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Magnetic properties of NiFe2O4 thin films grown on La0.7Sr0.3MnO3-buffered Si substrate
Vacuum 86 (2011) 340e343
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Magnetic properties of NiFe2O4 thin films grown on La0.7Sr0.3MnO3-buffered Si substrate J.J. Tong a, Q.X. Liu a, *, Y.P. Jiang a, X.G. Tang a, Y.C. Zhou b, J. Chen c a
School of Physics & Optoelectronic Engineering, Guangdong University of Technology, Guangzhou Guangzhou Higher Education Mega Center 510006, PR China Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China c Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2011 Received in revised form 18 July 2011 Accepted 19 July 2011
Nickel ferrite NiFe2O4 (NFO) thin films have been prepared on a Si substrate (NFO/Si) and La0.7Sr0.3MnO3 (LSMO)-coated Si (100) substrate (NFO/LSMO/Si) by RF magnetron sputtering. The microstructures and magnetic properties of the two films were systematically investigated. X-ray diffraction (XRD) and atomic force microscopy (AFM) revealed that highly (331)-oriented NFO films with a smooth surface were grown on the LSMO/Si substrate. The magnetization of the films was measured at room temperature. It showed a clear hysteresis loop in both samples, with the magnetic field applied in the plane. However, no hysteresis loop is seen with the magnetic field applied perpendicular to the film plane. This indicates the presence of an anisotropy favoring the orientation of the magnetization in the direction parallel to the film plane. A study of magnetization hysteresis loop measurements indicates that the LSMO buffer layer may improve the magnetic properties of NFO thin films, and that the saturation magnetization increases from 4.15 104 to 3.5 105 A/m. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Nickel ferrite thin films RF magnetron sputtering Magnetic films
1. Introduction Nanocrystalline spinel ferrites with the general formula AFe2O4 (A ¼ Mn, Co, Ni, Cu or Zn) are very important magnetic materials because of their interesting magnetic and electrical properties with their added chemical and thermal stabilities. These materials are technologically important and have been used in many applications, including magnetic recording media and magnetic fluids for the storage and/or retrieval of information, magnetic resonance imaging (MRI) enhancement, catalysis, magnetically guided drug delivery, sensors and pigments [1,2]. Nickel ferrite NiFe2O4 nanoparticles, as one of most common spinel ferrites, have been considerably studied using physical or chemical methods in order to understand and tailor the magnetic properties of spinel ferrites. These have an inverse spinel structure showing ferromagnetism that originates from the magnetic moment of anti-parallel spins between Fe3þ ions at tetrahedral sites and Ni2þ ions at octahedral sites [3]. The magnetic structure consists of two antiferromagnetically coupled sub-lattices. The first sub-lattice is formed by ferromagnetically ordered Fe3þ ions occupying the tetragonal A sites of the spinel AB2O4 structure, while
* Corresponding author. E-mail address: [email protected] (Q.X. Liu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.060
the second sub-lattice contains ferromagnetically ordered Ni2þ and Fe3þ ions occupying the octahedral B sites. Therefore a detailed understanding of relationship between film structure, microstructure and their correlation with function parameters (such as magnetic property) is definitely needed [4e10]. As always the choice of substrate profoundly affects the structure and properties of the films. Si substrates offer the apparent feasibility of integrating with the current semiconductor devices [11]. For deposition on Si substrates, several different types of buffer layer have been studied, including, for example, SiO2 [12], (Ba,Sr,Pb) TiO3 [13e15]. Up to now, there are few papers reporting the effect of a La0.7Sr0.3MnO3 (LSMO) buffer layer on the magnetic properties of NiFe2O4 (NFO) thin films. LSMO exhibits colossal magnetoresistive properties, which make it a potential candidate as a material for sensors and memories. LSMO is also a metallic oxide with a resistivity of approximately 105 U-m at room temperature [16,17]. Several approaches to the synthesis of spinel ferrite thin films have been proven successful, including pulsed-laser deposition (PLD) [18], radio frequency (RF) magnetron sputtering [19], and the sol-gel method [20]. In this work, NFO thin films were prepared by RF magnetron sputtering on Si (100) substrates and LSMOcoated Si (100) substrates, respectively. The effects of LSMO buffer layer on the structures and magnetic properties of NFO thin films were studied.
341
2. Experimental
(311)
J.J. Tong et al. / Vacuum 86 (2011) 340e343
The stoichiometric targets of NiFe2O4 (NFO) and La0.7Sr0.3MnO3 (LSMO) were calcined by the conventional solid state reaction method, and then NFO films were deposited on Si substrates directly by RF-magnetron sputtering （China SKY Technology development Co. Ltd, CAS，JGP-450A）at 550 C. On the other hand, NFO thin films were also deposited on LSMO-coated substrates to study the influence of LSMO on the structures and magnetic properties, with LSMO acting as a buffer layer. The deposition chamber was evacuated to a pressure less than 2.67 104 Pa before preparing the thin films and then the gas mixture of Ar and O2 was fed into the chamber to keep the total pressure to 1.2 Pa in the chamber during the deposition. The ratio of Ar:O2 was kept to 10:1. Details of the deposition conditions are given in Table 1. After deposition all the films were annealed in air at 650 C for 30 min and cooled down slowly to room temperature. After post deposition annealing the crystalline structure of the films was studied by X-ray diffraction (XRD) with Cu Ka radiation (l ¼ 1.5554 Å). The microstructure of the samples was investigated by atomic force microscopy (AFM，Japan SIINT，SPI3800N). Magnetic properties of the films were measured using a vibrating sample magnetometer (VSM， USA Quantum Design，PPMS-9) at room temperature [21,22].
*
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the films. Fig. 1(a) shows the X-ray diffraction (XRD) patterns of the NFO thin films deposited on a Si (100) substrate crystallized at 650 C for 30 min in the air in which a highly (331)-oriented structure was obtained. For the films deposited on LSMO/Si substrates (see Fig. 1(b)), two evident sets of diffraction peaks were observed which could be ascribed to the spinel NFO phase and the pseudocubic perovskite LSMO phase. No additional or intermediate phase peaks are seen [17,23]. On the other hand, a highly (311)oriented structure was present, and the strong (311) peak corresponds to the crystalline space interval of about 2.643 Å. By comparing Fig. 1(a) and (b), the relative peak intensities of SIðh00Þ=SIðhklÞ for NFO and NFO/LSMO thin films are found to be 0.09 and 0.52, respectively. This indicates a remarkably strong (311) grain orientation for NFO/LSMO composite film. Here, we define the mismatch degree asjas ae j=ae 100%, where ae and as denote the in-plane lattice constants of the deposited film and the substrate, respectively. According to the above mismatch degree definition, the mismatch degree of 6.02% between NFO (311) films and LSMO (a ¼ 0.387 nm) is small. These orientations of the films may be attributed to the small lattice mismatch between the NFO (311) film and LSMO layer [24]. When the films were deposited on the Si substrate directly at high temperature and residual oxygen pressure, it is presumed that an amorphous SiO2 layer is formed below the films and the crystallinity of the films is well maintained,
# LSMO phase
(b) NFO/LSMO/Si # (214)
*
# (024)
(331)
* *
# (202)
(222) (400)
# (012)
Intensity (arb.units)
* NFO phase
(a) NFO/Si 10
20
30
40
50
60
2θ(deg) Fig. 1. X-ray diffraction patterns of NFO thin films deposited on Si substrate. There is no bufferlayer in (a), while there is an LSMO buffer layer in (b).
even after annealing under oxygen. When a crystallized film is grown on an amorhous substrate, there is also a preferred growth orientation due to surface free energy. The microscopic surface morphology of the NFO film and the NFO/LSMO composite film are analyzed by using a single-mode AFM. 3D AFM images over a scan area of 10 10 mm2 are shown in Fig. 2(a) and (b). The AFM micrographs indicate that the films presented in this work are well crystallized, very dense, and quite
Table 1 Deposition conditions of NFO thin films and LSMO buffer layers by RF-magnetron sputtering method. Deposition parameters Deposition temperature ( C) Film thickness (nm) Base vacuum (Pa) Sputtering power (W) Deposition time (min) Working pressure (Pa) Ar vs. O2 ration Annealing temperature ( C)
NiFe2O4 550 35 2.67 104 170 60 1.2 10:1 650
La0.7Sr0.3MnO3 550 200 2.67 104 80 60 1.2 10:1 650
Fig. 2. AFM images (10 10 mm2) in 3D for NFO films deposited on (a) Si, (b) LSMO/Si substrate.
342
J.J. Tong et al. / Vacuum 86 (2011) 340e343
NFO/LSMO Pure NFO Pure LSMO
200
H // Film
0 -1
Magnetization (kAm )
Magnetization (kAm-1)
400
-200
80 40 0 -40 -80 -0.03
0.00
0.03
Magnetic field (T)
-400 -4
-2
0
2
4
Magnetic field (T) Fig. 3. Magnetic hysteresis loop measurement at room temperature for (a) NFO/LSMO composite films, (b) pure NFO films and (c) pure LSMO films.
smooth. The root-mean-square (rms) roughness of NFO film and NFO/LSMO composite film are 0.80 nm and 0.95 nm, respectively. Fig. 3 shows the magnetization hysteresis loops (MeH) of the samples at room temperature when a maximum magnetic field of 3 T is applied, with the magnetic field in the plane. Clear magnetic hysteresis loops were obtained for (a) NFO/LSMO composite film, (b) pure NFO film and (c) pure LSMO film. They were corrected by subtracting the diamagnetic contribution from the substrate. Magnetic properties were not observed for both samples with the magnetic field applied perpendicular to the film plane. However, the loops were obtained at room temperature by applying a magnetic field parallel to the surface of the films (see Fig. 3). This indicates the presence of an anisotropy favoring the orientation of the magnetization in the direction parallel to the film plane. The pure LSMO film exhibited a weak magnetic moment at room temperature. Compared with the magnetic property of the pure NFO films, a dramatic increase of magnetization is observed in the NFO/LSMO composite films. The values of saturation magnetization
4
2.4x10
Fe 2p 1/2
Fe 2p3/2
711.3
724.5 4
Counts / s
2.2x10
719.0 4
2.0x10
4
1.8x10
4
1.6x10
730
720
710
700
Binding energy (eV) Fig. 4. X-ray photoelectron spectrums of the Fe 2p lines for NFO/LSMO composite thin films.
(Ms) are 4.15 104 A/m for NFO thin films and 3.5 105 A/m for NFO/LSMO thin films. According to previous investigations, the ferromagnetism enhancement in the composite films may be ascribed to the following two reasons [25]. One is the formation of Fe2þ ions [26], and the other comes from the possibility of atomic rearrangements in the spinel structure [23,27]. In order to identify the origin of the magnetism of the films, X-ray photoelectron spectroscopy (XPS) was performed to investigate the Fe oxidation. It is well known that the Fe 2p core level splits into 2p1/ 2 and 2p3/2 components. A representative scan of the Fe 2p line is shown in Fig. 4. The position of this line is expected to be 711.0 eV for Fe3þ and 706.5 eV for Fe2þ. From Fig. 4, we observe that the peak of Fe 2p3/2 is located at 711.3 eV. Meanwhile the position of the satellite peak is expected at 719.0 eV for Fe3þ [28,29]. According to these two characteristics, we deduce that the oxidation state of Fe in our NFO/ LSMO films is Fe3þ and that there is no evidence for Fe2þ within a resolution of a few atomic percent [25,26]. Consequently, we confirm that an enhanced magnetic moment in the composite films should be mainly due to the atomic rearrangements in NFO films. 4. Conclusions In conclusion, the highly (311)-oriented NFO films were deposited on Si substrates by RF magnetron sputtering method. The influence of LSMO as a buffer layer was studied. The films with cubic phase were grown on cubic substrates with no evidence of parasitic phases or inter-diffusion, and the surface has roughness of 0.9 nm. More important, compared with the films without the buffer layer, the NFO/LSMO composite films have a larger magnetic moment (3.5 105 A/m), which is 9 times larger than that for the pure NFO film (4.15 104 A/m). The enhanced magnetic moment may be attributed to the cationic inversion in NFO. Acknowledgements We wish to thank Dr Z.G. Zheng for his help in the VSM measurement at South China University of Technology. This work was supported by the National Natural Science Foundation of China (Grant No. 11032010 and 10774030), and Guangdong Provincial Natural Science Foundation of China (Grant No. 10151009001000050). References [1] Sugimoto M. J Am Ceram Soc 1999;82:269. [2] Safarik I, Safarikova M. Magnetic nanoparticles and biosciences. In: Hofmann H, Rahman Z, Schubert U, editors. Nanostructured materials. Vienna: Springer; 2002. p. 1. [3] Kinemuchi Y, Ishizaka K, Suematsu H, Jiang W, Yatsui K. Thin Solid Films 2002; 407:109. [4] Spaldin NA, Pickett WE. J Solid State Chem 2003;176:615. [5] Fitzgerald AG, Muir G. Surf Interface Anal 1986;8:247. [6] Suran G, Heurtel A. J Appl Phys 1972;43:536. [7] Johnson MT, Kotula PG, Carter CB. J Cryst Growth 1999;206:299. [8] Muralidharan S, Saraswathy V, Berchmans LJ, Thangavel K, Ann KY. Sens Actuators B 2010;145:225. [9] Rigato F, Estrade S, Arbiol J, Peiro F, Luders U, Marti X, et al. Mater Sci Eng B 2007;144:43. [10] Dixit G, Singh JP, Srivastava RC, Agrawal HM, Choudhary RJ, Gupta A. Surf Interface Anal 2010;42:151. [11] Wu J, Wang J. J Am Ceram Soc 2010;93:1422. [12] Mohallem NDS, Seara LM. Appl Surf Sci 2003;214:143. [13] Deng C, Zhang Y, Ma J, Lin Y, Nan CW. Acta Mater 2008;56:405. [14] Ryu H, Murugavel P, Lee JH, Chae SC, Noh TW. Appl Phys Lett 2006;89:102907. [15] Luders U, Bibes M, Bobo JF, Cantoni M, Bertacco R, Fontcuberta J. Phys Rev B 2005;71:134419. [16] Urushibara A, Moritomo Y, Arima T, Asamitsu A, Kido G, Tokura Y. Phys Rev B 1995;51:14103. [17] Wang ZJ, Usuki H, Kumagai T, Kokawa H. J Cryst Growth 2006;293:68. [18] Xi XW, Chen YJ, Zhang XY. Vacuum 2004;75:161. [19] Luders U, Bibes M, Bobo JF, Fontcuberta J. Appl Phys A 2005;80:427. [20] Gunjakar JL, More AM, Shinde VR, Lokhande CD. J Alloys Compd 2008;465:468.
J.J. Tong et al. / Vacuum 86 (2011) 340e343 [21] Tang XG, Liu QX, Jiang YP, Zheng RK, Chan HLW. J Appl Phys 2006;100: 114105. [22] Gao J, Cui Y, Yang Z. Mater Sci Eng B 2004;110:111. [23] Chinnasamy CN, Narayanasamy A, Ponpandian N, Chattopadhyay K, Shinoda K, Jeyadevan B, et al. Phys Rev B 2001;63:184108. [24] Arnaud H, Astrid A, Patrick A, Bruno V, Jeanine T, Alain L, et al. Mater Chem.J 2000;10:549.
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[25] Wu Y, Wan J, Huang C, Weng Y, Zhao S, Liu J, et al. Appl Phys Lett 2008;93: 192915. [26] Eerenstein W. Science 2005;307:1203. [27] Luders U, Barthelemy A, Bibes M, Bouzehouane K, Fusil S, Jacquet E, et al. Adv Mater 2006;18:1733. [28] Graat PCJ, Somers MAJ. Appl Surf Sci 1996;100:36. [29] Niedrig TS, Weiss W, Schlogl R. Phys Rev B 1995;52:17449.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Magnetic properties of NiFe2O4 thin films grown on La0.7Sr0.3MnO3-buffered Si substrate J.J. Tong a, Q.X. Liu a, *, Y.P. Jiang a, X.G. Tang a, Y.C. Zhou b, J. Chen c a
School of Physics & Optoelectronic Engineering, Guangdong University of Technology, Guangzhou Guangzhou Higher Education Mega Center 510006, PR China Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China c Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2011 Received in revised form 18 July 2011 Accepted 19 July 2011
Nickel ferrite NiFe2O4 (NFO) thin films have been prepared on a Si substrate (NFO/Si) and La0.7Sr0.3MnO3 (LSMO)-coated Si (100) substrate (NFO/LSMO/Si) by RF magnetron sputtering. The microstructures and magnetic properties of the two films were systematically investigated. X-ray diffraction (XRD) and atomic force microscopy (AFM) revealed that highly (331)-oriented NFO films with a smooth surface were grown on the LSMO/Si substrate. The magnetization of the films was measured at room temperature. It showed a clear hysteresis loop in both samples, with the magnetic field applied in the plane. However, no hysteresis loop is seen with the magnetic field applied perpendicular to the film plane. This indicates the presence of an anisotropy favoring the orientation of the magnetization in the direction parallel to the film plane. A study of magnetization hysteresis loop measurements indicates that the LSMO buffer layer may improve the magnetic properties of NFO thin films, and that the saturation magnetization increases from 4.15 104 to 3.5 105 A/m. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Nickel ferrite thin films RF magnetron sputtering Magnetic films
1. Introduction Nanocrystalline spinel ferrites with the general formula AFe2O4 (A ¼ Mn, Co, Ni, Cu or Zn) are very important magnetic materials because of their interesting magnetic and electrical properties with their added chemical and thermal stabilities. These materials are technologically important and have been used in many applications, including magnetic recording media and magnetic fluids for the storage and/or retrieval of information, magnetic resonance imaging (MRI) enhancement, catalysis, magnetically guided drug delivery, sensors and pigments [1,2]. Nickel ferrite NiFe2O4 nanoparticles, as one of most common spinel ferrites, have been considerably studied using physical or chemical methods in order to understand and tailor the magnetic properties of spinel ferrites. These have an inverse spinel structure showing ferromagnetism that originates from the magnetic moment of anti-parallel spins between Fe3þ ions at tetrahedral sites and Ni2þ ions at octahedral sites [3]. The magnetic structure consists of two antiferromagnetically coupled sub-lattices. The first sub-lattice is formed by ferromagnetically ordered Fe3þ ions occupying the tetragonal A sites of the spinel AB2O4 structure, while
* Corresponding author. E-mail address: [email protected] (Q.X. Liu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.060
the second sub-lattice contains ferromagnetically ordered Ni2þ and Fe3þ ions occupying the octahedral B sites. Therefore a detailed understanding of relationship between film structure, microstructure and their correlation with function parameters (such as magnetic property) is definitely needed [4e10]. As always the choice of substrate profoundly affects the structure and properties of the films. Si substrates offer the apparent feasibility of integrating with the current semiconductor devices [11]. For deposition on Si substrates, several different types of buffer layer have been studied, including, for example, SiO2 [12], (Ba,Sr,Pb) TiO3 [13e15]. Up to now, there are few papers reporting the effect of a La0.7Sr0.3MnO3 (LSMO) buffer layer on the magnetic properties of NiFe2O4 (NFO) thin films. LSMO exhibits colossal magnetoresistive properties, which make it a potential candidate as a material for sensors and memories. LSMO is also a metallic oxide with a resistivity of approximately 105 U-m at room temperature [16,17]. Several approaches to the synthesis of spinel ferrite thin films have been proven successful, including pulsed-laser deposition (PLD) [18], radio frequency (RF) magnetron sputtering [19], and the sol-gel method [20]. In this work, NFO thin films were prepared by RF magnetron sputtering on Si (100) substrates and LSMOcoated Si (100) substrates, respectively. The effects of LSMO buffer layer on the structures and magnetic properties of NFO thin films were studied.
341
2. Experimental
(311)
J.J. Tong et al. / Vacuum 86 (2011) 340e343
The stoichiometric targets of NiFe2O4 (NFO) and La0.7Sr0.3MnO3 (LSMO) were calcined by the conventional solid state reaction method, and then NFO films were deposited on Si substrates directly by RF-magnetron sputtering （China SKY Technology development Co. Ltd, CAS，JGP-450A）at 550 C. On the other hand, NFO thin films were also deposited on LSMO-coated substrates to study the influence of LSMO on the structures and magnetic properties, with LSMO acting as a buffer layer. The deposition chamber was evacuated to a pressure less than 2.67 104 Pa before preparing the thin films and then the gas mixture of Ar and O2 was fed into the chamber to keep the total pressure to 1.2 Pa in the chamber during the deposition. The ratio of Ar:O2 was kept to 10:1. Details of the deposition conditions are given in Table 1. After deposition all the films were annealed in air at 650 C for 30 min and cooled down slowly to room temperature. After post deposition annealing the crystalline structure of the films was studied by X-ray diffraction (XRD) with Cu Ka radiation (l ¼ 1.5554 Å). The microstructure of the samples was investigated by atomic force microscopy (AFM，Japan SIINT，SPI3800N). Magnetic properties of the films were measured using a vibrating sample magnetometer (VSM， USA Quantum Design，PPMS-9) at room temperature [21,22].
*
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the films. Fig. 1(a) shows the X-ray diffraction (XRD) patterns of the NFO thin films deposited on a Si (100) substrate crystallized at 650 C for 30 min in the air in which a highly (331)-oriented structure was obtained. For the films deposited on LSMO/Si substrates (see Fig. 1(b)), two evident sets of diffraction peaks were observed which could be ascribed to the spinel NFO phase and the pseudocubic perovskite LSMO phase. No additional or intermediate phase peaks are seen [17,23]. On the other hand, a highly (311)oriented structure was present, and the strong (311) peak corresponds to the crystalline space interval of about 2.643 Å. By comparing Fig. 1(a) and (b), the relative peak intensities of SIðh00Þ=SIðhklÞ for NFO and NFO/LSMO thin films are found to be 0.09 and 0.52, respectively. This indicates a remarkably strong (311) grain orientation for NFO/LSMO composite film. Here, we define the mismatch degree asjas ae j=ae 100%, where ae and as denote the in-plane lattice constants of the deposited film and the substrate, respectively. According to the above mismatch degree definition, the mismatch degree of 6.02% between NFO (311) films and LSMO (a ¼ 0.387 nm) is small. These orientations of the films may be attributed to the small lattice mismatch between the NFO (311) film and LSMO layer [24]. When the films were deposited on the Si substrate directly at high temperature and residual oxygen pressure, it is presumed that an amorphous SiO2 layer is formed below the films and the crystallinity of the films is well maintained,
# LSMO phase
(b) NFO/LSMO/Si # (214)
*
# (024)
(331)
* *
# (202)
(222) (400)
# (012)
Intensity (arb.units)
* NFO phase
(a) NFO/Si 10
20
30
40
50
60
2θ(deg) Fig. 1. X-ray diffraction patterns of NFO thin films deposited on Si substrate. There is no bufferlayer in (a), while there is an LSMO buffer layer in (b).
even after annealing under oxygen. When a crystallized film is grown on an amorhous substrate, there is also a preferred growth orientation due to surface free energy. The microscopic surface morphology of the NFO film and the NFO/LSMO composite film are analyzed by using a single-mode AFM. 3D AFM images over a scan area of 10 10 mm2 are shown in Fig. 2(a) and (b). The AFM micrographs indicate that the films presented in this work are well crystallized, very dense, and quite
Table 1 Deposition conditions of NFO thin films and LSMO buffer layers by RF-magnetron sputtering method. Deposition parameters Deposition temperature ( C) Film thickness (nm) Base vacuum (Pa) Sputtering power (W) Deposition time (min) Working pressure (Pa) Ar vs. O2 ration Annealing temperature ( C)
NiFe2O4 550 35 2.67 104 170 60 1.2 10:1 650
La0.7Sr0.3MnO3 550 200 2.67 104 80 60 1.2 10:1 650
Fig. 2. AFM images (10 10 mm2) in 3D for NFO films deposited on (a) Si, (b) LSMO/Si substrate.
342
J.J. Tong et al. / Vacuum 86 (2011) 340e343
NFO/LSMO Pure NFO Pure LSMO
200
H // Film
0 -1
Magnetization (kAm )
Magnetization (kAm-1)
400
-200
80 40 0 -40 -80 -0.03
0.00
0.03
Magnetic field (T)
-400 -4
-2
0
2
4
Magnetic field (T) Fig. 3. Magnetic hysteresis loop measurement at room temperature for (a) NFO/LSMO composite films, (b) pure NFO films and (c) pure LSMO films.
smooth. The root-mean-square (rms) roughness of NFO film and NFO/LSMO composite film are 0.80 nm and 0.95 nm, respectively. Fig. 3 shows the magnetization hysteresis loops (MeH) of the samples at room temperature when a maximum magnetic field of 3 T is applied, with the magnetic field in the plane. Clear magnetic hysteresis loops were obtained for (a) NFO/LSMO composite film, (b) pure NFO film and (c) pure LSMO film. They were corrected by subtracting the diamagnetic contribution from the substrate. Magnetic properties were not observed for both samples with the magnetic field applied perpendicular to the film plane. However, the loops were obtained at room temperature by applying a magnetic field parallel to the surface of the films (see Fig. 3). This indicates the presence of an anisotropy favoring the orientation of the magnetization in the direction parallel to the film plane. The pure LSMO film exhibited a weak magnetic moment at room temperature. Compared with the magnetic property of the pure NFO films, a dramatic increase of magnetization is observed in the NFO/LSMO composite films. The values of saturation magnetization
4
2.4x10
Fe 2p 1/2
Fe 2p3/2
711.3
724.5 4
Counts / s
2.2x10
719.0 4
2.0x10
4
1.8x10
4
1.6x10
730
720
710
700
Binding energy (eV) Fig. 4. X-ray photoelectron spectrums of the Fe 2p lines for NFO/LSMO composite thin films.
(Ms) are 4.15 104 A/m for NFO thin films and 3.5 105 A/m for NFO/LSMO thin films. According to previous investigations, the ferromagnetism enhancement in the composite films may be ascribed to the following two reasons [25]. One is the formation of Fe2þ ions [26], and the other comes from the possibility of atomic rearrangements in the spinel structure [23,27]. In order to identify the origin of the magnetism of the films, X-ray photoelectron spectroscopy (XPS) was performed to investigate the Fe oxidation. It is well known that the Fe 2p core level splits into 2p1/ 2 and 2p3/2 components. A representative scan of the Fe 2p line is shown in Fig. 4. The position of this line is expected to be 711.0 eV for Fe3þ and 706.5 eV for Fe2þ. From Fig. 4, we observe that the peak of Fe 2p3/2 is located at 711.3 eV. Meanwhile the position of the satellite peak is expected at 719.0 eV for Fe3þ [28,29]. According to these two characteristics, we deduce that the oxidation state of Fe in our NFO/ LSMO films is Fe3þ and that there is no evidence for Fe2þ within a resolution of a few atomic percent [25,26]. Consequently, we confirm that an enhanced magnetic moment in the composite films should be mainly due to the atomic rearrangements in NFO films. 4. Conclusions In conclusion, the highly (311)-oriented NFO films were deposited on Si substrates by RF magnetron sputtering method. The influence of LSMO as a buffer layer was studied. The films with cubic phase were grown on cubic substrates with no evidence of parasitic phases or inter-diffusion, and the surface has roughness of 0.9 nm. More important, compared with the films without the buffer layer, the NFO/LSMO composite films have a larger magnetic moment (3.5 105 A/m), which is 9 times larger than that for the pure NFO film (4.15 104 A/m). The enhanced magnetic moment may be attributed to the cationic inversion in NFO. Acknowledgements We wish to thank Dr Z.G. Zheng for his help in the VSM measurement at South China University of Technology. This work was supported by the National Natural Science Foundation of China (Grant No. 11032010 and 10774030), and Guangdong Provincial Natural Science Foundation of China (Grant No. 10151009001000050). References [1] Sugimoto M. J Am Ceram Soc 1999;82:269. [2] Safarik I, Safarikova M. Magnetic nanoparticles and biosciences. In: Hofmann H, Rahman Z, Schubert U, editors. Nanostructured materials. Vienna: Springer; 2002. p. 1. [3] Kinemuchi Y, Ishizaka K, Suematsu H, Jiang W, Yatsui K. Thin Solid Films 2002; 407:109. [4] Spaldin NA, Pickett WE. J Solid State Chem 2003;176:615. [5] Fitzgerald AG, Muir G. Surf Interface Anal 1986;8:247. [6] Suran G, Heurtel A. J Appl Phys 1972;43:536. [7] Johnson MT, Kotula PG, Carter CB. J Cryst Growth 1999;206:299. [8] Muralidharan S, Saraswathy V, Berchmans LJ, Thangavel K, Ann KY. Sens Actuators B 2010;145:225. [9] Rigato F, Estrade S, Arbiol J, Peiro F, Luders U, Marti X, et al. Mater Sci Eng B 2007;144:43. [10] Dixit G, Singh JP, Srivastava RC, Agrawal HM, Choudhary RJ, Gupta A. Surf Interface Anal 2010;42:151. [11] Wu J, Wang J. J Am Ceram Soc 2010;93:1422. [12] Mohallem NDS, Seara LM. Appl Surf Sci 2003;214:143. [13] Deng C, Zhang Y, Ma J, Lin Y, Nan CW. Acta Mater 2008;56:405. [14] Ryu H, Murugavel P, Lee JH, Chae SC, Noh TW. Appl Phys Lett 2006;89:102907. [15] Luders U, Bibes M, Bobo JF, Cantoni M, Bertacco R, Fontcuberta J. Phys Rev B 2005;71:134419. [16] Urushibara A, Moritomo Y, Arima T, Asamitsu A, Kido G, Tokura Y. Phys Rev B 1995;51:14103. [17] Wang ZJ, Usuki H, Kumagai T, Kokawa H. J Cryst Growth 2006;293:68. [18] Xi XW, Chen YJ, Zhang XY. Vacuum 2004;75:161. [19] Luders U, Bibes M, Bobo JF, Fontcuberta J. Appl Phys A 2005;80:427. [20] Gunjakar JL, More AM, Shinde VR, Lokhande CD. J Alloys Compd 2008;465:468.
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