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Synthesis and characterization of chemically ordered FePt magnetic nano-particles
ARTICLE IN PRESS Physica B 405 (2010) 3205–3207
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and characterization of chemically ordered FePt magnetic nano-particles K. Srinivasa Rao a, T. Balaji a,, Y. Lingappa b, M.R.P. Reddy a, Arbind kumar a, T.L. Prakash a a b
Centre for Materials for Electronics Technology (C-MET), IDA phase-III, Cherlapally, Hyderabad 500 051, India Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, India
a r t i c l e in fo
abstract
Article history: Received 3 December 2009 Received in revised form 31 December 2009 Accepted 2 February 2010
Monodispersed FePt alloy magnetic nano-particles are prepared by reduction of platinum acetyl acetonate and iron acetyl acetonate salts together in the presence of oleic acid and oleyl amine stabilizers by polyol process. The particle size of FePt is in the range of 2–3 nm confirmed by transmission electron microscopy (TEM). As-synthesized FePt nano-particles are chemically disordered with face centre cubic (fcc) structure where as after vacuum annealing these particles changed to face centre tetragonal (fct) ordered structure confirmed by the X-ray diffraction technique. Magnetic coercivity of 5.247 KOe was observed for fct structure. & 2010 Elsevier B.V. All rights reserved.
Keywords: FePt nano-particles Fct structure Magnetic properties
1. Introduction Recently, there has been given keen attention on chemically ordered FePt and CoPt nano-magnetic alloys with L1o crystalline phase, which are the key candidates for future ultra high density magnetic recording media [1]. However, synthesis of respective nanostructures with control over size, chemical composition and size distribution still remains a major challenge. The first successful demonstration of monodispersed FePt nano-particles has been accomplished by solution phase chemical synthesis [2]. Calculations predict that the high anisotropy of L1o FePt nanoparticles lead to room temperature ferromagnetic stability for particle size as small as 3 nm [3–5]. Unfortunately, as-prepared FePt nano-particles are disordered face centre cubic structure, which has low magneto-crystalline anisotropy and cannot be applied as the recording media or permanent magnets. Additional thermal energy is necessary for the transformation of fcc phase to ordered fct phase. However, the vacuum annealing treatment makes particle agglomeration, which causes degrading magnetic properties. Hence, more attention and control are necessary during the annealing process to avoid particle agglomeration. The fct phase has a high uniaxial magneto-crystalline anisotropy and good chemical stability [2]. Thus in the recent time, a huge number of research works have been devoted towards FePt alloys both as nano-particles and thin films [6–8].
Corresponding author. Tel.: + 91 04 27267309; fax: + 91 40 27261658.
E-mail address: [email protected] (T. Balaji). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.007
In this paper, we reported the synthesis of FePt nano-particles of size below 3 nm by the simultaneous chemical reduction of Pt (acac)2 and Fe(acac)3 by ethylene glycol in presence of oleic acid and oleyl amine stabilizers. The as-prepared FePt nanoparticles have chemically disordered fcc structure and can be transformed into chemically ordered fct structure after vacuum annealing. The fct FePt nano-particles exhibit hard magnetic properties.
2. Experimental All the chemicals used for the synthesis are purchased from different sources. Platinum acetyl acetonate is from Alfa Aesar and the stabilizers such as oleic acid, oleyl amine and iron acetyl acetonate are from Sigma Aldrich. Solvents such as ethylene glycol, hexan and ethanol are purchased from SD. Fine. FePt nanoparticles are synthesized by employing polyol process. Fe and Pt salts are mixed in known quantity of ethylene glycol in a round bottom flask and placed in the oil bath. Oleic acid and oleyl amine are added to the reaction mixture at a temperature of 100 1C. The chemical reaction was carried out at a temperature of 200 1C with a constant heating rate. Gentle mechanical stirring is provided through out the reaction under argon atmosphere. The change in solution color from red to black indicates the formation of FePt nano-particles. After cooling the solution to room temperature, ethyl alcohol is added to the reaction mixture for FePt precipitation. The FePt nano-particles are separated using centrifugation and then dried in vacuum oven at 50 1C for 10 h.
ARTICLE IN PRESS K. Srinivasa Rao et al. / Physica B 405 (2010) 3205–3207
(111)
3206
1600
FePt nano-particles crystal structure is studied by using the XRD (Bruker, D8 advanced). Particle size is confirmed by the transmission electron microscope (Philips, CM 200) where as magnetic properties are studied by using the vibratory sample magnetometer (ADE Magnetics, DMS4).
1400
(220) (202)
(112)
400
(201)
600
(200) (002)
800
(110)
1000
(001)
Intensity
1200
3. Results and discussions
200 0 20
30
40
50 60 2Theta (Degree)
70
80
Fig. 1. The XRD profile of FePt nano-particles after vacuum annealing at 650 1C for 3 h.
Fig. 2. The TEM image of as-prepared FePt nanoparticles.
Among the different salts reduction processes, polyol process has been chosen for FePt nano-particles synthesis. The advantage of this process is simple, highly reproducible and applicable for large scale production. During chemical reduction process, the stabilizers oleic acid and oleyl amine protect FePt particles from agglomeration and also from particle surface oxidation. Vacuum annealing is an important external heat source for the conversion of fcc FePt nano-particles to fct hard magnetic particles. Dai et al. [9] have shown that with high temperature annealing ( 4600 1C), a particle coalescence occurs, which causes an increase in the particle size. In this study, we have employed 650 1C annealing temperature for 3 h to convert fully ordered fct crystal structure. The ambient atmosphere (vacuum, 10 6 Torr) used during the annealing process helps to avoid oxidation of FePt nano-particles. Also, the uniform spreading of FePt nano-powder particles in the sample holder of vacuum annealing chamber helps in such a way that all fcc FePt particles absorb the thermal energy to convert to fct magnetically ordered structure. The crystal phase purity analysis of as-prepared and vaccum annealed FePt nano-particles are carried out using the XRD. This study confirmed that as-synthesized and vaccum annealed FePt nano-particles are with face centre cubic (fcc) and face centre tetragonal crystal structures, respectively. Fig. 1 explains the XRD profile of FePt nano-particles after vacuum annealing at 650 1C for 3 h. From the XRD profile, it is seen that the superlattice reflections (0 0 1) and (1 1 0) are very clear. The splitting of (2 0 0)/(0 0 2) peak signifies the tetragonal crystal structure. It also proves that there is no evidence of impurity peaks such as iron oxide even at high temperature annealing treatments. It tells that in the chemical synthesis, pure FePt alloy nano-particles are only formed.
Fig. 3. Hysteresis loop of vacuum annealed FePt nano-particles at 650 1C for 3 h.
ARTICLE IN PRESS K. Srinivasa Rao et al. / Physica B 405 (2010) 3205–3207
Fig. 2 shows the TEM image of as-prepared FePt nano-particles. From this figure, it is observed that the particle size of FePt is in the range 2–3 nm. The particles are well-dispersed and separated from each other with approximately uniform size and shape. It will explain that our method has good control over FePt nanoparticle synthesis, size and morphology. Fig. 3 shows the hysteresis loop of vacuum annealed FePt nano-particles at 650 1C for 3 h. It can be seen that the hysteresis curve is quite rectangular with magnetic coercivity of 5.247 KOe at room temperature. The as-prepared FePt nano-particles are fcc crystal structure with super paramagnetic behavior at room temperature. Both the XRD and magnetic property studies explain the transformation from as-prepared chemically disordered fcc to chemically ordered fct crystalline structure.
4. Conclusions Monodispersed Fept nano-particles are synthesized by chemical reduction using ethylene glycol as the reducing reagent. It is observed that after vacuum annealing at 650 1C for 3 h, the magnetic coercivity of FePt nano-particles is as high as 5.247 KOe. The TEM analysis confirms that the FePt particles are well separated, having size in the range 2–3 nm.
3207
Acknowledgement We would like to thank Department of Science and Technology (DST) for supporting this work by sanctioning a Grant-in-aid project.
References [1] D. Weller, A. Moser, IEEE Tras. Magn. 35 (1999) 4423. [2] S. Sun, C.B. Murray, D. Weller, L. Floks, A. Moser, Science 287 (2000) 1989. [3] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. toney, M. schwickert, J.U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [4] R. Skomski, J.M.D. Coey, Permanent Magnetism, Institute of Physics, Bristol, 1999. [5] D.J. Sellmyer, R. Skomski (Eds.), Advanced Magnetic Nanostructures, Springer, Berlin, 2006. [6] G. Brown, B. Kraczek, A. Janotti, G.M. Schulthess Stocks, D.D. Johnson, Phys. Rev. B 68 (2003) 052405. [7] B. Stahl, J. Ellrich, R. Theissmann, M. Ghafari, S. Bhattacharya, H. Hahn, N.S. Gajbhiye, D. Kramer, R.N. Vishwanathan, J. Weissmuller, H. Gleiter, Phys. Rev. B. 67 (2003) 014422. [8] J.A. Christodoulides, M.J. Bonder, Y. Huang, Y. Zhang, S. Stoyanov, G.C. Hadjipanayis, A. Simopoulos, D. Weller, Phys. Rev. 68 (2003) 54428. [9] Z.R. Dai, S. Sun, Z.L. Wang, Nanoletters 1 (2001) 443.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and characterization of chemically ordered FePt magnetic nano-particles K. Srinivasa Rao a, T. Balaji a,, Y. Lingappa b, M.R.P. Reddy a, Arbind kumar a, T.L. Prakash a a b
Centre for Materials for Electronics Technology (C-MET), IDA phase-III, Cherlapally, Hyderabad 500 051, India Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, India
a r t i c l e in fo
abstract
Article history: Received 3 December 2009 Received in revised form 31 December 2009 Accepted 2 February 2010
Monodispersed FePt alloy magnetic nano-particles are prepared by reduction of platinum acetyl acetonate and iron acetyl acetonate salts together in the presence of oleic acid and oleyl amine stabilizers by polyol process. The particle size of FePt is in the range of 2–3 nm confirmed by transmission electron microscopy (TEM). As-synthesized FePt nano-particles are chemically disordered with face centre cubic (fcc) structure where as after vacuum annealing these particles changed to face centre tetragonal (fct) ordered structure confirmed by the X-ray diffraction technique. Magnetic coercivity of 5.247 KOe was observed for fct structure. & 2010 Elsevier B.V. All rights reserved.
Keywords: FePt nano-particles Fct structure Magnetic properties
1. Introduction Recently, there has been given keen attention on chemically ordered FePt and CoPt nano-magnetic alloys with L1o crystalline phase, which are the key candidates for future ultra high density magnetic recording media [1]. However, synthesis of respective nanostructures with control over size, chemical composition and size distribution still remains a major challenge. The first successful demonstration of monodispersed FePt nano-particles has been accomplished by solution phase chemical synthesis [2]. Calculations predict that the high anisotropy of L1o FePt nanoparticles lead to room temperature ferromagnetic stability for particle size as small as 3 nm [3–5]. Unfortunately, as-prepared FePt nano-particles are disordered face centre cubic structure, which has low magneto-crystalline anisotropy and cannot be applied as the recording media or permanent magnets. Additional thermal energy is necessary for the transformation of fcc phase to ordered fct phase. However, the vacuum annealing treatment makes particle agglomeration, which causes degrading magnetic properties. Hence, more attention and control are necessary during the annealing process to avoid particle agglomeration. The fct phase has a high uniaxial magneto-crystalline anisotropy and good chemical stability [2]. Thus in the recent time, a huge number of research works have been devoted towards FePt alloys both as nano-particles and thin films [6–8].
Corresponding author. Tel.: + 91 04 27267309; fax: + 91 40 27261658.
E-mail address: [email protected] (T. Balaji). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.007
In this paper, we reported the synthesis of FePt nano-particles of size below 3 nm by the simultaneous chemical reduction of Pt (acac)2 and Fe(acac)3 by ethylene glycol in presence of oleic acid and oleyl amine stabilizers. The as-prepared FePt nanoparticles have chemically disordered fcc structure and can be transformed into chemically ordered fct structure after vacuum annealing. The fct FePt nano-particles exhibit hard magnetic properties.
2. Experimental All the chemicals used for the synthesis are purchased from different sources. Platinum acetyl acetonate is from Alfa Aesar and the stabilizers such as oleic acid, oleyl amine and iron acetyl acetonate are from Sigma Aldrich. Solvents such as ethylene glycol, hexan and ethanol are purchased from SD. Fine. FePt nanoparticles are synthesized by employing polyol process. Fe and Pt salts are mixed in known quantity of ethylene glycol in a round bottom flask and placed in the oil bath. Oleic acid and oleyl amine are added to the reaction mixture at a temperature of 100 1C. The chemical reaction was carried out at a temperature of 200 1C with a constant heating rate. Gentle mechanical stirring is provided through out the reaction under argon atmosphere. The change in solution color from red to black indicates the formation of FePt nano-particles. After cooling the solution to room temperature, ethyl alcohol is added to the reaction mixture for FePt precipitation. The FePt nano-particles are separated using centrifugation and then dried in vacuum oven at 50 1C for 10 h.
ARTICLE IN PRESS K. Srinivasa Rao et al. / Physica B 405 (2010) 3205–3207
(111)
3206
1600
FePt nano-particles crystal structure is studied by using the XRD (Bruker, D8 advanced). Particle size is confirmed by the transmission electron microscope (Philips, CM 200) where as magnetic properties are studied by using the vibratory sample magnetometer (ADE Magnetics, DMS4).
1400
(220) (202)
(112)
400
(201)
600
(200) (002)
800
(110)
1000
(001)
Intensity
1200
3. Results and discussions
200 0 20
30
40
50 60 2Theta (Degree)
70
80
Fig. 1. The XRD profile of FePt nano-particles after vacuum annealing at 650 1C for 3 h.
Fig. 2. The TEM image of as-prepared FePt nanoparticles.
Among the different salts reduction processes, polyol process has been chosen for FePt nano-particles synthesis. The advantage of this process is simple, highly reproducible and applicable for large scale production. During chemical reduction process, the stabilizers oleic acid and oleyl amine protect FePt particles from agglomeration and also from particle surface oxidation. Vacuum annealing is an important external heat source for the conversion of fcc FePt nano-particles to fct hard magnetic particles. Dai et al. [9] have shown that with high temperature annealing ( 4600 1C), a particle coalescence occurs, which causes an increase in the particle size. In this study, we have employed 650 1C annealing temperature for 3 h to convert fully ordered fct crystal structure. The ambient atmosphere (vacuum, 10 6 Torr) used during the annealing process helps to avoid oxidation of FePt nano-particles. Also, the uniform spreading of FePt nano-powder particles in the sample holder of vacuum annealing chamber helps in such a way that all fcc FePt particles absorb the thermal energy to convert to fct magnetically ordered structure. The crystal phase purity analysis of as-prepared and vaccum annealed FePt nano-particles are carried out using the XRD. This study confirmed that as-synthesized and vaccum annealed FePt nano-particles are with face centre cubic (fcc) and face centre tetragonal crystal structures, respectively. Fig. 1 explains the XRD profile of FePt nano-particles after vacuum annealing at 650 1C for 3 h. From the XRD profile, it is seen that the superlattice reflections (0 0 1) and (1 1 0) are very clear. The splitting of (2 0 0)/(0 0 2) peak signifies the tetragonal crystal structure. It also proves that there is no evidence of impurity peaks such as iron oxide even at high temperature annealing treatments. It tells that in the chemical synthesis, pure FePt alloy nano-particles are only formed.
Fig. 3. Hysteresis loop of vacuum annealed FePt nano-particles at 650 1C for 3 h.
ARTICLE IN PRESS K. Srinivasa Rao et al. / Physica B 405 (2010) 3205–3207
Fig. 2 shows the TEM image of as-prepared FePt nano-particles. From this figure, it is observed that the particle size of FePt is in the range 2–3 nm. The particles are well-dispersed and separated from each other with approximately uniform size and shape. It will explain that our method has good control over FePt nanoparticle synthesis, size and morphology. Fig. 3 shows the hysteresis loop of vacuum annealed FePt nano-particles at 650 1C for 3 h. It can be seen that the hysteresis curve is quite rectangular with magnetic coercivity of 5.247 KOe at room temperature. The as-prepared FePt nano-particles are fcc crystal structure with super paramagnetic behavior at room temperature. Both the XRD and magnetic property studies explain the transformation from as-prepared chemically disordered fcc to chemically ordered fct crystalline structure.
4. Conclusions Monodispersed Fept nano-particles are synthesized by chemical reduction using ethylene glycol as the reducing reagent. It is observed that after vacuum annealing at 650 1C for 3 h, the magnetic coercivity of FePt nano-particles is as high as 5.247 KOe. The TEM analysis confirms that the FePt particles are well separated, having size in the range 2–3 nm.
3207
Acknowledgement We would like to thank Department of Science and Technology (DST) for supporting this work by sanctioning a Grant-in-aid project.
References [1] D. Weller, A. Moser, IEEE Tras. Magn. 35 (1999) 4423. [2] S. Sun, C.B. Murray, D. Weller, L. Floks, A. Moser, Science 287 (2000) 1989. [3] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. toney, M. schwickert, J.U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [4] R. Skomski, J.M.D. Coey, Permanent Magnetism, Institute of Physics, Bristol, 1999. [5] D.J. Sellmyer, R. Skomski (Eds.), Advanced Magnetic Nanostructures, Springer, Berlin, 2006. [6] G. Brown, B. Kraczek, A. Janotti, G.M. Schulthess Stocks, D.D. Johnson, Phys. Rev. B 68 (2003) 052405. [7] B. Stahl, J. Ellrich, R. Theissmann, M. Ghafari, S. Bhattacharya, H. Hahn, N.S. Gajbhiye, D. Kramer, R.N. Vishwanathan, J. Weissmuller, H. Gleiter, Phys. Rev. B. 67 (2003) 014422. [8] J.A. Christodoulides, M.J. Bonder, Y. Huang, Y. Zhang, S. Stoyanov, G.C. Hadjipanayis, A. Simopoulos, D. Weller, Phys. Rev. 68 (2003) 54428. [9] Z.R. Dai, S. Sun, Z.L. Wang, Nanoletters 1 (2001) 443.