Effects of a 10 T External Magnetic Field on the Thermal

Jun 10, 2008 - Center for Advanced Materials and Nanotechnology, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel, and Department...
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Effects of a 10 T External Magnetic Field on the Thermal Decomposition of Fe, Ni, and Co Acetyl Acetonates S. V. Pol,† V. G. Pol,*,† A. Gedanken,† M. G. Sung,‡ and S. Asai‡ Center for AdVanced Materials and Nanotechnology, Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel, and Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed March 4, 2008. ReVised Manuscript ReceiVed March 27, 2008 The current investigation is centered on the thermal decomposition (700 °C) of acetyl acetonates of Ni, Co, and Fe in a closed reactor that was conducted by employing an external magnetic field (MF) of 10 T. Interestingly, reactions of Co and Ni acetyl acetonates under a 10 T MF produce Co and Ni nanoparticles (NPs) coated with carbon, while Fe acetyl acetonate produces Fe3O4 uncoated with carbon. Additionally, it is observed that all the as-formed magnetic particles tend to align in one dimension along applied MF; thus, this process can be used to fabricate large arrays of magnetic nanoparticles. The effect of an applied MF to synthesizemorphologicallyandcompositionallydifferentproductsfromcorrespondingprecursorswiththeirmesoscopicorganization is the key theme of the present paper, explained with a plausible mechanism.

Introduction Nanoscience, a growing field of research, the use of individual nanocrystals for growing one-, two-, and three-dimensional (3D) superstructures and the investigation of the collective properties of artificial quantum dot solids,1 has recently emerged. The fabrication of nanometer-ordered entities is considered to be the key for applications in data storage, functional devices, communications, and nanotechnology. The randomly distributed nanostructures with size fluctuations and without periodicity have significant limitations regarding their application. Thus, the ability to systematically manipulate these nanocrystals is an important goal in modern materials science. The nanoscale magnetic materials have attracted intensive interest because oftheirpotentialapplicationinhighdensitymagneticrecording,magnetic sensors, and addressing some basic issues about magnetic phenomena in low dimensional systems.2,3 Various approaches have been developed to prepare nanoscale magnetic materials.4–7 One of the ways to arrange magnetically susceptible material is by employing an external magnetic field (MF). However, little attention has been paid to study the effect of an external MF on the nucleation and growth processes of magnetic materials. It is quite simple to apply an external MF at room temperature reactions, but it is difficult to carry out a high-temperature (HT) reaction under the application of a 10 T MF without spoiling the magnets. We have developed an experimental setup whereby the reactor and the ceramic oven are systematically assembled to safely carry out a HT reaction under the influence of a high (10 T) MF in order to study the assembly behavior of the formed magnetic nanocrystallites. * Corresponding author, [email protected]. † Bar-Ilan University. ‡ Nagoya University.

(1) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (2) Jun, Y.; Jung, Y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615. (3) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (4) Srivastava, D. N.; Perkas, N.; Gedanken, A.; Felner, I. J. Phys. Chem. B 2002, 106, 1878. (5) Wu, Y. H.; Qiao, P. W.; Qiu, J. J.; Chong, T.; Low, T. S. Nano Lett. 2002, 2, 161. (6) Tripp, S. L.; Pusztay, S. V.; Ribbe, A. E.; Wei, A. J. Am. Chem. Soc. 2002, 124, 7914. (7) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874.

There are few reports on the study of the effect of an external MF on the movement and self-assembly activities of magnetic nanocrystallites. The growth and assemblage of cobalt nanocrystallites under an external MF of 0.25 T is known.8 The external MF-induced growth of single crystalline Fe3O4 nanowires is reported by Wang et al. along the easy magnetic [111] or [110] axes.9 Nanowires of SrFe12O19 with diameters of 100 nm and lengths of 2.5 mm have been successfully synthesized in a hydrothermal cell at 180 °C with an applied MF of 0.35 T.10 Only arborescent aggregates are obtained in the electrochemical growth of iron under a MF of 0.6 T.11 The surface morphology and growth mechanism of YBa2Cu3O7 films by chemical vapor deposition in a MF of 2-8 T are identified.12 Abu-Much et al. recently demonstrated the formation of a three-dimensional microstructure of a Fe3O4-poly(vinyl alcohol) composite by evaporating the hydrosol under a weak MF.13 From a literature survey, it is confirmed that the application of 10 T MF is the highest of all the previous reports on the organization of nanomaterials formed at 700 °C. The effect of an applied external 10 T MF on corresponding precursors to synthesize morphologically and compositionally different products with their mesoscopic organization is the key theme of the present paper. A solvent-free, single-step, scalable, and straightforward approach is demonstrated for the anisotropic orientation of Co-C, Ni-C, and Fe3O4-C nanostructures. With a 10 T external MF, the thermal decomposition (700 °C) of acetyl acetonates of Ni, Co, and Fe in a closed reactor yielded Co and Ni NPs coated with carbon and Fe3O4 particles uncoated with carbon under their autogenic pressure at elevated temperature. The probable mechanism is also developed.

Experimental Section Synthesis of Ni-C, Co-C, and Fe3O4-C Nanostructures. High-purity acetyl acetylacetonates ((C5H7O2)2) of Fe(II) (Aldrich), Co(II) (Aldrich), and Ni(II) (Alfa Aesar) were purchased and used as received. In the fabrication of Ni-C, Co-C, and Fe3O4-C (8) Niu, H.; Chen, Q.; Zhu, H.; Lin, Y.; Zhang, X. J. Mater. Chem. 2003, 13, 1803. (9) Wang, J.; Chen, Q.; Zheng, C.; Hou, B. AdV. Mater. 2004, 16, 137. (10) Wang, J.; Zeng, C. J. Cryst. Growth 2004, 270, 729. (11) Bodea, S.; Vignon, L.; Ballou, R.; Molho, P. Phys. ReV. Lett. 1999, 83, 2612.

10.1021/la800683m CCC: $40.75  2008 American Chemical Society Published on Web 06/10/2008

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Figure 1. The representative digital image of (a) the SS reactor filled with iron(II) acetyl acetonate under inert atmosphere, (b) the homemade ceramic oven accommodating the SS reactor, and (c) the SS reactor filled with the desired precursor, placed in the bore at the center, where the MF is 10 T.

nanostructures, 2 g of the acetyl acetonate of Ni, Co, and Fe were introduced separately into a 5 cm3 stainless steel (SS) reactor at room temperature in a nitrogen-filled glovebox in order to avoid probable oxidation due to air. The 1/2 in. diameter union part was initially sealed with a cap on one side. The partially filled SS reactor was closed tightly with the other cap and placed at the center of the tube furnace for uniform heating. The parts for making the SS reactor were purchased from the Swagelok Company. A ceramic oven containing the SS reactor filled with 2 g of iron(II) acetyl acetonate was positioned in the bore of 10 T magnets (Figure 1c), where the gradient of the MF was the smallest. The ceramic oven was heated to 700 °C at a rate of 30 °C/min and maintained at 700 °C for 3 h. The chemical dissociation and transformation reaction took place under the autogenic pressure of the precursors. The oven-containing SS reactor heated to 700 °C was gradually cooled to 50 °C. The MF simultaneously decreased from 10 to 0 T within 90 min. Similar reactions are carried out for Ni and Co acetyl acetonates. The effect of an external MF on the magnetically susceptible carbonaceous14,15 materials was recently demonstrated by us. Characterization. Various analytical techniques were used to identify and characterize the as-prepared products of Ni, Co, and Fe acetyl acetonate reactions under a MF. To understand the purity of the crystal structure (the interatomic distance and angle) of the asprepared products, the X-ray diffraction patterns were measured with a Bruker AXS D* Advance Powder X-ray diffractometer (using Cu KR ) 1.5418 Å radiation). Since the acetyl acetonate contains C, H, and O, in addition to metal, C, H, N, S, and O analyses (EA1110 analyzer) were carried out for the as-prepared products. The morphology, grain size, and chemical composition in a selected area were measured for the Ni-C, Co-C, and Fe3O4-C nanostructures by employing transmission electron microscopy (HR-TEM, JEOL 2010). The morphology of the materials was investigated by highresolution scanning electron microscopy (HR-SEM, JSM, 7000 F). A room temperature magnetic susceptibility measurement of Co-C, Ni-C, and Fe3O4-C samples is conducted by employing a vibrating sample magnetometer. A small amount of material, ∼20 mg, is inserted into a plastic capsule. Cotton wool was placed on the top of the sample, and the capsule was sealed. This prevents the particles from any movement caused by the vibration of a VSM sample holder rod or by a variation in the applied magnetic field during measurements. (12) Ma, Y.; Watanabe, K.; Awaji, S.; Motokawa, M. J. Cryst. Growth 2001, 233, 483. (13) Abu-Much, R.; Meridor, U.; Frydman, A.; Gedanken, A. J. Phys. Chem B 2006, 110, 8194. (14) Pol, V. G.; Pol, S. V.; Gedanken, A.; Sung, M-G.; Shigeo, A. Carbon 2004, 42, 2738. (15) Pol, V. G.; Pol, S. V.; Calderon-Moreno, J. M.; Sung, M. G.; Asai, S.; Gedanken, A. Carbon 2006, 14, 1913.

Figure 2. XRD patterns of as-prepared (a) Co-C, (b) Ni-C, and (c) Fe3O4-C samples.

Results and Discussion The structural information for the formation of Co-C, Ni-C, and Fe3O4-C is obtained from X-ray diffraction (XRD), morphological data are acquired from scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and compositional analyses are carried out with EDX. The diffraction pattern of the as-prepared Co-C sample (Figure 2a) can be indexed to the face-centered cubic (space group Fm3jm) Co, and matches well with PDF No. 89-4307. The diffraction peaks observed at 2θ ) 44.22, 51. 35, and 75.86 are assigned as (111), (200), and (220) reflection lines. The XRD pattern of the as-prepared Ni-C sample is presented in Figure 2b. The diffraction peaks, observed at 2θ ) 44.49, 51.85, and 76.38, are assigned as (111), (200), and (220) reflection lines of the face-centered cubic phase of Ni (space group Fm3jm). These values are in good agreement with the diffraction peaks, peak intensities and cell parameters of crystalline Ni (PDF No. 4-850). The XRD pattern of the as-prepared Fe3O4-C sample is presented in Figure 2c. The major diffraction peaks are observed

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Figure 3. Scanning electron micrographs of (a) Co-C, (b) Ni-C, (c) Fe3O4--C, and (d) HR-SEM of Fe3O4-C samples. All samples are the products obtained under the application of the 10 T MF.

at 2θ ) 30.10, 35.45, 37.08, 43.08, 53.45, 56.98, 62.57, and 74.02 and are assigned as (220), (311), (222), (400), (422), (511), (440), and (533) reflection lines. These values are in good agreement with the diffraction peaks, peak intensities and cell parameters of crystalline Fe3O4 (PDF No. 82-1533). However, since the XRD of γ-Fe2O3 is very similar to that of magnetite, by employing Mo¨ssbauer spectroscopy at 298 K, it was confirmed that the formed product is Fe3O419 and not γ-Fe2O3. The additional diffraction peaks confirming the presence of graphite are seen at 2θ ) 26.45 only for Ni-C and Co-C samples. The SEM image shown in Figure 3a indicates the spherical morphology of the particles in the Co-C sample. The as-prepared Co-C particles have a tendency to form agglomerates that make a 1D array. The aggregation is attributed to their magnetic nature. The formed Co-C arrays are several micrometers long (Figure 3a). Similar 1D arrangements are observed for Ni-C particles produced applying 10 T MF. The several 1D layers (Figure 3b) of Ni-C particles piled up to outline 2D arrangements. For the Fe3O4-C sample, 10-200 µm long 1D arrays with a diameter of several micrometers (Figure 3c) were observed under 10 T MF. Even the smallest broken part of the Fe3O4-C sample showed a 1D arrangement of the comprised particles. The high-resolution SEM (HR-SEM) image (Figure 3d) illustrates that ∼8 µm diameter aggregates of pyramid-shaped particles are chained together and form several chains aligned parallel to each other. These chains are comprised of Fe3O4-C particles. A thorough morphological depiction was carried out for all the three samples, employing TEM measurements. The transmission electron micrograph of a Co-C sample is shown in Figure 4a. The thermal dissociation of Co acetyl acetonate favors the formation of the dark particles of Co, which is surrounded by carbon layers. We assume that the thermal decomposition of [(C5H7O2)2]Co leads to dissociation into carbon, hydrogen, oxygen, and cobalt atoms at 700 °C. The Co atom

forms the Co core. The particle diameters range from 30 to 120 nm. The high-resolution image (Figure 4b) taken on a single particle revealed that the 25 nm dark core of Co is encapsulated in 6 nm carbon layers. The electron diffraction (ED) pattern taken on the Co-C particle is depicted in Figure 4c. The inner diffused ring belongs to graphite for the 002 planes. For the Ni-C sample, the spherical dark Ni core is surrounded by faint carbon layers. In a few places, short carbon nanotubes are grown using Ni as a catalyst. The diameters of the Ni particles range from 50 to 70 nm (Figure 4d). The high-resolution image (Figure 4e) taken on a single Ni-C particle revealed that the 35 nm dark core of Ni is encapsulated in 10 nm graphitic carbon layers. A similar ED pattern is obtained for Co-C and Ni-C (Figure 4f) samples and assigned to the respective planes. Surprisingly, in the Fe3O4-C sample the TEM picture illustrates trigonal or square-shaped particles, which are not coated with a nanolayer of carbon (Figure 4g). This particular picture (Figure 4g) is intentionally focused on the Fe3O4 particles, while image 4h focuses on the carbon bodies. The rejected carbon from the Fe3O4 surfaces under 10 T MF is random in shape and size. The as-prepared carbon during the thermal decomposition of iron(II) acetyl acetonate is mostly disordered, compared to Ni and Co, though the reaction temperatures (700 °C) were the same for all the three reactions. In order to understand the graphitization of formed carbon layers around the metal or oxides, HR-TEM measurements of the carbon were carried out for the Co-C, Ni-C, and Fe3O4-C samples. The measured interlayer spacing between these graphitic planes was 0.34 and 0.33 nm for the Co-C (Figure 5a) and Ni-C (Figure 5b) samples, respectively. The carbon accompanying the Fe3O4 was formed as disordered carbon, which is not found on the surfaces of the Fe3O4 particles (Figure 5c). For the Co-C and Ni-C samples, these values are very close to those of the graphitic (0.34 nm) layers. It should be noted that

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Figure 4. Transmission electron micrographs of (a) a Co-C sample, (b) HR-TEM of a single Co-C particle, (c) electron diffraction on Co-C particles, (d) TEM of a Ni-C sample, (e) HR-TEM of a single Ni-C particle, (f) ED on Ni-C particles, (g) TEM of a Fe3O4-C sample deliberately taken on Fe3O4 particles, (h) TEM purposely taken on C particles (insert shows HR-TEM of carbon particles), and (i) ED on Fe3O4-C particles.

the graphitic ordering is much better in Ni-C and Co-C samples than in the Fe3O4-C samples, verifying that Co or Ni metals are better catalysts for inducing graphitic order than oxides of Fe. During the TEM measurements of Co-C (Figure 6a), Ni-C (Figure 6b), and Fe3O4sC (Figure 6c) samples, additional EDS (energy dispersive X-ray analysis) was carried out to check the purity of the samples. In all three EDS spectra, Cu were seen, which originated from the sample-supported copper grid. In addition to carbon, Co and Ni were observed in the Co-C and Ni-C samples, respectively. A very small signal (1 wt %) for oxygen was also noted for both samples. No additional impurities were observed, confirming the purity of the formed Co-C and Ni-C samples. In the Fe3O4-C sample, in addition to carbon and Fe, a strong signal of oxygen was also noted. This means that although all the reactions are carried out under similar reaction conditions with similar precursors of Co, Ni, and Fe, only Fe tends to oxidize heavily compared to Ni and Co. The magnetizations versus magnetic field curves were measured for Co-C, Ni-C, and Fe3O4-C samples (Figure 7) in the range of -14000 to 14000 Oe. The measured saturation

magnetization for Co-C sample was 55 emu/g (Figure 7a), with 195 Oe coercivity and 4.2 emu/g remanent magnetization. The Ni-C sample exhibits typical hysteresis with a saturation magnetization of 22.4 emu/g (Figure 7b). A 102 Oe coercive field and remanent magnetization of 2.2 emu/g was obtained for the Ni-C sample. The Fe3O4-C sample exhibits typical hysteresis with a saturation magnetization of 40 emu/g (Figure 7c). For Fe3O4-C sample, the coercive field was 110 Oe and the remanent magnetization is 7 emu/g. Therefore, we ensure that all abovementioned compounds are ferromagnetically ordered. Considering that all three samples contain some amount of carbon, for the 100% of Co, Ni, and Fe3O4, the magnetization would be even higher. As a representative case, Fe3O4-C sample contains 34.87% carbon, the remaining 65.13% of Fe3O4 yields a value of 61.41 emu/g for 100% Fe3O4. Our obtained Ms value for pure Fe3O4 was smaller than the microwave-synthesized 8-9 nm pure Fe3O4 nanoparticles (Ms of ∼65 emu/g).17a Additionally, (16) Pol, S. V.; Pol, V. G.; Felner, I.; Gedanken, A. Eur. J. Inorg. Chem. 2007, 2089–2096.

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Figure 5. HR-TEM at the edge of the (a) Co-C, (b) Ni-C, and (c) from Fe3O4-C particles representing the nature of formed carbon.

Figure 6. EDS of (a) Co-C, (b) Ni-C, and (c) Fe3O4-C samples.

nanomagnetic particles are expected to exhibit reduced magnetization due to the large percentage of surface spins with disordered magnetization orientation.17b In fact, the magnetization is smaller than the bare particles, indicating that there is no magnetic proximity effect between the magnetic (Ni, Co, or Fe3O4) particles and carbon, which is consistent with previous experiments.17c Without the applied MF, the thermal decomposition of three magnetic acetates (Co, Ni, and Fe) at their autogenic pressure yielded graphitic carbon16 coated on nanosized Ni, Co, and Fe3O4 cores. The mechanism is confirmed in the topic regarding “why are Ni and Co obtained in the metallic form,16 while Fe tends to form Fe3O4” on the basis of enthalpy change (∆H0) and free energy changes (∆G 0), to form the respective metals from the corresponding oxide. A positive ∆G 0 is obtained for the formation of Fe16 from Fe(II) and, unlike Ni and Co, indicates that, thermodynamically, the reduction to the metallic state is not favorable. On the other hand, the ∆G 0 value is negative for the formation of metallic cobalt and Ni. Now, the question remaining (17) (a) Kodoma, H. R.; Berkovitz, A. E.; Mcniff, E. J., Jr.; Foner, S. Phys. ReV. Lett. 1996, 77, 3942. (b) Hohne, M.Zeise.; Esquinazi, P. Carbon 2004, 42, 3109. (c) Hong, R. Y.; Pan, T. T.; Li, H. Z. J. Magn. Magnet. Mater. 2006, 303, 60. (d) Ayache, J.; Oberlin, A.; Inagaki, M. Carbon 1990, 28, 353.

Figure 7. Magnetization vs magnetic field curves for (a) Co-C, (b) Ni-C, and (c) Fe3O4-C samples.

unanswered is why the carbon shell was rejected from the Fe3O4 surface while it remained coated on the surface of Co and Ni under the application of a 10 T MF? To learn about the decomposition products of iron(II) acetyl acetonate, thermogravimetric analysis (TGA) coupled with mass spectroscopy analysis was carried out up to 700 °C under inert atmosphere. The results are presented in Figure 8. The total weight losses are observed in three steps. The green curve indicates percent weight loss with respect to temperature, while the blue curve represents its derivative. The major (∼66%) weight loss is observed below 300 °C, minor (4%) at 350 °C, and the additional 7.7 wt % loss occurs over a range of 597-643 °C. The major

Thermal Decomposition of Metal Acetyl Acetonates

Figure 8. TGA curve of iron(II) acetyl acetonate carried out under inert atmosphere.

decomposition of the iron(II) acetyl acetonate might have occurred below 600 °C. However, below 600 °C we did not receive any crystalline products. That is the reason why the RAPET reactions are conducted at 700 °C. In the case of Fe acetyl acetonate, the reactant possesses a +2 oxidation state, while the product has an oxidation state of 2.66. This means that part of the original Fe2+ is oxidized. This indicates that the formation of Fe3O4 occurs first and then the formation of the carbon sheets. The TGA recorded a total weight loss of 77.7% below 700 °C under a flow of nitrogen, leaving a 22.3% product. These 22.3% weights are identical to the iron content in the precursor formula that remains in the TGA pan. On dissociation of the iron(II) acetyl acetonate, C2H2 (m/e ) 25), C3H6 (42), CO2 (44), C (12), and acetone, CH3COCH3 (58) escape, mostly below 300 °C, as confirmed by mass spectroscopy. Once again, 4% at 350 °C and 7.7% at ∼650 °C weight losses are observed in the mass spectrum as C (m/e ) 12). The detected m/e ) 12 might have originated from the dissociation of hydrocarbons under the electron bombardment inside the mass spectrometer (MS) and not from the gaseous carbon. The dissociation of iron(II) acetyl acetonate in a closed RAPET system at 700 °C confirmed the weight loss of 53%, keeping ∼47% of a solid product consisting of Fe3O4 (29.86%) and carbon (17.16%). The rest of the weight loss is due to the formation of hydrocarbons and carbon oxides. The products of the dissociation reaction float in the gas phase and solidify right after their formation. The matter is what solidifies first, depending on either kinetic or thermodynamic stability under an externally applied MF. It is possible that the order is dictated by thermodynamics, namely, Fe3O4 is formed at a lower temperature than the dissociation of the hydrocarbons, to form carbon. The arrangement of parallel stacking of condensed aromatic rings is called basic structural units (BSUs). The texture of different graphitic bodies is classified based on the arrangement of the BSUs such as concentric, radial, and random. It is known that when there are some solid impurities present,17d the carbon layers tend to align by arranging their basal planes parallel18 to the surface of the solid materials. Under an applied 10 T MF, the magnetic rejection of carbon from the surface of Fe3O4, forming separated carbon sheets, occurs, while the carbon is attracted to Ni and Co which constitute the core and carbon forms the shell. From this result, we believe that the binding energy of the in plane of BSUs on particles would be larger than that of edge of BSUs on the particles. This matter would be related to the binding force (energy) of BSUs on solid particle surfaces, namely, metal or oxide surfaces. When the binding (18) Inagaki, M. Carbon 1997, 35, 711.

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force is smaller than the alignment energy, the BSUs align along the magnetic line. Thus, the binding force of BSUs on Co and Ni is larger than that of the BSUs on Fe3O4. Ferromagnetic Fe, Ni, and Co are well-known as catalysts for the growth of carbon around it.19 In the present system with a 10 T MF, a carbon shell is formed around the Ni and Co. In the 10 T MF, the Fe3O4 particle becomes a permanent magnet, and thus, a magnetic line would be formed perpendicular to the surface of the Fe3O4 particle, and an aromatic substance would align along the magnetic line. Such a stable arrangement would eventually lead to carbon sheets separated from the Fe3O4 particle. The rejection of carbon occurs due to magnetic forces between the magnetic Fe3O4 particles and the approaching carbon particles. The carbon surface is also known to be either paramagnetic18 or ferromagnetic (an equal number of sp2 and sp3 hybridized carbon atoms).20 The carbon formed upon this dissociation has a strong interaction with the magnetic Fe3O4 particles, leading to its rejection from the surface of the substrate. Upon cooling, the Fe3O4-C product fabricated in a MF at 700 °C/3 h causes the formation of Fe3O4 particles, which solidify first and reject any approaching magnetic carbon moiety. Similar carbon rejection processes occurred from the Co/ZrO2 nanocomposite21 and the paramagnetic MoO2 surfaces.22 The pyramid-shaped Fe3O4 particles magnetize each another by a dipolar interaction in arbitrary directions. In an externally applied 10 T MF, the pyramidal particles tend to align along the magnetic line of force and favor the formation of linear chains. Magnetization makes all single domain particles orient along the magnetic line of force. As aresult,dipole-directedself-assemblythroughdipolarinteraction8 along the magnetic line of force could occur, leading to the formation of linear chains. The magnetic moment, magnetic field and magnetic interactions on Fe3O4 crystals increase due to an applied MF. Under the attraction of an external MF, 1D entities are formed through pyramidal particle connections (Figure 3c,d), and the paramagnetic carbon diffuses to form separated sheets. The 1D chains are nearly parallel, which could be the result of magnetic attraction, resulting in the chains aligning parallel to the magnetic line of force. A similar effect of 10 T MF on the alignment of Co-C and Ni-C is anticipated. It is worth mentioning that without the MF, acetyl acetonates of Co, Ni,23 and Fe produced completely core-shell morphologies of Co-C, Ni-C, and Fe3O4-C, respectively. In the control reactions, under a low MF (>1 T), we did not observe any changes in morphology, compared to 0 T MF. Therefore, high (10 T) MF is essential to occur morphological changes in in situ formed nanomagnetic materials, which further leads to form 1D arrays. In conclusion, three magnetically susceptible precursors (acetyl acetonates of Ni, Co, and Fe) were thermally (700 °C) dissociated in a closed reactor, separately under 10 T external MF, enabling us to learn about their formed morphologically and compositionally different products. Moreover, it was observed that all the as-formed magnetic particles tend to align in 1D along with applied MF. The plausible mechanism is furnished for the synthesis of morphologically different products with their mesoscopic organization due to the applied external magnetic field. LA800683M (19) Flahaut, E.; Govindaraj, A.; Peigney, A.; Laurent, C.; Rousset, A.; Rao, C. N. R. Chem. Phys. Lett. 1999, 300, 236. (20) Ovchinnikov, A. A.; Shamovsky, I. L. THEOCHEM-J. Mol. Struct. 1991, 83, 133. (21) Pol, V. G.; Pol, S. V.; Gedanken, A.; Kessler, V. G.; Seisenbaeva, G. A.; Sung, M.; Asai, S. J. Phys. Chem. B 2005, 109, 6121. (22) Pol, V. G.; Pol, S. N.; Kessler, V. G.; Seisenbaeva, G. A.; Sung, M-G.; Asai, S.; Gedanken, A. J. Phys. Chem. B 2004, 108, 6322. (23) Pol, S. V.; Pol, V. G.; Frydman, A.; Churilov, G. N.; Gedanken, A. J. Phys. Chem. B 2005, 109, 9495.