- Email: [email protected]
Linear-chain assemblies of iron oxide nanoparticles
Accepted Manuscript Linear-chain assemblies of iron oxide nanoparticles Prasanta Dhak, Min-Kwan Kim, Jae Hyeok Lee, Miyoung Kim, Sang-Koog Kim PII: DOI: Reference:
S0304-8853(17)30745-X http://dx.doi.org/10.1016/j.jmmm.2017.02.050 MAGMA 62511
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 February 2016 15 October 2016 25 February 2017
Please cite this article as: P. Dhak, M-K. Kim, J.H. Lee, M. Kim, S-K. Kim, Linear-chain assemblies of iron oxide nanoparticles, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm. 2017.02.050
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Linear-chain assemblies of iron oxide nanoparticles Prasanta Dhak, Min-Kwan Kim, Jae Hyeok Lee, Miyoung Kim, and Sang-Koog Kim* National Creative Research Initiative Center for Spin Dynamics and Spin-Wave Devices, and Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea We synthesized iron oxide nanoparticles using a simple hydrothermal approach and found several types of segments of their linear-chain self-assemblies as observed by field emission scanning electron microscopy.
X-ray diffraction and transmission electron microscopy
measurements confirm a well-defined single-phase FCC structure. Vibrating sample magnetometry measurements exhibit a ferromagnetic behavior. Micromagnetic numerical simulations show magnetic vortex states in the nanosphere model. Also, calculations of binding energies for different numbers of particles in the linear-chain assemblies explain a possible mechanism responsible for the self-assemblies of segments of the linear chains of nanoparticles. This work offers a step towards linear-chain self-assemblies of iron oxide nanoparticles and the effect of magnetic vortex states in individual nanoparticles on their binding energy.
Keywords: Fe3O4 nanosphere; Linear-chain assemblies; 3D magnetic vortex; Magnetic binding energy
*Author to whom all correspondence should be addressed; e-mail: [email protected]
1
1. Introduction Magnetic nanoparticles possess a wide range of unique properties with great potential for optical, electronic, biomedical, and computing applications. They represent one of the most exciting fields in nanotechnology, specifically as relates to magnetic memory devices and biomedical research purposes [1]. In magnetic nanoparticles, a very specific spin configuration known as a magnetic vortex, in its different shapes and dimensions, is particularly promising for its bio-, information-storage and -processing technology applications [2, 3]. For example, twobit-per-dot media for magnetic storage could be obtained, provided that a magnetic vortex’s four different states can be manipulated [4]. Additionally, some researchers have reported the potential of artificial structures of periodically arranged magnetic vortices to show exotic physical phenomena. Indeed, one-dimensional (1D) and two-dimensional (2D) dipolar-coupledvortex arrays and their vortex-gyration band structures have already been demonstrated as information-guiding media [5]. Additionally, artificial hexagonal arrays of vortices surrounded by perpendicular domains normal to the plane could be manipulated to 2D skyrmion crystals without Dzyaloshinskii-Moriya (DM) interaction [6]. With regard to the three-dimensional (3D) vortex, single-domain magnetic particles are the usual targets for bio applications; for the dedicated purposes of spintronics, magnetic recording, and nanomedicine, 3D vortex nanoparticles are of particular interest [3,7]. Notwithstanding the recent insights into the exotic physical phenomena of both artificial structures and vortex-state nanoparticles, the formation of magnetic nanostructures with 3D magnetic vortices remains elusive. Few researchers have reported the 3D vortex structures in soft magnetic materials [8,9]. Also, there are few reports available in open literature regarding magnetization reversals in self-assembled chains of soft magnetic nanospheres investigated using 2
micromagnetic modeling based on the Landau-Lifshitz-Gilbert equations [10,11]. It is interesting that the self-assembly of isolated or aggregated magnetic nanoparticles is a promising way to realize novel metastructures using magnetic vortices as elementary units for combining a 3D magnetic vortex with artificial structures. Due to the extremely small size of magnetic nanoparticles, conventional means of self-assembly are quite limited, and exact understanding of the assembly mechanism of nanoparticles becomes essential to the successful formation of magnetic vortex nanoparticle self-assemblies. Taking a bottom-up approach, the starting point for understanding vortex-state nanoparticle assembly is to investigate the formation mechanism of small assemblies of vortex nanoparticles. Apart from the self-assembly phenomena, analysis of small assemblies of magnetic vortex particles could prove central to material science and condensed matter physics, providing key insight into the frustratingly elusive underlying non-equilibrium phenomena, including nucleation in magnetic systems [12-14]. The first step of understanding the formation of a particular cluster system is to calculate the ground states as a function of particle morphology [15,16]. Such morphology of minimal-energy clusters is interpreted in a thermodynamic and kinetic perspective. Still though, much work on model development in terms of thermal entropy and symmetry remains to be done, especially for weakly interacting particles in the energetically stable state. For example, one requirement is to investigate how the interaction between vortex nanoparticles influences the free energy and dynamic protocols that produce small assemblies. However, to achieve this ambitious goal, it is essential to control the fabrication of specific structures using nanoparticles as elementary units. To date, most magnetic nanoparticle applications have focused on spherical primary nanoparticles or nanoparticle assemblies with aspect ratios close to 1, while utilization of magnetic nanowires and linear-chain assemblies of
3
magnetic nanoparticles has been very much limited. However, 1D magnetic structures have the potential to open up new applications in biomedicine, as their high aspect ratio results in a much larger dipole moment, allowing their manipulation with lower magnetic field strengths. Flexible long chains of magnetic particles could also be of importance across a wide range of applied materials technologies. Additionally, it is important to achieve structural precision for optimization of properties and functions. Electronic and plasmonic coupling between metallic nanoparticles has been known to yield novel electronic and optical properties. Such coupling is critically dependent on structural parameters such as inter-particle spacing and the spatial organization of individual nanoparticles. In comparison with higher-order nanostructures, 1D nanoparticle chains are more expedient building blocks for circuits in nanoelectronics, optoelectronics, and biosensors. Minimizing structural irregularity is essential: a large gap can break the coupling along a chain, and branching can cause a short circuit. Therefore, the investigation of magnetic particle assembly in linear chain-like structures is of great interest among concerned researchers. Several methods, often utilizing polymer templates to direct the assembly, have been employed to form nanoparticle chains [11,17]. For example, there have been many studies on nanoparticle self-assembly at interfaces within and on the surfaces of block co-polymers. Our current focus is template-free self-assembly of magnetic nanoparticles, which approach offers the potential for control and tunability of the self-assembly process without the use of templates. In this study, we successfully synthesized high-uniformity 200 nm monodisperse iron oxide nanoparticles and discovered interesting linear-chain self-assemblies that can be enforced from the vortex state of each iron oxide nanosphere. The employed modified hydrothermal method was adjusted in respect of reaction temperature, time and precursor concentration. The 4
micro-magnetic simulation results confirmed the vortex state present in our model system. An extensive literature survey showed that there are only a limited number of studies on linear-chain self-assembly of iron oxide nanoparticles in the vortex state. Our statistical analysis revealed a clear order of priority according to the assembly pattern and the number of assembled particles. Interestingly, all of the experimental results of the field emission scanning electron microscopy (FESEM) analysis exactly agree with the simulated results obtained from the energy densities of the respective assembly patterns. We believe that this study provides valuable insight into the interplay between particles’ assembly patterns and their magnetic vortex configurations. 2. Sample preparation and characterization Fe3O4 nanospheres of 200 nm average diameter were synthesized using a conventional hydrothermal process [18]. For the synthesis, 1.35 g, 5 mmol of FeCl3·6H2O was dissolved in 40 ml ethylene glycol to form a clear solution. Next, 3.6 g of sodium acetate and 1 g of polyethylene glycol were added to the solution, as illustrated in the flowchart of Fig. 1. The mixture was stirred vigorously for 30 minutes followed by sealed in a Teflon-lined stainless-steel autoclave of 50 ml capacity. The autoclave was heated to 200ºC, maintained for 8 hours, and allowed to cool to room temperature. The black products were washed several times with ethanol and dried at 60ºC for 6 hours. A phase analysis of the synthesized magnetite nanoparticles was performed using an X’pert Pro Phillips x-ray diffractometer with a Cu Kα target (λ =1.5418 Å) in a wide range of Bragg angles 2θ (20°≤2θ≤70°) at a scanning rate of 2°(2θ)/min. The XRD spectrum of a Si crystal was used as the standard for calibration of the scanning angles. The size and morphology of the assynthesized iron oxide nanoparticles were observed using a TEM [JEM-3000F, JEOL] with an
5
acceleration voltage of 200 kV. Samples for the TEM analysis were prepared by depositing a few drops of the respective nanoparticle solutions, ultrasonically dispersed in water for 30 minutes on separate carbon-coated copper grids. In order to study the size and surface morphology of the iron oxide nanoparticles, FESEM was performed using JSM 5410LV. To further determine the composition of the synthesized material, energy-dispersive X-ray (EDX) analysis was performed on the sample using an Oxford Instrument INCA attached to the scanning electron microscope (SEM). The magnetic properties of the iron oxide nanoparticles were studied using a vibrating sample magnetometer (VSM) (7404 VSM, Lake Shore) with a maximum field of 1.0 T and a field step of 5 Oe. In order to obtain the assembly pattern and verify the vortex phenomena of the iron oxide nanoparticles, an FESEM analysis was conducted in the following designed system. First, since magnetic particles tend to aggregate in solution, we tried to make dispersible particles instantaneously by vortexing and sonication. Partially dispersed solution was then rapidly loaded onto a Si wafer by the drop-casting method and dried at room temperature for 2 hours. When ferromagnetic particles interact each other in the medium without any other external forces, magnetic interactions act as a key factor in determining their arrangement. When a drop of magnetic nanoparticle solution with dispersion concentration of 2 mg mL-1 is deposited on the substrate, magnetic interactions between those particles can affect assembly of nanoparticles as reported in our previous article [20], while avoiding the influence of other, incidental forces. Dissimilar to arrangements of as synthesized iron oxide nanoparticles, certain types of assembly patterns are formed during aggregation on the substrate. We believe that these originated from the vortex states of the individual particles. 3. Results and discussion 6
Figure 2 plots the XRD pattern obtained from the randomly oriented iron oxide nanoparticles measured at room temperature. Careful analysis of the X-ray data confirmed the single-phase formation of iron oxide particles. No other secondary phases were detected. All of the peaks were indexed properly, as shown in Fig. 2. The material showed a cubic spinel structure with space group P4132 (213). Figure 3 shows the TEM micrographs for iron oxide nanoparticles. Figure 3(a) clearly indicates that all of the particles were well dispersed and spherical in shape, with an average particle diameter of about 200 nm. The high-resolution TEM (HRTEM) image shows the lattice imaging of the (311) plane, where inter-planer distance d = 2.5Å in Figs. 3(b) and 3(c). In Fig. 3(d) the fast-Fourier transform (FFT) of the lattice image shown in Fig. 3(c) represents a single phase FCC structure of the embedded iron oxide particles. The HRTEM-identified plane correlates well with that of the XRD pattern. Figure 4(a) and inset of figure 4(a) provide the FESEM micrographs of the iron oxide microsphere with 200 nm average diameter. The image confirms the spherical morphology and lesser agglomeration present in our studied materials. In Fig. 4(b), the EDX spectra of the iron oxide nanoparticles show satisfactory results in good agreement with the iron oxide composition. Figure 5 plots the magnetization curves measured at 300 K for the 200 nm diameter iron oxide nanoparticles. The magnetic saturation value for our sample was 77.17 emu g-1, which is comparable with the standard magnetization value reported for iron oxide nanoparticles of 200 nm diameter [18]. Figure 6 shows the one-to-five linear assembly configurations of iron oxide nanoparticles denoted as a1, a2, a3, a4 and a5, respectively. A few of the arrangements might not form a 7
perfectly straight line, minute particle displacement is ignored and is considered as almost a straight line. We examined the spin configuration of each assembled linear-chain cluster and then calculated the exchange and dipolar interaction energies involved. Next, we performed micromagnetic simulations to calculate the ground states of the spin configurations and the magnetic interaction binding energies, using the Finite Element MicroMagnEtics (FEMME) code (version 5.0.8). The following Fe3O4 input parameters were applied: diameter of nanosphere (2r) = 200 nm, saturation magnetization Ms = 4.85 x 105 A/m, exchange stiffness Aex = 1.3 x 10-11 J/m, and magnetocrystalline anisotropy K1 = -1.36 x 104 J/m3 [19]. The formation of a 3D magnetic vortex in a sphere has been well established both theoretically and experimentally [9,20]. The ground states of the spheres were obtained through relaxation from their saturated state. In the 200 nm iron oxide nanospheres, single magnetic vortex states were well established. In such magnetic configurations, the spins achieve a complete flux closure around the vortex core, which is aligned in the +x direction. This exotic variation of the static magnetic spin configuration is generally determined by the competition between magnetostatic energy and exchange energy. The exchange interaction is very strong in the atomic-scale range, and thus tends to impart uniformly to the magnetized domains. The dipolar interaction, contrastingly, is rather weak, but works in the micrometer range, thereby effecting spiral spin configurations inside the nanoparticles. In the case of iron oxide nanoparticles, we must also consider the cubic anisotropy energy, which, similarly to exchange interaction, is very strong in the atomic-scale range. As a result of the anisotropy interaction, the calculated vortex core was aligned in the <111> direction, which is the easy axis of iron oxide. Additionally, this vortex state, as the ground state, remained in each particle within the assembled clusters. The outward core orientations were aligned parallel to the orientation of the 8
vortex core in the case of the linear geometry. It is clear from Fig. 7 that in the assembled particles, the internal local magnetization is modified in the proximity of the individual core. In the case of the double particles a2, the broadening of the core region in each sphere is greater than that for the single particle a1, and the width of the contact area between the two particles is larger, indicating modification of the local internal magnetizations. In the cases of more than two particles, which is to say, a3, a4 and a5, there is more core broadening of the middle particle compared with the end particles in the linear-chain assemblies of the nanoclusters. The core magnetizations were modified in the contact area of the individual particles in order to reduce the exchange interaction energy inside each particle. Next, we calculated the magnetic interaction energies of all of the clustered nanoparticles as shown in Fig. 8, so as to understand the driving force behind magnetic nanoparticle assembly. The binding energy can be represented as (I)
∆EB = ET − N × ES
where ET is the total magnetic energy in each given chain, N is the number of particles in the chain, and ES is the magnetic energy of an isolated single particle. In exploring each contribution of the magnetic interactions to the total binding energy, we separately calculated the exchange (
∆Eexch ), dipolar ( ∆Edip ), and anisotropic ( ∆Eani ) binding energies. It is well established that the magnetic interaction is almost ten times larger than the Van der Waals [21] and electrostatic interactions [22]. In this study, only those magnetic interactions that are essential factors determining the clustering of magnetic particles having a single 3D vortex structure were considered. The inter-particle exchange interaction was not considered here, owing to its atomic scale and atomic-scale separations between individual particles as observed 9
by FESEM. Figure 8 shows the binding energy calculations in terms of ∆Eexch , ∆Edip and ∆Eani for the a1, a2, a3, a4 and a5 nanoclusters depicted in Fig. 6. With increasing particle number, the total magnetic binding energy decreases. One of our recent investigation showed that the exchange binding interaction is the dominant factor in the assembly of nanoparticles with a 3D magnetic vortex, and that the dipolar binding interaction inhibits increasing numbers of particles in the linear configuration [20]. Also, it was interesting to observe that the value ∆Eani increases with increasing numbers of magnetic nanoparticles participating in the linear-chain formation. This observation could be explained by the fact that there exist four easy axes energetically equivalent to each other in the Fe3O4 nanosphere, which implies that there are three other easy axes also in the linear-chain direction. Magneto-crystalline anisotropy binding interaction hinders the formation of the linear chain in a specific easy axis, due to the existence of freedom at the other three binding sites. From the present binding energy calculation, an important feature was revealed: the intra-exchange and dipolar energies oppositely contribute to the formation of nanoparticle assemblies. Here, the dipolar interaction contributes oppositely to assembling of individual particles from assembling; the intra-exchange interaction, contrastingly, prefers to assemble them by modification of each particle’s internal 3D structure. This phenomenon, might be counter-intuitive, but observed as in our earlier report [20]. 4. Summary We demonstrated the linear assembly of 200 nm iron oxide nanoparticles and the relation to the 3D magnetic vortex structure and binding energy. From the micromagnetic simulation analysis, it is clear that the intra-exchange interaction has an important role in modifying the internal spin configuration of the core of an iron oxide nanosphere. In this regard, it is helpful to
10
reduce the magnetic binding energy necessary for magnetic nanoparticle assembly. We believe that this study provides valuable insights into the interplay between particles’ assembly patterns and their spin-vortex magnetic properties. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).
References [1] B.V. Waeyenberge, A. Puzic, H. Stoll et al., Nature 444 (2006) 461. [2] W. Rave, K. Fabian, A. Hubert, J. Magn. Magn. Mater. 190 (1998) 332. [3] L. M. Lacroix, Se. Lachaize, F. Hue et al., Nano letters 12 (2012) 3245. [4] J. Miltat, A. Thiaville, Science 298 (2002) 555. [5] D.S. Han, H.B. Jeong, S.K. Kim, Appl. Phys. Lett. 103 (2013) 112406. [6] L. Sun, R.X. Cao, B.F Miao et al., Phys. Rev. Lett. 110 (2013) 167201. [7] Y. Yang, X.L. Liu, J. Yi et al., J. Appl. Phys. 111 (2012) 044303. [8] R.E. Dunin-Borkowski, R.K.K. Chong, T. Kasama et al., Inst. Phys. Conf. Ser. 179 (2003) 451. [9] M.J. Hÿtch, R.E. Dunin-Borkowski, M.R. Scheinfein et al., Phys. Rev. Lett. 91 (2003) 257207. [10] T. Kasama P. Barpanda, R.E. Dunin-Borkowski et al., J. Appl. Phys. 99 (2006) 08G103. [11] R.E.C. Schmidtke, R. Zierold, A. Feld et al., Langmuir 30 (2014) 11190−11196. 11
[12] D.R. Nelson, F. Spaepen, Solid State Phys. 42 (1989) 1. [13] J.P.K. Doye, D.J. Wales, Science 271 (1996) 484. [14] F.H. Stillinger, T.A. Weber, Science 225 (1984) 983. [15] N. Arkus, V. N. Manoharan, M. P. Brenner, Phys. Rev. Lett. 103 (2009) 118303. [16] G. Meng, N. Arkus, M.P Brenner et al., Science 327 (2010).560. [17] P.J. Krommenhoek, J.B. Tracy, Part. Part. Syst. Charact. 30 (2013) 759. [18] X.L.H. Deng, Q. Peng, X. Wang et al., Angew. Chem. Int. Ed. 44 (2005) 2782. [19] L.L. Afremov, A.V. Panov, Phys. Met. Metallogra 86 (1998) 269. [20] M.K. Kim, P. Dhak, H.Y. Lee et al., Appl. Phys. Lett. 105 (2014) 232402. [21] C.E. Wilmer, K.J. Bishop, S. Soh et al., Small 5 (2009) 1600. [22] Y. Levin, Rep. Prog. Phys. 65 (2002) 1577.
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List of Figures Fig. 1. Schematic representation of synthesis of single-crystal iron oxide nanoparticles Fig. 2. XRD pattern of as-synthesized iron oxide nanoparticles Fig. 3. TEM images of (a) aggregated particles, (b) single-particle, (c) HRTEM lattice image and (d) FFT of the lattice image shown in (c) Fig. 4. (a) FESEM and (b) EDX of iron oxide particles Fig. 5. Room-temperature magnetic hysteresis loop of aggregated iron oxide nanoparticles of 200 nm in diameter Fig. 6. FESEM images of segments of linear-chain assemblies of iron oxide nanoparticles Fig. 7. Simulation results of magnetization distributions of single and linearly assembled iron oxide nanoparticles Fig. 8. Magnetic binding energy terms for different numbers of iron oxide nanoparticles in linear chain.
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Figures:
Fig. 1
Fig. 2
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Fig. 3
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Fig. 4
Fig. 5
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Fig. 6 Fig. 6
Fig. 7
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Fig. 8
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Research highlights:
•
Hydrothermal synthesis of pure phase 200 nm Fe3O4 nanoparticles.
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Studies of linear-chain assemblies of iron oxide nanosphere by FESEM.
•
Micromagnetic simulations showed the presence of 3D vortex states.
•
The B.E. for different numbers of particles in linear chain assemblies were calculated.
19
S0304-8853(17)30745-X http://dx.doi.org/10.1016/j.jmmm.2017.02.050 MAGMA 62511
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 February 2016 15 October 2016 25 February 2017
Please cite this article as: P. Dhak, M-K. Kim, J.H. Lee, M. Kim, S-K. Kim, Linear-chain assemblies of iron oxide nanoparticles, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm. 2017.02.050
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Linear-chain assemblies of iron oxide nanoparticles Prasanta Dhak, Min-Kwan Kim, Jae Hyeok Lee, Miyoung Kim, and Sang-Koog Kim* National Creative Research Initiative Center for Spin Dynamics and Spin-Wave Devices, and Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea We synthesized iron oxide nanoparticles using a simple hydrothermal approach and found several types of segments of their linear-chain self-assemblies as observed by field emission scanning electron microscopy.
X-ray diffraction and transmission electron microscopy
measurements confirm a well-defined single-phase FCC structure. Vibrating sample magnetometry measurements exhibit a ferromagnetic behavior. Micromagnetic numerical simulations show magnetic vortex states in the nanosphere model. Also, calculations of binding energies for different numbers of particles in the linear-chain assemblies explain a possible mechanism responsible for the self-assemblies of segments of the linear chains of nanoparticles. This work offers a step towards linear-chain self-assemblies of iron oxide nanoparticles and the effect of magnetic vortex states in individual nanoparticles on their binding energy.
Keywords: Fe3O4 nanosphere; Linear-chain assemblies; 3D magnetic vortex; Magnetic binding energy
*Author to whom all correspondence should be addressed; e-mail: [email protected]
1
1. Introduction Magnetic nanoparticles possess a wide range of unique properties with great potential for optical, electronic, biomedical, and computing applications. They represent one of the most exciting fields in nanotechnology, specifically as relates to magnetic memory devices and biomedical research purposes [1]. In magnetic nanoparticles, a very specific spin configuration known as a magnetic vortex, in its different shapes and dimensions, is particularly promising for its bio-, information-storage and -processing technology applications [2, 3]. For example, twobit-per-dot media for magnetic storage could be obtained, provided that a magnetic vortex’s four different states can be manipulated [4]. Additionally, some researchers have reported the potential of artificial structures of periodically arranged magnetic vortices to show exotic physical phenomena. Indeed, one-dimensional (1D) and two-dimensional (2D) dipolar-coupledvortex arrays and their vortex-gyration band structures have already been demonstrated as information-guiding media [5]. Additionally, artificial hexagonal arrays of vortices surrounded by perpendicular domains normal to the plane could be manipulated to 2D skyrmion crystals without Dzyaloshinskii-Moriya (DM) interaction [6]. With regard to the three-dimensional (3D) vortex, single-domain magnetic particles are the usual targets for bio applications; for the dedicated purposes of spintronics, magnetic recording, and nanomedicine, 3D vortex nanoparticles are of particular interest [3,7]. Notwithstanding the recent insights into the exotic physical phenomena of both artificial structures and vortex-state nanoparticles, the formation of magnetic nanostructures with 3D magnetic vortices remains elusive. Few researchers have reported the 3D vortex structures in soft magnetic materials [8,9]. Also, there are few reports available in open literature regarding magnetization reversals in self-assembled chains of soft magnetic nanospheres investigated using 2
micromagnetic modeling based on the Landau-Lifshitz-Gilbert equations [10,11]. It is interesting that the self-assembly of isolated or aggregated magnetic nanoparticles is a promising way to realize novel metastructures using magnetic vortices as elementary units for combining a 3D magnetic vortex with artificial structures. Due to the extremely small size of magnetic nanoparticles, conventional means of self-assembly are quite limited, and exact understanding of the assembly mechanism of nanoparticles becomes essential to the successful formation of magnetic vortex nanoparticle self-assemblies. Taking a bottom-up approach, the starting point for understanding vortex-state nanoparticle assembly is to investigate the formation mechanism of small assemblies of vortex nanoparticles. Apart from the self-assembly phenomena, analysis of small assemblies of magnetic vortex particles could prove central to material science and condensed matter physics, providing key insight into the frustratingly elusive underlying non-equilibrium phenomena, including nucleation in magnetic systems [12-14]. The first step of understanding the formation of a particular cluster system is to calculate the ground states as a function of particle morphology [15,16]. Such morphology of minimal-energy clusters is interpreted in a thermodynamic and kinetic perspective. Still though, much work on model development in terms of thermal entropy and symmetry remains to be done, especially for weakly interacting particles in the energetically stable state. For example, one requirement is to investigate how the interaction between vortex nanoparticles influences the free energy and dynamic protocols that produce small assemblies. However, to achieve this ambitious goal, it is essential to control the fabrication of specific structures using nanoparticles as elementary units. To date, most magnetic nanoparticle applications have focused on spherical primary nanoparticles or nanoparticle assemblies with aspect ratios close to 1, while utilization of magnetic nanowires and linear-chain assemblies of
3
magnetic nanoparticles has been very much limited. However, 1D magnetic structures have the potential to open up new applications in biomedicine, as their high aspect ratio results in a much larger dipole moment, allowing their manipulation with lower magnetic field strengths. Flexible long chains of magnetic particles could also be of importance across a wide range of applied materials technologies. Additionally, it is important to achieve structural precision for optimization of properties and functions. Electronic and plasmonic coupling between metallic nanoparticles has been known to yield novel electronic and optical properties. Such coupling is critically dependent on structural parameters such as inter-particle spacing and the spatial organization of individual nanoparticles. In comparison with higher-order nanostructures, 1D nanoparticle chains are more expedient building blocks for circuits in nanoelectronics, optoelectronics, and biosensors. Minimizing structural irregularity is essential: a large gap can break the coupling along a chain, and branching can cause a short circuit. Therefore, the investigation of magnetic particle assembly in linear chain-like structures is of great interest among concerned researchers. Several methods, often utilizing polymer templates to direct the assembly, have been employed to form nanoparticle chains [11,17]. For example, there have been many studies on nanoparticle self-assembly at interfaces within and on the surfaces of block co-polymers. Our current focus is template-free self-assembly of magnetic nanoparticles, which approach offers the potential for control and tunability of the self-assembly process without the use of templates. In this study, we successfully synthesized high-uniformity 200 nm monodisperse iron oxide nanoparticles and discovered interesting linear-chain self-assemblies that can be enforced from the vortex state of each iron oxide nanosphere. The employed modified hydrothermal method was adjusted in respect of reaction temperature, time and precursor concentration. The 4
micro-magnetic simulation results confirmed the vortex state present in our model system. An extensive literature survey showed that there are only a limited number of studies on linear-chain self-assembly of iron oxide nanoparticles in the vortex state. Our statistical analysis revealed a clear order of priority according to the assembly pattern and the number of assembled particles. Interestingly, all of the experimental results of the field emission scanning electron microscopy (FESEM) analysis exactly agree with the simulated results obtained from the energy densities of the respective assembly patterns. We believe that this study provides valuable insight into the interplay between particles’ assembly patterns and their magnetic vortex configurations. 2. Sample preparation and characterization Fe3O4 nanospheres of 200 nm average diameter were synthesized using a conventional hydrothermal process [18]. For the synthesis, 1.35 g, 5 mmol of FeCl3·6H2O was dissolved in 40 ml ethylene glycol to form a clear solution. Next, 3.6 g of sodium acetate and 1 g of polyethylene glycol were added to the solution, as illustrated in the flowchart of Fig. 1. The mixture was stirred vigorously for 30 minutes followed by sealed in a Teflon-lined stainless-steel autoclave of 50 ml capacity. The autoclave was heated to 200ºC, maintained for 8 hours, and allowed to cool to room temperature. The black products were washed several times with ethanol and dried at 60ºC for 6 hours. A phase analysis of the synthesized magnetite nanoparticles was performed using an X’pert Pro Phillips x-ray diffractometer with a Cu Kα target (λ =1.5418 Å) in a wide range of Bragg angles 2θ (20°≤2θ≤70°) at a scanning rate of 2°(2θ)/min. The XRD spectrum of a Si crystal was used as the standard for calibration of the scanning angles. The size and morphology of the assynthesized iron oxide nanoparticles were observed using a TEM [JEM-3000F, JEOL] with an
5
acceleration voltage of 200 kV. Samples for the TEM analysis were prepared by depositing a few drops of the respective nanoparticle solutions, ultrasonically dispersed in water for 30 minutes on separate carbon-coated copper grids. In order to study the size and surface morphology of the iron oxide nanoparticles, FESEM was performed using JSM 5410LV. To further determine the composition of the synthesized material, energy-dispersive X-ray (EDX) analysis was performed on the sample using an Oxford Instrument INCA attached to the scanning electron microscope (SEM). The magnetic properties of the iron oxide nanoparticles were studied using a vibrating sample magnetometer (VSM) (7404 VSM, Lake Shore) with a maximum field of 1.0 T and a field step of 5 Oe. In order to obtain the assembly pattern and verify the vortex phenomena of the iron oxide nanoparticles, an FESEM analysis was conducted in the following designed system. First, since magnetic particles tend to aggregate in solution, we tried to make dispersible particles instantaneously by vortexing and sonication. Partially dispersed solution was then rapidly loaded onto a Si wafer by the drop-casting method and dried at room temperature for 2 hours. When ferromagnetic particles interact each other in the medium without any other external forces, magnetic interactions act as a key factor in determining their arrangement. When a drop of magnetic nanoparticle solution with dispersion concentration of 2 mg mL-1 is deposited on the substrate, magnetic interactions between those particles can affect assembly of nanoparticles as reported in our previous article [20], while avoiding the influence of other, incidental forces. Dissimilar to arrangements of as synthesized iron oxide nanoparticles, certain types of assembly patterns are formed during aggregation on the substrate. We believe that these originated from the vortex states of the individual particles. 3. Results and discussion 6
Figure 2 plots the XRD pattern obtained from the randomly oriented iron oxide nanoparticles measured at room temperature. Careful analysis of the X-ray data confirmed the single-phase formation of iron oxide particles. No other secondary phases were detected. All of the peaks were indexed properly, as shown in Fig. 2. The material showed a cubic spinel structure with space group P4132 (213). Figure 3 shows the TEM micrographs for iron oxide nanoparticles. Figure 3(a) clearly indicates that all of the particles were well dispersed and spherical in shape, with an average particle diameter of about 200 nm. The high-resolution TEM (HRTEM) image shows the lattice imaging of the (311) plane, where inter-planer distance d = 2.5Å in Figs. 3(b) and 3(c). In Fig. 3(d) the fast-Fourier transform (FFT) of the lattice image shown in Fig. 3(c) represents a single phase FCC structure of the embedded iron oxide particles. The HRTEM-identified plane correlates well with that of the XRD pattern. Figure 4(a) and inset of figure 4(a) provide the FESEM micrographs of the iron oxide microsphere with 200 nm average diameter. The image confirms the spherical morphology and lesser agglomeration present in our studied materials. In Fig. 4(b), the EDX spectra of the iron oxide nanoparticles show satisfactory results in good agreement with the iron oxide composition. Figure 5 plots the magnetization curves measured at 300 K for the 200 nm diameter iron oxide nanoparticles. The magnetic saturation value for our sample was 77.17 emu g-1, which is comparable with the standard magnetization value reported for iron oxide nanoparticles of 200 nm diameter [18]. Figure 6 shows the one-to-five linear assembly configurations of iron oxide nanoparticles denoted as a1, a2, a3, a4 and a5, respectively. A few of the arrangements might not form a 7
perfectly straight line, minute particle displacement is ignored and is considered as almost a straight line. We examined the spin configuration of each assembled linear-chain cluster and then calculated the exchange and dipolar interaction energies involved. Next, we performed micromagnetic simulations to calculate the ground states of the spin configurations and the magnetic interaction binding energies, using the Finite Element MicroMagnEtics (FEMME) code (version 5.0.8). The following Fe3O4 input parameters were applied: diameter of nanosphere (2r) = 200 nm, saturation magnetization Ms = 4.85 x 105 A/m, exchange stiffness Aex = 1.3 x 10-11 J/m, and magnetocrystalline anisotropy K1 = -1.36 x 104 J/m3 [19]. The formation of a 3D magnetic vortex in a sphere has been well established both theoretically and experimentally [9,20]. The ground states of the spheres were obtained through relaxation from their saturated state. In the 200 nm iron oxide nanospheres, single magnetic vortex states were well established. In such magnetic configurations, the spins achieve a complete flux closure around the vortex core, which is aligned in the +x direction. This exotic variation of the static magnetic spin configuration is generally determined by the competition between magnetostatic energy and exchange energy. The exchange interaction is very strong in the atomic-scale range, and thus tends to impart uniformly to the magnetized domains. The dipolar interaction, contrastingly, is rather weak, but works in the micrometer range, thereby effecting spiral spin configurations inside the nanoparticles. In the case of iron oxide nanoparticles, we must also consider the cubic anisotropy energy, which, similarly to exchange interaction, is very strong in the atomic-scale range. As a result of the anisotropy interaction, the calculated vortex core was aligned in the <111> direction, which is the easy axis of iron oxide. Additionally, this vortex state, as the ground state, remained in each particle within the assembled clusters. The outward core orientations were aligned parallel to the orientation of the 8
vortex core in the case of the linear geometry. It is clear from Fig. 7 that in the assembled particles, the internal local magnetization is modified in the proximity of the individual core. In the case of the double particles a2, the broadening of the core region in each sphere is greater than that for the single particle a1, and the width of the contact area between the two particles is larger, indicating modification of the local internal magnetizations. In the cases of more than two particles, which is to say, a3, a4 and a5, there is more core broadening of the middle particle compared with the end particles in the linear-chain assemblies of the nanoclusters. The core magnetizations were modified in the contact area of the individual particles in order to reduce the exchange interaction energy inside each particle. Next, we calculated the magnetic interaction energies of all of the clustered nanoparticles as shown in Fig. 8, so as to understand the driving force behind magnetic nanoparticle assembly. The binding energy can be represented as (I)
∆EB = ET − N × ES
where ET is the total magnetic energy in each given chain, N is the number of particles in the chain, and ES is the magnetic energy of an isolated single particle. In exploring each contribution of the magnetic interactions to the total binding energy, we separately calculated the exchange (
∆Eexch ), dipolar ( ∆Edip ), and anisotropic ( ∆Eani ) binding energies. It is well established that the magnetic interaction is almost ten times larger than the Van der Waals [21] and electrostatic interactions [22]. In this study, only those magnetic interactions that are essential factors determining the clustering of magnetic particles having a single 3D vortex structure were considered. The inter-particle exchange interaction was not considered here, owing to its atomic scale and atomic-scale separations between individual particles as observed 9
by FESEM. Figure 8 shows the binding energy calculations in terms of ∆Eexch , ∆Edip and ∆Eani for the a1, a2, a3, a4 and a5 nanoclusters depicted in Fig. 6. With increasing particle number, the total magnetic binding energy decreases. One of our recent investigation showed that the exchange binding interaction is the dominant factor in the assembly of nanoparticles with a 3D magnetic vortex, and that the dipolar binding interaction inhibits increasing numbers of particles in the linear configuration [20]. Also, it was interesting to observe that the value ∆Eani increases with increasing numbers of magnetic nanoparticles participating in the linear-chain formation. This observation could be explained by the fact that there exist four easy axes energetically equivalent to each other in the Fe3O4 nanosphere, which implies that there are three other easy axes also in the linear-chain direction. Magneto-crystalline anisotropy binding interaction hinders the formation of the linear chain in a specific easy axis, due to the existence of freedom at the other three binding sites. From the present binding energy calculation, an important feature was revealed: the intra-exchange and dipolar energies oppositely contribute to the formation of nanoparticle assemblies. Here, the dipolar interaction contributes oppositely to assembling of individual particles from assembling; the intra-exchange interaction, contrastingly, prefers to assemble them by modification of each particle’s internal 3D structure. This phenomenon, might be counter-intuitive, but observed as in our earlier report [20]. 4. Summary We demonstrated the linear assembly of 200 nm iron oxide nanoparticles and the relation to the 3D magnetic vortex structure and binding energy. From the micromagnetic simulation analysis, it is clear that the intra-exchange interaction has an important role in modifying the internal spin configuration of the core of an iron oxide nanosphere. In this regard, it is helpful to
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reduce the magnetic binding energy necessary for magnetic nanoparticle assembly. We believe that this study provides valuable insights into the interplay between particles’ assembly patterns and their spin-vortex magnetic properties. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).
References [1] B.V. Waeyenberge, A. Puzic, H. Stoll et al., Nature 444 (2006) 461. [2] W. Rave, K. Fabian, A. Hubert, J. Magn. Magn. Mater. 190 (1998) 332. [3] L. M. Lacroix, Se. Lachaize, F. Hue et al., Nano letters 12 (2012) 3245. [4] J. Miltat, A. Thiaville, Science 298 (2002) 555. [5] D.S. Han, H.B. Jeong, S.K. Kim, Appl. Phys. Lett. 103 (2013) 112406. [6] L. Sun, R.X. Cao, B.F Miao et al., Phys. Rev. Lett. 110 (2013) 167201. [7] Y. Yang, X.L. Liu, J. Yi et al., J. Appl. Phys. 111 (2012) 044303. [8] R.E. Dunin-Borkowski, R.K.K. Chong, T. Kasama et al., Inst. Phys. Conf. Ser. 179 (2003) 451. [9] M.J. Hÿtch, R.E. Dunin-Borkowski, M.R. Scheinfein et al., Phys. Rev. Lett. 91 (2003) 257207. [10] T. Kasama P. Barpanda, R.E. Dunin-Borkowski et al., J. Appl. Phys. 99 (2006) 08G103. [11] R.E.C. Schmidtke, R. Zierold, A. Feld et al., Langmuir 30 (2014) 11190−11196. 11
[12] D.R. Nelson, F. Spaepen, Solid State Phys. 42 (1989) 1. [13] J.P.K. Doye, D.J. Wales, Science 271 (1996) 484. [14] F.H. Stillinger, T.A. Weber, Science 225 (1984) 983. [15] N. Arkus, V. N. Manoharan, M. P. Brenner, Phys. Rev. Lett. 103 (2009) 118303. [16] G. Meng, N. Arkus, M.P Brenner et al., Science 327 (2010).560. [17] P.J. Krommenhoek, J.B. Tracy, Part. Part. Syst. Charact. 30 (2013) 759. [18] X.L.H. Deng, Q. Peng, X. Wang et al., Angew. Chem. Int. Ed. 44 (2005) 2782. [19] L.L. Afremov, A.V. Panov, Phys. Met. Metallogra 86 (1998) 269. [20] M.K. Kim, P. Dhak, H.Y. Lee et al., Appl. Phys. Lett. 105 (2014) 232402. [21] C.E. Wilmer, K.J. Bishop, S. Soh et al., Small 5 (2009) 1600. [22] Y. Levin, Rep. Prog. Phys. 65 (2002) 1577.
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List of Figures Fig. 1. Schematic representation of synthesis of single-crystal iron oxide nanoparticles Fig. 2. XRD pattern of as-synthesized iron oxide nanoparticles Fig. 3. TEM images of (a) aggregated particles, (b) single-particle, (c) HRTEM lattice image and (d) FFT of the lattice image shown in (c) Fig. 4. (a) FESEM and (b) EDX of iron oxide particles Fig. 5. Room-temperature magnetic hysteresis loop of aggregated iron oxide nanoparticles of 200 nm in diameter Fig. 6. FESEM images of segments of linear-chain assemblies of iron oxide nanoparticles Fig. 7. Simulation results of magnetization distributions of single and linearly assembled iron oxide nanoparticles Fig. 8. Magnetic binding energy terms for different numbers of iron oxide nanoparticles in linear chain.
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Figures:
Fig. 1
Fig. 2
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Fig. 3
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Fig. 4
Fig. 5
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Fig. 6 Fig. 6
Fig. 7
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Fig. 8
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Research highlights:
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Hydrothermal synthesis of pure phase 200 nm Fe3O4 nanoparticles.
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Studies of linear-chain assemblies of iron oxide nanosphere by FESEM.
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Micromagnetic simulations showed the presence of 3D vortex states.
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The B.E. for different numbers of particles in linear chain assemblies were calculated.
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