Room Temperature Ferromagnetic (Fe1âxCox)3BO5 Nanorods

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Room Temperature Ferromagnetic (Fe1-xCox)3BO5 Nanorods Shuli He, Hongwang Zhang, Hui Xing, Kai Li, Hongfei Cui, Chenguang Yang, Shouheng Sun, and Hao Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl501193x • Publication Date (Web): 06 Jun 2014 Downloaded from http://pubs.acs.org on June 10, 2014

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Room Temperature Ferromagnetic (Fe1-xCox)3BO5 Nanorods Shuli He,1,2 Hongwang Zhang,2 Hui Xing,2 Kai Li,3 Hongfei Cui,1 Chenguang Yang,1 Shouheng Sun,4 and Hao Zeng2,* 1 Department of Physics, Capital Normal University, Beijing 100048, P. R. China 2 Department of Physics, University at Buffalo, SUNY, Buffalo, New York 14260, USA 3 Department of Chemistry, Capital Normal University, Beijing 100048, P. R. China 4 Department of Chemistry, Brown University, Providence, RI 02912, USA ABSTRACT: Cobalt-doped ferroferriborate ((Fe1-xCox)3BO5) nanorods (NRs) are synthesized by a one-pot high-temperature organic-solution-phase method. The aspect ratios of the NRs are tuned by the heating rate. These NRs form via anisotropic growth along twin boundaries of the multiply-twinned nuclei. Magnetic properties are dramatically modified by Co substitutional doping, changing from antiferromagnetic order at low temperatures to ferromagnetic above room temperature, with greatly enhanced magnetic ordering temperature. These anisotropic ferromagnetic NRs with high ordering temperature may provide a new platform for understanding nanomagnetism and for magnetic applications. KEYWORDS: ferromagnetic, antiferromagnetic, nanorods, ferroferriborate, chemical doping

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Monodisperse superparamagnetic (SPM) and ferromagnetic (FM) nanoparticles (NPs) have found many technological applications including data storage, 1-3 energy conversion, 4-9 and nanomedicine. 10-20 NP size, shape, crystal structure and composition are often used as tuning knobs to control NP magnetism (moment, anisotropy, coercivity and ordering temperature). Such fundamental understanding of the relationship between structural parameters and magnetism opens up unprecedented opportunities for tailoring nanoscale magnetic properties for advanced applications. Among various magnetic NPs studied, antiferromagnetic (AFM) NPs are an interesting class of magnetic materials due to their surface-induced magnetic property changes. For example, they often exhibit a net magnetization and blocking temperature at low temperatures due to non-compensated surface spins. 21-23 Exchange bias is also commonly observed in FM/AFM exchange coupled core/shell NPs, which is found to stabilize the magnetic domains against thermal fluctuations. 24-27 For broader understanding of nanomagnetism and for creation of new magnetic materials, it is interesting to investigate if the magnetism of an AFM NP can be tuned by engineering its material composition. In this work we report the synthesis of a new magnetic nanomaterial by substitutional doping of an AFM host NP. The ferroferriborate, Fe3BO5, is known to be AFM with a Néel temperature (TN) of about 110 K.28-34 By modifying the synthesis of Fe3BO5 nanorods (NRs) with a high temperature organic solution phase technique, 35 we produced Co-doped ferroferriborate ((Fe1xCox)3BO5)

(x = 0 to 0.2) NRs. In the present synthesis, the aspect ratios of these NRs were tuned

by controlling the heating rate during the nucleation stage, which is different from what is reported previously, 35 where the amount of borane t-butylamine (BBA) was used to control NR parameters. Electron microscopy analyses inferred that the NRs were obtained by anisotropic


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growth along twin boundaries of the multiply-twinned nanoparticle nuclei. We demonstrated that a small percentage of Co doping in an AFM host NP modified the magnetic interactions dramatically, changing its behavior from AFM to FM. Moreover, the magnetic ordering temperature was enhanced by several folds from 110 K to 560 K. Our work suggests that doping in nanoscale may provide an attractive avenue to develop new nanomaterials for magnetic applications. (Fe1-xCox)3BO5 NRs were synthesized by reductive decomposition of Fe(acac)3 and Co(acac)2 with predetermined ratio, in the presence of oleic acid (OA) and oleylamine (OAm). 35 The mixture was first kept at 180 °C for 1 h, followed by injection of BBA. NR growth was controlled by temperature ramping rate from 2 to 10 K⋅min-1 during the subsequent heating to reflux. After heated at 300 °C for 4 h, the solution was cooled down to room temperature and final products were purified by centrifugation (see supporting information for synthesis details).

Figure 1. The TEM images of the as-synthesized (Fe0.9Co0.1)3BO5 nanorods at heating rates of a) 2 K⋅⋅min-1, b) 5 K⋅⋅min-1, and c) 10 K⋅⋅min-1.


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Figure 2. The X-ray diffraction patterns of the as-synthesized (Fe0.9Co0.1)3BO5 nanorods at heating rates of a) 2 K⋅⋅min-1, b) 5 K⋅⋅min-1, and c) 10 K⋅⋅min-1. d) The standard XRD pattern of Fe3BO5 (JCPDS, card#01-073-2457). The shape and composition of (Fe1-xCox)3BO5 NRs were controlled independently by the heating rate and the amount of BBA, respectively. This is different from what has been reported in the synthesis of Fe3BO5 NRs where the NR shape and compositions were controlled by the amount of BBA added in the reaction solution35. Figure S1 and S2 confirm that varying the concentration of BBA not only leads to morphological change, but also results in the crystal structure changes. Figure 1 shows TEM images of (Fe0.9Co0.1)3BO5 NRs synthesized with the heating rate of 2, 5, and 10 K⋅⋅min-1 from 180 to 300 °C. We can see that as the heating rate increases from 2 to 10 K⋅⋅min-1, the average diameter of the NRs is reduced from 20 nm to 10 nm; while the length grows rapidly. This suggests that the heating rate is a sensitive knob to tune the aspect ratio of the (Fe0.9Co0.1)3BO5 NRs. Figure 2 shows XRD patterns of the NRs obtained


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from different heating rates during temperature ramping from 180 to 300 °C. The reduction in diameter of NRs with increasing heating rate from 5 to 10 K⋅⋅min-1 leads to broadening and overlap of neighboring peaks in the XRD patterns. Other than peak broadening due to the size effects, the peak positions of all samples are the same and nearly identical to those of the Fe3BO5 bulk crystal, suggesting that the NRs prepared from different heating rates have the same crystal structure. The composition of the NRs is consistent with the precursor ratio (see Figure S3 for details).


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Figure 3. The TEM images of the (Fe0.9Co0.1)3BO5 NPs taken at different growth stages, with the amount of BBA fixed at 0.55 mmol. a) 180 °C for 20 min, b) 240 ℃ for 20 min, c) 300 ℃ for 20 min, and d) 300 ℃ for 2 h. The growth of anisotropic (Fe1-xCox)3BO5 NRs were monitored by examining the TEM images of the intermediate products at different stages of reaction, as shown in Figure 3. At 180 °C, only spherical NPs, identified as (Fe1-xCox)3O4, are formed (Figure 3a). 35 Most of them incorporate multiply twinned boundary defects to minimize the surface energy, at the expense of increased internal strain, and are stable when their sizes exceed a critical value. 36-41 With the 6 Environment ACS Paragon Plus

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reaction temperature increased to 240 °C, spherical NPs are converted into anisotropic NRs (Figure 3b). Further raising the temperature to 300 °C results in the formation of more NRs (Figure 3c), and nearly 100% of NRs are obtained after prolonged heating for 2 h (Figure 3d).

Figure 4. HRTEM images of the tip of an as-synthesized (Fe0.9Co0.1)3BO5 Nanorod: a) side view and b) top view from the b-axis direction. Figure 4a, b show the typical HRTEM images of the side and top view of the tip of a NR. The lattice spacing of the planes perpendicular and parallel to the longitudinal direction of the NRs are 0.30 nm and 0.47 nm, corresponding to (010) and (101) planes of ludwigite crystals, 7 Environment ACS Paragon Plus

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respectively. This suggests that the NRs grow along the [010] direction, the b-axis of the rhombohedral crystal. The lattice spacing of the planes parallel to the edge of the tip is measured to be 0.26 nm, indexed to the (111) plane. Therefore the tip of the NRs is terminated by {111} facets. The top view of the NR tip revealed a multiply twinned structure (Figure 4b). The twinned structure of the NR tips is likely inherited from the twinned NP seeds. The twinned structure provides the constraint necessary for the evolution of NPs into NRs. Because a twin boundary is the highest energy site, crystallization preferentially occurs there. OA and OAm interact more strongly with {010} facets than with {111} facets. The preferred binding to {010} facets inhibits the growth along the [010] directions. 35 New atoms tend to diffuse towards the ends of NRs, leading to preferential crystallization of the {111} facets at the tips. During the growth process, the quasi-molten twin regions at the NR tips serve as an intermediate phase to facilitate the transport of atoms from the solution phase to the end surfaces. The growth mechanism of ludwigite NRs is expected to be similar to that of solution-liquid-solid (SLS) methods. With higher heating rate, atoms can diffuse more rapidly towards the twin boundaries from the solution, which enables the ends of the NRs to grow faster, and leads to the formation of NRs with large aspect ratio. As shown in Figure 1, when the heating rate is raised from 2 K/min to 10 K/min, the aspect ratio of the NRs increases greatly.


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Figure 5. a) Magnetic hysteresis loops of the (Fe0.9Co0.1)3BO5 nanorods (red circles: 10 K, and black squares: 300 K), and undoped Fe3BO5 nanorods (green triangles), displayed in the field range between -10 kOe and 10 kOe. The inset are hysteresis loops of the (Fe0.9Co0.1)3BO5 nanorods in the full field range between -50 kOe and 50 kOe. b) Magnetic moment as a function of temperature of the (Fe0.9Co0.1)3BO5 nanorods, showing a Curie temperature of 560 K. Magnetic behavior of pristine Fe3BO5 NRs shows a transition from paramagnetic to AFM at 110 K, consistent with their bulk behavior. 35 As shown in Figure 5a, room temperature hysteresis of undoped Fe3BO5 NRs indeed shows paramagnetic behavior (green triangles). Differently, the (Fe0.9Co0.1)3BO5 NRs exhibit FM behavior at 300 K (black squares), with the saturation magnetization (σs) of 8 emu/g, remnant magnetization of 4 emu/g, and coercivity (Hc) of 850 Oe. As the temperature is lowered to 10 K, the coercivity rises significantly to about 2 KOe (red circles), indicating that the incorporation of a moderate concentration of Co cations into Fe3BO5 greatly enhances the magnetic properties. The fact that the Co-doped Fe3BO5 NRs are FM at 300 K suggests that their magnetic ordering temperature is higher than room temperature. Tc of (Fe0.9Co0.1)3BO5 nanorods is determined to be 560 K, as measured from the magnetic moment vs temperature curve shown in Figure 5b. This value is 400% higher than TN 9 Environment ACS Paragon Plus

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of Fe3BO5, and is the highest ordering temperature ever reported in all magnetic ludwigite systems. The enhanced magnetism observed in Co-doped Fe3BO5 NRs is intriguing. Ludwigite Fe3BO5 is a mixed valence low dimensional semiconductor with complicated magnetic interactions. Its high temperature phase consists of 4 crystallographically distinct Fe sites, which can be viewed as two weakly-coupled subunits. 29, 32, 42-44 Upon cooling, it goes under a series of structural, charge ordering and magnetic phase transitions. Starting from an AFM Fe3BO5 lattice, with each Fe2+ ion being substituted by Co2+, it may contribute ~ 1 µB uncompensated moment since Co2+ has 1 less 3d electron compared to Fe2+. If Co2+ preferentially occupy particular lattice sites in one of the subunits, and these sites couple ferromagnetically, one can estimate σs to be ~6.5 emu/g for 10% Co-doped Fe3BO5, which is close to what we measured (8 emu/g). Further experimental and theoretical investigation is needed to verify the preferred Co sites and their exchange coupling. Substituting Fe cations with other transition metal ions showing different valence states and local magnetic moments should modify the magnetic behavior of Fe-based compounds. 45, 46 Considering that Fe3BO5 orders antiferromagnetically with TN ~ 110 K and Co3BO5 becomes ferromagnetic at 43 K, one expect that small Co substitution in Fe3BO5 would lead to a moderate change in its TN. Taking the example of cubic ferrites, where Fe is partially substituted by other transition metal elements such as Co, Ni and Mn, Tc all decreases. The 5-fold increased Tc observed in Co-doped Fe3BO5 NRs is thus quite unusual, which reflects significant modifications to the exchange interactions in this nanostructure and is not well understood at this stage. Nevertheless, this drastic change in the magnetic behavior suggests that doping in AFM


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nanostructures can be an attractive new approach to obtain novel materials with interesting magnetic properties. Examples of anisotropic NRs demonstrating above room temperature remanent magnetic moment are rare. They may offer opportunities for novel biological applications. For example, micron sized magnetic beads are often used to study the mechanical properties of biomolecules by exerting a force or torque using an external magnetic field. However, their sizes much larger than those of biomolecules inevitably distort the intrinsic behavior of biological systems.47 Our ferromagnetic NRs with sizes comparable to those of biomolecules can be used as labels to study the intrinsic mechanical properties such as protein folding and behavior of motor proteins, with minimized interference from the labels. Because of the anisotropic shape, a much weaker magnetic field or field gradient is needed to apply the required force or torque. In conclusion, (Fe1-xCox)3BO5 NRs are synthesized by thermal reaction of Fe(acac)3 and Co(acac)2 in the presence of BBA. The aspect ratios and compositions of the NRs are controlled independently, by the heating rate and BBA concentration, respectively. Electron microscopy analyses of the reaction intermediates indicate that the NRs are formed from the initial nucleation of multiply-twinned NP nuclei followed by anisotropic growth. Incorporation of Co dopants into Fe3BO5 NRs changes the magnetic behavior from AFM into FM, with their coercivities reaching 850 Oe at room temperature and 2 kOe at 10 K. The magnetic ordering temperature is greatly enhanced to 560 K. Our work provides a new avenue on tuning and understanding magnetism in nanostructures for magnetic applications.

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Supporting Information. The experimental details, EDX analysis of (Fe1-xCox)3BO5 NRs, TEM images and XRD of (Fe0.9Co0.1)3BO5 nanorods synthesized by varying the amount of BBA, magnetic hysteresis loops of (Fe1-xCox)3BO5 NRs with different Co doping concentration. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Xing Huang for HRTEM measurement and discussions on crystal structure. This work was supported by US NSF DMR1104994, the Research Foundation for the State University of New York, National Science Foundation of China (Grant No. 51171121), and by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), under its Vehicle Technologies Program, through the Ames Laboratory. The Ames Laboratory is operated by Iowa State University under contract DE-AC02-07CH11358.


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