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Magnetic anisotropy of L10-FePt film on (001) LaAlO3
Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic anisotropy of L10-FePt film on (001) LaAlO3 A.M. Zhang a,b, W.H. Zhu a, L. Zheng b, L. Huang b, J.L. Gao b, S.L. Tang b, X.S. Wu b,n a b
College of Science, Hohai University, Nanjing 210098, China National Lab of Solid State of Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 4 May 2012 Received in revised form 6 November 2012 Available online 13 December 2012
L10-FePt films are fabricated layer by layer using the magnetic sputtering technique at different growth temperatures. Structure, ordering process and magnetic properties of the films are studied. The film is partly ordered at the growth temperature of 300 1C, and reaches a completely ordered state at deposition temperature of 400 1C. With increasing the substrate temperature from 400 1C to 500 1C, the out-of-plane coercivity increases dramatically from 5.69 KOe to 16.3 KOe, and the magnetic anisotropy Ku increases from 2.0 107 erg/cm3 to 4.5 107 erg/cm3. The surface structure of the film varies with the growth temperature. We attribute the temperature dependence of magnetic properties to the decreased tetragonal distortion at high temperature. & 2012 Elsevier B.V. All rights reserved.
Keywords: L10-FePt film Magnetic anisotropy Lattice strain
1. Introduction L10-FePt is a material of considerable interest for very high density recording media owing to its high magnetocrystalline anisotropy [1–3]. Researchers have paid much attention on the physical and technical aspects for the application of L10-FePt films. Much effort has been made to obtain the L10-FePt film with well (001)-oriented texture at low temperature. Lattice strain, which may be introduced by doping the third atom, growing films on substrates or buffers, surrounded by oxide materials, etc., is usually thought to be one of the favorable factors of accelerating the chemical ordering of FePt alloy [4–8]. Bulk L10-FePt crystallizes in tetragonal symmetry with the lattice parameters of ao ¼3.852 A˚ and co ¼3.728 A˚ [9]. Tension strain resulted from the lattice mismatch between the film and the substrate may induce L10-FePt film growing along the direction of [001] at low temperature [3,5]. Lots of substrates and buffer layers, such as MgO, Pt, and Au have been used. Based on the configuration of the unit cell structure of L10-FePt alloy with the alternating Fe and Pt layer, together with stimulation of the monatomic layer deposition method of Shima et al. [10], multilayer deposition technique is widely used to prepare (001)-oriented L10-FePt films on glass and amorphous SiO2 capped Si substrate [11]. There are little attempt to deposit FePt films directly on a substrate with lattice parameter as less than ao, by multilayer deposition technique [11]. LaAlO3 (LAO) is usually considered to have a pseudo-cubic ˚ which is smaller structure with crystal constant of as ¼ 3.789 A, than that of L10-FePt, ao. Ordering process may occur at the
growth temperature of 300 1C for FePt film on LAO substrate [12]. Unfortunately, there is no systematic study on the order process for FePt on LAO with varying the growth temperature. In the present, FePt films are prepared on LAO with varying the growth temperature from room temperature (RT) to 500 1C by alternatively depositing Fe layer and Pt layer to study the ordering process. Structure and the magnetic properties of FePt films are reported in detail.
2. Experimental details [Fe/Pt]16 multilayers are grown on LaAlO3(001) (LAO) by magnetron sputtering with the base pressure of 5 10 5 Pa. The substrate temperature varies between room temperature and 500 1C. The background atmosphere of Ar pressure remains 0.55 Pa during the sample preparation. The sputtering power of the Fe and Pt layer is set to be 30 W and 15 W, respectively. The crystallographic structure of the film is characterized by X-ray diffraction (XRD) and the grazing incidence X-ray diffraction (GIXRD) [13] measured at Shanghai Synchrotron Radiation Facility (SSRF). Magnetic domain structure on the surface for all samples is detected by Magnetic Force Microscope (MFM). The surface morphology is observed by Atomic Force Microscope (AFM). Magnetic properties of the films are measured at room temperature by vibrating sample magnetometry (VSM) and Superconducting Quantum Interference Device (SQUID).
3. Results and discussions n
Corresponding author. Fax: þ86 25 8359 4402. E-mail address: [email protected] (X.S. Wu).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.016
Fig. 1 shows the XRD patterns of [Fe/Pt]16 multilayers on LAO substrates prepared by varying the growth temperature. As seen
(003)
(111)
(200) (002)
A.M. Zhang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
(001)
90
103
300°C
100 103
400°C
100 103
I 500°C
100 20
40
60 2θ (°)
(220) (202)
150°C
100 103
(110)
Intensity (CPS)
RT 100 103
80
Fig. 1. X-ray diffraction patterns of FePt films deposited on LAO substrate with temperature from 30 to 500 1C.
and LAO will increase with substrate temperature increasing, which may enhance the influence of LAO substrate. Considering the very close coefficients of thermal expansion for FePt (10.5 10 6/K) and LaAlO3 (10 10 6/K), the slight decrease of lattice constant a may result from improved compressive effect from LAO substrate when film is prepared at relatively high substrate temperature. As seen from the XRD patterns in Fig. 1, it is difficult to calculate the ordering degree S using the integral intensity as in the literature, since reflections of (00l) superlattice peaks from L10-FePt are very close to those from LAO substrate. Here, the ordering parameter S is defined as S2 ¼ ð1ðc=aÞÞ=1ðc=aÞSf , where ðc=aÞSf is the axial ratio for fully ordered phase which was determined to be 0.961 [14]. The values of the ordering degree S are 0.4, 0.98 and 0.97 for films grown at 300 1C, 400 1C and 500 1C, respectively. It shows that the FePt films have become completely ordered at deposition temperature above 300 1C, which is consistent with the XRD results. These results show that (001)-preferred L10-FePt film can be obtained on LAO substrate
3.86 3.85
a (angstrom)
300°C 3.84
400°C 500°C
3.83 3.82
interface
3.81 3.80 0
50
100
250 150 200 Depth (angstrom)
300
350
Fig. 2. Depth dependence of the in-plane parameter a obtained from the grazing incident XRD at Shanghai Synchrotron Radiation Facility (SSRF).
M*10 (emu/cm-3)
from Fig. 1, when the substrate temperature is below 300 1C, there are no obvious reflections from the FePt film in the accuracy of our experiments. Therefore, FePt alloy has not formed below 300 1C, while reflections from Fe and/or Pt may be too weak to be explored, which may be ascribed to the too small thickness of the ˚ At substrate temperature of 300 1C, reflecFe or Pt layer (8.5 A). tions of (111) and (200) textures corresponding to the ordering FePt alloy are observed at 2y of about 41.01 and 47.41, indicating the existence of the chemical ordering L10-FePt alloy. When substrate temperature goes up to 400 1C, reflections from the (00l) superlattice of ordering FePt are observed, besides a weak contribution from (202) reflection. With the deposition temperature rising up to 500 1C, (00l) reflections remain, although the intensity of reflections from (110) and (202) increases. We hereafter discuss the film structure based on the main grains in the film. The out-of-plane lattice constants, calculated from the (00l) ˚ 3.694 A˚ and 3.70 A˚ for films grown at 300 1C, peaks, are 3.820 A, 400 1C and 500 1C, respectively. The out-of-plane lattice constant for film deposited at 300 1C is more close to the lattice parameter of L10-FePt, ao, indicating that the lattice constant c lies in the plane. Depth dependence of the in-plane lattice parameters for films grown at 300 1C, 400 1C and 500 1C were obtained from the grazing incident XRD (GIXRD) measured at SSRF, which are shown ˚ in Fig. 2. The average in-plane lattice parameters are 3.802 A, ˚ and 3.851 A˚ for films grown at 300 1C, 400 1C and 500 1C, 3.854 A, respectively. The lattice constant c for film grown at 300 1C is checked to lie in the plane, while it is out of the plane for those grown at 400 1C and 500 1C. The lattice parameters of a-axis and c-axis are very close to those of the bulk, for films grown at 400 1C and 500 1C, which shows the films are almost fully-relaxed, which may be ascribed to the non-infiltration at the interface of FePt and LAO under the preparation conditions, although the lattice constant of LaAlO3 (LAO) substrate is smaller than that of FePt. These results show that phase transition of ordering FePt films on LAO has been finished at substrate temperature of 400 1C by multilayer sputtering technique [10,11]. The in-plane lattice constant a slightly decreases with deposition temperature increasing from 400 1C to 500 1C, which is consistent with the change of the out-of-plane lattice constant c. The ratios of c/a are 0.995, 0.958 and 0.960 for films grown at 300 1C, 400 1C and 500 1C, respectively. These results show that the tetragonal distortion increases with increasing the deposition temperature from 300 1C to 400 1C, along with the ordering degree S increasing. However, it slightly decreases with increasing the growth temperature from 400 1C to 500 1C. Infiltration effect at the interface of FePt
60 0 -60 60 0 -60 60 0 -60 60 0 -60
RT
150°C 300°C
400°C
-20k 80 40 0 -40 -80
-10k
0 10k Magnetic field (Oe)
20k
in-plane magnetic field out-of-plane magnetic field
500°C -60.0k
-40.0k
0.0 -20.0k 20.0k Magnetic field (Oe)
40.0k
60.0k
Fig. 3. Hysteresis loops of [Fe/Pt]16 superlattice grown on LaAlO3 (001) substrates with temperature from 30 to 500 1C.
A.M. Zhang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
at low temperature by multilayer sputtering method, which may be dominated by the magic effect of the interface in multilayers [11]. The room temperature M–H loops of samples are shown in Fig. 3. The ordering parameter S and the magnetic parameters are summarized in Table 1. The magnetic field is applied parallel and perpendicular to the film plane during the measurements individually. For films deposited below 300 1C, a soft magnetic nature of Fe is observed, indicating that ordering FePt alloy has not formed. For film grown at 300 1C, almost isotropic magnetic moment M is observed, with the out-of-plane coercivity rising up to 920 Oe. The relatively low coercivity may be related to the lower ordering degree and the random texture of the sample. At deposition temperature of 400 1C, the easy axis of magnetic moment M is in the normal direction of the film plane, which is quite consistent with the XRD patterns shown in Fig. 1, where (001)-oriented texture is preferred. With increasing the deposition temperature from 400 1C to 500 1C, the out-of-plane coercivity increases
Table 1 The ordering parameters S and the magnetic parameters of FePt film deposited on LaAlO3 substrate grown at temperature of 300 1C, 400 1C and 500 1C. Ts (1C)
S
Hc (KOe)
Ms (emu/cm3)
Ku (erg/cm3)
300 400 500
0.4 0.98 0.97
0.92 5.69 16.3
680 678 890
0.6 107 2.0 107 4.5 107
91
dramatically from 5.694 K Oe to 16.3 K Oe, along with the saturated magnetic moments (Ms) increasing from 680 emu/cm3 to 890 emu/cm3. The enhancement of the out-of-plane coercivity and saturated magnetic moment seem independent of the ordering degree which almost remains constant for film deposited at 400 1C and 500 1C. The magnetic anisotropy Ku is estimated from the formula Ku ¼Ms Hk/2, where Ms and Hk are the saturated magnetization and anisotropy field respectively. As seen from Table 1, the magnetic anisotropy Ku values are 0.6 107, 2.0 107, and 4.5 107 erg/cm3 for films grown at 300 1C, 400 1C, and 500 1C, respectively. The increasing Ku resulted from the promotion of the ordering degree with increasing the growth temperature from 300 1C to 400 1C. With the growth temperature rising up to 500 1C, the ordering degree is almost the same, which seems that it has no effect on Ku and Ms. X-ray diffraction results have shown that the ratio of lattice constant ratio c/a increases from 0.958 to 0.96. Slightly increased c/a will result in a considerable enhancement of magnetic anisotropy Ku [15], which is consistent with those studied in our previous work [12]. Moreover, the ferromagnetic phase of FePt is preferred with high c/a ratio while the antiferromagnetic phase becomes stable at a slightly low c/a ratio [16]. Enhancement of c/a may lead to a more stable ferromagnetic phase and increase the magnetic anisotropy Ku and saturated magnetic moment Ms. Therefore, the enhancement of the out-of-plane coercivity and saturated magnetic moment may be ascribed to the increase of magnetic anisotropy Ku and stability of ferromagnetic phase of FePt grown at 500 1C at which the tetragonal distortion of the film increases. The magnetic property
Fig. 4. Magnetic Force Microscope images (left) and the corresponding AFM images (right) of L10-FePt films grown at 400 1C (the above row) and 500 1C (the below row).
92
A.M. Zhang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
of the films may be also intimately linked with their fascinating morphology of films. Fig. 4 shows the surface magnetic domain structure and the corresponding surface morphology for samples deposited at 400 1C and 500 1C. As seen from the MFM images, perpendicular magnetic domains are obviously observed, corresponding to the (001)-preferred texture of L10-FePt, and become more obvious with temperature increasing to 500 1C. The film deposited at 400 1C is a selfsimilar fractal pattern seen from the AFM image. At 500 1C, the fractal breaks into the arbitrary-shaped islands with the average size of 0.2 mm, which corresponds to single-domain structure. And the grains are one-to-one correspondence to the domains in size and shape. The demagnetization of the single-domain particles will lead to a large coercivity if their uniaxial anisotropy energy is large [17]. The anisotropy Ku of film deposited at 500 1C is 4.5 107. Therefore, large Ku and single-domain structure result in the large coercivity. For the film deposited at 400 1C, the reduced c/a ratio will lead to lower Ku. In addition, continuous-fractal structure induces the demagnetization proceeding via the motion of pinned domain walls with a consequence of lower coercivity. Thus, the pinned domain walls in fractal structure and the reduced Ku lead to the small value of coercivity.
4. Conclusions FePt films are obtained on LaAlO3 (001) single crystal by multilayer magnetic sputtering at different temperatures. FePt alloy begins to order at about 300 1C, and (001)-preferred L10-FePt film with continuous-fractal structure is obtained at deposition temperature of 400 1C. For film deposited at 500 1C, they become single-domain FePt islands. The coercivity is drastically increased from 5.69 K Oe for film deposited at 400 1C to 16.3 K Oe for that deposited at 500 1C. The dramatic increase of coercivity is ascribed to the contributions of the single-domain structure and the increased magnetic isotropy Ku at high temperature. Results show that L10-FePt film with perpendicular magnetic anisotropy may be obtained on a substrate with the lattice parameter of as less than ao of L10-FePt by multilayer method. Acknowledgments Authors thank Prof. J.G. Lin from Taiwan University for her useful suggestion. This work is supported by the National Key Projects for Basic Researches of China (B12020129), the Natural Science Foundation of China (10974081, 10979017). We appreciate
the sample preparations and useful discussions from Professor S.M. Zhou, Tongji University. The authors thank beam line BL14B1 (Shanghai Synchrotron Radiation Facility) for providing beam time.
References [1] S. Sun, Recent advances in chemical synthesis, self-assembly, and applications of FePt nanoparticles, Advanced Materials 18 (2006) 393. [2] C.B. Rong, V. Nandwana, N. Poudyal, J.P. Liu, M.E. Kozlov, R.H. Baughman, Y. Ding, Z.L. Wang, Bulk FePt-based nanocomposite magnets with enhanced exchange coupling, Journal of Applied Physics 102 (2007) 023908. [3] C.B. Rong, J.P. Liu, Temperature and magnetic-field-induced phase transitions in Fe-rich FePt alloys, Applied Physics Letters 90 (2007) 222504. [4] J.K. Mei, F.T. Yuan, W.M. Liao, Y.D. Yao, H.M. Lin, H.Y. Lee, J.H. Hsu, Effect of initial stress/strain state on formation of (001) preferred orientation in L10 FePt thin films, Journal of Applied Physics 109 (2011) 07A737. [5] S.N. Hsiao, F.T. Yuan, H.W. Chang, H.W. Huang, S.K. Chen, H.Y. Lee, Effect of initial stress/strain state on order–disorder transformation of FePt thin films, Applied Physics Letters 94 (2009) 232505. [6] Chun Feng, Qian Zhan, Baohe Li, Jiao Teng, Minghua Li, Yong Jiang, Guanghua Yu, Magnetic properties and microstructure of FePt/Au multilayers with high perpendicular magnetocrystalline anisotropy, Applied Physics Letters 93 (2008) 152513. [7] J.S. Chen, B.C. Lim, J.F. Hu, B. Liu, G.M. Chow, G. Ju, Low temperature deposited L10 FePt–C (001) films with high coercivity and small grain size, Applied Physics Letters 91 (2007) 132506. [8] G.R. Trichy, D. Chakraborti, J. Narayan, J.T. Prater, Structure–magnetic property correlations in the epitaxial FePt system, Applied Physics Letters 92 (2008) 102504. [9] L.J. Cabri, D.R. Owens, J.H.G. Laflamme, Tulameenite, a new platinum–iron– copper mineral, from placers in the Tulameen River area, British Columbia, Canadian Mineralogist 12 (1973) 21–25. [10] T. Shima, T. Moriguchi, S. Mitani, K. Takanashi, Low-temperature fabrication of L10 ordered FePt alloy by alternate monatomic layer deposition, Applied Physics Letters 80 (2002) 288. [11] Yun-Chung Wu, Liang-WeiWang, Chih-Huang Lai, Low-temperature ordering of (001) granular FePt films by inserting ultrathin SiO2 layers, Applied Physics Letters 91 (2007) 072502. [12] A.M. Zhang, Z.X. Chen, W.Q. Zou, B. Lv, J.J. Ge, H.L. Cai, J. Du, X.S. Wu, S.J. Zhang, S.M. Zhou, Effects of substrate on structure and the magnetic properties of (001)-textured FePt films grown at low temperature, Journal of Applied Physics 111 (2012) 07A704. [13] X.S. Wu, Q.S. Bie, Z.S. Lin, A. Hu, H.R. Zhai, S.S. Jiang, X-ray small-angle reflection and high-angle diffraction studies on Co/Cu magnetic multilayers, Modern Physics Letters B 13 (1999) 325. [14] C. Feng, B.H. Li, G. Han, J. Teng, Y. Jiang, Q.L. Liu, G.H. Yu, Low-temperature ordering and enhanced coercivity of L10-FePt thin film promoted by a Bi underlayer, Applied Physics Letters 88 (2006) 232109. [15] Till Burkert, Olle Eriksson, Sergei I. Simak, Andrei V. Ruban, Biplab Sanyal, ¨ Lars Nordstrom, John M. Wills, Magnetic anisotropy of L10 FePt and Fe1 xMnxPt, Physical Review B 71 (2005) 134411. [16] Antje Dannenberg, Markus E. Gruner, AlfredHucht, Peter Entel, Surface energies of stoichiometric FePt and CoPt alloys and their implications for nanoparticle morphologies, Physical Review B 80 (2009) 245438. [17] H. Zijlstra, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, 3, NorthHolland, New York, 1982, p. 37.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic anisotropy of L10-FePt film on (001) LaAlO3 A.M. Zhang a,b, W.H. Zhu a, L. Zheng b, L. Huang b, J.L. Gao b, S.L. Tang b, X.S. Wu b,n a b
College of Science, Hohai University, Nanjing 210098, China National Lab of Solid State of Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
abstract
Article history: Received 4 May 2012 Received in revised form 6 November 2012 Available online 13 December 2012
L10-FePt films are fabricated layer by layer using the magnetic sputtering technique at different growth temperatures. Structure, ordering process and magnetic properties of the films are studied. The film is partly ordered at the growth temperature of 300 1C, and reaches a completely ordered state at deposition temperature of 400 1C. With increasing the substrate temperature from 400 1C to 500 1C, the out-of-plane coercivity increases dramatically from 5.69 KOe to 16.3 KOe, and the magnetic anisotropy Ku increases from 2.0 107 erg/cm3 to 4.5 107 erg/cm3. The surface structure of the film varies with the growth temperature. We attribute the temperature dependence of magnetic properties to the decreased tetragonal distortion at high temperature. & 2012 Elsevier B.V. All rights reserved.
Keywords: L10-FePt film Magnetic anisotropy Lattice strain
1. Introduction L10-FePt is a material of considerable interest for very high density recording media owing to its high magnetocrystalline anisotropy [1–3]. Researchers have paid much attention on the physical and technical aspects for the application of L10-FePt films. Much effort has been made to obtain the L10-FePt film with well (001)-oriented texture at low temperature. Lattice strain, which may be introduced by doping the third atom, growing films on substrates or buffers, surrounded by oxide materials, etc., is usually thought to be one of the favorable factors of accelerating the chemical ordering of FePt alloy [4–8]. Bulk L10-FePt crystallizes in tetragonal symmetry with the lattice parameters of ao ¼3.852 A˚ and co ¼3.728 A˚ [9]. Tension strain resulted from the lattice mismatch between the film and the substrate may induce L10-FePt film growing along the direction of [001] at low temperature [3,5]. Lots of substrates and buffer layers, such as MgO, Pt, and Au have been used. Based on the configuration of the unit cell structure of L10-FePt alloy with the alternating Fe and Pt layer, together with stimulation of the monatomic layer deposition method of Shima et al. [10], multilayer deposition technique is widely used to prepare (001)-oriented L10-FePt films on glass and amorphous SiO2 capped Si substrate [11]. There are little attempt to deposit FePt films directly on a substrate with lattice parameter as less than ao, by multilayer deposition technique [11]. LaAlO3 (LAO) is usually considered to have a pseudo-cubic ˚ which is smaller structure with crystal constant of as ¼ 3.789 A, than that of L10-FePt, ao. Ordering process may occur at the
growth temperature of 300 1C for FePt film on LAO substrate [12]. Unfortunately, there is no systematic study on the order process for FePt on LAO with varying the growth temperature. In the present, FePt films are prepared on LAO with varying the growth temperature from room temperature (RT) to 500 1C by alternatively depositing Fe layer and Pt layer to study the ordering process. Structure and the magnetic properties of FePt films are reported in detail.
2. Experimental details [Fe/Pt]16 multilayers are grown on LaAlO3(001) (LAO) by magnetron sputtering with the base pressure of 5 10 5 Pa. The substrate temperature varies between room temperature and 500 1C. The background atmosphere of Ar pressure remains 0.55 Pa during the sample preparation. The sputtering power of the Fe and Pt layer is set to be 30 W and 15 W, respectively. The crystallographic structure of the film is characterized by X-ray diffraction (XRD) and the grazing incidence X-ray diffraction (GIXRD) [13] measured at Shanghai Synchrotron Radiation Facility (SSRF). Magnetic domain structure on the surface for all samples is detected by Magnetic Force Microscope (MFM). The surface morphology is observed by Atomic Force Microscope (AFM). Magnetic properties of the films are measured at room temperature by vibrating sample magnetometry (VSM) and Superconducting Quantum Interference Device (SQUID).
3. Results and discussions n
Corresponding author. Fax: þ86 25 8359 4402. E-mail address: [email protected] (X.S. Wu).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.016
Fig. 1 shows the XRD patterns of [Fe/Pt]16 multilayers on LAO substrates prepared by varying the growth temperature. As seen
(003)
(111)
(200) (002)
A.M. Zhang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
(001)
90
103
300°C
100 103
400°C
100 103
I 500°C
100 20
40
60 2θ (°)
(220) (202)
150°C
100 103
(110)
Intensity (CPS)
RT 100 103
80
Fig. 1. X-ray diffraction patterns of FePt films deposited on LAO substrate with temperature from 30 to 500 1C.
and LAO will increase with substrate temperature increasing, which may enhance the influence of LAO substrate. Considering the very close coefficients of thermal expansion for FePt (10.5 10 6/K) and LaAlO3 (10 10 6/K), the slight decrease of lattice constant a may result from improved compressive effect from LAO substrate when film is prepared at relatively high substrate temperature. As seen from the XRD patterns in Fig. 1, it is difficult to calculate the ordering degree S using the integral intensity as in the literature, since reflections of (00l) superlattice peaks from L10-FePt are very close to those from LAO substrate. Here, the ordering parameter S is defined as S2 ¼ ð1ðc=aÞÞ=1ðc=aÞSf , where ðc=aÞSf is the axial ratio for fully ordered phase which was determined to be 0.961 [14]. The values of the ordering degree S are 0.4, 0.98 and 0.97 for films grown at 300 1C, 400 1C and 500 1C, respectively. It shows that the FePt films have become completely ordered at deposition temperature above 300 1C, which is consistent with the XRD results. These results show that (001)-preferred L10-FePt film can be obtained on LAO substrate
3.86 3.85
a (angstrom)
300°C 3.84
400°C 500°C
3.83 3.82
interface
3.81 3.80 0
50
100
250 150 200 Depth (angstrom)
300
350
Fig. 2. Depth dependence of the in-plane parameter a obtained from the grazing incident XRD at Shanghai Synchrotron Radiation Facility (SSRF).
M*10 (emu/cm-3)
from Fig. 1, when the substrate temperature is below 300 1C, there are no obvious reflections from the FePt film in the accuracy of our experiments. Therefore, FePt alloy has not formed below 300 1C, while reflections from Fe and/or Pt may be too weak to be explored, which may be ascribed to the too small thickness of the ˚ At substrate temperature of 300 1C, reflecFe or Pt layer (8.5 A). tions of (111) and (200) textures corresponding to the ordering FePt alloy are observed at 2y of about 41.01 and 47.41, indicating the existence of the chemical ordering L10-FePt alloy. When substrate temperature goes up to 400 1C, reflections from the (00l) superlattice of ordering FePt are observed, besides a weak contribution from (202) reflection. With the deposition temperature rising up to 500 1C, (00l) reflections remain, although the intensity of reflections from (110) and (202) increases. We hereafter discuss the film structure based on the main grains in the film. The out-of-plane lattice constants, calculated from the (00l) ˚ 3.694 A˚ and 3.70 A˚ for films grown at 300 1C, peaks, are 3.820 A, 400 1C and 500 1C, respectively. The out-of-plane lattice constant for film deposited at 300 1C is more close to the lattice parameter of L10-FePt, ao, indicating that the lattice constant c lies in the plane. Depth dependence of the in-plane lattice parameters for films grown at 300 1C, 400 1C and 500 1C were obtained from the grazing incident XRD (GIXRD) measured at SSRF, which are shown ˚ in Fig. 2. The average in-plane lattice parameters are 3.802 A, ˚ and 3.851 A˚ for films grown at 300 1C, 400 1C and 500 1C, 3.854 A, respectively. The lattice constant c for film grown at 300 1C is checked to lie in the plane, while it is out of the plane for those grown at 400 1C and 500 1C. The lattice parameters of a-axis and c-axis are very close to those of the bulk, for films grown at 400 1C and 500 1C, which shows the films are almost fully-relaxed, which may be ascribed to the non-infiltration at the interface of FePt and LAO under the preparation conditions, although the lattice constant of LaAlO3 (LAO) substrate is smaller than that of FePt. These results show that phase transition of ordering FePt films on LAO has been finished at substrate temperature of 400 1C by multilayer sputtering technique [10,11]. The in-plane lattice constant a slightly decreases with deposition temperature increasing from 400 1C to 500 1C, which is consistent with the change of the out-of-plane lattice constant c. The ratios of c/a are 0.995, 0.958 and 0.960 for films grown at 300 1C, 400 1C and 500 1C, respectively. These results show that the tetragonal distortion increases with increasing the deposition temperature from 300 1C to 400 1C, along with the ordering degree S increasing. However, it slightly decreases with increasing the growth temperature from 400 1C to 500 1C. Infiltration effect at the interface of FePt
60 0 -60 60 0 -60 60 0 -60 60 0 -60
RT
150°C 300°C
400°C
-20k 80 40 0 -40 -80
-10k
0 10k Magnetic field (Oe)
20k
in-plane magnetic field out-of-plane magnetic field
500°C -60.0k
-40.0k
0.0 -20.0k 20.0k Magnetic field (Oe)
40.0k
60.0k
Fig. 3. Hysteresis loops of [Fe/Pt]16 superlattice grown on LaAlO3 (001) substrates with temperature from 30 to 500 1C.
A.M. Zhang et al. / Journal of Magnetism and Magnetic Materials 332 (2013) 89–92
at low temperature by multilayer sputtering method, which may be dominated by the magic effect of the interface in multilayers [11]. The room temperature M–H loops of samples are shown in Fig. 3. The ordering parameter S and the magnetic parameters are summarized in Table 1. The magnetic field is applied parallel and perpendicular to the film plane during the measurements individually. For films deposited below 300 1C, a soft magnetic nature of Fe is observed, indicating that ordering FePt alloy has not formed. For film grown at 300 1C, almost isotropic magnetic moment M is observed, with the out-of-plane coercivity rising up to 920 Oe. The relatively low coercivity may be related to the lower ordering degree and the random texture of the sample. At deposition temperature of 400 1C, the easy axis of magnetic moment M is in the normal direction of the film plane, which is quite consistent with the XRD patterns shown in Fig. 1, where (001)-oriented texture is preferred. With increasing the deposition temperature from 400 1C to 500 1C, the out-of-plane coercivity increases
Table 1 The ordering parameters S and the magnetic parameters of FePt film deposited on LaAlO3 substrate grown at temperature of 300 1C, 400 1C and 500 1C. Ts (1C)
S
Hc (KOe)
Ms (emu/cm3)
Ku (erg/cm3)
300 400 500
0.4 0.98 0.97
0.92 5.69 16.3
680 678 890
0.6 107 2.0 107 4.5 107
91
dramatically from 5.694 K Oe to 16.3 K Oe, along with the saturated magnetic moments (Ms) increasing from 680 emu/cm3 to 890 emu/cm3. The enhancement of the out-of-plane coercivity and saturated magnetic moment seem independent of the ordering degree which almost remains constant for film deposited at 400 1C and 500 1C. The magnetic anisotropy Ku is estimated from the formula Ku ¼Ms Hk/2, where Ms and Hk are the saturated magnetization and anisotropy field respectively. As seen from Table 1, the magnetic anisotropy Ku values are 0.6 107, 2.0 107, and 4.5 107 erg/cm3 for films grown at 300 1C, 400 1C, and 500 1C, respectively. The increasing Ku resulted from the promotion of the ordering degree with increasing the growth temperature from 300 1C to 400 1C. With the growth temperature rising up to 500 1C, the ordering degree is almost the same, which seems that it has no effect on Ku and Ms. X-ray diffraction results have shown that the ratio of lattice constant ratio c/a increases from 0.958 to 0.96. Slightly increased c/a will result in a considerable enhancement of magnetic anisotropy Ku [15], which is consistent with those studied in our previous work [12]. Moreover, the ferromagnetic phase of FePt is preferred with high c/a ratio while the antiferromagnetic phase becomes stable at a slightly low c/a ratio [16]. Enhancement of c/a may lead to a more stable ferromagnetic phase and increase the magnetic anisotropy Ku and saturated magnetic moment Ms. Therefore, the enhancement of the out-of-plane coercivity and saturated magnetic moment may be ascribed to the increase of magnetic anisotropy Ku and stability of ferromagnetic phase of FePt grown at 500 1C at which the tetragonal distortion of the film increases. The magnetic property
Fig. 4. Magnetic Force Microscope images (left) and the corresponding AFM images (right) of L10-FePt films grown at 400 1C (the above row) and 500 1C (the below row).
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of the films may be also intimately linked with their fascinating morphology of films. Fig. 4 shows the surface magnetic domain structure and the corresponding surface morphology for samples deposited at 400 1C and 500 1C. As seen from the MFM images, perpendicular magnetic domains are obviously observed, corresponding to the (001)-preferred texture of L10-FePt, and become more obvious with temperature increasing to 500 1C. The film deposited at 400 1C is a selfsimilar fractal pattern seen from the AFM image. At 500 1C, the fractal breaks into the arbitrary-shaped islands with the average size of 0.2 mm, which corresponds to single-domain structure. And the grains are one-to-one correspondence to the domains in size and shape. The demagnetization of the single-domain particles will lead to a large coercivity if their uniaxial anisotropy energy is large [17]. The anisotropy Ku of film deposited at 500 1C is 4.5 107. Therefore, large Ku and single-domain structure result in the large coercivity. For the film deposited at 400 1C, the reduced c/a ratio will lead to lower Ku. In addition, continuous-fractal structure induces the demagnetization proceeding via the motion of pinned domain walls with a consequence of lower coercivity. Thus, the pinned domain walls in fractal structure and the reduced Ku lead to the small value of coercivity.
4. Conclusions FePt films are obtained on LaAlO3 (001) single crystal by multilayer magnetic sputtering at different temperatures. FePt alloy begins to order at about 300 1C, and (001)-preferred L10-FePt film with continuous-fractal structure is obtained at deposition temperature of 400 1C. For film deposited at 500 1C, they become single-domain FePt islands. The coercivity is drastically increased from 5.69 K Oe for film deposited at 400 1C to 16.3 K Oe for that deposited at 500 1C. The dramatic increase of coercivity is ascribed to the contributions of the single-domain structure and the increased magnetic isotropy Ku at high temperature. Results show that L10-FePt film with perpendicular magnetic anisotropy may be obtained on a substrate with the lattice parameter of as less than ao of L10-FePt by multilayer method. Acknowledgments Authors thank Prof. J.G. Lin from Taiwan University for her useful suggestion. This work is supported by the National Key Projects for Basic Researches of China (B12020129), the Natural Science Foundation of China (10974081, 10979017). We appreciate
the sample preparations and useful discussions from Professor S.M. Zhou, Tongji University. The authors thank beam line BL14B1 (Shanghai Synchrotron Radiation Facility) for providing beam time.
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