- Email: [email protected]
Structure and magnetic properties of CoFe2O4 and Fe3O4 nanoparticles
Materials Science and Engineering C 27 (2007) 1415 – 1417 www.elsevier.com/locate/msec
Structure and magnetic properties of CoFe2O4 and Fe3O4 nanoparticles L. Chitu a,⁎, M. Jergel a , E. Majkova a , S. Luby a , I. Capek b , A. Satka c , J. Ivan d , J. Kovac e , M. Timko e a
Institute of Physics SAS, 84511 Bratislava, Slovakia Polymer Institute SAS, 84236 Bratislava, Slovakia International Laser Center and Faculty of Electrical Engineering and Informatics SUT, 81219 Bratislava, Slovakia d Institute of Materials and Machine Mechanics SAS, 83102 Bratislava, Slovakia e Institute of Experimental Physics SAS, 04501 Kosice, Slovakia b
c
Received 6 May 2006; received in revised form 21 July 2006; accepted 21 July 2006 Available online 14 November 2006
Abstract The Fe3O4 and CoFe2O4 nanoparticles of radius 3.2 ± 0.3 and 3.8 ± 0.3 nm, respectively, were synthesized by the high-temperature solution phase reaction of metal acetylacetonates. Nanoparticles with the spherical shape and well-developed crystalline structure are superparamagnetic at room temperature. The CoFe2O4 showed high coercivity up to 1.7 T at low temperatures and a step-like change of magnetization at 138 K, which might point at the Verwey transition. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe3O4; CoFe2O4; Nanoparticles; Magnetic particles
1. Introduction
2. Experiment
The Fe3O4 and CoFe2O4 nanoparticles are intensively studied because of their interesting magnetic properties and wide applications in the information storage systems, magnetic nanodevices, ferrofluids and in medical diagnostics [1,2]. In all applications, the nanoparticle preparation method is of primary importance for the particle size distribution, shape, surface characteristics and magnetic properties. Up to now various synthesis procedures were developed for the production of monodispersed oxide magnetic particles with diameter below 20 nm [3–5]. Among them the synthesis from organic salts [3] is a powerful technique which produces particles with desired size and small size distribution. In this work we report on the structure and magnetic properties of and Fe3O4 and CoFe2O4 nanoparticles synthesized from organic salts.
Fe3O4 and CoFe2O4 nanoparticles were synthesized by a high-temperature solution phase reaction of metal acetylacetonates (Fe(acac)3, Co(acac)2) with 1,2-hexadecanediol, oleic acid and oleylamine in phenyl ether [3]. For the synthesis of Fe3O4 nanoparticles the reactants (1.4 g Fe(acac)3, 5.2 g 1,2-hexadecanediol, 3.4 g oleic acid and 3.2 g oleylamine) were dissolved in phenyl ether (40 ml) and magnetically stirred under an argon flow. In the argon ambient, the mixture was heated first up to 200 °C for 30 min, then up to 265 °C for another 30 min and then cooled to room temperature. After adding ethanol, a black particulate product precipitated. It was diluted in toluene in the presence of surfactant (oleic acid and oleylamine). Nanoparticles were precipitated with ethanol, centrifuged to remove the solvent and redispersed into toluene. The CoFe2O4 nanoparticles were prepared under similar conditions. The size, shape and ordering of nanoparticles were studied by transmission electron microscopy (TEM, JEM 100C) and scanning electron microscopy (SEM, Leo 1550). The crystalline structure of nanoparticles was studied by grazing incidence Xray diffraction (GI XRD) using the D8 DISCOVER SSS diffractometer (Bruker) and by electron diffraction (ED). The
⁎ Corresponding author. E-mail address: [email protected] (L. Chitu). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.07.036
1416
L. Chitu et al. / Materials Science and Engineering C 27 (2007) 1415–1417
Fig. 1. GIXRD diffraction patterns (incidence angle of 1°) and ED micrograph of Fe3O4 and CoFe2O4 nanoparticles prepared by drying a droplet deposited onto Si/Si3N4 substrate and carbon coated grid.
elemental analysis was performed by the high sensitivity energy dispersive X-ray spectroscopy (EDS). The magnetization versus magnetic field (H ≤ ± 5 T) and temperature (4.2–278 K) were measured using the Vibrating Sample Magnetometer and diluted solution of nanoparticles in a capillary.
Fe3O4 and CoFe2O4 structures belong to the inverse spinel group with the general formula A(B2)O4. Scherrer equation applied to the main diffraction peak provides the size of coherently scattering domains of approx. 6 nm and 7 nm for Fe3O4 and CoFe2O4, respectively. The EDS spectroscopy confirmed the presence of Co in CoFe2O4 particles with the average Fe/Co ratio 2:1. Particles are nearly spherical with the radius of 3.2 ± 0.3 nm for Fe3O4 and 3.8 ± 0.27 nm for CoFe2O4 and they form hexagonally ordered arrays (Fig. 2). The particle size coincides well with that of coherently scattering domains, which suggests that the particles form a structurally homogeneous monocrystalline-like entities. The magnetization versus temperature M(T) curves were measured in the temperature range 4.2–250 K in zero-fieldcooled (ZFC) and field-cooled (FC) regimes (Fig. 3a,b). For ZFC measurements the sample was cooled down to 4.2 K. The field (between 5 and 100 mT) was turned on and the M(T) curve was measured as the sample was heated from 4.2 K up to 250 K. For FC experiments the magnetic field was applied at 250 K and the magnetization was measured as the sample was cooled down to
3. Results and discussion Fig. 1 shows GI XRD patterns of Fe3O4 and CoFe2O4 nanoparticles together with the respective ED patterns. The
Fig. 2. SEM picture of arrays of (a) Fe3O4 and (b) CoFe2O4 nanoparticles prepared by drying a droplet deposited onto Si/Si3N4 substrate.
Fig. 3. Magnetization versus temperature dependence for field-cooled (FC) and zero-field-cooled (ZFC) experiments with Fe3O4 (a: ∇—10 mT, ○—5 mT) and CoFe2O4 (b: ∇—100 mT, ○—50 mT) nanoparticles.
L. Chitu et al. / Materials Science and Engineering C 27 (2007) 1415–1417
1417
atoms in the Fe–O matrix increase magnetic anisotropy of the material [3,5,6]. In Fig. 4a,b the M(H) dependences measured for the field cooled (2 T) Fe3O4 and CoFe2O4 assemblies are presented as well. For Fe3O4 there is observed an increase of Hc from 0.022 to 0.030 T. A significant increase of Hc from 1.0 T to 1.75 T was found for CoFe2O4 being close to the maximum value 2 T reported [3]. This behavior also points at the increase of magnetic anisotropy in the CoFe2O4 system. A step-like change in ZFC M(T) dependence observed for CoFe2O4 around 136 K could point at the change of magnetic properties probably induced by the Verwey transition recently observed also for Fe nanoparticles [7]. A detail analysis of this effect will be published later. 4. Conclusion We have prepared Fe3O4 and CoFe2O4 nanoparticles of radius 3.2 ± 0.3 and 3.8 ± 0.3 nm, respectively, by a hightemperature solution phase reaction of metal acetylacetonates. The nanoparticles have spherical shape, well developed crystalline structure and they are superparamagnetic at room temperature, the blocking temperatures being 22 K for Fe3O4, and 204 K for CoFe2O4. For CoFe2O4 particles the coercivity up to 1.75 T was found. The ZFC M(T) dependence of CoFe2O4 could point a the Verwey transition at 138 K. Acknowledgements
Fig. 4. Magnetization versus field dependence (a) for Fe3O4 nanoparticles: solid line 280 K,-□- 4.2 K, low field M(H) dependence at 4.2 K is shown in the inset (-□- at 4.2 K ,-∇- at 4.2 K after cooling in magnetic field of 2 T); (b) for CoFe2O4 nanoparticles: -□- 280 K, solid line 4.2 K and -∇- after cooling in magnetic field of 2 T.
4.2 K. For both systems the ZFC/FC magnetization curves show irreversible behavior typical for superparamagnetic nanoparticles. For Fe3O4 particles the irreversibility occurs below 160 K and the blocking temperature TB = 22 K. For CoFe2O4 particles the irreversibility starts at 214 K and TB = 204 K. Above TB the nanoparticle assembly is superparamagnetic and below TB it is ferromagnetic. This is well documented by the M(H) curves measured below and above the TB (Fig. 4a,b). For Fe3O4 the coercivity Hc = 0.022 T whereas for CoFe2O4 Hc = 1.0 T at 4.2 K. The coercivity behavior indicates that Co
The work was supported by Science and Technology Assistance Agency Grant No. APVT-20-029804, Center of Excellence SAS project CE-PI I/2/2005, by Scientific Grant Agency VEGA 2/2041/25 and 2/4101/24. The CNR/SAS Common Program Project No. 12 (2004–2006) is acknowledged as well. References [1] D. Kim, Y. Zhang, J. Kehr, T. Klason, B. Bjelke, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 256. [2] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341. [3] Sh. Sun, H. Zeng, D.B. Robinson, S. Raoux, Ph.M. Rice, Sh.X. Wang, G. Li, J. Am. Chem. Soc. 126 (2004) 273. [4] A.T. Ngo, P. Bonville, M.P. Pileni, Eur. Phys. J. B9 (1999) 583. [5] T. Meron, Y. Rosenberg, Y. Lereah, G. Markovich, J. Magn. Magn. Mater. 292 (2005) 11. [6] T.Y. Kim, M.S. Lee, Y.I. Kom, C.S. Lee, J.C. Park, D. Kim, J. Phys. D: Appl. Phys. 36 (2003) 1451. [7] A. Slawska-Waniewska, A. Roig, M. Gich, L. Casas, K. Racka, N. Nedelko, E. Molins, Phys. Rev., B 70 (2004) 054412.
Structure and magnetic properties of CoFe2O4 and Fe3O4 nanoparticles L. Chitu a,⁎, M. Jergel a , E. Majkova a , S. Luby a , I. Capek b , A. Satka c , J. Ivan d , J. Kovac e , M. Timko e a
Institute of Physics SAS, 84511 Bratislava, Slovakia Polymer Institute SAS, 84236 Bratislava, Slovakia International Laser Center and Faculty of Electrical Engineering and Informatics SUT, 81219 Bratislava, Slovakia d Institute of Materials and Machine Mechanics SAS, 83102 Bratislava, Slovakia e Institute of Experimental Physics SAS, 04501 Kosice, Slovakia b
c
Received 6 May 2006; received in revised form 21 July 2006; accepted 21 July 2006 Available online 14 November 2006
Abstract The Fe3O4 and CoFe2O4 nanoparticles of radius 3.2 ± 0.3 and 3.8 ± 0.3 nm, respectively, were synthesized by the high-temperature solution phase reaction of metal acetylacetonates. Nanoparticles with the spherical shape and well-developed crystalline structure are superparamagnetic at room temperature. The CoFe2O4 showed high coercivity up to 1.7 T at low temperatures and a step-like change of magnetization at 138 K, which might point at the Verwey transition. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe3O4; CoFe2O4; Nanoparticles; Magnetic particles
1. Introduction
2. Experiment
The Fe3O4 and CoFe2O4 nanoparticles are intensively studied because of their interesting magnetic properties and wide applications in the information storage systems, magnetic nanodevices, ferrofluids and in medical diagnostics [1,2]. In all applications, the nanoparticle preparation method is of primary importance for the particle size distribution, shape, surface characteristics and magnetic properties. Up to now various synthesis procedures were developed for the production of monodispersed oxide magnetic particles with diameter below 20 nm [3–5]. Among them the synthesis from organic salts [3] is a powerful technique which produces particles with desired size and small size distribution. In this work we report on the structure and magnetic properties of and Fe3O4 and CoFe2O4 nanoparticles synthesized from organic salts.
Fe3O4 and CoFe2O4 nanoparticles were synthesized by a high-temperature solution phase reaction of metal acetylacetonates (Fe(acac)3, Co(acac)2) with 1,2-hexadecanediol, oleic acid and oleylamine in phenyl ether [3]. For the synthesis of Fe3O4 nanoparticles the reactants (1.4 g Fe(acac)3, 5.2 g 1,2-hexadecanediol, 3.4 g oleic acid and 3.2 g oleylamine) were dissolved in phenyl ether (40 ml) and magnetically stirred under an argon flow. In the argon ambient, the mixture was heated first up to 200 °C for 30 min, then up to 265 °C for another 30 min and then cooled to room temperature. After adding ethanol, a black particulate product precipitated. It was diluted in toluene in the presence of surfactant (oleic acid and oleylamine). Nanoparticles were precipitated with ethanol, centrifuged to remove the solvent and redispersed into toluene. The CoFe2O4 nanoparticles were prepared under similar conditions. The size, shape and ordering of nanoparticles were studied by transmission electron microscopy (TEM, JEM 100C) and scanning electron microscopy (SEM, Leo 1550). The crystalline structure of nanoparticles was studied by grazing incidence Xray diffraction (GI XRD) using the D8 DISCOVER SSS diffractometer (Bruker) and by electron diffraction (ED). The
⁎ Corresponding author. E-mail address: [email protected] (L. Chitu). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.07.036
1416
L. Chitu et al. / Materials Science and Engineering C 27 (2007) 1415–1417
Fig. 1. GIXRD diffraction patterns (incidence angle of 1°) and ED micrograph of Fe3O4 and CoFe2O4 nanoparticles prepared by drying a droplet deposited onto Si/Si3N4 substrate and carbon coated grid.
elemental analysis was performed by the high sensitivity energy dispersive X-ray spectroscopy (EDS). The magnetization versus magnetic field (H ≤ ± 5 T) and temperature (4.2–278 K) were measured using the Vibrating Sample Magnetometer and diluted solution of nanoparticles in a capillary.
Fe3O4 and CoFe2O4 structures belong to the inverse spinel group with the general formula A(B2)O4. Scherrer equation applied to the main diffraction peak provides the size of coherently scattering domains of approx. 6 nm and 7 nm for Fe3O4 and CoFe2O4, respectively. The EDS spectroscopy confirmed the presence of Co in CoFe2O4 particles with the average Fe/Co ratio 2:1. Particles are nearly spherical with the radius of 3.2 ± 0.3 nm for Fe3O4 and 3.8 ± 0.27 nm for CoFe2O4 and they form hexagonally ordered arrays (Fig. 2). The particle size coincides well with that of coherently scattering domains, which suggests that the particles form a structurally homogeneous monocrystalline-like entities. The magnetization versus temperature M(T) curves were measured in the temperature range 4.2–250 K in zero-fieldcooled (ZFC) and field-cooled (FC) regimes (Fig. 3a,b). For ZFC measurements the sample was cooled down to 4.2 K. The field (between 5 and 100 mT) was turned on and the M(T) curve was measured as the sample was heated from 4.2 K up to 250 K. For FC experiments the magnetic field was applied at 250 K and the magnetization was measured as the sample was cooled down to
3. Results and discussion Fig. 1 shows GI XRD patterns of Fe3O4 and CoFe2O4 nanoparticles together with the respective ED patterns. The
Fig. 2. SEM picture of arrays of (a) Fe3O4 and (b) CoFe2O4 nanoparticles prepared by drying a droplet deposited onto Si/Si3N4 substrate.
Fig. 3. Magnetization versus temperature dependence for field-cooled (FC) and zero-field-cooled (ZFC) experiments with Fe3O4 (a: ∇—10 mT, ○—5 mT) and CoFe2O4 (b: ∇—100 mT, ○—50 mT) nanoparticles.
L. Chitu et al. / Materials Science and Engineering C 27 (2007) 1415–1417
1417
atoms in the Fe–O matrix increase magnetic anisotropy of the material [3,5,6]. In Fig. 4a,b the M(H) dependences measured for the field cooled (2 T) Fe3O4 and CoFe2O4 assemblies are presented as well. For Fe3O4 there is observed an increase of Hc from 0.022 to 0.030 T. A significant increase of Hc from 1.0 T to 1.75 T was found for CoFe2O4 being close to the maximum value 2 T reported [3]. This behavior also points at the increase of magnetic anisotropy in the CoFe2O4 system. A step-like change in ZFC M(T) dependence observed for CoFe2O4 around 136 K could point at the change of magnetic properties probably induced by the Verwey transition recently observed also for Fe nanoparticles [7]. A detail analysis of this effect will be published later. 4. Conclusion We have prepared Fe3O4 and CoFe2O4 nanoparticles of radius 3.2 ± 0.3 and 3.8 ± 0.3 nm, respectively, by a hightemperature solution phase reaction of metal acetylacetonates. The nanoparticles have spherical shape, well developed crystalline structure and they are superparamagnetic at room temperature, the blocking temperatures being 22 K for Fe3O4, and 204 K for CoFe2O4. For CoFe2O4 particles the coercivity up to 1.75 T was found. The ZFC M(T) dependence of CoFe2O4 could point a the Verwey transition at 138 K. Acknowledgements
Fig. 4. Magnetization versus field dependence (a) for Fe3O4 nanoparticles: solid line 280 K,-□- 4.2 K, low field M(H) dependence at 4.2 K is shown in the inset (-□- at 4.2 K ,-∇- at 4.2 K after cooling in magnetic field of 2 T); (b) for CoFe2O4 nanoparticles: -□- 280 K, solid line 4.2 K and -∇- after cooling in magnetic field of 2 T.
4.2 K. For both systems the ZFC/FC magnetization curves show irreversible behavior typical for superparamagnetic nanoparticles. For Fe3O4 particles the irreversibility occurs below 160 K and the blocking temperature TB = 22 K. For CoFe2O4 particles the irreversibility starts at 214 K and TB = 204 K. Above TB the nanoparticle assembly is superparamagnetic and below TB it is ferromagnetic. This is well documented by the M(H) curves measured below and above the TB (Fig. 4a,b). For Fe3O4 the coercivity Hc = 0.022 T whereas for CoFe2O4 Hc = 1.0 T at 4.2 K. The coercivity behavior indicates that Co
The work was supported by Science and Technology Assistance Agency Grant No. APVT-20-029804, Center of Excellence SAS project CE-PI I/2/2005, by Scientific Grant Agency VEGA 2/2041/25 and 2/4101/24. The CNR/SAS Common Program Project No. 12 (2004–2006) is acknowledged as well. References [1] D. Kim, Y. Zhang, J. Kehr, T. Klason, B. Bjelke, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 256. [2] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341. [3] Sh. Sun, H. Zeng, D.B. Robinson, S. Raoux, Ph.M. Rice, Sh.X. Wang, G. Li, J. Am. Chem. Soc. 126 (2004) 273. [4] A.T. Ngo, P. Bonville, M.P. Pileni, Eur. Phys. J. B9 (1999) 583. [5] T. Meron, Y. Rosenberg, Y. Lereah, G. Markovich, J. Magn. Magn. Mater. 292 (2005) 11. [6] T.Y. Kim, M.S. Lee, Y.I. Kom, C.S. Lee, J.C. Park, D. Kim, J. Phys. D: Appl. Phys. 36 (2003) 1451. [7] A. Slawska-Waniewska, A. Roig, M. Gich, L. Casas, K. Racka, N. Nedelko, E. Molins, Phys. Rev., B 70 (2004) 054412.