- Email: [email protected]
Microstructural and magnetic properties of passivated Co nanoparticle films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
Microstructural and magnetic properties of passivated Co nanoparticle films G.H. Wena,b, R.K. Zhenga, K.K. Funga, X.X. Zhanga,* a
Department of Physics and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China b National Laboratory of Super Hard Materials, Jilin University, Jilin, China Received 19 May 2003; received in revised form 31 August 2003
Abstract Co nanoparticle films were prepared by plasma–gas-condensation-type particle beam deposition system. Highresolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of B20 nm, and each of the Co nanoparticles is covered with an B3 nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5 T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 75.30.Gw; 75.50.Vv; 75.60.d Keywords: Nanoparticles; Exchange bias effect; Coercive field; Remanence
1. Introduction Nanostructured magnetic materials have been intensively studied for years due to their fundamental scientific interests and potential applications [1–8]. It is well known that a ferromagnetic particle will contain only one single magnetic domain when it is smaller than a critical size. It therefore behaves like a single large spin and *Corresponding author. Tel.: +852-2358-7493; fax: +8522358-1652. E-mail address: [email protected] (X.X. Zhang).
shows some unique properties in contrast to its bulk material. Particularly, the magnetic coercivity of some magnetic nanoparticles has been found to be greatly enhanced [9,10]. It has always been of great interest to fabricate monodispersed magnetic nanoparticles, since this makes magnetic nanoparticles much easy to handle and to be characterized from viewpoints of both fundamental physics and technical application without complications arising from size distribution. There have been several reports on the successful synthesization of monodispersed magnetic nanoparticles by solution-phase metal
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.09.008
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
408
salt reduction [11–14]. More interestingly, these monodispersed magnetic nanoparticles can selfassembly into ordered superlattices [15]. This makes a great breakthrough in fabricating functional nanodevices. However, it proves to be more difficult to prepare monodispersed nanoparticles by the traditional gas condensation method because the synthesis process is more complex. There are only a few reports on the fabrication of monodispersed nanoparticles by means of gas condensation [16,17]. One way of obtaining monodispersed magnetic nanoparticles by the gas condensation method is to combine a size-selective apparatus with it. Monodispersed nanoparticles of several kinds of magnetic materials have been successfully synthesized in this way [18–21]. In this work, we present the results on the structural and magnetic properties of cobalt nanoparticle films fabricated by using the gas condensation method with a size-selective apparatus.
2. Experiment Fig. 1 shows a schematic diagram of the main features of our nanoparticle film deposition system, which is mainly composed up of three parts. From left to right they are source, size selection and deposition chambers. A nozzle and a skimmer are embodied orderly as illustrated. The source chamber is a typical gas condensation
system mounted with a magnetron sputteringgun. The pressures in source, size selection and deposition chambers are set at successively degressive values, 0.6, 0.04 and 1E4 Torr, by means of gradient pumping during deposition. Cobalt nanoparticles were first formed in the source chamber, and then were accelerated by the pressure gradient. The velocity of the cobalt nanoparticles is estimated to be very high according Ref. [22]. The particles with the same size are selected when they fly through the nozzle and skimmer, and finally deposit on a substrate in the deposition chamber. To achieve a high yield of cobalt nanoparticles, the cobalt target is thermally insulated from the water-cooled magnetron sputtering-gun in this work. This can increase the deposition rate of cobalt nanoparticle films highly. The cobalt nanoparticles formed in this way have a high emittance. When they land on the substrate with a very high velocity, they form a very dense nanoparticle film. Cobalt nanoparticle films were studied by a JEOL 2010 transmission electron microscope (TEM) and a Quantum Design SQUID magnetometer. The cobalt nanoparticle film for magnetic measurements was deposited on a polyimide film and had a thickness of about 2 mm. During measurements, magnetic field was applied parallel to the film plane. The specimen for TEM observation was deposited separately on holey carbon film-coated copper grids with a much thinner thickness under the same conditions as the sample prepared for magnetic measurements.
Ar
3. Results and discussion
V Nozzle Skimmer Sample Holder
TMP
MBP TMP Fig. 1. Schematic diagram of the monosized nanoparticle film growth system. TMP and MBP represent the turbomolecular and mechanical booster pumps.
As shown in Fig. 2, the average diameter of the Co nanoparticles is about 20 nm with a narrow size distribution, and all Co nanoparticles have a coreshell structure. The corresponding selected area electron diffraction (SAED) pattern was indexed and is shown in the inset of Fig. 2. It is clear that the SAED pattern consists of two sets of facecentered cubic (fcc) rings. The sharp, strong rings are from Co, while the broad and weak diffraction rings are contributions of CoO. This indicates that the CoO crystallite grains are much smaller than
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
the Co crystallite. On calculating from the SAD patterns, the cell parameters of Co and the CoO ( respectively, which is conare 3.55 and 4.25 A, sistent with previous reports [21]. The high-resolution TEM (HRTEM) image exhibits the core-shell structured nanoparticles in detail, as shown in Fig. 3. It is clear that the Co core is enclosed by about a 3 nm CoO crystalline shell. The inset diffraction patterns at the top right corner are obtained by fast Fourier transformation (FFT) of the square area marked in an HRTEM image, which includes the Co core and the CoO shell simultaneously. Both Co and CoO are [0 1 1] orientated and f1% 1 1gCo ==f1% 1 1% gCoO : Similar relationships can also be deduced for other facets. The plane spacings computed according to HRTEM image are consistent with the results from the SAD patterns. Hysteresis loops of the cobalt nanoparticle films measured at 6 K after being zero-field-cooled (ZFC) and field-cooled (FC) in a magnetic field of 5 T are shown in Fig. 4; the ZFC hysteresis loop is symmetrical, but the FC hysteresis loop is clearly shifted to the direction opposite to the FC cooling field. The bias field He ¼ ðHc1 FC Hc2 Þ=2 is about 9850 Oe (750), and the coercive
Fig. 3. The HRTEM image taken at 200 kV shows the coreshell structure in detail. The inset diffraction patterns at topright corner were obtained by FFT of an HRTEM lattice image for the Co core and the CoO shell simultaneously. The diffraction spots from Co are enclosed by a square and indexed with a normal font; those from CoO are enclosed by a circle and indexed with an italic font. Both Co and CoO are [0 1 1] oriented.
0.015 0.012
ZFC FC
0.009 0.006
M (emu)
Fig. 2. The bright field image taken at 200 kV shows that the size of the Co nanoparticles is about 20 nm and the size distribution is nearly uniform. The inset shows the corresponding SAED diffraction patterns exhibiting two fcc series of rings from Co (sharp, strong) and CoO (broad, weak), respectively.
409
0.003
FC
Hc2
ZFC
Hc2
0.000 -0.003
ZFC
FC
-0.006
Hc1
Hc1
-0.009 -0.012 -0.015 -60 -50 -40 -30 -20 -10 0
10 20 30 40 50 60
H (kOe) Fig. 4. Hysteresis loops measured at 6 K after ZFC and FC processes.
FC FC Hc2 Þ=2 is about 9900 Oe field HcFC ¼ ðHc1 (750). This value is quite larger than the coercive field HcZFC in ZFC hysteresis loop, which is about 6250 Oe (750). Another remarkable difference between the FC and ZFC hysteresis loop is that the magnetization at 5 T in the FC loop is
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
410
60
ZFC
10
Hc
55
ZFC
50
He
M r / M 5T ( % )
2
FC
, Hc and He (kOe)
8
Hc
FC
Hc
6 4
45 40 35 30 25 20
0 0
20
40
60
80
100 120 140 160 180 200
T (K) Fig. 5. The temperature dependence of He ; HcFC and HcZFC :
clearly larger than that in ZFC loop. The magnetic properties of the cobalt nanoparticle film have typical features of exchange bias effect, which was first observed by Meiklejohn and Bean and has been extensively studied [23–29]. So it is believed that the bias field originates from the interfacial exchange coupling between the ferromagnetic cobalt core and antiferromagnetic CoO shell of cobalt nanoparticles. And the difference of the magnetization at 5 T in FC and ZFC loop is probably due to aligned uncompensated spins after FC in the antiferromagnetic CoO shell. The temperature dependences of the He ; HcFC and HcZFC were also studied. As shown in Fig. 5, both He and HcFC decrease rapidly as temperature increases. This is in agreement with some experimental results reported previously [30,31], and has been proposed by some theoretical models [25,32]. He is undetectable at 150 K and above, while HcFC and HcZFC are still apparent and have the same value. This means that the exchange bias effect in the cobalt nanoparticle film disappears at a temperature close to 150 K that is rather lower than the Ne! el temperature of bulk CoO (TN ¼ 293 K). A similar result has also been observed by others and has been attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell at a temperature rather lower than TN of the shell, which is composed of very small crystallites [33]. This is consistent with the
15 -25 0
25 50 75 100 125 150 175 200 225 250 275
T(K) Fig. 6. The temperature dependence of the ratio of remanence to magnetization at 5 T in a ZFC film.
microstructure of the cobalt nanoparticles, as shown in Fig. 3. The HcZFC shows different behavior with changing temperature. It varies smoothly at low temperatures, but decreases fast when He tends to zero. The ZFC hysteresis loops also show some other prominent features. At low temperature the HcZFC is much larger than that of Ag-coated Co particles and Co particles in granular alloys, with nearly the same size but without the CoO shell [33,34]. And the remanence at low temperature is quite low. As shown in Fig. 6, the ratio of remanence to saturation magnetization at 5 T is only 26% at 6 K; it increases as the temperature increases and then reaches a maximum of 55% at 100 K. It decreases rapidly from 100 to 150 K when temperature increases further. It is worth noting that this value is quite close to the temperature where He disappears. These features are totally different from the properties proposed for a system composed of well-separated and randomly oriented magnetic nanoparticles, which is an analogue of the cobalt nanoparticle film. In the cobalt nanoparticle film the Co core is well separated by the CoO shell, and interactions between the particles can be neglected. In this system the remanent magnetization should be about half of the saturate magnetization at low temperature and the coercive field should have a linear dependence on T 1=2 [35–37].
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
0.0010
FC M (emu)
0.0008
300K
0.0006 0.0004 0.0002
ZFC
H = 200 Oe
0.0000 (a) 0.0134
FC
0.0132
H = 50 kOe
0.0130
M (emu)
The unusual magnetic properties of the cobalt nanoparticle film after ZFC are attributed to its unique core-shell structure. It is proposed that the exchange bias effect already exists in each Co nanoparticle consisting of the film, although it is not field cooled. This is reasonable because the Co core is single domain and can provide a stable magnetic field to the CoO shell in contact below its blocking temperature. And the blocking temperature of the Co core is clearly larger than the blocking temperature of the CoO shell, as proved by the results in Fig. 5. Thus each Co particle is field cooled after the temperature decreases, although an external magnetic field is absent. So an exchange bias effect already exists in each Co particle, which increases HcZFC greatly. But the He will be macroscopically smeared out by the random orientation of the Co cores. The exchange bias field within each Co particle tends to maintain the original random orientation, which decreases the remanence of the cobalt nanoparticle film. When the exchange bias effect nearly disappears, HcZFC will decrease, while the remanence will increase to the expected value, about half of the saturate magnetization. This model agrees with the experimental results well. Thermomagnetic curves of the cobalt nanoparticle films measured in both ZFC and FC way in a field of 200 Oe are shown in Fig. 7. The ZFC magnetization is nearly zero and both ZFC and FC magnetization are nearly constant at low temperature. This could be due to the exchange bias effect within each Co particle, which prevents the magnetic moment of each Co particle from rotating away from its original orientation. Above the temperature where the exchange bias effect disappears (near 150 K), the ZFC magnetization starts to increase rapidly with temperature and finally reaches a maximum around 300 K. It is revealed that the blocking temperature of the Co nanoparticle film is about 300 K, above which both thermomagnetic curves and magnetization curves show no hysteresis. The ZFC and FC thermomagnetic curves measured in a field of 5 T are also shown in Fig. 7. The ZFC curve and FC curve diverge below 135 K. The difference between the two curves below 135 K might be caused by the alignment of uncompensated spins in the anti-
411
135K
0.0128 0.0126 0.0124
ZFC
0.0122 0.0120 (b)
0
50 100 150 200 250 300 350
T (K)
Fig. 7. Magnetization of the film as a function of temperature measured in a ZFC and FC manner in 200 Oe and 50 kOe.
ferromagnetic CoO shell through exchange coupling with a ferromagnetic cobalt core in such a high magnetic field, the same as the difference in Fig. 4 because this temperature is quite close to the onset temperature of the exchange bias effect as determined in Figs. 5 and 6. In conclusion, films composed of Co nanoparticles with a very narrow size distribution were prepared using the gas condensation method together with a size-selective apparatus. TEM images show that Co nanoparticles have an average size of about 20 nm with a core-shell structure, where Co core is covered with a CoO layer of B3 nm thickness. The exchange bias effect was observed in the film after being FC, which could arise from the interfacial exchange coupling of the Co core and the CoO shell. It is also found that exchange bias effect exists in each single Co nanoparticle, although the film was not field cooled. This is exhibited by several prominent properties observed in the film after being ZFC,
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such as quite large coercive field, small remanence and their abnormal dependence on temperature in comparing with theoretical prospects.
Acknowledgements This work was supported by grants from RGC of HKSAR China (HKUST6165/01P, HKUS T6140/00P).
References [1] J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997) 283. [2] J. Zhang, C. Boyd, W. Luo, Phys. Rev. Lett. 77 (1996) 390. [3] H. Mamiya, I. Nakatani, T. Furubayashi, Phys. Rev. Lett. 80 (1998) 177. [4] J. Tejada, R.F. Ziolo, X.X. Zhang, Chem. Mater. 8 (1996) 1784. [5] J. Ulrich, R. Zobl, K. Unterrainer, G. Strasser, E. Gornik, Appl. Phys. Lett. 76 (2000) 19. [6] N. Mais, J.P. Reithmaier, A. Forchel, M. Kohls, L. Spanhel, G. Muller, Appl. Phys. Lett. 75 (1999) 2005. [7] R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359. [8] J.I. Mart!ın, J. Nogu!es, K. Lui, J.L. Vicent, I.K. Schuller, J. Magn. Magn. Mater. 256 (2003) 449. [9] A. Gavrin, C.L. Chien, J. Appl. Phys. 67 (1990) 938. [10] G. Xiao, C.L. Chien, Appl. Phys. Lett. 51 (1987) 1280. [11] S. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325. [12] C. Petit, A. Taleb, P. Pileni, J. Phys. Chem. 103 (1999) 1805. [13] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [14] V.F. Puntes, K.M. Krishnan, P. Alivisatos, Appl. Phys. Lett. 78 (2001) 2187.
[15] Z.L. Wang, Adv. Mater. 10 (1998) 13; Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153. [16] S. Rubin, M. Holdenried, H. Micklitz, Eur. Phys. J. B 5 (1998) 23. [17] J.M. Meldrim, Y. Qiang, Y. Liu, H. Haberland, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 7013. [18] M. Oda, N. Saegusa, Jpn. J. Appl. Phys. Part 2 24 (1985) L702. [19] S. Yamamuro, K. Sumiyama, K. Suzuki, J. Appl. Phys. 85 (1999) 483. [20] P. Zhang, F. Zuo, F.K. Urban III, A. Khabari, P. Griffithsm, A. Hosseini-Tehrani, J. Magn. Magn. Mater. 225 (2001) 337. [21] D.L. Peng, K. Dumiyata, T.J. Konno, T. Hihara, S. Yamamura, Phys. Rev. B 60 (1999) 2093. [22] P.M. Denby, D.A. Eastham, Appl. Phys. Lett. 79 (2000) 2477. [23] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. [24] D. Mauri, H.C. Siegmann, P.S. Bagus, E. Kay, J. Appl. Phys. 62 (1987) 3047. [25] A.P. Malozemoff, Phys. Rev. B 35 (1987) 3679. [26] N.C. Koon, Phys. Rev. Lett. 78 (1997) 4865. [27] T.C. Schulthess, W.H. Butler, J. Appl. Phys. 85 (1999) 5510. [28] J. Nogu!es, I.K. schuller, J. Magn. Magn. Mater. 192 (1999) 203. [29] R.L. Stamps, J. Phys. D 33 (2000) R247. [30] C. Tsang, K. Lee, J. Appl. Phys. 53 (1982) 2605. [31] T. Ambrose, C.L. Chien, J. Appl. Phys. 83 (1998) 6822. [32] M.D. Stiles, R.D. McMichael, Phys. Rev. B 60 (1999) 12950. [33] S. Gangopadhyay, G.C. Hadjipanayis, C.M. Sorenesen, K.J. Klabunde, J. Appl. Phys. 73 (1993) 6964. [34] Gang Xiao, Jian-Qing Wang, J. Appl. Phys. 75 (1994) 6604. [35] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [36] A.H. Morrish, Physical Principles of Magnetism, Wiley, New York, 1965. [37] I.S. Jacobs, C.P. Bean, in: G.T. Rado, H. Suhl (Eds.), Magnetism III, Academic Press, New York, 1963, p. 275.
Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
Microstructural and magnetic properties of passivated Co nanoparticle films G.H. Wena,b, R.K. Zhenga, K.K. Funga, X.X. Zhanga,* a
Department of Physics and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China b National Laboratory of Super Hard Materials, Jilin University, Jilin, China Received 19 May 2003; received in revised form 31 August 2003
Abstract Co nanoparticle films were prepared by plasma–gas-condensation-type particle beam deposition system. Highresolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of B20 nm, and each of the Co nanoparticles is covered with an B3 nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5 T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 75.30.Gw; 75.50.Vv; 75.60.d Keywords: Nanoparticles; Exchange bias effect; Coercive field; Remanence
1. Introduction Nanostructured magnetic materials have been intensively studied for years due to their fundamental scientific interests and potential applications [1–8]. It is well known that a ferromagnetic particle will contain only one single magnetic domain when it is smaller than a critical size. It therefore behaves like a single large spin and *Corresponding author. Tel.: +852-2358-7493; fax: +8522358-1652. E-mail address: [email protected] (X.X. Zhang).
shows some unique properties in contrast to its bulk material. Particularly, the magnetic coercivity of some magnetic nanoparticles has been found to be greatly enhanced [9,10]. It has always been of great interest to fabricate monodispersed magnetic nanoparticles, since this makes magnetic nanoparticles much easy to handle and to be characterized from viewpoints of both fundamental physics and technical application without complications arising from size distribution. There have been several reports on the successful synthesization of monodispersed magnetic nanoparticles by solution-phase metal
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.09.008
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
408
salt reduction [11–14]. More interestingly, these monodispersed magnetic nanoparticles can selfassembly into ordered superlattices [15]. This makes a great breakthrough in fabricating functional nanodevices. However, it proves to be more difficult to prepare monodispersed nanoparticles by the traditional gas condensation method because the synthesis process is more complex. There are only a few reports on the fabrication of monodispersed nanoparticles by means of gas condensation [16,17]. One way of obtaining monodispersed magnetic nanoparticles by the gas condensation method is to combine a size-selective apparatus with it. Monodispersed nanoparticles of several kinds of magnetic materials have been successfully synthesized in this way [18–21]. In this work, we present the results on the structural and magnetic properties of cobalt nanoparticle films fabricated by using the gas condensation method with a size-selective apparatus.
2. Experiment Fig. 1 shows a schematic diagram of the main features of our nanoparticle film deposition system, which is mainly composed up of three parts. From left to right they are source, size selection and deposition chambers. A nozzle and a skimmer are embodied orderly as illustrated. The source chamber is a typical gas condensation
system mounted with a magnetron sputteringgun. The pressures in source, size selection and deposition chambers are set at successively degressive values, 0.6, 0.04 and 1E4 Torr, by means of gradient pumping during deposition. Cobalt nanoparticles were first formed in the source chamber, and then were accelerated by the pressure gradient. The velocity of the cobalt nanoparticles is estimated to be very high according Ref. [22]. The particles with the same size are selected when they fly through the nozzle and skimmer, and finally deposit on a substrate in the deposition chamber. To achieve a high yield of cobalt nanoparticles, the cobalt target is thermally insulated from the water-cooled magnetron sputtering-gun in this work. This can increase the deposition rate of cobalt nanoparticle films highly. The cobalt nanoparticles formed in this way have a high emittance. When they land on the substrate with a very high velocity, they form a very dense nanoparticle film. Cobalt nanoparticle films were studied by a JEOL 2010 transmission electron microscope (TEM) and a Quantum Design SQUID magnetometer. The cobalt nanoparticle film for magnetic measurements was deposited on a polyimide film and had a thickness of about 2 mm. During measurements, magnetic field was applied parallel to the film plane. The specimen for TEM observation was deposited separately on holey carbon film-coated copper grids with a much thinner thickness under the same conditions as the sample prepared for magnetic measurements.
Ar
3. Results and discussion
V Nozzle Skimmer Sample Holder
TMP
MBP TMP Fig. 1. Schematic diagram of the monosized nanoparticle film growth system. TMP and MBP represent the turbomolecular and mechanical booster pumps.
As shown in Fig. 2, the average diameter of the Co nanoparticles is about 20 nm with a narrow size distribution, and all Co nanoparticles have a coreshell structure. The corresponding selected area electron diffraction (SAED) pattern was indexed and is shown in the inset of Fig. 2. It is clear that the SAED pattern consists of two sets of facecentered cubic (fcc) rings. The sharp, strong rings are from Co, while the broad and weak diffraction rings are contributions of CoO. This indicates that the CoO crystallite grains are much smaller than
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
the Co crystallite. On calculating from the SAD patterns, the cell parameters of Co and the CoO ( respectively, which is conare 3.55 and 4.25 A, sistent with previous reports [21]. The high-resolution TEM (HRTEM) image exhibits the core-shell structured nanoparticles in detail, as shown in Fig. 3. It is clear that the Co core is enclosed by about a 3 nm CoO crystalline shell. The inset diffraction patterns at the top right corner are obtained by fast Fourier transformation (FFT) of the square area marked in an HRTEM image, which includes the Co core and the CoO shell simultaneously. Both Co and CoO are [0 1 1] orientated and f1% 1 1gCo ==f1% 1 1% gCoO : Similar relationships can also be deduced for other facets. The plane spacings computed according to HRTEM image are consistent with the results from the SAD patterns. Hysteresis loops of the cobalt nanoparticle films measured at 6 K after being zero-field-cooled (ZFC) and field-cooled (FC) in a magnetic field of 5 T are shown in Fig. 4; the ZFC hysteresis loop is symmetrical, but the FC hysteresis loop is clearly shifted to the direction opposite to the FC cooling field. The bias field He ¼ ðHc1 FC Hc2 Þ=2 is about 9850 Oe (750), and the coercive
Fig. 3. The HRTEM image taken at 200 kV shows the coreshell structure in detail. The inset diffraction patterns at topright corner were obtained by FFT of an HRTEM lattice image for the Co core and the CoO shell simultaneously. The diffraction spots from Co are enclosed by a square and indexed with a normal font; those from CoO are enclosed by a circle and indexed with an italic font. Both Co and CoO are [0 1 1] oriented.
0.015 0.012
ZFC FC
0.009 0.006
M (emu)
Fig. 2. The bright field image taken at 200 kV shows that the size of the Co nanoparticles is about 20 nm and the size distribution is nearly uniform. The inset shows the corresponding SAED diffraction patterns exhibiting two fcc series of rings from Co (sharp, strong) and CoO (broad, weak), respectively.
409
0.003
FC
Hc2
ZFC
Hc2
0.000 -0.003
ZFC
FC
-0.006
Hc1
Hc1
-0.009 -0.012 -0.015 -60 -50 -40 -30 -20 -10 0
10 20 30 40 50 60
H (kOe) Fig. 4. Hysteresis loops measured at 6 K after ZFC and FC processes.
FC FC Hc2 Þ=2 is about 9900 Oe field HcFC ¼ ðHc1 (750). This value is quite larger than the coercive field HcZFC in ZFC hysteresis loop, which is about 6250 Oe (750). Another remarkable difference between the FC and ZFC hysteresis loop is that the magnetization at 5 T in the FC loop is
ARTICLE IN PRESS G.H. Wen et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 407–412
410
60
ZFC
10
Hc
55
ZFC
50
He
M r / M 5T ( % )
2
FC
, Hc and He (kOe)
8
Hc
FC
Hc
6 4
45 40 35 30 25 20
0 0
20
40
60
80
100 120 140 160 180 200
T (K) Fig. 5. The temperature dependence of He ; HcFC and HcZFC :
clearly larger than that in ZFC loop. The magnetic properties of the cobalt nanoparticle film have typical features of exchange bias effect, which was first observed by Meiklejohn and Bean and has been extensively studied [23–29]. So it is believed that the bias field originates from the interfacial exchange coupling between the ferromagnetic cobalt core and antiferromagnetic CoO shell of cobalt nanoparticles. And the difference of the magnetization at 5 T in FC and ZFC loop is probably due to aligned uncompensated spins after FC in the antiferromagnetic CoO shell. The temperature dependences of the He ; HcFC and HcZFC were also studied. As shown in Fig. 5, both He and HcFC decrease rapidly as temperature increases. This is in agreement with some experimental results reported previously [30,31], and has been proposed by some theoretical models [25,32]. He is undetectable at 150 K and above, while HcFC and HcZFC are still apparent and have the same value. This means that the exchange bias effect in the cobalt nanoparticle film disappears at a temperature close to 150 K that is rather lower than the Ne! el temperature of bulk CoO (TN ¼ 293 K). A similar result has also been observed by others and has been attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell at a temperature rather lower than TN of the shell, which is composed of very small crystallites [33]. This is consistent with the
15 -25 0
25 50 75 100 125 150 175 200 225 250 275
T(K) Fig. 6. The temperature dependence of the ratio of remanence to magnetization at 5 T in a ZFC film.
microstructure of the cobalt nanoparticles, as shown in Fig. 3. The HcZFC shows different behavior with changing temperature. It varies smoothly at low temperatures, but decreases fast when He tends to zero. The ZFC hysteresis loops also show some other prominent features. At low temperature the HcZFC is much larger than that of Ag-coated Co particles and Co particles in granular alloys, with nearly the same size but without the CoO shell [33,34]. And the remanence at low temperature is quite low. As shown in Fig. 6, the ratio of remanence to saturation magnetization at 5 T is only 26% at 6 K; it increases as the temperature increases and then reaches a maximum of 55% at 100 K. It decreases rapidly from 100 to 150 K when temperature increases further. It is worth noting that this value is quite close to the temperature where He disappears. These features are totally different from the properties proposed for a system composed of well-separated and randomly oriented magnetic nanoparticles, which is an analogue of the cobalt nanoparticle film. In the cobalt nanoparticle film the Co core is well separated by the CoO shell, and interactions between the particles can be neglected. In this system the remanent magnetization should be about half of the saturate magnetization at low temperature and the coercive field should have a linear dependence on T 1=2 [35–37].
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0.0010
FC M (emu)
0.0008
300K
0.0006 0.0004 0.0002
ZFC
H = 200 Oe
0.0000 (a) 0.0134
FC
0.0132
H = 50 kOe
0.0130
M (emu)
The unusual magnetic properties of the cobalt nanoparticle film after ZFC are attributed to its unique core-shell structure. It is proposed that the exchange bias effect already exists in each Co nanoparticle consisting of the film, although it is not field cooled. This is reasonable because the Co core is single domain and can provide a stable magnetic field to the CoO shell in contact below its blocking temperature. And the blocking temperature of the Co core is clearly larger than the blocking temperature of the CoO shell, as proved by the results in Fig. 5. Thus each Co particle is field cooled after the temperature decreases, although an external magnetic field is absent. So an exchange bias effect already exists in each Co particle, which increases HcZFC greatly. But the He will be macroscopically smeared out by the random orientation of the Co cores. The exchange bias field within each Co particle tends to maintain the original random orientation, which decreases the remanence of the cobalt nanoparticle film. When the exchange bias effect nearly disappears, HcZFC will decrease, while the remanence will increase to the expected value, about half of the saturate magnetization. This model agrees with the experimental results well. Thermomagnetic curves of the cobalt nanoparticle films measured in both ZFC and FC way in a field of 200 Oe are shown in Fig. 7. The ZFC magnetization is nearly zero and both ZFC and FC magnetization are nearly constant at low temperature. This could be due to the exchange bias effect within each Co particle, which prevents the magnetic moment of each Co particle from rotating away from its original orientation. Above the temperature where the exchange bias effect disappears (near 150 K), the ZFC magnetization starts to increase rapidly with temperature and finally reaches a maximum around 300 K. It is revealed that the blocking temperature of the Co nanoparticle film is about 300 K, above which both thermomagnetic curves and magnetization curves show no hysteresis. The ZFC and FC thermomagnetic curves measured in a field of 5 T are also shown in Fig. 7. The ZFC curve and FC curve diverge below 135 K. The difference between the two curves below 135 K might be caused by the alignment of uncompensated spins in the anti-
411
135K
0.0128 0.0126 0.0124
ZFC
0.0122 0.0120 (b)
0
50 100 150 200 250 300 350
T (K)
Fig. 7. Magnetization of the film as a function of temperature measured in a ZFC and FC manner in 200 Oe and 50 kOe.
ferromagnetic CoO shell through exchange coupling with a ferromagnetic cobalt core in such a high magnetic field, the same as the difference in Fig. 4 because this temperature is quite close to the onset temperature of the exchange bias effect as determined in Figs. 5 and 6. In conclusion, films composed of Co nanoparticles with a very narrow size distribution were prepared using the gas condensation method together with a size-selective apparatus. TEM images show that Co nanoparticles have an average size of about 20 nm with a core-shell structure, where Co core is covered with a CoO layer of B3 nm thickness. The exchange bias effect was observed in the film after being FC, which could arise from the interfacial exchange coupling of the Co core and the CoO shell. It is also found that exchange bias effect exists in each single Co nanoparticle, although the film was not field cooled. This is exhibited by several prominent properties observed in the film after being ZFC,
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such as quite large coercive field, small remanence and their abnormal dependence on temperature in comparing with theoretical prospects.
Acknowledgements This work was supported by grants from RGC of HKSAR China (HKUST6165/01P, HKUS T6140/00P).
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