePt films tend to have a (111) preferred orientation. In view of the technological interest in L10 FePt films for above applications, there is a need to control the orientation of the c-axis. Various underlayers or substrates, such as MgO [1–4], Ag [5], CrRu [6], and oxidized Si [7] have been successfully used to promote the L10 FePt (001) texture in the case of perpendicular
* Corresponding author. Tel.: +65 6790 5504; fax: +65 6791 1859. E-mail address: [email protected] (E. Liu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.080
magnetic recording. Substrates such as MgO (110) [8], Cu (100) [9] and SrTiO3 (110) [10] single-crystal substrates have been used to promote the L10 FePt (220) or FePt (200) texture. From a practical point of view, single-crystal substrates are expensive which limit their applications. In a previous work, we reported that the preferred orientation and easy axis of magnetic anisotropy of FePt films could be controlled with Cr90Mo10 underlayer [11] at relatively low temperatures. In this study, we show that the crystallographic orientation and easy axis of magnetic anisotropy in L10 FePt films can be controlled with Cu additive.
2. Experimental Films with a structure of (FePt)1 x Cux (20 nm)/Pt (4 nm)/Cr90Mo10 (30 nm)/Glass substrate were prepared by DC magnetron sputtering. The base pressure was lower than 110 8 Torr. The deposition temperature for all the
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
271
layers was fixed at 350 8C. The targets used were Cr90Mo10 and Fe50Pt50 alloys, and pure Pt and Cu. The substrate holder was rotated at approximately 40 rpm to ensure the film uniformity during deposition. The working gas pressures were 3 mTorr for the Cr90Mo10 underlayer, and 10 mTorr for the Pt and (FePt)1 x Cux layers, respectively. The Cu contents in the films were 0, 20 and 40 vol.% achieved by varying the sputtering power applied on the Cu target, whereas keeping that for FePt fixed, which corresponded to the Cu atomic ratios of 0%, 8% and 20%, respectively, in the films, as determined by X-ray photoelectron spectroscopy (XPS) (ULTRA, Kratos). The thicknesses of (FePt)1 x Cux layers (x=vol.%) and Cr90Mo10 layers were fixed at 20 and 30 nm, respectively. A 4-nm thick Pt buffer layer between Cr90Mo10 underlayers and FePt magnetic layers was used to suppress the diffusion of Cr into the FePt layer [12]. The magnetic properties were measured using vibrating sample magnetometer (VSM). The film crystallographic texture was measured with high-resolution X-ray diffraction (HRXRD) (Philips X’Pert) using a Cu K a radiation. The microstructural properties of the films were analyzed by transmission electron microscopy (TEM) (JEOL-2010).
3. Results and discussion Fig. 1 shows the XRD 2h scans of the FePt thin films deposited on the Cr90Mo10 underlayers with various Cu contents. The pure FePt film depicts a (001) preferred orientation (Spectrum a). With 20 vol.% Cu, the FePt (001) peak is weakened drastically, and the peak between 458 and 508 becomes dominant (Spectrum b) and shifts to a lower angle. It is well known that the three diffraction peaks of L10 FePt (002), (200) and fcc FePt (200) locate at 48.978, 47.358 and 47.668, respectively. The weak FePt (001) peak (near 23.98), the peak near
Fig. 1. XRD spectra of the FePt films grown on Cr90Mo10 underlayers (a) without Cu doping, (b) with 20 vol.% Cu, and (c) with 40 vol.% Cu.
Fig. 2. Out-of-plane and in-plane hysteresis loops of FePt films grown on Cr90Mo10 underlayers with various Cu contents.
47.358 of Spectrum b and the high in-plane coercivity value of the film (N3300 Oe) (as shown in Fig. 4) suggest that the FePt (200) peak dominate. When 40 vol.% Cu is doped, the FePt (001) peak almost disappears. In addition, the peak between 458 and 508 shifts to a higher angle at 47.68 (Spectrum c), which depicts a smaller lattice constant of the FePt film. The absence of the FePt (001) peak and the smaller lattice constant illustrate the formation of the FePtCu alloy [13]. The XRD spectra show that the preferred orientations of the FePt films have been changed from FePt (001) to FePt (200) as 20 and 40 vol.% Cu are doped. The changes of the preferred orientation in the FePt films are also reflected by their hysteresis loops. Fig. 2 shows both the in-plane and the out-of-plane hysteresis loops of all samples. The pure FePt film shows an out-ofplane magnetic anisotropy. As Cu is doped, the films show an in-plane magnetic anisotropy. The hysteresis loops illustrate that the magnetic easy axis of the FePt films has been successfully changed from out-of-plane to
272
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
in-plane when Cu is added. The variation of magnetic easy axis with Cu content is consistent with the evolution of the crystallographic orientation for the FePt films as reflected by the XRD spectra in Fig. 1. In order to study the microstructural properties of the Cu-doped FePt films and the mechanism of the change of preferred orientation in the FePt films with Cu doping, the TEM bright-field images and corresponding selected area electron diffraction (SAED) patterns of the films are taken (Fig. 3). The image of the pure FePt film does not reveal the clear grain boundaries, and the grains are found to have formed a continuous microstructure. As 20 and 40 vol.% Cu are doped, the films show a mixture of particle-like and continuous microstructures. Combining the XRD results as discussed above with the TEM results, we hypothesize that the continuous microstructure helps keep the epitaxial strain energy along the film normal direction and induce the growth of the FePt films with a (001) preferred orientation, while the particle-like microstructure releases the strain energy. Hence, the FePt film is not contracted and does not
Fig. 3. TEM bright field images and corresponding selected area diffraction patterns of FePt films grown on Cr90Mo10 underlayers with various Cu contents.
Fig. 4. In-plane and out-of-plane coercivities and saturation magnetization (Ms) as a function of Cu content.
form the (001)-oriented L10 grains when Cu is doped. The films with Cu doped show a (200) preferred orientation rather than any other orientation such as (111) which is the most close-packed plane of both the fcc and the fct phases. It might be explained by the competition of the grain-boundary energy, surface free energy and epitaxial-strain energy [12]. In this case, the grain boundary energy may be most important with Cu doping. However, the mechanism needs to be further understood. From the SAED pattern of pure FePt film, a sharp superlattice (110) reflection which results from the (001)oriented grains is observed, indicating that the film is well ordered. The appearance of the FePt (110) reflection and the absence of the FePt (001) reflection which results from (100)- or (010)-oriented FePt grains indicate that the c-axis (magnetic easy axis) is perpendicular to the film plane. As 20 vol.% Cu is doped, the weak (110) and (100) superlattice reflections indicate a lower ordering degree. When 40 vol.% Cu is doped, only the fundamental FePt reflections are observed, indicating a further decrease of the ordering degree of the film. The in-plane and out-of-plane coercivities (Hc) and saturation magnetization (Ms) as a function of Cu content are shown in Fig. 4. The saturation magnetization of the FePt films decreases with increased Cu content. It is well known that the Ms in FePt film is from the Fe atoms. Thus, the reduction of the Ms should be due to the reduction of the Fe volume fraction in the Cu-doped FePt films. The in-plane coercivity increases whereas the out-of-plane one decreases with increasing Cu content from 0 to 20 vol.%, which confirms the switch of magnetic easy axis from the perpendicular to the longitudinal direction. Both the in-plane and the outof-plane coercivities decrease with increasing Cu content from 20 to 40 vol.%, which may be due to the lowered ordering degree of the FePt film containing 40 vol.% Cu.
Y.F. Ding et al. / Surface & Coatings Technology 198 (2005) 270–273
273
4. Conclusions
Acknowledgement
The microstructure and magnetic properties of Cudoped FePt films were studied with the following conclusions:
One of the authors, Y.F. Ding, would like to thank Nanyang Technological University (Singapore) for providing the scholarship for this research.
(1) The pure FePt film grown on the Cr90Mo10 underlayer showed a (001) preferred orientation with out-of-plane magnetic anisotropy. When 20 and 40 vol.% Cu were doped, the FePt films illustrated a (200) preferred orientation with inplane magnetic anisotropy. (2) The pure FePt film had a continuous microstructure, while the Cu-doped FePt films got a mixture of particle-like and continuous microstructures. The change of the preferred orientation in the FePt films caused by Cu doping might be due to the competition between grain-boundary energy, surface free energy, and epitaxial-strain energy. (3) The Cu-doped FePt films had a lower Ms value than that of the pure FePt films. The experimental results suggest that Cu additive be a promising method for fabrication of in-plane oriented FePt films.
References [1] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [2] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [3] M.G. Kim, S.C. Shin, K. Kang, Appl. Phys. Lett. 80 (2002) 3802. [4] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [5] Y.-N. Hsu, S. Jeong, D. Laughlin, D.N. Lambeth, J. Appl. Phys. 89 (2001) 7068. [6] Y. Xu, J.S. Chen, J.P. Wang, Appl. Phys. Lett. 80 (2002) 3325. [7] H. Zeng, M.L. Yan, N. Powers, D.J. Sellmyer, Appl. Phys. Lett. 80 (2002) 2350. [8] T. Shima, T. Seki, K. Takanashi, Y.K. Takahashi, K. Hono, J. Magn. Magn. Mater. 272–276 (2004) 557. [9] C.J. Sun, G.M. Chow, J.P. Wang, Appl. Phys. Lett. 82 (2003) 1902. [10] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (8) (1996) 5967. [11] Y.F. Ding, J.S. Chen, E. Liu, J. Magn. Magn. Mater. (in press). [12] J.S. Chen, Y.F. Xu, J.P. Wang, J. Appl. Phys. 93 (2003) 1661. [13] T. Maeda, T. Kai, A. Kikitsu, T. Nagasa, J. Akiyama, Appl. Phys. Lett. 80 (2002) 2147.