- Email: [email protected]
CoFe2 composite nano-particles
Journal of Alloys and Compounds 567 (2013) 73–76
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Exchange-spring effect in CoFe2O4/CoFe2 composite nano-particles Yue Zhang a, Zhi Yang b, Benpeng Zhu a, Shi Chen a, Xiaofei Yang a, Rui Xiong b,c, Yong Liu b,⇑ a
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China c Key Laboratory for the Green Preparation and Application of Functional Materials of Ministry of Education, Hubei University, Wuhan 430062, china b
a r t i c l e
i n f o
Article history: Received 26 November 2012 Received in revised form 19 February 2013 Accepted 7 March 2013 Available online 21 March 2013 Keywords: Composite materials Nano-structured materials Magentization Magnetic measurements
a b s t r a c t CoFe2O4/CoFe2 composite nano-particles were synthesized by heating CoFe2O4 nano-particles in reducing atmosphere at 300 °C and 400 °C. Their composition, microstructure and magnetic properties were investigated. It was found that the product of reducing CoFe2O4 at 300 °C is composed by CoFe2O4 nano-particles and a small amount of CoFe2 nano-alloy. Based on the negative values of Henkel plots and the twophase behavior in magnetic hysteresis loop the dominant inter-particle dipole coupling was confirmed. But in the process of reducing CoFe2O4 at 400 °C, surface layers of CoFe2 were formed on the CoFe2O4 nano-particles, and exchange coupling appeared at the interface of both phases, which was confirmed from the positive values of Henkel plots. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable attention has been paid to exchange-spring effect in soft magnetic (SM)/hard magnetic (HM) composite nano-materials [1]. This effect is attributed to exchange coupling at SM/HM interface and indicates a reversible rotation of magnetic moments in the SM phase by the magnetic field that is defined by the anisotropy field of the HM phase [2,3]. In exchange-spring magnet, the high saturation magnetization (MS) of the SM phase and the large coercivity (HC) of the HM phase are both utilized, and magnetic energy product (BH)max can also be enhanced greatly [1,2]. To form a strong exchange coupling at the HM/SM interface, sizes of the composite phases should be in nano-meter scale [1], including nano-films and powder/ceramic systems composed by nano-particles. The exchange-spring films have application foreground in perpendicular recording [4], for example, the out-ofplane coercivity of FePt/Fe exchange-spring film can be manipulated by changing the thickness of Fe layer [5–10]. Besides the composite nano-films of metal or alloy, in recent years, considerable attention has been paid to the exchange-spring effect in HM/SM composite powder/ceramic systems, such as CoFe2O4/Fe3O4 [11], CoFe2O4/ZnFe2O4 [12], Ba-ferrite/Ni0.8Zn0.2Fe2O4 [13], Ba-ferrite/Fe3O4 [14], and CoFe2O4/CoFe2 [3,15–17]. Among these composite materials, CoFe2O4/CoFe2 has some special properties: Firstly, with respect to the HM CoFe2O4, the SM CoFe2 ⇑ Corresponding author. Address: Department of Physics, Wuhan University, Wuhan 430072, China. Tel.: +86 27 68752482; fax: +86 27 68752569. E-mail address: [email protected] (Y. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.078
has a much larger MS (about 230 emu/g [16]); secondly, CoFe2O4/ CoFe2 composite powders can be easily prepared by directly reducing CoFe2O4 nano-particles; finally, with respect to some HM metal or alloy, CoFe2O4 has a good chemical stability [18]. Therefore, the exchange-spring effect in CoFe2O4/CoFe2 composite materials has attracted attention recently [3,15–17]. For example, Soares et al. have reported their work concerning the exchange-spring behavior in CoFe2O4/CoFe2 core/shell nano-particles, in their work, the thickness of CoFe2 shell is manipulated via changing the reaction time for reducing CoFe2O4 at 300 °C, and a critical thickness for the exchange-spring behavior is found [3,17]. In the present work, CoFe2O4/CoFe2 composite nano-particles were synthesized by combustion method with subsequent heat treatment in reducing atmosphere, and the impact of heat-treatment temperature on the exchange-spring effect has been studied in detail.
2. Experimental methods The CoFe2O4 nano-particles were firstly synthesized by the combustion method reported in Ref. [19]. Then the obtained powders were annealed at 300 °C and 400 °C for 0.5 h in Ar/H2 mixed atmosphere (5 vol.% H2) under a pressure of 3 atm. The products of the heat treatment at 300 °C and 400 °C are named as sample-300 and sample-400, respectively. The phases, morphologies and microstructure of both samples were characterized by using X-ray diffraction (XRD) (D8 Advanced, Cu Ka radiation), transmission electron microscope (TEM) (JEM2010) and high resolution transmission electron microscope (HRTEM) (JEM2010 FEF). A vibrating-sample magnetometer (VSM) on a physical property measurement system (PPMS-9, Quantum Design) was utilized to measure magnetic hysteresis loops and Henkel plots. The hysteresis loops were collected at 300 K and 10 K with applied field changing between 50,000 Oe and 50,000 Oe. The Henkel plots (dm) were collected by measuring the magnetizing
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Fig. 1. The XRD patterns for the products of reducing CoFe2O4 at 300 °C and 400 °C.
Mr(H) and demagnetizing Md(H) remanent magnetization [3]: The Mr(H) plot was measured after a field H(H > 0) was applied and removed, this procedure was repeated with the gradual increase in field till the sample was saturated magnetized. After that, the Md(H) was measured by reducing the field from the positive saturated one to a negative one H(H < 0) and then removing it, this procedure was also repeated till the sample was saturated magnetized under the negative field. Finally, the data of Mr(H) and Md(H) were normalized by the saturation remanent magnetization: mr = Mr(H)/Mr(Hsat) and md = Md(H)/Md(Hsat), and the dm = md (1 2mr).
3. Results and discussion Fig. 1 shows the XRD patterns of both samples, it can be seen that sample-300 is composed by CoFe2O4 and a small amount of CoFe2, but for sample-400, the peak intensity for CoFe2 is clearly
enhanced, indicating a great increase in the amount of CoFe2. By using the data of peak positions and full-width at half maximum (the broading from instrument has been removed by the method reported in Ref. [20]), the average crystallite sizes (Dm) for both phases were determined by Scherrer formula. For sample-300, the Dm of CoFe2O4 and CoFe2 are 25.1 nm and 20.4 nm, respectively, but for sample-400, the Dm of CoFe2O4 and CoFe2 increased to 39.3 nm and 32.7 nm, respectively. The TEM images for sample-300 and sample-400 are shown in Fig. 2a and b, respectively. It can be seen that both samples are composed by aggregated nano-particles. For sample-300, the sizes of most particles are between 20 and 30 nm, consistent with the sizes determined from the XRD data. Since we did not observe particles with sizes larger than 40 nm, it is reasonable to think that CoFe2O4 and CoFe2 may exist individually. But in sample-400, it is noticed that particles are tightly aggregated and the sizes of many particles are clearly larger than the crystallite sizes determined by XRD, which hints the complicated microstructure in the particles due to the likely coupling of CoFe2O4 and CoFe2. To confirm this coupling, the microstructure in the particles was observed via HRTEM, as shown in Fig. 2c and d. In Fig. 2c one can see that the particle can be divided into two parts: an inner core and a rough surface layer with the thickness of 30–40 nm, and the nano-particles are connected via some superficial material. In Fig. 2d, the image with a higher resolution, one can see the complicated microstructure in a particle, and the districts separated by lines with different colors show different crystalline properties. In the inner regions enclosed by the yellow and blue lines, a lattice image with the interplanar distance of 0.48 nm was clearly observed, showing the (1 1 1) plane of CoFe2O4 [16]. Besides, the lattice image with the interplanar distance of 0.29 nm was also shown in some outer regions, and a rough surface layer with the thickness of about 30 nm can be seen between the red and yellow
Fig. 2. The TEM images for the products of reducing CoFe2O4 at (a) 300 °C and (b) 400 °C; (c) and (d): the HRTEM images for the products of reducing CoFe2O4 at 400 °C.
Y. Zhang et al. / Journal of Alloys and Compounds 567 (2013) 73–76
75
Fig. 4. The Henkel plots for the products of reducing CoFe2O4 at 300 °C and 400 °C.
Fig. 3. The magnetic hysteresis loops measured at 10 K and 300 K for the products of reducing CoFe2O4 at (a) 300 °C and (b) 400 °C.
Table 1 The magnetic properties for the products of reducing CoFe2O4 at 300 °C and 400 °C. Sample
T (K)
Sample-300
10 300
MS (emu/g) 67.8 65.0
10,168 1614
Sample-400
10 300
151.2 145.4
1574 730
Fig. 5. The dD(H)/dH–H plots for the products of reducing CoFe2O4 at 300 °C and 400 °C.
HC (Oe)
lines. The lattice image in this surface layer is not clear but still detectable, and the interplanar distance is also 0.29 nm. These additional lattice images indicate the reduction of CoFe2O4 to CoFe2 near particle surface [16], and the formed CoFe2 surface layers are coupled to the inner CoFe2O4 nano-crystals. The magnetic hysteresis loops for sample-300 and sample-400 are shown in Fig. 3a and b, respectively. The magnetic parameters of MS and HC were determined via the methods in Ref. [20] and are shown in Table. 1. It can be seen that both samples exhibit ferromagnetic-like behavior at 300 K. But with respect to sample-300, sample-400 has a much larger MS and a clearly smaller HC, which is attributed to the larger amount of SM CoFe2 in sample-400. At 10 K, the difference of magnetic properties between them becomes much bigger: For sample-300, the HC increases to be larger than 10,000 Oe, and steps are clearly shown in the loop, indicating the dominant role of the HM CoFe2O4 and the lack of exchange coupling at the interfaces of CoFe2O4 and CoFe2 [2]; For sample400, the loop has a single shape with a much smaller HC, which may be ascribed to the formation of exchange coupling at the interfaces. However, the possibility for covering up the steps due to the small hysteresis should also be taken into account (just like the
hysteresis loop measured at 300 K for sample-300). Therefore, the Henkel plots were collected for a precise analysis on the inter-particle magnetic interaction. Fig. 4 shows the Henkel plots for both samples. For sample-300, the dm plots have negative values under almost all fields, indicating the dominant inter-particle dipole interactions. This result is consistent with the appearance of the steps in the magnetic hysteresis loop. However, the Henkel plots of sample-400 are clearly different: As the applied fields are weaker than 2000 Oe, the dm plots are negative with larger absolute values, indicating the stronger inter-particle dipole interactions, which may be attributed to the larger magnetization from the SM CoFe2 and the smaller interparticle distance due to the tight aggregation of the nano-particles. However, as the applied fields become larger than 3000 Oe, the dm values become positive, indicating the appearance for the interparticle exchange coupling. As the particles in sample-400 are composed by inner CoFe2O4 nano-crystals and CoFe2 surface layers, it is reasonable to think that the exchange coupling at the interface of both phases may have the major contribution. From the inset figure of Fig. 3 in Ref. [3], one can see that the high-field dm plots can also become positive by prolonging the time for reducing CoFe2O4 nano-particles at a lower temperature (300 K), but the absolute values for the negative low-field dm become smaller. Therefore, it is concluded that in the composite powders of the nano-sized CoFe2O4 and CoFe2, the inter-phase exchange coupling can be formed either by prolonging heating
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time or by raising heating temperature. What is different is that after the heat treatment at the higher temperature, the inter-particle dipole interactions can also be greatly enhanced. The magnetic field required for nucleating irreversible magnetization reversal (Hn) is also a critical parameter for exchange-spring effect, and the Hn can be experimentally determined from the field for the peak in dD(H)/dH plots, where D(H) = [Mr Md(H)]/2Mr [3]. The dD(H)/dH versus H curves were shown in Fig. 5. As shown in this figure, the Hn of sample-300 is about 1500 Oe, but for sample-400, it shifts to 1000 Oe. It is noticed that the Hn decreases with the increase in the sizes of the SM CoFe2 phase, this is consistent with the result reported in Ref. [3]. 4. Conclusions In conclusion, the nano-sized composite particles of CoFe2O4 and CoFe2 were synthesized by reducing CoFe2O4 nano-particles at 300 °C and 400 °C. In the samples heated at 300 °C, the CoFe2O4 and CoFe2 nano-particles exist individually, and the inter-particle dipole interactions are the dominant, but in the samples heated at 400 °C, surface layers of CoFe2 were formed on the CoFe2O4 nano-particles, and the amount of the CoFe2 is greatly increased, as a result, the saturation magnetization is enhanced, the coercivity is reduced and inter-particle exchange interactions appear. Acknowledgments The authors would like to acknowledge the financial support from the Chinese National Foundation of Natural Science (Nos. 51172166, 51202078, 11205116), the Fundamental Research
Funds for the Central Universities (No. 111042), Funding of Wuhan University (No. 2012202020203) and the Huazhong University of Science and Technology (No. 01-18-185011). References [1] S.D. Bader, Rev. Mod. Phys. 78 (2006) 1–15. [2] E.F. Kneller, IEEE Trans. Magn. 27 (1991) 3588–3600. [3] J.M. Soares, F.A.O. Cabral, J.H. de Araújo, F.L.A. Machado, Appl. Phys. Lett. 98 (2011) 072502. [4] D. Suess, T. Schrefl, S. Fähler, M. Kirschner, G. Hrkac, F. Dorfbauer, J. Fidler, Appl. Phys. Lett. 87 (2005) 012504. [5] J.L. Tsai, H.T. Tzeng, B.F. Liu, Thin Solid Films 518 (2010) 7271–7274. [6] L.S. Huang, J.F. Hu, J.S. Chen, J. Magn. Magn. Meter. 324 (2012) 1242–1247. [7] B. Ma, H. Wang, H. Zhao, C. Sun, R. Acharya, J. Appl. Phys. 109 (2011) 083907. [8] J.L. Tsai, H.T. Tzeng, G.B. Lin, Appl. Phys. Lett. 96 (2010) 032505. [9] D. Goll, A. Breitling, L. Gu, P.A. van Aken, W. Sigle, J. Appl. Phys. 104 (2008) 083903. [10] G. Asti, M. Ghidini, R. Pellicelli, C. Pernechele, M. Solzi, F. Albertini, F. Casoli, S. Fabbrici, L. Pareti, Phys. Rev. B 73 (2006) 094406. [11] C.L. Fei, Y. Zhang, Z. Yang, Y. Liu, R. Xiong, J. Shi, X.F. Ruan, J. Magn. Magn. Meter. 323 (2011) 1811–1816. [12] O. Masala, D. Hoffman, N. Sundaram, K. Page, T. Proffen, G. Lawes, R. Seshadri, Solid State Sci. 8 (2006) 1015–1022. [13] D. Roy, C. Shivakumara, P.S. AnilKumar, J. Appl. Phys. 321 (2009) L11–L14. [14] D. Roy, P.S. Anil Kumar, J. Appl. Phys. 106 (2009) 073902. [15] K. Zhang, O. Amponsah, M. Arslan, T. Holloway, D. Sinclair, W. Cao, M. Bahoura, A.K. Pradhan, J. Magn. Magn. Meter. 324 (2012) 1938–1944. [16] G.C.P. Leite, E. FChagas, R. Pereira, R.J. Prado, A.J. Terezo, M. Alzamora, E. Baggio-Saitovitch, J. Magn. Magn. Meter. 324 (2012) 2711–2716. [17] J.M. Soares, V.B. Galdino, O.L.A. Conceição, M.A. Morales, J.H. deAraújo, F.L.A. Machado, J. Magn. Magn. Mater. 326 (2013) 81–84. [18] D. Zhao, X. Wu, H. Guan, E. Han, J. Supercrit. Fluids 42 (2007) 226–233. [19] Y. Liu, Y. Zhang, J.D. Feng, C.F. Li, J. Shi, R. Xiong, J. Exp. Nanosci. 4 (2009) 159– 168. [20] M.Y. Rafique, L.Q. Pan, Q. Javed, M.Z. Iqbal, L.H. Yang, J. Nanopart. Res. 14 (2012) 1189.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Exchange-spring effect in CoFe2O4/CoFe2 composite nano-particles Yue Zhang a, Zhi Yang b, Benpeng Zhu a, Shi Chen a, Xiaofei Yang a, Rui Xiong b,c, Yong Liu b,⇑ a
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China c Key Laboratory for the Green Preparation and Application of Functional Materials of Ministry of Education, Hubei University, Wuhan 430062, china b
a r t i c l e
i n f o
Article history: Received 26 November 2012 Received in revised form 19 February 2013 Accepted 7 March 2013 Available online 21 March 2013 Keywords: Composite materials Nano-structured materials Magentization Magnetic measurements
a b s t r a c t CoFe2O4/CoFe2 composite nano-particles were synthesized by heating CoFe2O4 nano-particles in reducing atmosphere at 300 °C and 400 °C. Their composition, microstructure and magnetic properties were investigated. It was found that the product of reducing CoFe2O4 at 300 °C is composed by CoFe2O4 nano-particles and a small amount of CoFe2 nano-alloy. Based on the negative values of Henkel plots and the twophase behavior in magnetic hysteresis loop the dominant inter-particle dipole coupling was confirmed. But in the process of reducing CoFe2O4 at 400 °C, surface layers of CoFe2 were formed on the CoFe2O4 nano-particles, and exchange coupling appeared at the interface of both phases, which was confirmed from the positive values of Henkel plots. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable attention has been paid to exchange-spring effect in soft magnetic (SM)/hard magnetic (HM) composite nano-materials [1]. This effect is attributed to exchange coupling at SM/HM interface and indicates a reversible rotation of magnetic moments in the SM phase by the magnetic field that is defined by the anisotropy field of the HM phase [2,3]. In exchange-spring magnet, the high saturation magnetization (MS) of the SM phase and the large coercivity (HC) of the HM phase are both utilized, and magnetic energy product (BH)max can also be enhanced greatly [1,2]. To form a strong exchange coupling at the HM/SM interface, sizes of the composite phases should be in nano-meter scale [1], including nano-films and powder/ceramic systems composed by nano-particles. The exchange-spring films have application foreground in perpendicular recording [4], for example, the out-ofplane coercivity of FePt/Fe exchange-spring film can be manipulated by changing the thickness of Fe layer [5–10]. Besides the composite nano-films of metal or alloy, in recent years, considerable attention has been paid to the exchange-spring effect in HM/SM composite powder/ceramic systems, such as CoFe2O4/Fe3O4 [11], CoFe2O4/ZnFe2O4 [12], Ba-ferrite/Ni0.8Zn0.2Fe2O4 [13], Ba-ferrite/Fe3O4 [14], and CoFe2O4/CoFe2 [3,15–17]. Among these composite materials, CoFe2O4/CoFe2 has some special properties: Firstly, with respect to the HM CoFe2O4, the SM CoFe2 ⇑ Corresponding author. Address: Department of Physics, Wuhan University, Wuhan 430072, China. Tel.: +86 27 68752482; fax: +86 27 68752569. E-mail address: [email protected] (Y. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.078
has a much larger MS (about 230 emu/g [16]); secondly, CoFe2O4/ CoFe2 composite powders can be easily prepared by directly reducing CoFe2O4 nano-particles; finally, with respect to some HM metal or alloy, CoFe2O4 has a good chemical stability [18]. Therefore, the exchange-spring effect in CoFe2O4/CoFe2 composite materials has attracted attention recently [3,15–17]. For example, Soares et al. have reported their work concerning the exchange-spring behavior in CoFe2O4/CoFe2 core/shell nano-particles, in their work, the thickness of CoFe2 shell is manipulated via changing the reaction time for reducing CoFe2O4 at 300 °C, and a critical thickness for the exchange-spring behavior is found [3,17]. In the present work, CoFe2O4/CoFe2 composite nano-particles were synthesized by combustion method with subsequent heat treatment in reducing atmosphere, and the impact of heat-treatment temperature on the exchange-spring effect has been studied in detail.
2. Experimental methods The CoFe2O4 nano-particles were firstly synthesized by the combustion method reported in Ref. [19]. Then the obtained powders were annealed at 300 °C and 400 °C for 0.5 h in Ar/H2 mixed atmosphere (5 vol.% H2) under a pressure of 3 atm. The products of the heat treatment at 300 °C and 400 °C are named as sample-300 and sample-400, respectively. The phases, morphologies and microstructure of both samples were characterized by using X-ray diffraction (XRD) (D8 Advanced, Cu Ka radiation), transmission electron microscope (TEM) (JEM2010) and high resolution transmission electron microscope (HRTEM) (JEM2010 FEF). A vibrating-sample magnetometer (VSM) on a physical property measurement system (PPMS-9, Quantum Design) was utilized to measure magnetic hysteresis loops and Henkel plots. The hysteresis loops were collected at 300 K and 10 K with applied field changing between 50,000 Oe and 50,000 Oe. The Henkel plots (dm) were collected by measuring the magnetizing
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Y. Zhang et al. / Journal of Alloys and Compounds 567 (2013) 73–76
Fig. 1. The XRD patterns for the products of reducing CoFe2O4 at 300 °C and 400 °C.
Mr(H) and demagnetizing Md(H) remanent magnetization [3]: The Mr(H) plot was measured after a field H(H > 0) was applied and removed, this procedure was repeated with the gradual increase in field till the sample was saturated magnetized. After that, the Md(H) was measured by reducing the field from the positive saturated one to a negative one H(H < 0) and then removing it, this procedure was also repeated till the sample was saturated magnetized under the negative field. Finally, the data of Mr(H) and Md(H) were normalized by the saturation remanent magnetization: mr = Mr(H)/Mr(Hsat) and md = Md(H)/Md(Hsat), and the dm = md (1 2mr).
3. Results and discussion Fig. 1 shows the XRD patterns of both samples, it can be seen that sample-300 is composed by CoFe2O4 and a small amount of CoFe2, but for sample-400, the peak intensity for CoFe2 is clearly
enhanced, indicating a great increase in the amount of CoFe2. By using the data of peak positions and full-width at half maximum (the broading from instrument has been removed by the method reported in Ref. [20]), the average crystallite sizes (Dm) for both phases were determined by Scherrer formula. For sample-300, the Dm of CoFe2O4 and CoFe2 are 25.1 nm and 20.4 nm, respectively, but for sample-400, the Dm of CoFe2O4 and CoFe2 increased to 39.3 nm and 32.7 nm, respectively. The TEM images for sample-300 and sample-400 are shown in Fig. 2a and b, respectively. It can be seen that both samples are composed by aggregated nano-particles. For sample-300, the sizes of most particles are between 20 and 30 nm, consistent with the sizes determined from the XRD data. Since we did not observe particles with sizes larger than 40 nm, it is reasonable to think that CoFe2O4 and CoFe2 may exist individually. But in sample-400, it is noticed that particles are tightly aggregated and the sizes of many particles are clearly larger than the crystallite sizes determined by XRD, which hints the complicated microstructure in the particles due to the likely coupling of CoFe2O4 and CoFe2. To confirm this coupling, the microstructure in the particles was observed via HRTEM, as shown in Fig. 2c and d. In Fig. 2c one can see that the particle can be divided into two parts: an inner core and a rough surface layer with the thickness of 30–40 nm, and the nano-particles are connected via some superficial material. In Fig. 2d, the image with a higher resolution, one can see the complicated microstructure in a particle, and the districts separated by lines with different colors show different crystalline properties. In the inner regions enclosed by the yellow and blue lines, a lattice image with the interplanar distance of 0.48 nm was clearly observed, showing the (1 1 1) plane of CoFe2O4 [16]. Besides, the lattice image with the interplanar distance of 0.29 nm was also shown in some outer regions, and a rough surface layer with the thickness of about 30 nm can be seen between the red and yellow
Fig. 2. The TEM images for the products of reducing CoFe2O4 at (a) 300 °C and (b) 400 °C; (c) and (d): the HRTEM images for the products of reducing CoFe2O4 at 400 °C.
Y. Zhang et al. / Journal of Alloys and Compounds 567 (2013) 73–76
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Fig. 4. The Henkel plots for the products of reducing CoFe2O4 at 300 °C and 400 °C.
Fig. 3. The magnetic hysteresis loops measured at 10 K and 300 K for the products of reducing CoFe2O4 at (a) 300 °C and (b) 400 °C.
Table 1 The magnetic properties for the products of reducing CoFe2O4 at 300 °C and 400 °C. Sample
T (K)
Sample-300
10 300
MS (emu/g) 67.8 65.0
10,168 1614
Sample-400
10 300
151.2 145.4
1574 730
Fig. 5. The dD(H)/dH–H plots for the products of reducing CoFe2O4 at 300 °C and 400 °C.
HC (Oe)
lines. The lattice image in this surface layer is not clear but still detectable, and the interplanar distance is also 0.29 nm. These additional lattice images indicate the reduction of CoFe2O4 to CoFe2 near particle surface [16], and the formed CoFe2 surface layers are coupled to the inner CoFe2O4 nano-crystals. The magnetic hysteresis loops for sample-300 and sample-400 are shown in Fig. 3a and b, respectively. The magnetic parameters of MS and HC were determined via the methods in Ref. [20] and are shown in Table. 1. It can be seen that both samples exhibit ferromagnetic-like behavior at 300 K. But with respect to sample-300, sample-400 has a much larger MS and a clearly smaller HC, which is attributed to the larger amount of SM CoFe2 in sample-400. At 10 K, the difference of magnetic properties between them becomes much bigger: For sample-300, the HC increases to be larger than 10,000 Oe, and steps are clearly shown in the loop, indicating the dominant role of the HM CoFe2O4 and the lack of exchange coupling at the interfaces of CoFe2O4 and CoFe2 [2]; For sample400, the loop has a single shape with a much smaller HC, which may be ascribed to the formation of exchange coupling at the interfaces. However, the possibility for covering up the steps due to the small hysteresis should also be taken into account (just like the
hysteresis loop measured at 300 K for sample-300). Therefore, the Henkel plots were collected for a precise analysis on the inter-particle magnetic interaction. Fig. 4 shows the Henkel plots for both samples. For sample-300, the dm plots have negative values under almost all fields, indicating the dominant inter-particle dipole interactions. This result is consistent with the appearance of the steps in the magnetic hysteresis loop. However, the Henkel plots of sample-400 are clearly different: As the applied fields are weaker than 2000 Oe, the dm plots are negative with larger absolute values, indicating the stronger inter-particle dipole interactions, which may be attributed to the larger magnetization from the SM CoFe2 and the smaller interparticle distance due to the tight aggregation of the nano-particles. However, as the applied fields become larger than 3000 Oe, the dm values become positive, indicating the appearance for the interparticle exchange coupling. As the particles in sample-400 are composed by inner CoFe2O4 nano-crystals and CoFe2 surface layers, it is reasonable to think that the exchange coupling at the interface of both phases may have the major contribution. From the inset figure of Fig. 3 in Ref. [3], one can see that the high-field dm plots can also become positive by prolonging the time for reducing CoFe2O4 nano-particles at a lower temperature (300 K), but the absolute values for the negative low-field dm become smaller. Therefore, it is concluded that in the composite powders of the nano-sized CoFe2O4 and CoFe2, the inter-phase exchange coupling can be formed either by prolonging heating
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Y. Zhang et al. / Journal of Alloys and Compounds 567 (2013) 73–76
time or by raising heating temperature. What is different is that after the heat treatment at the higher temperature, the inter-particle dipole interactions can also be greatly enhanced. The magnetic field required for nucleating irreversible magnetization reversal (Hn) is also a critical parameter for exchange-spring effect, and the Hn can be experimentally determined from the field for the peak in dD(H)/dH plots, where D(H) = [Mr Md(H)]/2Mr [3]. The dD(H)/dH versus H curves were shown in Fig. 5. As shown in this figure, the Hn of sample-300 is about 1500 Oe, but for sample-400, it shifts to 1000 Oe. It is noticed that the Hn decreases with the increase in the sizes of the SM CoFe2 phase, this is consistent with the result reported in Ref. [3]. 4. Conclusions In conclusion, the nano-sized composite particles of CoFe2O4 and CoFe2 were synthesized by reducing CoFe2O4 nano-particles at 300 °C and 400 °C. In the samples heated at 300 °C, the CoFe2O4 and CoFe2 nano-particles exist individually, and the inter-particle dipole interactions are the dominant, but in the samples heated at 400 °C, surface layers of CoFe2 were formed on the CoFe2O4 nano-particles, and the amount of the CoFe2 is greatly increased, as a result, the saturation magnetization is enhanced, the coercivity is reduced and inter-particle exchange interactions appear. Acknowledgments The authors would like to acknowledge the financial support from the Chinese National Foundation of Natural Science (Nos. 51172166, 51202078, 11205116), the Fundamental Research
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