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CoO “lines” with large exchange bias
Physics Letters A 307 (2003) 69–75 www.elsevier.com/locate/pla
Magnetic anisotropy in carbon encapsulated Co/CoO “lines” with large exchange bias Hong Bi a , Shandong Li b , Xiqun Jiang a,∗ , Youwei Du b , Changzheng Yang a a Department of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China b National Laboratory of Solid State Microstructure, and Department of Physics, Nanjing University, Nanjing 210093, PR China
Received 27 August 2002; accepted 28 November 2002 Communicated by J. Fluoquet
Abstract Carbon encapsulated cobalt nanoparticles were prepared by Co(II) reduction and post-annealing in inert argon gas. Cobalt nanoparticles form parallel “lines” with approximately equal space of 35 µm on the single-crystal silica substrate in the presence of a magnetic field of 3 kGs. Magnetic measurement results indicate that this system exhibits a strong magnetic anisotropy. Along the line direction, it was ferromagnetic with coercive force of 85 Oe, while perpendicular to the substrate plane, only a narrow hysteresis loop was observed. The samples were oxidized at 573 K for various times under air atmosphere. Coercivity was enhanced due to the exchange coupling between ferromagnetic Co and antiferromagnetic CoO. Compared with the random dispersed Co/CoO nanoparticles that was prepared under the same annealing and oxidation condition, a larger exchange bias was obtained in the magnetic aligned specimen. The mechanism of coercivity enhancement and larger exchange bias was discussed. 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Tt; 75.30.Et Keywords: Magnetic anisotropy; Co/CoO; Exchange coupling; Coercivity; Alignment
1. Introduction Recently, there have been immense interests in building advanced materials using nanoscale building blocks due to the special properties in nanoparticles [1–7]. When the size of particles decreases into nanoscale which is near or smaller than the charac* Corresponding author.
E-mail addresses: [email protected] (H. Bi), [email protected] (X. Jiang).
teristic length such as magnetic exchange coupling length, superconductor electron coherent length, light wavelength, etc., the finite size effect, quantum size effect, surface and interface effect are taking place leading to anomalous physical and chemical properties. Such nanoparticles are ideal building blocks for two- and three-dimensional functional nanostructured materials, which can be used extensively in biological detection, magnetism, microelectronics and photonics. For example, colloidal metal or semiconductor quantum dots can be used as fluorescent biological
0375-9601/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-9601(02)01678-X
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H. Bi et al. / Physics Letters A 307 (2003) 69–75
labels or probes in DNA detection [4,5,8]. Magnetic nanoparticles buried in a dielectric or nonmagnetic metal matrix may exhibit oscillatory coupling and giant magnetoresistance properties [3,9], and a tightly packed assembly of exchange-decoupled magnetic nanoparticles may serve as future ultrahigh-density data storage media [10]. Also, it has been observed that spin-dependent tunneling in self-assembled cobalt nanocrystal superlattice [11], and two-dimensional arrays of spin-dependent tunnel junctions show promise for nonvolatile memory applications [12]. Recently, state-of-the-art nanofabrication has succeeded in making three-dimensional semiconductor photonic crystals with a bandgap at 1.5 µm [13,14]. Meiklejohn and Bean first discovered the exchange anisotropy effect, which originates from a strong exchange coupling between the ferromagnetic (FM) Co core and the antiferromagnetic (AFM) CoO layer in oxide-coated Co particles [15]. Much attention has been focused on the magnetic properties and the unidirectional exchange anisotropy in oxide-passivated magnetic transition-metal particles including Fe, Co, Ni because of their potential application [16,17]. Cobalt nanocrystals system was often chosen to study because it displays a wealth of size-dependent structural, magnetic, electronic, and catalytic properties [18]. Compared the magnetic properties of Co with those of Fe and Ni, Co presents a high uniaxial anisotropy and has potential application as highdensity data storage material. Co/CoO system exhibits relatively high coercivity, large exchange bias, and magnetoresistance effect. Much recently, one-, two-, and three-dimensional self-assembled arrays of cobalt nanoparticles have been successfully obtained by chemical solution phase method [19–21]. It is observed that various shapes of cobalt assemblies can be obtained by changing the applied external magnetic field. For example, regular stripes of cobalt nanocrystals are formed by applying a magnetic field parallel to the substrate during the deposition process [19], onedimensional ordered, densely packed assemblies have also been prepared using cobalt nanocrystals with irregular shape and large particle sizes (10–20 nm) under an external magnetic field [20]. The formation of these different structures is attributed to the orientation of the easy axes and ferromagnetic domain formation in cobalt nanoparticles with different sizes under the applied magnetic field.
Similarly, Davidov et al. reported a multifunctional material fabricated by immersing magnetic beads in oil, and these magnetic particles form parallel “lines” in the direction of the external magnetic field [22]. The total length of these lines may exceed 3 mm, their average width is roughly 5–10 µm due to aggregation. Changing the direction of the in-plane magnetic field leads to formation of new lines in the same direction with relatively fast response time less than 1 s. In this magnetic “lines” system, an anisotropy reflection of polarized microwaves has been observed, which indicates that these “lines” are somewhat conducting. And the conducting “lines” can be switched into a nonconductive state by an external field, suggesting it can be used as a tunable polarizer for microwaves and milliwaves. In this Letter, a magnetic functional material, carbon encapsulated Co/CoO magnetic “lines” was prepared showing a strong magnetic anisotropy and high exchange bias field.
2. Experimental procedure A typical procedure is as follows: 2.0 g styrene/ maleic acid anhydride random copolymer (SMA) was dissolved in 50 ml N, N -dimethylamide (DMF) at 353 K in a three-neck bottle, then 1.20 g CoCl2 · 6H2 O was put into it, the solution was kept stirring under 353 K for 2 h, then it was heated to 393 K with N2 bubbling up. 1.0 g NaBH4 was pre-dissolved in 50 ml DMF, then the reducing reagent was dropped into the solution quickly, the black-blue transparent solution turned into black viscous colloid very fast, the reaction was kept more 30 minutes under the N2 protection, then the black colloid was cooled to 323 K naturally and filtered. The black waxy product dried in air at 323 K over 72 h. The resulting powder was washed by anhydrous alcohol for three times to clean remanent chemicals and by-product NaCl, then dried in air at 323 K again. The purified and dried powder was annealed in a vacuum furnace with pressure less than 3 × 10−3 Pa. The sample was heated in a low heating rate. At about 673 K, the decomposed gas arising from the decomposition of organic matrix was evacuated continuously. At the end of the decomposition, inert argon gas was filled to 0.96 atm, and the temperature was kept at 833 K for 8 h. The annealed specimen was redispersed in anhydrous alcohol solution un-
H. Bi et al. / Physics Letters A 307 (2003) 69–75
der ultrasonic vibration, and centrifugally separated. Then it was dropped on the smooth surface of a singlecrystal silica substrate in the presence of a magnetic field of 3 kGs parallel to the substrate surface. The solution was slowly evaporated at room temperature. In situ oxidation at 573 K for various times was adopted in order to enhance the coercivity. The microstructure of the annealed nanoparticles has been characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS, Thermo NORAN). The morphology of the annealed sample on the Si substrate was obtained by using scanning electron microscopy (SEM). Magnetic properties of the samples were measured by highsensitivity vibrating sample magnetometer (VSM) with the resolution of 10−6 emu. Exchange bias was measured in the presence of a cooling field of 10 kGs from 320 K to 100 K.
3. Results and discussion 3.1. Formation of the magnetic “lines” The HRTEM image (Fig. 1, left) of an individual nanoparticle confirms that the Co nanocrystal in the size of 30 nm is surrounded by an amorphous carbon
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layer after annealing. It arises from the polymer SMA carbonized at the temperature of 833 K. Fringes inside the nanoparticle with spacing of 1.77 Å correspond to {200} lattice spacing of the fcc-Co phase. The EDS spectrum (Fig. 1, right) shows carbon, cobalt and copper are present in the annealed sample grid. Cu is from copper grid, and carbon is in part from carbon film on the surface of the copper grid, and partially from the polymer carbonization. From the EDS, the content of Co is much more than that of oxygen in the sample, and considering that no CoO, Co2 O3 , Co3 O4 or Co2 C phases were observed in the XRD (Fig. 3(1)), it can be concluded that carbon layer prevent the cobalt core from oxidation and no cobalt carbide formed during annealing in the inert atmosphere. Fig. 2 shows the SEM images of the magnetic lines consisting of above-mentioned carbon encapsulated cobalt nanoparticles on a Si substrate under an external field. An in-plane applied magnetic field aligned the magnetic particles along parallel “lines” (more or less equally spaced) in the direction of the external field. The average length of these lines may exceed 300 µm, and their average width is roughly 5– 10 µm. It is difficult to achieve this regular structure due to many restrictive conditions, such as the magnitude of aligned magnetic field, concentration of particles in solution, the evaporating rate, and the smoothness of the substrate surface. Under the op-
Fig. 1. HRTEM image and EDS spectrum of cobalt nanoparticle surrounded by a carbon layer after annealing.
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Fig. 2. SEM images of the magnetic lines prepared in the presence of a magnetic field parallel to the substrate surface. The inset gives a magnified image of a small section of the “line”.
timum condition of the external field of 3 kGs parallel to the surface substrate, suitable solution with reasonable evaporating rate, and narrow distributed fine particles, regular arranged magnetic lines were achieved. Formation of such lines in the presence of an external field is analogous to the situation in ferrofluids as well as in other magnetic nanoparticle system [23,24]. Thanks to the interaction between the dipole moments induced by the external filed, along the direction of field the nanoparticles attract mutually, however, perpendicular to the direction of external field repulsive forces exist. As a consequence, with the slow evaporation of the solution, the nanoparticles contact with each other, form a magnetic “line” in the direction of the field. Along the perpendicular direction, narrow size-distributed nanoparticles repulse mutually leading to more or less equally spaced lines. The formation of parallel “lines” has been simulated by Meakin and Skjeltorp via using Monte Carlo method [25].
3.2. Magnetic anisotropy and exchange bias of the magnetic lines It is well known that cobalt nanoparticles can be oxidized forming a CoO shell around the Co core, coercivity is expected to be enhanced by exchange coupling interaction between AFM CoO and FM Co [26,27]. In our system, the aligned magnetic lines were in situ oxidized at 573 K for various times in order to promote coercivity. XRD patterns shown in Fig. 3 reveal the structural evolution of samples during the air oxidation process. Before the air oxidation, the crystalline compositions of the sample (Fig. 3(1)) include face-centered cubic (fcc)-Co (61.1 vol%), c.p. hexagonal (hcp)-Co (38.9 vol%) and NaCl phases. It is in agreement with the fact: there are two stable structures of bulk cobalt material, fcc-Co and hcp-Co. Although fcc-Co is more stable than hcp-Co in higher temperature of 723 K, in practice, many cobalt nanoparticles display mixed structures with hcp and fcc character at room temperature [28]. NaCl is a remanent by-product
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Table 1 Variation of the coercivity at room temperature with the oxidation time. (Hc )0 represents the coercivity of the original sample with zero oxidation time
Fig. 3. The XRD spectra of nanoparticles (1) before the air oxidation and (2) after the air oxidation. ( NaCl, hcp-Co, • fcc-Co, CoO).
Time (min)
Hc (Oe)
Hc /(Hc )0
0 30 60 90 120 150 210 300
39.3 48.7 60.9 84.2 62.6 54.4 41.7 40.6
1.00 1.24 1.55 2.14 1.59 1.38 1.06 1.03
was obtained, however, perpendicular to the surface substrate only a narrow loop was observed. This fact suggests that these magnetic lines exhibit a strong magnetic anisotropy. It can be explained in terms of magnetism and microstructure of the nanoparticles. The coercivity of materials can be expressed by modified Brown’s equation: Hc = α
Fig. 4. The hysteresis loops of magnetic lines both along the lines direction and perpendicular to the substrate surface.
of the reaction. When the sample was oxidized in air at 573 K for various times, the composition change into hcp-Co, NaCl and CoO (Fig. 3(2)), showing the coexistence of hcp-Co phase and CoO phase. Owing to the small quantity of magnetic particles on the substrate, a high sensitive VSM with resolution of 10−6 emu was used to measure their magnetic properties. Shown in Fig. 4 are the hysteresis loops of magnetic lines both along the lines direction and perpendicular to the substrate surface. It can be seen that along the direction of the lines a standard loop
2K1 − Neff MS , MS
(1)
where α is microstructure parameter of magnet which includes the influence of defects and orientation of particles on the coercivity. Neff represents the effective demagnetization factor that includes the selfdemagnetization interaction of particles themselves and dipolar interaction between particles. In the field of 3 kGs, the magnetization M of both directions reaches to saturation in the same value (see Fig. 4). Therefore, the large coercivity along the line’s direction can be attributed to the reduction of Neff . It is clear that along the line direction the particles aligned in the same direction, the dipolar interaction among the particles results in the reduction of Neff . In other words, the magnetic lines arise from induced assembly of nanoparticles in presence of a magnetic field, the magnetic anisotropy results from induction of the field. The induced-assembly magnetic “lines” were in situ oxidized at 573 K for various time from 30 to 300 min under air atmosphere. VSM measurement results (shown in Table 1) display that along the line’s direction the coercivity of all oxidized samples is enhanced due to the oxidation. The coercivity reaches to the maximum when the sample was oxi-
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dized for 90 min. When the oxidation time is longer than 150 min, the variation of coercivity is small due to the passivation of Co core that is covered by a thick CoO layer and carbon layer. However, along the perpendicular direction only a little increase of coercivity took place. So we focus our discussion on the magnetic properties along the line direction. Two kinds of cobalt oxides of CoO and Co3 O4 can be usually achieved via oxidation of the cobalt nanoparticles [29]. In the products of cobalt oxides, to obtain CoO is difficult due to the metastable medial valence of Co2+ , a rigid oxidation condition should be adopted. In the Co/CoO nanoparticles system, the enhanced coercivity has been widely investigated, which originated from the exchange coupling interaction between AFM CoO and FM Co [30]. It is clear that volume fraction of CoO increases with increase of the oxidation time [31]. Therefore, the variation of coercivity in Table 1 is attributed to the change of relative volume fraction of FM Co and AFM CoO. The relationship between coercivity and grain sizes of AFM and FM has been reviewed by Nogués [30]. AFM-FM exchange coupling is an interface effect, which is related to the interface area and the width of AFM and FM. With increase of oxidation time, the volume fraction and the width of AFM CoO, the interface area between AFM CoO and FM Co increase. The reversal of FM spins should drag more and more AFM spins due to the AFM-FM exchange coupling, as a result, increasingly enhanced coercivity is achieved. However, for the excessively oxidized sample, the volume fraction of FM Co decreases so that the effective AFM-FM exchange coupling is deteriorated leading to a lower coercivity. In other words, the maximum coercivity of this system corresponds to the suitable volume fraction of AFM CoO and FM Co. It is impossible that the coercivity reduction results from the Co grain growth due to long time annealing. On the one hand, the Co grain is encapsulated by CoO and carbon layer, while annealing at 573 K, it is difficult for Co atoms to diffuse to grow into a larger grain. It has been demonstrated by XRD that the grain size of the long time annealed sample is close to that of the initial sample. On the other hand, the average grain size of the samples is ca. 30 nm, while the single domain size of Co is larger than 30 nm. Therefore, Co grain size is in the range of single domain, they present single domain magnetic behavior.
Fig. 5. The hysteresis loop of the sample oxidized at 573 K for 90 min showing a large exchange bias field.
In this Letter, we report a very interesting result related to exchange bias in our aligned system, i.e., a two times larger exchange bias field than the random dispersed nanoparticles is obtained. Exchange bias is often observed cooling the AFM-FM couple in the presence of a static magnetic field from a temperature above TN (Néel temperature) of AFM, but below TC (Curie temperature) of FM (TN < T < TC ) to temperatures T < TN [30,32]. The optimum oxidized sample was cooling in the presence of a magnetic field of 10 kGs from 320 K to 100 K, and its hysteresis loop was measured at 100 K (shown in Fig. 5). It can be seen that a large exchange bias field of −585.6 Oe exists at 100 K. On the contrary, the original random dispersed Co nanoparticles oxidized under the same condition shows a smaller exchange bias field of −288 Oe. It is interesting that the assembled magnetic “lines” present a two times exchange bias field than that of the random oriented powder. However, there are few reports to explain this fact. The mechanism is not yet clear. This interesting phenomenon may be associated with the coupling interaction between the lines. Owing to the special structure of parallel magnetic lines, moreover, the measurement direction is along the direction of lines, the magnetic coupling interaction between the lines results in that the reversal of FM spins along line direction is hampered mutually, unlike in the random system this action is averaged out by random dispersion of the nanoparticles. Consequently, a further field is required to change the direction of FM spins, i.e., in this or-
H. Bi et al. / Physics Letters A 307 (2003) 69–75
dered system a larger exchange bias is achieved. The collective effect presents due to the magnetic coupling between the lines. The detailed mechanism is still in progress. The induced-assembly magnetic lines exhibit a strong magnetic anisotropy and a large exchange bias. It can be expected that it have potential applications as a magnetic anisotropy device.
4. Conclusions Carbon encapsulated Co nanoparticles were assembled forming approximately equal spaced magnetic lines induced by magnetic field of 3 kGs. The oxidized samples consist of Co and CoO showing characteristics of coercivity enhancement and large exchange bias. In contrast with the random dispersed nanoparticles, the exchange bias field of these assembled lines is promoted more than two times. These magnetic lines have potential applications in magnetic anisotropy devices.
Acknowledgements This work is financially supported by “Natural Science Foundation of Anhui Provincial Educational Committee” under grant no. 2001hg008, and “The National Key Project For Basic Research” (G1999064508). References [1] G. Schmid, L.F. Chi, Adv. Mater. 10 (1998) 515. [2] G. Schmid, M. Baumle, N. Beyer, Angew. Chem. 39 (2000) 181. [3] F.J. Himpsel, J.E. Ortega, G.J. Mankey, R.F. Willis, Adv. Phys. 47 (1998) 511. [4] M. Bruchez, M. Moronne Jr., P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [5] J.T. Hu, L.S. Li, W.D. Yang, L. Manna, L.W. Wang, A.P. Alivisatos, Science 292 (2001) 2060.
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[6] Y. Huang, X.F. Duan, Q.Q. Wei, C.M. Lieber, Science 291 (2001) 630. [7] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [8] T.A. Taton, C.A. Mirkin, R.L. Letsinger, Science 289 (2000) 1757. [9] F.J. Himpsel, K.N. Altmann, G.J. Mankey, J.E. Ortega, D.Y. Petrovykh, J. Magn. Magn. Mater. 200 (1999) 456. [10] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423. [11] C.T. Black, C.B. Murray, R.L. Sandstrom, S.H. Sun, Science 290 (2000) 1131. [12] J.S. Maadera, E.F. Gallagher, K. Robinson, J. Nowak, Appl. Phys. Lett. 70 (1997) 3050. [13] S.Y. Lin, J.G. Fleming, D.L. Hetherington, B.K. Smith, R. Biswas, K.M. Ho, M.M. Sigalas, W. Zubrzycki, S.R. Kurtz, J. Bur, Nature 394 (1998) 251. [14] S. Noda, K. Tomoda, N. Yamamoto, A. Chutinan, Science 289 (2000) 604. [15] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413; W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904. [16] S. Gangopadhyay, G.C. Hadjipanayis, B. Dale, C.M. Sorensen, K.J. Klabunde, V. Papaefthymiou, A. Kostikas, Phys. Rev. B 45 (1992) 9778. [17] F.J. Darnell, J. Appl. Phys. 32 (1961) 186S. [18] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [19] C. Petit, J. Legrand, V. Russier, M.P. Pileni, J. Appl. Phys. 91 (2002) 1502. [20] J.S. Yin, Z.L. Wang, Nanostruct. Mater. 11 (1999) 845. [21] S.H. Sun, C.B. Murrary, J. Appl. Phys. 85 (1999) 4325. [22] T. Ji, V.G. Lirtsman, Y. Avny, D. Davidov, Adv. Mater. 13 (2001) 1253. [23] M. Golosovsky, Y. Saado, D. Davidov, Appl. Phys. Lett. 75 (1999) 4168. [24] F. Caruso, A.S. Susha, M. Giersig, H. Möhwald, Adv. Mater. 11 (1999) 950. [25] P. Meakin, A.T. Skjeltorp, Adv. Phys. 42 (1993) 1. [26] D.L. Peng, K. Sumiyama, T.J. Konno, T. Hihara, S. Yamamuro, Phys. Rev. B 60 (1999) 2093. [27] T.C. Schulthess, W.H. Butler, J. Magn. Magn. Mater. 198–199 (1999) 321. [28] S. Sun, C.B. Murray, H. Doyle, Mater. Res. Symp. Proc. 577 (1999) 385. [29] D. Barreca, M. Fabrizio, C. Piccirillo, L. Armelao, E. Tonclello, Chem. Mater. 13 (2001) 588. [30] J. Nogués, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [31] M. Gruyters, D. Riegel, J. Appl. Phys. 88 (2000) 6610. [32] J.J. Host, J.A. Block, K. Parvin, V.P. Dravid, J.L. Alpers, T. Sezen, R. LaDuca, J. Appl. Phys. 83 (1998) 793.
Magnetic anisotropy in carbon encapsulated Co/CoO “lines” with large exchange bias Hong Bi a , Shandong Li b , Xiqun Jiang a,∗ , Youwei Du b , Changzheng Yang a a Department of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China b National Laboratory of Solid State Microstructure, and Department of Physics, Nanjing University, Nanjing 210093, PR China
Received 27 August 2002; accepted 28 November 2002 Communicated by J. Fluoquet
Abstract Carbon encapsulated cobalt nanoparticles were prepared by Co(II) reduction and post-annealing in inert argon gas. Cobalt nanoparticles form parallel “lines” with approximately equal space of 35 µm on the single-crystal silica substrate in the presence of a magnetic field of 3 kGs. Magnetic measurement results indicate that this system exhibits a strong magnetic anisotropy. Along the line direction, it was ferromagnetic with coercive force of 85 Oe, while perpendicular to the substrate plane, only a narrow hysteresis loop was observed. The samples were oxidized at 573 K for various times under air atmosphere. Coercivity was enhanced due to the exchange coupling between ferromagnetic Co and antiferromagnetic CoO. Compared with the random dispersed Co/CoO nanoparticles that was prepared under the same annealing and oxidation condition, a larger exchange bias was obtained in the magnetic aligned specimen. The mechanism of coercivity enhancement and larger exchange bias was discussed. 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Tt; 75.30.Et Keywords: Magnetic anisotropy; Co/CoO; Exchange coupling; Coercivity; Alignment
1. Introduction Recently, there have been immense interests in building advanced materials using nanoscale building blocks due to the special properties in nanoparticles [1–7]. When the size of particles decreases into nanoscale which is near or smaller than the charac* Corresponding author.
E-mail addresses: [email protected] (H. Bi), [email protected] (X. Jiang).
teristic length such as magnetic exchange coupling length, superconductor electron coherent length, light wavelength, etc., the finite size effect, quantum size effect, surface and interface effect are taking place leading to anomalous physical and chemical properties. Such nanoparticles are ideal building blocks for two- and three-dimensional functional nanostructured materials, which can be used extensively in biological detection, magnetism, microelectronics and photonics. For example, colloidal metal or semiconductor quantum dots can be used as fluorescent biological
0375-9601/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-9601(02)01678-X
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H. Bi et al. / Physics Letters A 307 (2003) 69–75
labels or probes in DNA detection [4,5,8]. Magnetic nanoparticles buried in a dielectric or nonmagnetic metal matrix may exhibit oscillatory coupling and giant magnetoresistance properties [3,9], and a tightly packed assembly of exchange-decoupled magnetic nanoparticles may serve as future ultrahigh-density data storage media [10]. Also, it has been observed that spin-dependent tunneling in self-assembled cobalt nanocrystal superlattice [11], and two-dimensional arrays of spin-dependent tunnel junctions show promise for nonvolatile memory applications [12]. Recently, state-of-the-art nanofabrication has succeeded in making three-dimensional semiconductor photonic crystals with a bandgap at 1.5 µm [13,14]. Meiklejohn and Bean first discovered the exchange anisotropy effect, which originates from a strong exchange coupling between the ferromagnetic (FM) Co core and the antiferromagnetic (AFM) CoO layer in oxide-coated Co particles [15]. Much attention has been focused on the magnetic properties and the unidirectional exchange anisotropy in oxide-passivated magnetic transition-metal particles including Fe, Co, Ni because of their potential application [16,17]. Cobalt nanocrystals system was often chosen to study because it displays a wealth of size-dependent structural, magnetic, electronic, and catalytic properties [18]. Compared the magnetic properties of Co with those of Fe and Ni, Co presents a high uniaxial anisotropy and has potential application as highdensity data storage material. Co/CoO system exhibits relatively high coercivity, large exchange bias, and magnetoresistance effect. Much recently, one-, two-, and three-dimensional self-assembled arrays of cobalt nanoparticles have been successfully obtained by chemical solution phase method [19–21]. It is observed that various shapes of cobalt assemblies can be obtained by changing the applied external magnetic field. For example, regular stripes of cobalt nanocrystals are formed by applying a magnetic field parallel to the substrate during the deposition process [19], onedimensional ordered, densely packed assemblies have also been prepared using cobalt nanocrystals with irregular shape and large particle sizes (10–20 nm) under an external magnetic field [20]. The formation of these different structures is attributed to the orientation of the easy axes and ferromagnetic domain formation in cobalt nanoparticles with different sizes under the applied magnetic field.
Similarly, Davidov et al. reported a multifunctional material fabricated by immersing magnetic beads in oil, and these magnetic particles form parallel “lines” in the direction of the external magnetic field [22]. The total length of these lines may exceed 3 mm, their average width is roughly 5–10 µm due to aggregation. Changing the direction of the in-plane magnetic field leads to formation of new lines in the same direction with relatively fast response time less than 1 s. In this magnetic “lines” system, an anisotropy reflection of polarized microwaves has been observed, which indicates that these “lines” are somewhat conducting. And the conducting “lines” can be switched into a nonconductive state by an external field, suggesting it can be used as a tunable polarizer for microwaves and milliwaves. In this Letter, a magnetic functional material, carbon encapsulated Co/CoO magnetic “lines” was prepared showing a strong magnetic anisotropy and high exchange bias field.
2. Experimental procedure A typical procedure is as follows: 2.0 g styrene/ maleic acid anhydride random copolymer (SMA) was dissolved in 50 ml N, N -dimethylamide (DMF) at 353 K in a three-neck bottle, then 1.20 g CoCl2 · 6H2 O was put into it, the solution was kept stirring under 353 K for 2 h, then it was heated to 393 K with N2 bubbling up. 1.0 g NaBH4 was pre-dissolved in 50 ml DMF, then the reducing reagent was dropped into the solution quickly, the black-blue transparent solution turned into black viscous colloid very fast, the reaction was kept more 30 minutes under the N2 protection, then the black colloid was cooled to 323 K naturally and filtered. The black waxy product dried in air at 323 K over 72 h. The resulting powder was washed by anhydrous alcohol for three times to clean remanent chemicals and by-product NaCl, then dried in air at 323 K again. The purified and dried powder was annealed in a vacuum furnace with pressure less than 3 × 10−3 Pa. The sample was heated in a low heating rate. At about 673 K, the decomposed gas arising from the decomposition of organic matrix was evacuated continuously. At the end of the decomposition, inert argon gas was filled to 0.96 atm, and the temperature was kept at 833 K for 8 h. The annealed specimen was redispersed in anhydrous alcohol solution un-
H. Bi et al. / Physics Letters A 307 (2003) 69–75
der ultrasonic vibration, and centrifugally separated. Then it was dropped on the smooth surface of a singlecrystal silica substrate in the presence of a magnetic field of 3 kGs parallel to the substrate surface. The solution was slowly evaporated at room temperature. In situ oxidation at 573 K for various times was adopted in order to enhance the coercivity. The microstructure of the annealed nanoparticles has been characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS, Thermo NORAN). The morphology of the annealed sample on the Si substrate was obtained by using scanning electron microscopy (SEM). Magnetic properties of the samples were measured by highsensitivity vibrating sample magnetometer (VSM) with the resolution of 10−6 emu. Exchange bias was measured in the presence of a cooling field of 10 kGs from 320 K to 100 K.
3. Results and discussion 3.1. Formation of the magnetic “lines” The HRTEM image (Fig. 1, left) of an individual nanoparticle confirms that the Co nanocrystal in the size of 30 nm is surrounded by an amorphous carbon
71
layer after annealing. It arises from the polymer SMA carbonized at the temperature of 833 K. Fringes inside the nanoparticle with spacing of 1.77 Å correspond to {200} lattice spacing of the fcc-Co phase. The EDS spectrum (Fig. 1, right) shows carbon, cobalt and copper are present in the annealed sample grid. Cu is from copper grid, and carbon is in part from carbon film on the surface of the copper grid, and partially from the polymer carbonization. From the EDS, the content of Co is much more than that of oxygen in the sample, and considering that no CoO, Co2 O3 , Co3 O4 or Co2 C phases were observed in the XRD (Fig. 3(1)), it can be concluded that carbon layer prevent the cobalt core from oxidation and no cobalt carbide formed during annealing in the inert atmosphere. Fig. 2 shows the SEM images of the magnetic lines consisting of above-mentioned carbon encapsulated cobalt nanoparticles on a Si substrate under an external field. An in-plane applied magnetic field aligned the magnetic particles along parallel “lines” (more or less equally spaced) in the direction of the external field. The average length of these lines may exceed 300 µm, and their average width is roughly 5– 10 µm. It is difficult to achieve this regular structure due to many restrictive conditions, such as the magnitude of aligned magnetic field, concentration of particles in solution, the evaporating rate, and the smoothness of the substrate surface. Under the op-
Fig. 1. HRTEM image and EDS spectrum of cobalt nanoparticle surrounded by a carbon layer after annealing.
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Fig. 2. SEM images of the magnetic lines prepared in the presence of a magnetic field parallel to the substrate surface. The inset gives a magnified image of a small section of the “line”.
timum condition of the external field of 3 kGs parallel to the surface substrate, suitable solution with reasonable evaporating rate, and narrow distributed fine particles, regular arranged magnetic lines were achieved. Formation of such lines in the presence of an external field is analogous to the situation in ferrofluids as well as in other magnetic nanoparticle system [23,24]. Thanks to the interaction between the dipole moments induced by the external filed, along the direction of field the nanoparticles attract mutually, however, perpendicular to the direction of external field repulsive forces exist. As a consequence, with the slow evaporation of the solution, the nanoparticles contact with each other, form a magnetic “line” in the direction of the field. Along the perpendicular direction, narrow size-distributed nanoparticles repulse mutually leading to more or less equally spaced lines. The formation of parallel “lines” has been simulated by Meakin and Skjeltorp via using Monte Carlo method [25].
3.2. Magnetic anisotropy and exchange bias of the magnetic lines It is well known that cobalt nanoparticles can be oxidized forming a CoO shell around the Co core, coercivity is expected to be enhanced by exchange coupling interaction between AFM CoO and FM Co [26,27]. In our system, the aligned magnetic lines were in situ oxidized at 573 K for various times in order to promote coercivity. XRD patterns shown in Fig. 3 reveal the structural evolution of samples during the air oxidation process. Before the air oxidation, the crystalline compositions of the sample (Fig. 3(1)) include face-centered cubic (fcc)-Co (61.1 vol%), c.p. hexagonal (hcp)-Co (38.9 vol%) and NaCl phases. It is in agreement with the fact: there are two stable structures of bulk cobalt material, fcc-Co and hcp-Co. Although fcc-Co is more stable than hcp-Co in higher temperature of 723 K, in practice, many cobalt nanoparticles display mixed structures with hcp and fcc character at room temperature [28]. NaCl is a remanent by-product
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Table 1 Variation of the coercivity at room temperature with the oxidation time. (Hc )0 represents the coercivity of the original sample with zero oxidation time
Fig. 3. The XRD spectra of nanoparticles (1) before the air oxidation and (2) after the air oxidation. ( NaCl, hcp-Co, • fcc-Co, CoO).
Time (min)
Hc (Oe)
Hc /(Hc )0
0 30 60 90 120 150 210 300
39.3 48.7 60.9 84.2 62.6 54.4 41.7 40.6
1.00 1.24 1.55 2.14 1.59 1.38 1.06 1.03
was obtained, however, perpendicular to the surface substrate only a narrow loop was observed. This fact suggests that these magnetic lines exhibit a strong magnetic anisotropy. It can be explained in terms of magnetism and microstructure of the nanoparticles. The coercivity of materials can be expressed by modified Brown’s equation: Hc = α
Fig. 4. The hysteresis loops of magnetic lines both along the lines direction and perpendicular to the substrate surface.
of the reaction. When the sample was oxidized in air at 573 K for various times, the composition change into hcp-Co, NaCl and CoO (Fig. 3(2)), showing the coexistence of hcp-Co phase and CoO phase. Owing to the small quantity of magnetic particles on the substrate, a high sensitive VSM with resolution of 10−6 emu was used to measure their magnetic properties. Shown in Fig. 4 are the hysteresis loops of magnetic lines both along the lines direction and perpendicular to the substrate surface. It can be seen that along the direction of the lines a standard loop
2K1 − Neff MS , MS
(1)
where α is microstructure parameter of magnet which includes the influence of defects and orientation of particles on the coercivity. Neff represents the effective demagnetization factor that includes the selfdemagnetization interaction of particles themselves and dipolar interaction between particles. In the field of 3 kGs, the magnetization M of both directions reaches to saturation in the same value (see Fig. 4). Therefore, the large coercivity along the line’s direction can be attributed to the reduction of Neff . It is clear that along the line direction the particles aligned in the same direction, the dipolar interaction among the particles results in the reduction of Neff . In other words, the magnetic lines arise from induced assembly of nanoparticles in presence of a magnetic field, the magnetic anisotropy results from induction of the field. The induced-assembly magnetic “lines” were in situ oxidized at 573 K for various time from 30 to 300 min under air atmosphere. VSM measurement results (shown in Table 1) display that along the line’s direction the coercivity of all oxidized samples is enhanced due to the oxidation. The coercivity reaches to the maximum when the sample was oxi-
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dized for 90 min. When the oxidation time is longer than 150 min, the variation of coercivity is small due to the passivation of Co core that is covered by a thick CoO layer and carbon layer. However, along the perpendicular direction only a little increase of coercivity took place. So we focus our discussion on the magnetic properties along the line direction. Two kinds of cobalt oxides of CoO and Co3 O4 can be usually achieved via oxidation of the cobalt nanoparticles [29]. In the products of cobalt oxides, to obtain CoO is difficult due to the metastable medial valence of Co2+ , a rigid oxidation condition should be adopted. In the Co/CoO nanoparticles system, the enhanced coercivity has been widely investigated, which originated from the exchange coupling interaction between AFM CoO and FM Co [30]. It is clear that volume fraction of CoO increases with increase of the oxidation time [31]. Therefore, the variation of coercivity in Table 1 is attributed to the change of relative volume fraction of FM Co and AFM CoO. The relationship between coercivity and grain sizes of AFM and FM has been reviewed by Nogués [30]. AFM-FM exchange coupling is an interface effect, which is related to the interface area and the width of AFM and FM. With increase of oxidation time, the volume fraction and the width of AFM CoO, the interface area between AFM CoO and FM Co increase. The reversal of FM spins should drag more and more AFM spins due to the AFM-FM exchange coupling, as a result, increasingly enhanced coercivity is achieved. However, for the excessively oxidized sample, the volume fraction of FM Co decreases so that the effective AFM-FM exchange coupling is deteriorated leading to a lower coercivity. In other words, the maximum coercivity of this system corresponds to the suitable volume fraction of AFM CoO and FM Co. It is impossible that the coercivity reduction results from the Co grain growth due to long time annealing. On the one hand, the Co grain is encapsulated by CoO and carbon layer, while annealing at 573 K, it is difficult for Co atoms to diffuse to grow into a larger grain. It has been demonstrated by XRD that the grain size of the long time annealed sample is close to that of the initial sample. On the other hand, the average grain size of the samples is ca. 30 nm, while the single domain size of Co is larger than 30 nm. Therefore, Co grain size is in the range of single domain, they present single domain magnetic behavior.
Fig. 5. The hysteresis loop of the sample oxidized at 573 K for 90 min showing a large exchange bias field.
In this Letter, we report a very interesting result related to exchange bias in our aligned system, i.e., a two times larger exchange bias field than the random dispersed nanoparticles is obtained. Exchange bias is often observed cooling the AFM-FM couple in the presence of a static magnetic field from a temperature above TN (Néel temperature) of AFM, but below TC (Curie temperature) of FM (TN < T < TC ) to temperatures T < TN [30,32]. The optimum oxidized sample was cooling in the presence of a magnetic field of 10 kGs from 320 K to 100 K, and its hysteresis loop was measured at 100 K (shown in Fig. 5). It can be seen that a large exchange bias field of −585.6 Oe exists at 100 K. On the contrary, the original random dispersed Co nanoparticles oxidized under the same condition shows a smaller exchange bias field of −288 Oe. It is interesting that the assembled magnetic “lines” present a two times exchange bias field than that of the random oriented powder. However, there are few reports to explain this fact. The mechanism is not yet clear. This interesting phenomenon may be associated with the coupling interaction between the lines. Owing to the special structure of parallel magnetic lines, moreover, the measurement direction is along the direction of lines, the magnetic coupling interaction between the lines results in that the reversal of FM spins along line direction is hampered mutually, unlike in the random system this action is averaged out by random dispersion of the nanoparticles. Consequently, a further field is required to change the direction of FM spins, i.e., in this or-
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dered system a larger exchange bias is achieved. The collective effect presents due to the magnetic coupling between the lines. The detailed mechanism is still in progress. The induced-assembly magnetic lines exhibit a strong magnetic anisotropy and a large exchange bias. It can be expected that it have potential applications as a magnetic anisotropy device.
4. Conclusions Carbon encapsulated Co nanoparticles were assembled forming approximately equal spaced magnetic lines induced by magnetic field of 3 kGs. The oxidized samples consist of Co and CoO showing characteristics of coercivity enhancement and large exchange bias. In contrast with the random dispersed nanoparticles, the exchange bias field of these assembled lines is promoted more than two times. These magnetic lines have potential applications in magnetic anisotropy devices.
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