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Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 343–347
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Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules Dmitry S. Ilyushenkov a,, Veniamin I. Kozub a, Denis A. Yavsin a, Vladimir M. Kozhevin a, Irina N. Yassievich a, Thanh Trung Nguyen b, Ekkes H. Bruck b, Sergei A. Gurevich a a
Centre of Nanoheterostructure Physics, A. F. Ioffe Physico-Technical Institute of the Russian Academy of Sciences, Polytekchnicheskaya 26, 194021 Saint-Petersburg, Russian Federation b Van der Waals–Zeeman Institute, University of Amsterdam, Valckenierstaat 65, NL-1018 XE Amsterdam, The Netherlands
a r t i c l e in fo
abstract
Article history: Received 14 July 2008 Available online 2 October 2008
We report on structural and magnetic properties of granular films consisting of 2.5 nm Ni nanoparticles. The films are fabricated by the original laser electrodispersion technique, which allows producing nearly monodisperse and amorphous particles. Atomic force microscopy (AFM) study shows that in 8 nm thickness films the particles are self-assembled in clusters with the lateral size 100–150 nm and the height of about 8 nm. Performed by SQUID, the films magnetization measurements reveal superparamagnetic behaviour, characteristic for an ensemble of non-interacting single domain magnetic particulates. It is found that the magnetic moment of the particulate is equal to that of about 3000 individual Ni nanoparticles and the blocking temperature is close to room temperature. Defined from magnetic measurements, the size of single domain particulates correlates well with the size of the clusters determined from AFM images. We propose that exchange interaction plays an important role in the formation of the particulates by aligning the magnetic moments of the individual Ni nanoparticles inside the clusters. Presence of magnetic clusters with high blocking temperature makes the fabricated films potentially useful for high-density magnetic data storage applications. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.b 75.75.+a Keywords: Nanostructure Amorphous monodisperse nanoparticle Magnetic properties of nanoparticle
1. Introduction Metal nanostructures have attracted a lot of attention due to their properties, which could be substantially different from those of bulk metals [1,2]. In the last few years, various structures of this kind have been fabricated and their properties were tested showing novel and interesting features suitable for memory, medical diagnostics, and catalytic applications [3–8]. Several approaches exist to fabricate the structures consisting of metal nanoparticles with the sizes in the range from several to hundreds of nanometers [9–14]. Most of the modern techniques for metal nanostructure formation provide the crystalline nanoparticles that tend to coagulate when brought close to each other and, therefore, the formation of the structures with small interparticle distances, i.e. the structures with high particle density, is hardly possible. On the other hand, the structures of high particle density often possess most useful properties [15]. In this paper, we present Ni nanostructure films prepared by means of laser electrodispersion (LED) technique [16] and discuss
Corresponding author. Tel./fax: +7 812 2927392.
E-mail address: [email protected] (D.S. Ilyushenkov). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.09.024
the results of study of their structural, electrical, and magnetic properties. Our study has shown that the fabricated films consist of amorphous 2.5 nm Ni nanoparticles with low size dispersion (less than 10%). Amorphous state of the metal is, most probably, the reason for high particle stability against coagulation. The formation of these films proceeds via particle self-assembling in pyramidal clusters. Magnetic measurements have shown that the size of these clusters correlates with the size of magnetic domains within the system. Thus the particle self-organization has strong impact on the magnetic properties of these films and results in essential increase of the irreversible temperature. High particle density with stability against coagulation and enhanced magnetic properties governed by particle self-organization lead to the possibility to design high-density magnetic storage systems based on metallic nanostructures. In the paper, we present a discussion of such behaviour and will show that it is related to a direct exchange between the granules within the clusters. The direct exchange naturally implies the presence of relatively good electric contacts between the granules. Indeed, an important feature of the films is relatively high DC conductivity, which evidences the presence of electric contacts between the granules.
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2. Experimental details Ni nanoparticle films have been prepared by the LED technique [16], which has much in common with laser ablation. A beam from pulsed YAG:Nd3+ laser (wavelength 1.06 mm, pulse duration 30 ns, repetition rate 28 Hz, and pulse energy 0.3 J) has been focused on the surface of bulk Ni target.The liquid metallic drops of sub-micron size are first generated. These drops are charged in the laser torch plasma. The fission of sub-micron drops resulting from the capillary instability is of cascade type and stops sharply when the size of the daughter droplets diminishes to a few nanometers. The droplets have narrow size dispersion. This flux is forced to the substrate by the electric field and the deposited structure is self-assembled in a densely packed layer(s), due to Coulomb interaction of charged particles near the substrate surface. The final stage of the structure formation is free oxidation in ambient atmosphere. Structure characterization of prepared Ni nanoparticle films has been produced with the PHILIPS EM420 transmission electron microscope to obtain transmission electron microscopy (TEM) images and electron diffraction patterns. The NT-MDT SMENA atomic force microscope has been used to obtain AFM images. Measurements of temperature-dependent films resistivity have been performed by using the JANIS CCS-150 Cryostat. Magnetic properties of the films have been studied with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS5S). To obtain zero field cooling (ZFC) and Field Cooling (FC) curves the samples have been first cooled down from room temperature to 5 K in zero applied field (H ¼ 0). The magnetization has been then recorded on increasing the temperature from 5 to 350 K in applied magnetic field and further on decreasing the temperature to 5 K in the presence of the same field. The applied magnetic fields have been ranged
between 20 and +20 mT. In all experiments, the magnetic field has been applied along the plane of the film.
3. Results and discussion 3.1. Structural properties The main feature of the fabricated films is that they consist of amorphous Ni nanoparticles of very well-defined size. Fig. 1(a) shows TEM image of the film with small deposition time resulting in the formation of sub-monolayer on a smooth substrate surface. In this image individual nanoparticles could easily be resolved and it is seen that the film consists of nearly spherical Ni particles randomly distributed over the substrate surface. Close inspection of this image shows that neighboring nanoparticles still preserve their individual shape, this feature indicating the absence of particle coagulation, which is typically not the case for other fabrication techniques [17–20]. Handling TEM data gives the average particle size equal to 2.5 nm (which corresponds to 600 Ni atoms per particle), while the particle size dispersion is very narrow, less than 10% (Fig. 1(b)). Shown in Fig. 1(c) is the electron diffraction pattern obtained on the film directly in TEM. This pattern is non-structured halo, which indicates that the nanoparticles are in the amorphous state. With an increase of the film thickness, we observed a step-like growth (see Fig. 2(a)). This behaviour can be explained as follows. At the rise of each step, the pyramids composed of nanoparticles are formed. The height of these pyramids is fixed to 8 nm. Further deposition results in filling the space between the pyramids, and during this process the measured nominal film thickness remains constant
Fig. 1. Structural properties of Ni granular films fabricated by laser electrodispersion: (a) TEM view of the sub-monolayer film; (b) size dispersion of Ni particles; and (c) TEM electron diffraction pattern.
Fig. 2. (a) Dependence of the film thickness versus deposition time and (b) AFM image of 5 5mm2 surface area of multilayer 8 nm thickness film.
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(plateaus in Fig. 2(a)). At the end of each plateau the film surface becomes flat, after which the process repeats. Fig. 2(b) shows the AFM image of the surface of 8 nm thickness film, where clusters of particles (‘‘pyramids’’) with the height of 8 nm and the lateral size of about 100–150 nm are clearly seen. Taking the particle size equal to 2.5 nm, one can derive that the number of Ni nanoparticles in the cluster is NE3 103. Due to particle self-organization, appearing, in particular, in cluster formation, the dependence of the film thickness versus deposition time, measured by ‘‘blunt’’ AFM tip, has step-like form (see Fig. 2(a)). 3.2. Magnetic properties Magnetic properties of the films with a thickness 8 nm were studied by SQUID with magnetic field applied parallel to the film plane. Fig. 3 shows the ZFC and FC magnetization curves [21]. The temperature at which these curves separate, irreversible temperature (or blocking temperature), Tirr, is given as a function of magnetic field in Fig. 4. As one can see in this figure, in sufficiently low fields the irreversible temperature exceeds 200 K. The film magnetization (M) was also measured as a function of the applied magnetic field at different temperatures. In the range of temperatures and magnetic fields corresponding to the area below the irreversibility line magnetization demonstrates hysteresis accompanied by the appearance of remanent magnetization (Mr) and coercitivity (Hc). Above the irreversible line there is no hysteresis (see insets of Fig. 4). The temperature dependence of the coercive field Hc(T) is plotted in Fig. 5. Note that in the fabricated films the hysteresis can be observed up to room temperature, 300 K. The observed magnetic behaviour is understood on the basis of Ne´el’s model of an ensemble of non-interacting monodomain magnetic particulates that led to the concept of superparamagnetism [22]. In the frame of this model, the dependence of the irreversible temperature on the magnetic field is given by Ref. [22]: 2 mH T irr ðHÞ ¼ T irr;0 1 (1) 50kB T irr;0 where m is the magnetic moment of the single domain particulate and kB is Boltzmann’s constant. This dependence describes well the experimental data on Tirr(H).
Fig. 4. Field dependence of the irreversibility temperature. Insets: typical hysteresis loops measured below and above the irreversibility line.
Fig. 5. Temperature dependence of the coercive field as determined from hysteresis loops.
The behaviour demonstrated in Fig. 4 is fitted by the law of Eq. (1) with the following fitting parameters: Tirr,0 ¼ 253 K and m ¼ 0.93 106 mB (mB is Bohr magneton). The estimation of Tirr,0 and of the particulate moment can also be obtained from the temperature dependence of the coercitivity field, which is presented in Fig. 5. Based on the calculations given in Ref. [22] one has " 1=2 # T Hc ðTÞ ¼ Hc;0 1 (2) T irr;0 where Hc;0 ¼ 50kB T irr;0 =m
Fig. 3. ZFC and FC magnetization curves measured in different fields.
(3)
Straight dashed line is plotted in the inset of Fig. 5 according to Eq. (2) with Tirr,0 ¼ 300 K and Hc,0 ¼ 11.3 kA/m. Taking into account Eq. (3) the particulate moment is estimated as m ¼ 1.5 106 mB. These values of the irreversible temperature and magnetic moment agree well with those extracted from the magnetic field dependence of Tirr,0 given by Eq. (1). The agreement between the two estimations is a brilliant one, having in mind the degree of accuracy of our procedures.
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The estimated value of the particulate moment is obviously much bigger than the moment of the individual Ni nanoparticle. Indeed, Ni nanoparticle having a diameter of 2.5 nm contains about 600 atoms. In bulk Ni the moment per atom is equal to 0.6 mB, whereas it is slightly larger, 0.7 mB, for nanoparticles [23]. Therefore, the moment of Ni nanoparticle is about 400 mB. Thus, in order to explain the above results one should assume that in the studied 8 nm thickness film the individual Ni nanoparticles are assembled in particulates constructed of about 3 103 Ni nanoparticles. This number correlates well with the size of the clusters revealed by AFM (see Fig. 2(b)). Furthermore, the magnetic moments of all the particles within the clusters should be aligned to form monodomain particulates. The alignment of nanoparticle moments is determined, possibly, by exchange interaction between closely positioned individual nanoparticles [24]. Our results unambiguously demonstrate that we deal with large aggregates of the granules, which can only be a result of exchange interactions between the different granules. By using the values of particulate volume and irreversible temperature Tirr,0 extracted from the experiment, it is possible to estimate the anisotropy constant of monodomain particulate formed in the film due to particle self-assembling. The relation between these parameters is given by Ref. [22]: T irr;0 ¼
KV 25kB
(4)
Making use of this relation, the anisotropy constant of the monodomain particulate is estimated as K ¼ 4 104 erg/cm3 ¼ 4 103 J/m3. It would be instructive to compare this number with that revealed from theoretical consideration of monodomain cluster formation in thin films consisting of uniformly arranged ferromagnetic nanoparticles [24]. According to this consideration monodomain clusters are formed due to exchange interaction between nanoparticles, and the cluster size is about the correlation length of magnetic ordering. In this case, the anisotropy constant of the monodomain cluster K is proportional to the anisotropy constant of individual nanoparticle Kp: K ¼ K p N 1=2
(5)
where N is the number of nanoparticles in the cluster. Substituting in Eq. (5) the experimental number of particles in the cluster N ¼ 3 103 as well as the value Kp ¼ 3 105 erg/cm3 ¼ 3 104 J/ m3 determined for crystalline Ni nanoparticles [25] one obtains K ¼ 6 103 erg/cm3. The difference between this estimation and the above derived value K ¼ 4 104 erg/cm3 may be associated with two factors. First, the anisotropy constant of Ni nanoparticles employed in the estimation may not be correct for amorphous nanoparticles, which are the subjects in this work. The second possible reason is that the size of monodomain particulates in the studied films is smaller than the correlation length of magnetic ordering. The second assumption is strongly supported by the fact that Ne´el’s model describes well the experimental data shown in Figs. 4 and 5. The point is that this model deals with an ensemble weakly interacting monodomain magnetic particulates. This model can be applicable to nanostructure films only in the case when the size of the cluster composed of nanoparticles is smaller than the correlation length of magnetic ordering. It is significant that the observed value of irreversible temperature is sufficiently big. Typically, in the films consisting of randomly positioned spherical magnetic nanoparticles, this temperature does not exceed 10–20 K [26,27]. Using special techniques for film formation, which offer tight control of nanoparticle positions, the irreversible temperature can be substantially increased, up to the room temperature [28].
Contrary to this, in the structures considered in this paper high value of irreversible temperature is provided by nanoparticle selfassembling in clusters, which play the role of non-interacting monodomain magnetic particulates. 3.3. Electric properties Based on the values of the conductivity one concludes that the contacts still contain a tunnel interface or are formed by narrow constrictions. This fact as our further theoretical considerations based on the experimental results allows concluding that there is a direct exchange interaction between the particles. Although this interaction is relatively weak in comparison to the direct exchange between different atomic planes within the granules, it is still stronger than coupling of the granules magnetic moments to the (random) anisotropy field within the granules and thus the formation of the aggregates with nearly collinear magnetic moments (‘‘domains’’) is possible. To estimate the intergranular conductance for the closely packed aggregates, we start from the value of resistance for relatively thick closely packed films containing about 16 layers of granules. For such a film the resistance is equal to 400 O/square. Correspondingly, the average intergranular resistance can be estimated as 400 16 ¼ 6400 O. It is less than the elementary quantum resistance h/e2, but only by a factor around 4. If the contacts between the granules were perfect, the number of quantum channels between the granules was around (d/lF)2, where lF is the Fermi wavelength. Correspondingly, the intergranular resistance would be around (h/e2)/(d/lF)2, which is much less than the estimated value. Thus, we conclude that the contacts between the granules are either of a point-like type, or, which is more probable, have tunnelling interface. Nevertheless, the estimated intergranular conductance seems to be large enough to ensure effective exchange coupling between the granules, which supports our considerations. At the same time, the dependence of the film conductance on the width at small widths appears superlinear, which implies that the contacts between the different clusters are poor in comparison with the contacts of the granules within the clusters. It is this fact that explains superparamagnetic behaviour of different clusters, implying an absence of direct exchange between them.
4. Conclusions In conclusion, the LED allows fabricating the granular films consisting of amorphous Ni nanoparticles of well-defined size equal to 2.5 nm. The formation of these films proceeds via particle selfassembling in pyramidal clusters containing about 3 103 individual Ni nanoparticles. In these structures, Ni nanoparticles are closely positioned and do not coalescence keeping their individual properties. It has been demonstrated that the clusters are superparamagnetic, which implies that the contacts between the granules within the cluster ensure an effective direct exchange between the granules, while the contacts between different clusters are poor.
Acknowledgments Financial support by ISTC, RFBR, Russian Academy of Sciences, and by the grant for Science Schools is acknowledged. References [1] O. Fruchart, C. R. Phys. 6 (2005) 3. [2] F.E. Kruis, H. Fissan, A. Peled, Aerosol Sci. 29 (1998) 511.
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S.Y. Chou, M.S. Wei, P.R. Krauss, P.B. Fischer, J. Appl. Phys. 76 (1994) 6673. S.Y. Chou, Proc. IEEE 85 (1997) 652. P.R. Krauss, S.Y. Chou, J. Vac. Sci. Technol. B 13 (1995) 2850. E.H. Anderson, C.M. Horwitz, H.I. Smith, Appl. Phys. Lett. 43 (1983) 874. C.O. Bozler, C.T. Harris, S. Rabe, D.D. Rathman, M.A. Hollis, H.I. Smith, J. Vac. Sci. Technol. B 12 (1994) 629. W. Wu, B. Cui, X.-Y. Sun, W. Zhang, L. Zhuang, L. Kong, S.Y. Chou, J. Vac. Sci. Technol. B 16 (1998) 3825. D. Kumar, S.J. Pennycook, A. Lupini, G. Duscher, A. Tiwari, J. Narayan, Appl. Phys. Lett. 81 (2002) 4204. H.C. Schwarzer, W. Peukert, Chem. Eng. Technol. 25 (2002) 657. X.L. Dong, Z.D. Zhang, S.R. Jin, B.K. Kim, J. Appl. Phys. 86 (1999) 6701. S.H. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204. P.A. Dresco, V.S. Zaitsev, R.J. Gambino, B. Chu, Langmuir 15 (1999) 1945. M. Va´zquez, C. Luna, M.P. Morales, R. Sanz, C.J. Serna, C. Mijangos, Physica B 354 (2004) 71. T.N. Rostovshchikova, V.V. Smirnov, V.M. Kozhevin, D.A. Yavsin, M.A. Zabelin, I.N. Yassievich, S.A. Gurevich, Appl. Catal. A: Gen. 296 (2005) 70. V.M. Kozhevin, D.A. Yavsin, V.M. Kouznetsov, V.M. Busov, V.M. Mikushkin, S.Yu. Nikonov, S.A. Gurevich, A. Kolobov, J. Vac. Sci. Technol. B 18 (2000) 1402. C. Note, S. Kosmella, J. Koetz, Colloids Surf. A: Physicochem. Eng. Aspects 290 (2006) 150.
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[18] F. Cui, J. Zhang, T. Cui, K. Han, B. Xie, Q. Lin, B. Yang, Colloids Surf. A: Physicochem. Eng. Aspects 278 (2006) 39. [19] W. Peukert, H.-C. Schwarzer, F. Stenger, Chem. Eng. Proc. 44 (2005) 245. [20] A. Singhal, G. Skandan, A. Wang, N. Glumac, B.H. Kear, R.D. Hunt, Nanostruct. Mater. 11 (1999) 545. [21] Y. Shiratsuchi, Y. Endo, M. Yamamoto, J. Magn. Magn. Mater. 310 (2007) e756. [22] C.L. Dennis, R.P. Borges, L.D. Buda, U. Endes, J.F. Gregg, M. Hehn, E. Jouuelet, K. Ounadjjela, I. Peter, I.L. Prejbeam, M.J. Thornton, J. Phys.: Condens. Matter 14 (2002) R1175. [23] S.E. Apsel, J.W. Emmert, J. Deng, L.A. Bloomfield, Phys. Rev. Lett. 76 (1996) 1441. [24] D.S. Ilyushchenkov, V.I. Kozub, I.N. Yassievich, Phys. Solid State 10 (2007) 1944. [25] C. Fonseca, G.F. Goya, R.F. Jardim, R. Muccillo, N.L.V. Carreno, E. Longo, E.R. Leite, Phys. Rev. B 66 (2002) 104406. [26] M. Yoon, Y. Kim, Y.M. Kim, V. Volkov, H.J. Song, Y.J. Park, I.-W. Park, Mater. Chem. Phys. 91 (2005) 104. [27] G.G. Couto, J.J. Klein, W.H. Schreiner, D.H. Mosca, A.J.A. de Oliveira, A.J.G. Zarbin, J. Colloid Interface Sci. 311 (2007) 461. [28] O. Fruchart, M. Klaua, J. Barthel, J. Kirschner, Phys. Rev. Lett. 83 (1999) 2769.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of self-assembled nanostructure films on the base of amorphous Ni granules Dmitry S. Ilyushenkov a,, Veniamin I. Kozub a, Denis A. Yavsin a, Vladimir M. Kozhevin a, Irina N. Yassievich a, Thanh Trung Nguyen b, Ekkes H. Bruck b, Sergei A. Gurevich a a
Centre of Nanoheterostructure Physics, A. F. Ioffe Physico-Technical Institute of the Russian Academy of Sciences, Polytekchnicheskaya 26, 194021 Saint-Petersburg, Russian Federation b Van der Waals–Zeeman Institute, University of Amsterdam, Valckenierstaat 65, NL-1018 XE Amsterdam, The Netherlands
a r t i c l e in fo
abstract
Article history: Received 14 July 2008 Available online 2 October 2008
We report on structural and magnetic properties of granular films consisting of 2.5 nm Ni nanoparticles. The films are fabricated by the original laser electrodispersion technique, which allows producing nearly monodisperse and amorphous particles. Atomic force microscopy (AFM) study shows that in 8 nm thickness films the particles are self-assembled in clusters with the lateral size 100–150 nm and the height of about 8 nm. Performed by SQUID, the films magnetization measurements reveal superparamagnetic behaviour, characteristic for an ensemble of non-interacting single domain magnetic particulates. It is found that the magnetic moment of the particulate is equal to that of about 3000 individual Ni nanoparticles and the blocking temperature is close to room temperature. Defined from magnetic measurements, the size of single domain particulates correlates well with the size of the clusters determined from AFM images. We propose that exchange interaction plays an important role in the formation of the particulates by aligning the magnetic moments of the individual Ni nanoparticles inside the clusters. Presence of magnetic clusters with high blocking temperature makes the fabricated films potentially useful for high-density magnetic data storage applications. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.07.b 75.75.+a Keywords: Nanostructure Amorphous monodisperse nanoparticle Magnetic properties of nanoparticle
1. Introduction Metal nanostructures have attracted a lot of attention due to their properties, which could be substantially different from those of bulk metals [1,2]. In the last few years, various structures of this kind have been fabricated and their properties were tested showing novel and interesting features suitable for memory, medical diagnostics, and catalytic applications [3–8]. Several approaches exist to fabricate the structures consisting of metal nanoparticles with the sizes in the range from several to hundreds of nanometers [9–14]. Most of the modern techniques for metal nanostructure formation provide the crystalline nanoparticles that tend to coagulate when brought close to each other and, therefore, the formation of the structures with small interparticle distances, i.e. the structures with high particle density, is hardly possible. On the other hand, the structures of high particle density often possess most useful properties [15]. In this paper, we present Ni nanostructure films prepared by means of laser electrodispersion (LED) technique [16] and discuss
Corresponding author. Tel./fax: +7 812 2927392.
E-mail address: [email protected] (D.S. Ilyushenkov). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.09.024
the results of study of their structural, electrical, and magnetic properties. Our study has shown that the fabricated films consist of amorphous 2.5 nm Ni nanoparticles with low size dispersion (less than 10%). Amorphous state of the metal is, most probably, the reason for high particle stability against coagulation. The formation of these films proceeds via particle self-assembling in pyramidal clusters. Magnetic measurements have shown that the size of these clusters correlates with the size of magnetic domains within the system. Thus the particle self-organization has strong impact on the magnetic properties of these films and results in essential increase of the irreversible temperature. High particle density with stability against coagulation and enhanced magnetic properties governed by particle self-organization lead to the possibility to design high-density magnetic storage systems based on metallic nanostructures. In the paper, we present a discussion of such behaviour and will show that it is related to a direct exchange between the granules within the clusters. The direct exchange naturally implies the presence of relatively good electric contacts between the granules. Indeed, an important feature of the films is relatively high DC conductivity, which evidences the presence of electric contacts between the granules.
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2. Experimental details Ni nanoparticle films have been prepared by the LED technique [16], which has much in common with laser ablation. A beam from pulsed YAG:Nd3+ laser (wavelength 1.06 mm, pulse duration 30 ns, repetition rate 28 Hz, and pulse energy 0.3 J) has been focused on the surface of bulk Ni target.The liquid metallic drops of sub-micron size are first generated. These drops are charged in the laser torch plasma. The fission of sub-micron drops resulting from the capillary instability is of cascade type and stops sharply when the size of the daughter droplets diminishes to a few nanometers. The droplets have narrow size dispersion. This flux is forced to the substrate by the electric field and the deposited structure is self-assembled in a densely packed layer(s), due to Coulomb interaction of charged particles near the substrate surface. The final stage of the structure formation is free oxidation in ambient atmosphere. Structure characterization of prepared Ni nanoparticle films has been produced with the PHILIPS EM420 transmission electron microscope to obtain transmission electron microscopy (TEM) images and electron diffraction patterns. The NT-MDT SMENA atomic force microscope has been used to obtain AFM images. Measurements of temperature-dependent films resistivity have been performed by using the JANIS CCS-150 Cryostat. Magnetic properties of the films have been studied with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS5S). To obtain zero field cooling (ZFC) and Field Cooling (FC) curves the samples have been first cooled down from room temperature to 5 K in zero applied field (H ¼ 0). The magnetization has been then recorded on increasing the temperature from 5 to 350 K in applied magnetic field and further on decreasing the temperature to 5 K in the presence of the same field. The applied magnetic fields have been ranged
between 20 and +20 mT. In all experiments, the magnetic field has been applied along the plane of the film.
3. Results and discussion 3.1. Structural properties The main feature of the fabricated films is that they consist of amorphous Ni nanoparticles of very well-defined size. Fig. 1(a) shows TEM image of the film with small deposition time resulting in the formation of sub-monolayer on a smooth substrate surface. In this image individual nanoparticles could easily be resolved and it is seen that the film consists of nearly spherical Ni particles randomly distributed over the substrate surface. Close inspection of this image shows that neighboring nanoparticles still preserve their individual shape, this feature indicating the absence of particle coagulation, which is typically not the case for other fabrication techniques [17–20]. Handling TEM data gives the average particle size equal to 2.5 nm (which corresponds to 600 Ni atoms per particle), while the particle size dispersion is very narrow, less than 10% (Fig. 1(b)). Shown in Fig. 1(c) is the electron diffraction pattern obtained on the film directly in TEM. This pattern is non-structured halo, which indicates that the nanoparticles are in the amorphous state. With an increase of the film thickness, we observed a step-like growth (see Fig. 2(a)). This behaviour can be explained as follows. At the rise of each step, the pyramids composed of nanoparticles are formed. The height of these pyramids is fixed to 8 nm. Further deposition results in filling the space between the pyramids, and during this process the measured nominal film thickness remains constant
Fig. 1. Structural properties of Ni granular films fabricated by laser electrodispersion: (a) TEM view of the sub-monolayer film; (b) size dispersion of Ni particles; and (c) TEM electron diffraction pattern.
Fig. 2. (a) Dependence of the film thickness versus deposition time and (b) AFM image of 5 5mm2 surface area of multilayer 8 nm thickness film.
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(plateaus in Fig. 2(a)). At the end of each plateau the film surface becomes flat, after which the process repeats. Fig. 2(b) shows the AFM image of the surface of 8 nm thickness film, where clusters of particles (‘‘pyramids’’) with the height of 8 nm and the lateral size of about 100–150 nm are clearly seen. Taking the particle size equal to 2.5 nm, one can derive that the number of Ni nanoparticles in the cluster is NE3 103. Due to particle self-organization, appearing, in particular, in cluster formation, the dependence of the film thickness versus deposition time, measured by ‘‘blunt’’ AFM tip, has step-like form (see Fig. 2(a)). 3.2. Magnetic properties Magnetic properties of the films with a thickness 8 nm were studied by SQUID with magnetic field applied parallel to the film plane. Fig. 3 shows the ZFC and FC magnetization curves [21]. The temperature at which these curves separate, irreversible temperature (or blocking temperature), Tirr, is given as a function of magnetic field in Fig. 4. As one can see in this figure, in sufficiently low fields the irreversible temperature exceeds 200 K. The film magnetization (M) was also measured as a function of the applied magnetic field at different temperatures. In the range of temperatures and magnetic fields corresponding to the area below the irreversibility line magnetization demonstrates hysteresis accompanied by the appearance of remanent magnetization (Mr) and coercitivity (Hc). Above the irreversible line there is no hysteresis (see insets of Fig. 4). The temperature dependence of the coercive field Hc(T) is plotted in Fig. 5. Note that in the fabricated films the hysteresis can be observed up to room temperature, 300 K. The observed magnetic behaviour is understood on the basis of Ne´el’s model of an ensemble of non-interacting monodomain magnetic particulates that led to the concept of superparamagnetism [22]. In the frame of this model, the dependence of the irreversible temperature on the magnetic field is given by Ref. [22]: 2 mH T irr ðHÞ ¼ T irr;0 1 (1) 50kB T irr;0 where m is the magnetic moment of the single domain particulate and kB is Boltzmann’s constant. This dependence describes well the experimental data on Tirr(H).
Fig. 4. Field dependence of the irreversibility temperature. Insets: typical hysteresis loops measured below and above the irreversibility line.
Fig. 5. Temperature dependence of the coercive field as determined from hysteresis loops.
The behaviour demonstrated in Fig. 4 is fitted by the law of Eq. (1) with the following fitting parameters: Tirr,0 ¼ 253 K and m ¼ 0.93 106 mB (mB is Bohr magneton). The estimation of Tirr,0 and of the particulate moment can also be obtained from the temperature dependence of the coercitivity field, which is presented in Fig. 5. Based on the calculations given in Ref. [22] one has " 1=2 # T Hc ðTÞ ¼ Hc;0 1 (2) T irr;0 where Hc;0 ¼ 50kB T irr;0 =m
Fig. 3. ZFC and FC magnetization curves measured in different fields.
(3)
Straight dashed line is plotted in the inset of Fig. 5 according to Eq. (2) with Tirr,0 ¼ 300 K and Hc,0 ¼ 11.3 kA/m. Taking into account Eq. (3) the particulate moment is estimated as m ¼ 1.5 106 mB. These values of the irreversible temperature and magnetic moment agree well with those extracted from the magnetic field dependence of Tirr,0 given by Eq. (1). The agreement between the two estimations is a brilliant one, having in mind the degree of accuracy of our procedures.
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The estimated value of the particulate moment is obviously much bigger than the moment of the individual Ni nanoparticle. Indeed, Ni nanoparticle having a diameter of 2.5 nm contains about 600 atoms. In bulk Ni the moment per atom is equal to 0.6 mB, whereas it is slightly larger, 0.7 mB, for nanoparticles [23]. Therefore, the moment of Ni nanoparticle is about 400 mB. Thus, in order to explain the above results one should assume that in the studied 8 nm thickness film the individual Ni nanoparticles are assembled in particulates constructed of about 3 103 Ni nanoparticles. This number correlates well with the size of the clusters revealed by AFM (see Fig. 2(b)). Furthermore, the magnetic moments of all the particles within the clusters should be aligned to form monodomain particulates. The alignment of nanoparticle moments is determined, possibly, by exchange interaction between closely positioned individual nanoparticles [24]. Our results unambiguously demonstrate that we deal with large aggregates of the granules, which can only be a result of exchange interactions between the different granules. By using the values of particulate volume and irreversible temperature Tirr,0 extracted from the experiment, it is possible to estimate the anisotropy constant of monodomain particulate formed in the film due to particle self-assembling. The relation between these parameters is given by Ref. [22]: T irr;0 ¼
KV 25kB
(4)
Making use of this relation, the anisotropy constant of the monodomain particulate is estimated as K ¼ 4 104 erg/cm3 ¼ 4 103 J/m3. It would be instructive to compare this number with that revealed from theoretical consideration of monodomain cluster formation in thin films consisting of uniformly arranged ferromagnetic nanoparticles [24]. According to this consideration monodomain clusters are formed due to exchange interaction between nanoparticles, and the cluster size is about the correlation length of magnetic ordering. In this case, the anisotropy constant of the monodomain cluster K is proportional to the anisotropy constant of individual nanoparticle Kp: K ¼ K p N 1=2
(5)
where N is the number of nanoparticles in the cluster. Substituting in Eq. (5) the experimental number of particles in the cluster N ¼ 3 103 as well as the value Kp ¼ 3 105 erg/cm3 ¼ 3 104 J/ m3 determined for crystalline Ni nanoparticles [25] one obtains K ¼ 6 103 erg/cm3. The difference between this estimation and the above derived value K ¼ 4 104 erg/cm3 may be associated with two factors. First, the anisotropy constant of Ni nanoparticles employed in the estimation may not be correct for amorphous nanoparticles, which are the subjects in this work. The second possible reason is that the size of monodomain particulates in the studied films is smaller than the correlation length of magnetic ordering. The second assumption is strongly supported by the fact that Ne´el’s model describes well the experimental data shown in Figs. 4 and 5. The point is that this model deals with an ensemble weakly interacting monodomain magnetic particulates. This model can be applicable to nanostructure films only in the case when the size of the cluster composed of nanoparticles is smaller than the correlation length of magnetic ordering. It is significant that the observed value of irreversible temperature is sufficiently big. Typically, in the films consisting of randomly positioned spherical magnetic nanoparticles, this temperature does not exceed 10–20 K [26,27]. Using special techniques for film formation, which offer tight control of nanoparticle positions, the irreversible temperature can be substantially increased, up to the room temperature [28].
Contrary to this, in the structures considered in this paper high value of irreversible temperature is provided by nanoparticle selfassembling in clusters, which play the role of non-interacting monodomain magnetic particulates. 3.3. Electric properties Based on the values of the conductivity one concludes that the contacts still contain a tunnel interface or are formed by narrow constrictions. This fact as our further theoretical considerations based on the experimental results allows concluding that there is a direct exchange interaction between the particles. Although this interaction is relatively weak in comparison to the direct exchange between different atomic planes within the granules, it is still stronger than coupling of the granules magnetic moments to the (random) anisotropy field within the granules and thus the formation of the aggregates with nearly collinear magnetic moments (‘‘domains’’) is possible. To estimate the intergranular conductance for the closely packed aggregates, we start from the value of resistance for relatively thick closely packed films containing about 16 layers of granules. For such a film the resistance is equal to 400 O/square. Correspondingly, the average intergranular resistance can be estimated as 400 16 ¼ 6400 O. It is less than the elementary quantum resistance h/e2, but only by a factor around 4. If the contacts between the granules were perfect, the number of quantum channels between the granules was around (d/lF)2, where lF is the Fermi wavelength. Correspondingly, the intergranular resistance would be around (h/e2)/(d/lF)2, which is much less than the estimated value. Thus, we conclude that the contacts between the granules are either of a point-like type, or, which is more probable, have tunnelling interface. Nevertheless, the estimated intergranular conductance seems to be large enough to ensure effective exchange coupling between the granules, which supports our considerations. At the same time, the dependence of the film conductance on the width at small widths appears superlinear, which implies that the contacts between the different clusters are poor in comparison with the contacts of the granules within the clusters. It is this fact that explains superparamagnetic behaviour of different clusters, implying an absence of direct exchange between them.
4. Conclusions In conclusion, the LED allows fabricating the granular films consisting of amorphous Ni nanoparticles of well-defined size equal to 2.5 nm. The formation of these films proceeds via particle selfassembling in pyramidal clusters containing about 3 103 individual Ni nanoparticles. In these structures, Ni nanoparticles are closely positioned and do not coalescence keeping their individual properties. It has been demonstrated that the clusters are superparamagnetic, which implies that the contacts between the granules within the cluster ensure an effective direct exchange between the granules, while the contacts between different clusters are poor.
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