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Modifying the morphology and magnetic properties of magnetite nanoparticles using swift heavy ion irradiation
Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modifying the morphology and magnetic properties of magnetite nanoparticles using swift heavy ion irradiation Shubha Gokhale a,⇑, Subhalakshmi Lamba a, Neha Kumari a, Bhupendra Singh a, D.K. Avasthi b, S.K. Kulkarni c a
School of Sciences, Indira Gandhi National Open University, New Delhi 110068, India Inter University Accelerator Centre, P.O. Box 10502, New Delhi 110067, India c Indian Institute of Science Education Research, Dr. Homi Bhabha Road, Pune 411008, India b
a r t i c l e
i n f o
Article history: Received 21 February 2014 Received in revised form 28 April 2014 Accepted 28 April 2014 Available online 24 May 2014 Keywords: Magnetite nanoparticles SHI induced modifications Magnetic anisotropy
a b s t r a c t Magnetite (Fe3O4) nanospheres of 8–11 nm diameter synthesized using a chemical co-precipitation method were deposited as thin films on different substrates using spin coating. The thin films were irradiated with Ag ions at 100 MeV energy. Comparison of unirradiated, as synthesized Fe3O4 nanoparticulate thin film and ion irradiated film shows that irradiation causes dramatic changes in the morphology, structure and magnetic properties. Monte Carlo simulations carried out on this system indicate that the origin of the changes in the magnetic properties lies in the enhanced magnetic anisotropy energy density and reorientation of magnetic easy axis. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles find wide applications in technology area like memory devices, sensors and as therapeutic tools [1–3]. Magnetite (Fe3O4) particles are a preferred choice due to their non-toxic and stable nature in comparison with other magnetic nanoparticles like cobalt or nickel [4]. The magnetic properties of nanoparticles can be tailored by modifying their shape. In recent years ion beam irradiation has emerged as a powerful tool to impart shape modification in the nanoparticles [5–7]. In the present work we have investigated the effect of swift heavy ion (SHI) irradiation on magnetite nanoparticles. There have been some reports earlier on the effects of ion beam irradiation on the semiconductor/metal/metal oxide nanoparticles embedded in a matrix like silica [8–10]. Here we report for the first time the effect of ion beam irradiation on free standing magnetite nanoparticles, dispersed on a substrate in the form of a thin film. We have used 100 MeV silver ion beam dose of 1 1014 ions/cm2 in our investigations. The magnetite nanoparticles were synthesized by a chemical route and characterized using transmission electron microscopy (TEM) along with EDS for morphology, vibrating sample magnetometer (VSM) for magnetization properties and X-ray diffraction (XRD) for crystal structure determination. The irradiated sample results were compared with pristine (un-irradiated) samples. Pristine spherical magnetite particles of 8–11 nm ⇑ Corresponding author. Tel.: +91 1129572816; fax: +91 1129532167. E-mail address: [email protected] (S. Gokhale). http://dx.doi.org/10.1016/j.nimb.2014.04.020 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
diameter with good crystallinity suffered sputtering and melting due to interaction with the ion beam. Ion irradiation gives rise to shape modification in form of increased diameter in the plane of the thin film. This observation indicates that we have been able to achieve the shape modification of nanoparticles without embedding or encapsulating them with amorphous material like silica. After ion beam irradiation the crystallinity and the magnetic properties of the sample are found to be modified. To understand the observed post-irradiation changes in the magnetic properties, we have performed Monte Carlo simulations on the arrays of magnetite particles in a thin film like geometry (2d + h system). It reveals that the marked difference observed between in-plane and out-ofplane magnetization behaviour of the irradiated particles may be attributed to the increased anisotropy energy density and orientation of the magnetization easy axis in the out-of-plane direction. 2. Experimental Magnetite particles were synthesized using an aqueous route. 20 ml solution of 0.2 M FeCl3 was mixed with appropriate quantity of ammonia solution. This solution was stirred at 80 °C for 2.5 h. In another container 10 ml solution of 0.2 M FeSO4 was mixed with ammonia solution and ultrasonicated at room temperature for 5 min. After cooling the FeCl3 solution to room temperature, the two solutions were mixed and ultrasonicated for 35 min. The product in the form of black precipitate was washed twice with water and finally with ethanol. After this the sample was dried using vacuum dryer to obtain black powder of Fe3O4 nanoparticles.
S. Gokhale et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68
In order to facilitate various characterizations, free standing nanoparticles were dispersed in the form of thin films on different substrates. For TEM investigations the particles were dispersed on carbon coated copper grids, which were directly subjected to ion beam irradiation. For XRD studies, thin films of nanoparticles were spin coated on the glass slides, while for magnetic measurements, the films were dispersed on silicon substrates. All the samples were thoroughly dried before introducing them in the vacuum chamber for the irradiation experiment. For irradiation the samples were mounted on a copper sample holder (ladder) using a thermally conducting adhesive ensuring a good thermal contact, so that the sample temperature did not rise significantly during the experiment. The samples were irradiated using 100 MeV Ag ions at the fluence of 1 1014 ions/cm2 at room temperature using the 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi, India. The irradiation was carried out uniformly over an area of 1 cm2 by scanning the ion beam of 2 pnA current using an electromagnetic scanner. The incident ion beam was in the direction perpendicular to the plane of the thin film sample. At 100 MeV energy of Ag ions, the electronic energy imparted to Fe3O4 samples is 17.2 keV/nm length. The contribution of nuclear energy is negligible (<100 eV/nm) and hence can be ignored. The TEM analysis was carried out to discern the shape of the pre- and post-irradiated Fe3O4 nanoparticles using JEOL 2100 F microscope operated at 200 keV electron energy. The samples for TEM were prepared by placing a dilute drop of Fe3O4 particles in ethanol on carbon coated copper grids. The samples were carefully dried in air before introducing in the microscope. The structural analysis of the sample was carried out using X-ray diffraction (XRD) studies. For this purpose, approximately 100 nm thick film of nanoparticles was spin coated on glass substrates of 1 cm2 area and dried in vacuum dryer. XRD investigations were carried out with BRUKER D-8 advance X-ray diffractometer using Cu Ka (k = 1.54 Å) X-ray source. The magnetic measurements were carried out using vibrating sample magnetometer (VSM), MicroSense EV9. Magnetic field between 22,000 and 22,000 Oe was used and the samples were kept at room temperature. The samples in the form of thin films were prepared on 25 mm2 dimension pre-cleaned silicon wafer substrates for the VSM analysis. The magnetic field was applied in the parallel as well as in the perpendicular directions to the film plane for in-plane and out-of-plane measurements respectively. In all the analysis the samples prior to irradiation (pristine) and post-irradiation were prepared and characterized under similar conditions as mentioned above.
3. Results and discussion The TEM images of the pristine and ion beam irradiated Fe3O4 nanoparticles are shown in Fig. 1(a) and (b) respectively. Pristine sample exhibits aggregation of the nanoparticles throughout the film. There is no regular arrangement of the particles and a random assembly is visible. The individual particles are spherical in shape. The particle size estimated from the TEM micrographs is in the range of 8–11 nm. As will be discussed here, the ion beam irradiation affects the magnetite nanoparticle film in multiple ways. On one hand, it is observed that due to ion irradiation considerable sputtering of majority of the particles occurs, reducing the particle density significantly. By comparing the TEM images of pre- and post-irradiated samples, it can be estimated that only 4–5% particles remained on the substrate surface after irradiation. Any further irradiation resulted in the removal of almost all the particles (not shown here). This kind of sputtering effects of the ion beam
65
irradiation have been reported in other systems as well [11,12]. On the other hand, the particles remaining on the substrate are found to have undergone morphological changes induced by melting process. Such changes have been reported for other systems [13,14]. The particles shown in Fig. 1(b) clearly indicate two regions: a dark centre and relatively lighter outer ring. This indicates variation in the density in these two regions. Besides the single particles observed in Fig. 1(b), some regions also appeared to have 3–4 particles, partially melting together. The elemental analysis carried out using energy dispersive X-ray spectrometry (EDS) on a typical irradiated cluster comprising of three particles is shown in Fig. 1(c). The upper panel of this figure depicts this cluster with arrow indicating the part of the cluster used for line profiling. The length scale is indicated at the bottom of the elemental profile for iron and oxygen (on atomic percentage scale) shown in the lower panel. It indicates presence of iron and oxygen in both, the darker central and the paler outer, parts of the cluster, confirming the presence of iron oxide throughout; though, the oxygen content is little more than that expected for the stoichiometry of Fe3O4. It is possible that the energy deposition by the incident ions results in melting of nanoparticles, and the melted material could have solidified to form a larger cluster of magnetite having variation in density at periphery. The size of the single particles has also significantly increased vis-à-vis the pristine case. The darker region of the single particles is found to be in the range of 10–18 nm, whereas the outer (ringlike) lighter structure has the diameter in 20–30 nm range This increased dimensions could be attributed to either the flattening of the particles in the plane of the thin film, caused by Ag ion impact or to the melting of smaller spherical particles, which solidify together to assume a larger shape. Similar anisotropic shape modification induced by Xe ion irradiation in case of colloidal silica and ZnS nanospheres has been reported by Snoeks et al. [15]. The present result of achieving shape modification of magnetite nanoparticles by ion beam irradiation can prove to be a technological advancement in application areas like memory devices, since the deformation of the particles has been achieved without any intermediate step of coating an amorphous shell. The ion beam irradiation has affected the crystallinity of the nanoparticles as is evident from the X-ray diffraction patterns of pristine and irradiated samples shown in Fig. 2(a) and (b). The pattern for pristine sample exhibits sharp diffraction peaks indicating good crystallinity. The diffraction angles are in good agreement with the face centred cubic (FCC) crystal structure of Fe3O4 with lattice constant = 8.393 Å (diffraction pattern no. PCPDF#851436). The diffraction data of the irradiated sample shows quite noisy pattern due to small amount of sample left on the substrate. The diffraction peak positions are in agreement with the pristine sample data shown in Fig. 2(a), however there is change in the relative intensities of some diffraction peaks. In the pristine sample, the intensity of diffraction peak corresponding to (4 0 0) plane was about 25% of that corresponding to (3 1 1) peak, while in irradiated sample the intensities of these two peaks are almost same. This indicates crystallographic reorientation of irradiated sample in preferential cubic direction and could be correlated to the magnetic behaviour of the irradiated particles. Such reorientation of titanium nanocrystals caused by 350 MeV Au ion irradiation has been reported by Zizak et al. [16]. The VSM measurements carried out on the pristine and irradiated samples at room temperature are shown in Fig. 3(a) and (b) respectively. For both the samples the field (scanned between 22,000 Oe and +22,000 Oe) was applied in two directions viz. perpendicular (out-of-plane) (curve-(i)) and parallel to the plane (inplane) (curve-(ii)) of the films. For pristine samples, the saturation magnetization is about 38 emu/g. This value is smaller than the saturation magnetization
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Fig. 1. TEM micrographs of (a) pristine and (b) ion beam irradiated Fe3O4 nanoparticles. (c) Line profile using EDS carried out on a cluster of irradiated nanoparticles. Arrow in the upper panel indicates the part of cluster used for elemental analysis and lower panel indicates the elemental profile.
Fig. 2. X-ray diffraction patterns for (a) pristine and (b) irradiated Fe3O4 film.
reported for bulk magnetite (88.7 emu/g); however, it is reported to be dependent on the size of the nanoparticles and procedure adopted for the synthesis [17]. The pristine nanoparticles have small value of coercivity (about 15 Oe); Juan et al. have attributed such values to a mixed state comprising of superparamagnetic and ferromagnetic states within the particle size range of 9.8 nm and 16.5 nm [18]. Ion beam irradiation is found to have affected the magnetic properties of the magnetite nanoparticles significantly. The magnetization curves for both in-plane and out-of-plane fields do not attain the plateau associated with the saturation value as observed in the case of pristine sample. The maximum values of magnetization are reduced compared to those observed in case of pristine samples. Their coercivity has increased up to 58 Oe for in-plane and 43 Oe for out-of-plane field, which can be attributed to the
increased dimensions of the irradiated particles. Further, it is interesting to note that there is a significant difference between the inplane and out-of-plane magnetization, which seems to point at both, an orientation of magnetization axes of the particles in a preferred direction as well as an increase in the effective magnetic anisotropy energy density of the system. The preferred orientation of particles post-irradiation is evident from the variation in the relative intensities of different diffraction peaks observed in the XRD data. To investigate the magnetic behaviour of the pre- and post-irradiated particles, we performed Monte Carlo simulations on a model system. We modelled the pristine system as an array of 100 interacting single domain magnetic particles, each with a volume equal to that of a sphere of radius 5 nm, as estimated from the TE micrographs. The magnetization of each nanoparticle is M = MSV, where V is the
S. Gokhale et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68
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Fig. 3. Magnetization curves for (a) pristine and (b) ion irradiated Fe3O4 samples.
volume of a sphere of radius 5 nm and MS is the saturation magnetization of bulk Fe3O4. We assume these particles to possess a uniaxial anisotropy, with a random orientation of the anisotropy axis. The particles are confined within a box with a volume 10L 10L h, to mimic a thin film of thickness h (2d + h system) and we simulate the room temperature hysteresis by using a Monte Carlo simulation with the Metropolis algorithm as detailed elsewhere [19]. As is clear from the TEM studies, in the pristine state the particles tend to form clusters. Thus, we consider the following interactions between these particles: (i) the short range exchange interactions between particles in contact; and (ii) the long range dipolar interactions between all particles, estimated by the Lekner summation methods. In our calculations, we have taken film thickness parameter (h) to be 30 nm, L = 10 nm and the anisotropy energy density = 0.025 106 J/m3. The typical strength of the dipolar interaction energy is ED = M2/(100 L2h) and the exchange interaction energy is taken to be 0.1 ED Room temperature, out-of-plane and in-plane hysteresis plots for this sample are shown respectively in curves (iii) and (iv) of Fig. 3(a), where the simulated curves have been normalized to the saturation magnetization values observed experimentally in the pristine sample It is quite clear that for this value of the anisotropy energy density, and given that there is no preferential orientation of the easy direction, there is hardly any difference between the out-ofplane and in-plane magnetization. For comparison, the corresponding simulation data for the room temperature hysteresis of the post-irradiation sample, is shown in curves (iii) and (iv) of Fig. 3(b). Here we have considered an array of 30 single domain magnetic particles in a box of dimensions 10 L 10 L h, however with a reduced value of h = 3 nm. The reduction in the number of particles in the film is made in keeping with our interpretation that irradiation leads to the sputtering of nanoparticles from the film, resulting in fewer magnetic nanoparticles in the simulation volume and increased inter-particle distances. For this reason, in the post-irradiated sample, we do not expect any short range exchange interaction. As explained earlier, there also appears to be enlargement of the nanoparticle in the plane of the film. To account for this increased dimension in the plane of the film, we introduce reduction in the thickness parameter h in this case. It has already been reported for various systems that reduction in the thickness (of thin films) results into reorientation of easy axis of magnetization into preferential direction perpendicular to the film plane [20]. Hence we introduce a preferential orientation of the magnetization easy axes along the outof-plane direction and assume a higher value of the anisotropy energy density 0.1 106 J/m3. We assume that, though there are no exchange interactions in the sample as the particles are well separated, the dipolar interactions do play a role. In the simulated
curves we observe a separation between the out-of-plane and in-plane hysteresis and enhanced coercivity in the in-plane hysteresis curve compared to the out-of-plane curve, which agree with the trends observed experimentally. A comparison between in-plane and out-of-plane curves (for pristine and irradiated samples) seems to indicate that the experimentally observed changes in the magnetic behaviour in the sample are consistent with (i) an increase of the anisotropy energy density, which may be due a strain induced by irradiation and (ii) orientation of the magnetization easy axes along the out of plane direction. 4. Conclusions Swift heavy ion irradiation on magnetite nanoparticles results into deformation of the nanoparticles arising out to melting process. Sputtering of nanoparticles is also evident from the reduced number of particles in the transmission electron micrograph. The ion bombardment induces increase in the diameter of the particles in the plane of the film giving rise to magnetic anisotropy. Acknowledgements The authors thank Prof. Annapoorni, Delhi University, Delhi, India for fruitful discussions and facilitating VSM measurements. The authors would also like to thank Neeraj Maheshwari, IISER, Pune for carrying out XRD. S.K.K. thanks DST, India Nanomission Initiative Project SR/NM/NS-42/2009 and UGC, India for the continuous support. N.K. thanks IUAC, New Delhi, India for fellowship. The TEM characterization was carried out at the Advanced Instrumentation Research Facility, Jawaharlal Nehru University, New Delhi, India. References [1] S. Loth, S. Baumann, C.P. Lutz, D.M. Eigler, A.J. Heinrich, Science 335 (2012) 196–199. [2] A. McNally, Nat. Nanotechnol. 8 (2013) 315. [3] R. Qiao, C. Yang, M. Gao, J. Mater. Chem. 19 (2009) 6274. [4] W. Cai, J. Wan, J. Colloid Interface Sci. 305 (2007) 366. [5] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, K. Asokan, Radiat. Effect Defects Solids: Incorporating Plasma Sci. Plasma Technol. 167 (2012) 307. [6] J.J. Penninkhof, C. Graf, T. van Dillen, A.M. Vredenberg, A. van Bladderen, A. Polman, Adv. Mater. 17 (2005) 1484. [7] T. Blon, D. Chassaing, G. Ben Assayag, D. Hrabovsky, J.F. Bobo, J.C. Ousset, E. Snoeck, J. Magn. Magn. Mater. 272–276 (2004) e803. [8] L.L. Araujo, R. Giulian, D.J. Sprouster, C.S. Schnohr, D.J. Llewellyn, B. Johannessen, A.P. Byrne, M.C. Ridgway, Phys. Rev. B 85 (2012) 235417. [9] Y.K. Mishra, F. Singh, D.K. Avasthi, J.C. Pivin, D. Malinovska, E. Pippel, Appl. Phys. Lett. 91 (2007) 063103.
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[10] C. D’Orléans, J.P. Stoquert, C. Estournès, J.J. Grob, D. Muller, J.L. Guille, M. Richard-Plouet, C. Cerruti, F. Haas, Nucl. Instrum. Meth. Phys. Res. B 216 (2004) 372. [11] P.K. Kuiri, B. Joseph, H.P. Lenka, G. Sahu, J. Ghatak, D. Kanjilal, D.P. Mahapatra, Phys. Rev. Lett. 100 (2008) 245501. [12] R.C. Birtcher, S.E. Donnelly, S. Schlutig, Phys. Rev. Lett. 85 (2000) 4968. [13] Ch. Dufour, V. Khomenkov, G. Rizza, M. Toulemonde, J. Phys. D: Appl. Phys. 45 (2012) 065302. [14] T. van Dillen, A. Polman, C.M. van Kats, A. van Blaaderen, Appl. Phys. Lett. 83 (2003) 4315.
[15] E. Snoeks, A. van Blaaderen, T. van Dillen, C.M. van Kats, M.L. Brongersma, A. Polman, Adv. Mater. 2 (2000) 1511. [16] I. Zizak, N. Darowski, S. Klaumünzer, G. Schumacher, J.W. Gerlach, W. Assmann, Phys. Rev. Lett. 101 (2008) 065503. [17] D. Caruntu, G. Caruntu, C J O0 Connor, J. Phys. D: Appl. Phys. 40 (2007) 5801. [18] M. Jeun, S. Lee, J.K. Kang, A. Tomitaka, K.W. Kang, Y.Il Kim, Y. Takemura, K-Won Chung, J. Kwak, S. Bae, Appl. Phys. Lett. 100 (2012) 092406. [19] R. Sharma, C. Pratima, S. Lamba, S. Annapoorni, Pramana 65 (2005) 739. [20] A. Lisfi, J.C. Lodder, H. Wormeester, B. Poelsema, Phys. Rev. B66 (2002) 174420.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Modifying the morphology and magnetic properties of magnetite nanoparticles using swift heavy ion irradiation Shubha Gokhale a,⇑, Subhalakshmi Lamba a, Neha Kumari a, Bhupendra Singh a, D.K. Avasthi b, S.K. Kulkarni c a
School of Sciences, Indira Gandhi National Open University, New Delhi 110068, India Inter University Accelerator Centre, P.O. Box 10502, New Delhi 110067, India c Indian Institute of Science Education Research, Dr. Homi Bhabha Road, Pune 411008, India b
a r t i c l e
i n f o
Article history: Received 21 February 2014 Received in revised form 28 April 2014 Accepted 28 April 2014 Available online 24 May 2014 Keywords: Magnetite nanoparticles SHI induced modifications Magnetic anisotropy
a b s t r a c t Magnetite (Fe3O4) nanospheres of 8–11 nm diameter synthesized using a chemical co-precipitation method were deposited as thin films on different substrates using spin coating. The thin films were irradiated with Ag ions at 100 MeV energy. Comparison of unirradiated, as synthesized Fe3O4 nanoparticulate thin film and ion irradiated film shows that irradiation causes dramatic changes in the morphology, structure and magnetic properties. Monte Carlo simulations carried out on this system indicate that the origin of the changes in the magnetic properties lies in the enhanced magnetic anisotropy energy density and reorientation of magnetic easy axis. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanoparticles find wide applications in technology area like memory devices, sensors and as therapeutic tools [1–3]. Magnetite (Fe3O4) particles are a preferred choice due to their non-toxic and stable nature in comparison with other magnetic nanoparticles like cobalt or nickel [4]. The magnetic properties of nanoparticles can be tailored by modifying their shape. In recent years ion beam irradiation has emerged as a powerful tool to impart shape modification in the nanoparticles [5–7]. In the present work we have investigated the effect of swift heavy ion (SHI) irradiation on magnetite nanoparticles. There have been some reports earlier on the effects of ion beam irradiation on the semiconductor/metal/metal oxide nanoparticles embedded in a matrix like silica [8–10]. Here we report for the first time the effect of ion beam irradiation on free standing magnetite nanoparticles, dispersed on a substrate in the form of a thin film. We have used 100 MeV silver ion beam dose of 1 1014 ions/cm2 in our investigations. The magnetite nanoparticles were synthesized by a chemical route and characterized using transmission electron microscopy (TEM) along with EDS for morphology, vibrating sample magnetometer (VSM) for magnetization properties and X-ray diffraction (XRD) for crystal structure determination. The irradiated sample results were compared with pristine (un-irradiated) samples. Pristine spherical magnetite particles of 8–11 nm ⇑ Corresponding author. Tel.: +91 1129572816; fax: +91 1129532167. E-mail address: [email protected] (S. Gokhale). http://dx.doi.org/10.1016/j.nimb.2014.04.020 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
diameter with good crystallinity suffered sputtering and melting due to interaction with the ion beam. Ion irradiation gives rise to shape modification in form of increased diameter in the plane of the thin film. This observation indicates that we have been able to achieve the shape modification of nanoparticles without embedding or encapsulating them with amorphous material like silica. After ion beam irradiation the crystallinity and the magnetic properties of the sample are found to be modified. To understand the observed post-irradiation changes in the magnetic properties, we have performed Monte Carlo simulations on the arrays of magnetite particles in a thin film like geometry (2d + h system). It reveals that the marked difference observed between in-plane and out-ofplane magnetization behaviour of the irradiated particles may be attributed to the increased anisotropy energy density and orientation of the magnetization easy axis in the out-of-plane direction. 2. Experimental Magnetite particles were synthesized using an aqueous route. 20 ml solution of 0.2 M FeCl3 was mixed with appropriate quantity of ammonia solution. This solution was stirred at 80 °C for 2.5 h. In another container 10 ml solution of 0.2 M FeSO4 was mixed with ammonia solution and ultrasonicated at room temperature for 5 min. After cooling the FeCl3 solution to room temperature, the two solutions were mixed and ultrasonicated for 35 min. The product in the form of black precipitate was washed twice with water and finally with ethanol. After this the sample was dried using vacuum dryer to obtain black powder of Fe3O4 nanoparticles.
S. Gokhale et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68
In order to facilitate various characterizations, free standing nanoparticles were dispersed in the form of thin films on different substrates. For TEM investigations the particles were dispersed on carbon coated copper grids, which were directly subjected to ion beam irradiation. For XRD studies, thin films of nanoparticles were spin coated on the glass slides, while for magnetic measurements, the films were dispersed on silicon substrates. All the samples were thoroughly dried before introducing them in the vacuum chamber for the irradiation experiment. For irradiation the samples were mounted on a copper sample holder (ladder) using a thermally conducting adhesive ensuring a good thermal contact, so that the sample temperature did not rise significantly during the experiment. The samples were irradiated using 100 MeV Ag ions at the fluence of 1 1014 ions/cm2 at room temperature using the 15 UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi, India. The irradiation was carried out uniformly over an area of 1 cm2 by scanning the ion beam of 2 pnA current using an electromagnetic scanner. The incident ion beam was in the direction perpendicular to the plane of the thin film sample. At 100 MeV energy of Ag ions, the electronic energy imparted to Fe3O4 samples is 17.2 keV/nm length. The contribution of nuclear energy is negligible (<100 eV/nm) and hence can be ignored. The TEM analysis was carried out to discern the shape of the pre- and post-irradiated Fe3O4 nanoparticles using JEOL 2100 F microscope operated at 200 keV electron energy. The samples for TEM were prepared by placing a dilute drop of Fe3O4 particles in ethanol on carbon coated copper grids. The samples were carefully dried in air before introducing in the microscope. The structural analysis of the sample was carried out using X-ray diffraction (XRD) studies. For this purpose, approximately 100 nm thick film of nanoparticles was spin coated on glass substrates of 1 cm2 area and dried in vacuum dryer. XRD investigations were carried out with BRUKER D-8 advance X-ray diffractometer using Cu Ka (k = 1.54 Å) X-ray source. The magnetic measurements were carried out using vibrating sample magnetometer (VSM), MicroSense EV9. Magnetic field between 22,000 and 22,000 Oe was used and the samples were kept at room temperature. The samples in the form of thin films were prepared on 25 mm2 dimension pre-cleaned silicon wafer substrates for the VSM analysis. The magnetic field was applied in the parallel as well as in the perpendicular directions to the film plane for in-plane and out-of-plane measurements respectively. In all the analysis the samples prior to irradiation (pristine) and post-irradiation were prepared and characterized under similar conditions as mentioned above.
3. Results and discussion The TEM images of the pristine and ion beam irradiated Fe3O4 nanoparticles are shown in Fig. 1(a) and (b) respectively. Pristine sample exhibits aggregation of the nanoparticles throughout the film. There is no regular arrangement of the particles and a random assembly is visible. The individual particles are spherical in shape. The particle size estimated from the TEM micrographs is in the range of 8–11 nm. As will be discussed here, the ion beam irradiation affects the magnetite nanoparticle film in multiple ways. On one hand, it is observed that due to ion irradiation considerable sputtering of majority of the particles occurs, reducing the particle density significantly. By comparing the TEM images of pre- and post-irradiated samples, it can be estimated that only 4–5% particles remained on the substrate surface after irradiation. Any further irradiation resulted in the removal of almost all the particles (not shown here). This kind of sputtering effects of the ion beam
65
irradiation have been reported in other systems as well [11,12]. On the other hand, the particles remaining on the substrate are found to have undergone morphological changes induced by melting process. Such changes have been reported for other systems [13,14]. The particles shown in Fig. 1(b) clearly indicate two regions: a dark centre and relatively lighter outer ring. This indicates variation in the density in these two regions. Besides the single particles observed in Fig. 1(b), some regions also appeared to have 3–4 particles, partially melting together. The elemental analysis carried out using energy dispersive X-ray spectrometry (EDS) on a typical irradiated cluster comprising of three particles is shown in Fig. 1(c). The upper panel of this figure depicts this cluster with arrow indicating the part of the cluster used for line profiling. The length scale is indicated at the bottom of the elemental profile for iron and oxygen (on atomic percentage scale) shown in the lower panel. It indicates presence of iron and oxygen in both, the darker central and the paler outer, parts of the cluster, confirming the presence of iron oxide throughout; though, the oxygen content is little more than that expected for the stoichiometry of Fe3O4. It is possible that the energy deposition by the incident ions results in melting of nanoparticles, and the melted material could have solidified to form a larger cluster of magnetite having variation in density at periphery. The size of the single particles has also significantly increased vis-à-vis the pristine case. The darker region of the single particles is found to be in the range of 10–18 nm, whereas the outer (ringlike) lighter structure has the diameter in 20–30 nm range This increased dimensions could be attributed to either the flattening of the particles in the plane of the thin film, caused by Ag ion impact or to the melting of smaller spherical particles, which solidify together to assume a larger shape. Similar anisotropic shape modification induced by Xe ion irradiation in case of colloidal silica and ZnS nanospheres has been reported by Snoeks et al. [15]. The present result of achieving shape modification of magnetite nanoparticles by ion beam irradiation can prove to be a technological advancement in application areas like memory devices, since the deformation of the particles has been achieved without any intermediate step of coating an amorphous shell. The ion beam irradiation has affected the crystallinity of the nanoparticles as is evident from the X-ray diffraction patterns of pristine and irradiated samples shown in Fig. 2(a) and (b). The pattern for pristine sample exhibits sharp diffraction peaks indicating good crystallinity. The diffraction angles are in good agreement with the face centred cubic (FCC) crystal structure of Fe3O4 with lattice constant = 8.393 Å (diffraction pattern no. PCPDF#851436). The diffraction data of the irradiated sample shows quite noisy pattern due to small amount of sample left on the substrate. The diffraction peak positions are in agreement with the pristine sample data shown in Fig. 2(a), however there is change in the relative intensities of some diffraction peaks. In the pristine sample, the intensity of diffraction peak corresponding to (4 0 0) plane was about 25% of that corresponding to (3 1 1) peak, while in irradiated sample the intensities of these two peaks are almost same. This indicates crystallographic reorientation of irradiated sample in preferential cubic direction and could be correlated to the magnetic behaviour of the irradiated particles. Such reorientation of titanium nanocrystals caused by 350 MeV Au ion irradiation has been reported by Zizak et al. [16]. The VSM measurements carried out on the pristine and irradiated samples at room temperature are shown in Fig. 3(a) and (b) respectively. For both the samples the field (scanned between 22,000 Oe and +22,000 Oe) was applied in two directions viz. perpendicular (out-of-plane) (curve-(i)) and parallel to the plane (inplane) (curve-(ii)) of the films. For pristine samples, the saturation magnetization is about 38 emu/g. This value is smaller than the saturation magnetization
66
S. Gokhale et al. / Nuclear Instruments and Methods in Physics Research B 333 (2014) 64–68
Fig. 1. TEM micrographs of (a) pristine and (b) ion beam irradiated Fe3O4 nanoparticles. (c) Line profile using EDS carried out on a cluster of irradiated nanoparticles. Arrow in the upper panel indicates the part of cluster used for elemental analysis and lower panel indicates the elemental profile.
Fig. 2. X-ray diffraction patterns for (a) pristine and (b) irradiated Fe3O4 film.
reported for bulk magnetite (88.7 emu/g); however, it is reported to be dependent on the size of the nanoparticles and procedure adopted for the synthesis [17]. The pristine nanoparticles have small value of coercivity (about 15 Oe); Juan et al. have attributed such values to a mixed state comprising of superparamagnetic and ferromagnetic states within the particle size range of 9.8 nm and 16.5 nm [18]. Ion beam irradiation is found to have affected the magnetic properties of the magnetite nanoparticles significantly. The magnetization curves for both in-plane and out-of-plane fields do not attain the plateau associated with the saturation value as observed in the case of pristine sample. The maximum values of magnetization are reduced compared to those observed in case of pristine samples. Their coercivity has increased up to 58 Oe for in-plane and 43 Oe for out-of-plane field, which can be attributed to the
increased dimensions of the irradiated particles. Further, it is interesting to note that there is a significant difference between the inplane and out-of-plane magnetization, which seems to point at both, an orientation of magnetization axes of the particles in a preferred direction as well as an increase in the effective magnetic anisotropy energy density of the system. The preferred orientation of particles post-irradiation is evident from the variation in the relative intensities of different diffraction peaks observed in the XRD data. To investigate the magnetic behaviour of the pre- and post-irradiated particles, we performed Monte Carlo simulations on a model system. We modelled the pristine system as an array of 100 interacting single domain magnetic particles, each with a volume equal to that of a sphere of radius 5 nm, as estimated from the TE micrographs. The magnetization of each nanoparticle is M = MSV, where V is the
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Fig. 3. Magnetization curves for (a) pristine and (b) ion irradiated Fe3O4 samples.
volume of a sphere of radius 5 nm and MS is the saturation magnetization of bulk Fe3O4. We assume these particles to possess a uniaxial anisotropy, with a random orientation of the anisotropy axis. The particles are confined within a box with a volume 10L 10L h, to mimic a thin film of thickness h (2d + h system) and we simulate the room temperature hysteresis by using a Monte Carlo simulation with the Metropolis algorithm as detailed elsewhere [19]. As is clear from the TEM studies, in the pristine state the particles tend to form clusters. Thus, we consider the following interactions between these particles: (i) the short range exchange interactions between particles in contact; and (ii) the long range dipolar interactions between all particles, estimated by the Lekner summation methods. In our calculations, we have taken film thickness parameter (h) to be 30 nm, L = 10 nm and the anisotropy energy density = 0.025 106 J/m3. The typical strength of the dipolar interaction energy is ED = M2/(100 L2h) and the exchange interaction energy is taken to be 0.1 ED Room temperature, out-of-plane and in-plane hysteresis plots for this sample are shown respectively in curves (iii) and (iv) of Fig. 3(a), where the simulated curves have been normalized to the saturation magnetization values observed experimentally in the pristine sample It is quite clear that for this value of the anisotropy energy density, and given that there is no preferential orientation of the easy direction, there is hardly any difference between the out-ofplane and in-plane magnetization. For comparison, the corresponding simulation data for the room temperature hysteresis of the post-irradiation sample, is shown in curves (iii) and (iv) of Fig. 3(b). Here we have considered an array of 30 single domain magnetic particles in a box of dimensions 10 L 10 L h, however with a reduced value of h = 3 nm. The reduction in the number of particles in the film is made in keeping with our interpretation that irradiation leads to the sputtering of nanoparticles from the film, resulting in fewer magnetic nanoparticles in the simulation volume and increased inter-particle distances. For this reason, in the post-irradiated sample, we do not expect any short range exchange interaction. As explained earlier, there also appears to be enlargement of the nanoparticle in the plane of the film. To account for this increased dimension in the plane of the film, we introduce reduction in the thickness parameter h in this case. It has already been reported for various systems that reduction in the thickness (of thin films) results into reorientation of easy axis of magnetization into preferential direction perpendicular to the film plane [20]. Hence we introduce a preferential orientation of the magnetization easy axes along the outof-plane direction and assume a higher value of the anisotropy energy density 0.1 106 J/m3. We assume that, though there are no exchange interactions in the sample as the particles are well separated, the dipolar interactions do play a role. In the simulated
curves we observe a separation between the out-of-plane and in-plane hysteresis and enhanced coercivity in the in-plane hysteresis curve compared to the out-of-plane curve, which agree with the trends observed experimentally. A comparison between in-plane and out-of-plane curves (for pristine and irradiated samples) seems to indicate that the experimentally observed changes in the magnetic behaviour in the sample are consistent with (i) an increase of the anisotropy energy density, which may be due a strain induced by irradiation and (ii) orientation of the magnetization easy axes along the out of plane direction. 4. Conclusions Swift heavy ion irradiation on magnetite nanoparticles results into deformation of the nanoparticles arising out to melting process. Sputtering of nanoparticles is also evident from the reduced number of particles in the transmission electron micrograph. The ion bombardment induces increase in the diameter of the particles in the plane of the film giving rise to magnetic anisotropy. Acknowledgements The authors thank Prof. Annapoorni, Delhi University, Delhi, India for fruitful discussions and facilitating VSM measurements. The authors would also like to thank Neeraj Maheshwari, IISER, Pune for carrying out XRD. S.K.K. thanks DST, India Nanomission Initiative Project SR/NM/NS-42/2009 and UGC, India for the continuous support. N.K. thanks IUAC, New Delhi, India for fellowship. The TEM characterization was carried out at the Advanced Instrumentation Research Facility, Jawaharlal Nehru University, New Delhi, India. References [1] S. Loth, S. Baumann, C.P. Lutz, D.M. Eigler, A.J. Heinrich, Science 335 (2012) 196–199. [2] A. McNally, Nat. Nanotechnol. 8 (2013) 315. [3] R. Qiao, C. Yang, M. Gao, J. Mater. Chem. 19 (2009) 6274. [4] W. Cai, J. Wan, J. Colloid Interface Sci. 305 (2007) 366. [5] G. Dixit, J.P. Singh, R.C. Srivastava, H.M. Agrawal, K. Asokan, Radiat. Effect Defects Solids: Incorporating Plasma Sci. Plasma Technol. 167 (2012) 307. [6] J.J. Penninkhof, C. Graf, T. van Dillen, A.M. Vredenberg, A. van Bladderen, A. Polman, Adv. Mater. 17 (2005) 1484. [7] T. Blon, D. Chassaing, G. Ben Assayag, D. Hrabovsky, J.F. Bobo, J.C. Ousset, E. Snoeck, J. Magn. Magn. Mater. 272–276 (2004) e803. [8] L.L. Araujo, R. Giulian, D.J. Sprouster, C.S. Schnohr, D.J. Llewellyn, B. Johannessen, A.P. Byrne, M.C. Ridgway, Phys. Rev. B 85 (2012) 235417. [9] Y.K. Mishra, F. Singh, D.K. Avasthi, J.C. Pivin, D. Malinovska, E. Pippel, Appl. Phys. Lett. 91 (2007) 063103.
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