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Interdiffusion Induced Exchange Coupling of L10-FePd/#-Fe Magnetic Nanocomposites Alec Kirkeminde, and Shenqiang Ren Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl502167m • Publication Date (Web): 15 Jul 2014 Downloaded from http://pubs.acs.org on July 18, 2014

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Interdiffusion Induced Exchange Coupling of L10FePd/α-Fe Magnetic Nanocomposites Alec Kirkeminde and Shenqiang Ren* Department of Chemistry, University of Kansas, Lawrence, KS

* Correspondence and requests for materials should be addressed to S.R.([email protected]).

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Abstract

One-pot synthesis of FePd and FePd/Fe2O3 (core/shell) nanoparticles via interdiffusion is reported for the first time. It was found that the size of FePd particles and Fe2O3 shell thickness could be controlled by the ligand and iron precursor amounts, respectively. These FePd/Fe2O3 particles can be reductively annealed at 500 °C to produce exchanged coupled L10-FePd/α-Fe magnetic nanocomposites. The effect of the phosphine ligand on magnetic characteristics of synthesized particles and final annealed nanocomposite is discussed. Finally, it was found that the magnetic properties of the final L10-FePd/α-Fe nanocomposites could be tuned by Fe2O3 shell thickness and can reach a coercivity (Hc) of up to 2.4 kOe and a saturation magnetization (Ms) of 141 emu/g.

Keywords: Core/Shell Nanoparticles, Magnetism, Exchange Coupling, High Energy Density


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High magnetocrystalline anisotropy (magnetically hard) nanoparticles (NPs), have shown immense potential in green energy, power electronics and data storage applications.1-7 Many of these magnetically hard NPs are bimetallic alloys, such as FePt, MnPt, MnBi, and FePd.8-15 General syntheses of these alloy particles start with a reduction of the corresponding metal acetylacetonates (M(acac)) in the presence of organic ligands. This method is complicated by the different reduction potentials of the M(acac), leading to difficulty in controlling size and stoichiometry. Wang et al. has shown an unique way around this challenge by first creating cobalt core NPs and then in-pot annealing in iron via interdiffusion to synthesize FeCo NPs.16 While this clever workaround proved valid, the method has not been utilized in other bimetallic nanoparticle systems, such as FePd in this study. Synthesis of FePd NPs has seen increase interest recently as it utilizes Pd instead of its more expensive group mate Pt. In the past, the syntheses of FePd NPs are plagued with the reduction/decomposition, limiting their size control and reproducibility.17 Moreover, FePd NPs have been shown to be excellent candidates for use as a hard phase in creating exchange coupled nanocomposite magnets due to its L10-ordered phase exhibiting a high magnetic anisotropy constant (K) of 1.0 x 107 erg/cm.18 Alpha iron (α-Fe) is routinely used as a soft phase with FePd due to a thermodynamic stability of the phases existing together.1 Recently Yu et al. showed that they can create exchanged coupled FePd/α-Fe composite by reductive annealing of FePd aggregates and Fe3O4 nanorods.19 Teranishi et al. have shown that they can anisotropically grow Fe3O4 next to Pd particles and then high temperature reductive annealing to create the FePd with surrounding α-Fe matrix.6 In creating these exchange-coupled nanocomposites, it is vital to have control of phase placement, as the soft phase must be close enough to the hard phase to allow for


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pinning of its magnetization. Being able to precisely control the hard and soft phase placement and size will be instrumental to optimizing these exchanged coupled magnetic nanocomposites. Herein we report a versatile one-pot synthesis of FePd NPs with a controllable size from 4 to 11 nm via interfacial diffusion of Fe into Pd cores. More interestingly, upon addition of higher molar quantities of the Fe precursor, a Fe2O3 shell can be grown over the FePd NPs. The shell thickness can be tuned by addition of molar Fe amounts. These final FePd/Fe2O3 (core/shell) particles can then be reductively annealed to form exchanged coupled L10-FePd/α-Fe nanocomposite magnets. It is shown that the final magnetic properties can be controlled by amount of Fe added with highest Hc and Ms reaching 2.4 kOe and 141 emu/g, respectively. The synthesis of FePd NPs via interdiffusion starts with the creation of Pd cores. Using this method allows for the use of well-known Pd colloidal nanochemistry to control the size of initial Pd core particles and follow up with the annealing of Fe to enable the interfacial diffusion for the creation of FePd NPs. In this synthesis, the OA is used as a reducing agent for the Pd(acac)2 and TBP is used as the stabilizing ligand. Figure 1 show TEM images of the initial Pd core particles and the corresponding final FePd particles created after interdiffusion. Size of the Pd cores can be tuned by the different molar amount of TBP to Pd ratio. It is important to note that if no TBP is used, aggregated masses of 1 nm particles are produced (Figure S1 of Supporting Information), indicating that the phosphine ligand is essential for shape and size control. Figure 1a shows particles 0.5:1 TBP to Pd ratio, producing larger particles with a large standard deviation in their size and shape, as there is not enough phosphine ligand. As the amount of ligand increases from 1:1, 2:1, 4:1 (TBP:Pd), resulting particles show better shape control and decreasing size to 6 nm, 4 nm, and 2 nm respectively (Figure 1b, 1c and 1d). This trend follows what have been observed in the Pd system and of other systems such as nickel (Figure S2a,


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Supporting Information).20 Upon addition of one molar equivalent of Fe(CO)5, final FePd particles are created and are seen to grow in size due to Fe incorporation. Interdiffusion of the Fe atoms into the Pd cores can be modelled by thermally induced diffusion coefficient, which is presented in the supporting information. As such, the final size of FePd particles presented can be tuned from 4-7 nm, and can even reach 11 nm, albeit poor shape control due to lack of proper ligand amounts. More intriguingly, the concentration of Fe precursor loading dictates the core/shell (Pd/Fe) interdiffusion and the shell creation. It is seen that a Fe2O3 shell coating onto the interdiffusionformed FePd particle forms FePd/Fe2O3 (core/shell) nanostructures, under the addition of higher molar Fe content. It is believed when the interdiffusion of Fe shell into the Pd cores reaches the stoichiometric ratio of FePd, the excess Fe shell from the higher molar Fe content turns to the Fe2O3 phase. Figure 2 displays TEM images of particles with the FePd/Fe2O3 (core/shell) structure. All displayed particles in Figure 2 are created utilizing Pd cores created with a 2:1 TBP:Pd ratio. With using 1:1 Fe:Pd molar ratio, particles exhibit no shell as shown in Figure 2a. Upon increasing to 2:1 Fe:Pd, a shell appears with a thickness of ~1 nm (Figure 2b). When increasing the Fe content, it is seen that the shell thickness can be controlled from 2 to 3 nm with ratios 4:1 and 6:1 Fe:Pd. Upon reaching 6:1 Fe:Pd ratio, it is seen that pure Fe2O3 particles are being formed, indicating that 3 nm is the max shell thickness that can be achieved, most likely due to the strain of lattice mismatch of the FePd core and shell material. To confirm formation of FePd and Fe2O3 phase, both x-ray powder diffraction (XRD) and HRTEM lattice measurements were conducted. Collected XRD of the particles is displayed in Figure 2f. Starting from the Pd cores, to the final particles show a shift of the 2θ peak from Pd cores to final FePd particles when using 1:1 Fe:Pd ratio. When increasing the Fe ratio, a new peak arises, which corresponds to the


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Fe2O3 (Hematite) phase, indicating that the shell is indeed the Fe2O3 phase. Lattice of the shell material shows a spacing of 0.31 nm and the core displaying 0.27 nm spacing corresponding to the (220) plane of Fe2O3 and the (111) plane of FePd, reinforcing the conclusions of the XRD measurements above (Figure S3 of Supporting Information). It is interesting to note that this exact method can be also used for other bimetallic materials to produce cores/shell structures, such as FeNi/Fe2O3 (Figure S2 of Supporting Information) utilizing the exact stoichiometry with only replacing Pd(acac)2 with Ni(acac)2, proving this method’s versatility. When repeating the synthesis with trioctylphosphine (TOP) instead of TBP, no shells were formed. It is believed that TOP inhibits shell formation due to a steric effect of the longer chain, and that TBP is vital in depositing the final Fe2O3 shell. Elemental mapping of a FePd/Fe2O3 core/shell particle is presented in the inset of Figure 3a. It is seen that the Pd signal is localized in the middle of the particle and the Fe signal is present throughout the entire particle, extending past the Pd core. The energy-dispersive x-ray spectroscopy (EDS) line scans of core/shell particles, showing the typical core/shell elemental distribution, are also provided in Figure S4 of Supporting Information. This confirms that indeed the FePd is created in pot and that the shell is indeed the hematite Fe2O3 phase. To study the effects on using higher molar Fe quantities, the EDS technique was also utilized to determine relative Fe:Pd ratios of the FePd/Fe2O3. Figure 3a displays the EDS spectra, with intensity normalized to the Pd peak, of all the different Fe:Pd ratios used. The intensity of the Fe peak clearly grows as the Fe ratio increases. Quantifying these via EDS, although not completely quantitative, produces ratios of Fe57Pd42, Fe70Pd30, Fe76Pd24, Fe82Pd18, for the 1:1, 2:1, 4:1, 6:1 Fe:Pd ratios respectively. These trends help confirm the increase of Fe and give a handle on the real final stoichiometry of final systems. It is also important to note that all of the EDS spectra display a phosphine peak. Knowing that the


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phosphine comes from the TBP ligand used in the synthesis and that it can form Fe2P at higher temperatures, it was necessary to optimize the synthesis to reduce the amount of phosphine brought into the following reductive annealing step.21 Knowledge of the phosphine doping effect will be vital to future research utilizing this method, as the phosphine is required for control of the system, but in the final composite any formation of Fe2P is detrimental to the magnetic properties. To study the effects of phosphine doping due to residual ligand, the in-pot annealing temperatures of the 2:1 Fe:Pd system were varied from 190-290 °C. Figure 3b displays the TEM image of the particles formed at 290 °C. Particles formed at the various other temperatures can be found in Figure S5 of Supporting Information. It is observed that the particles still have the shell around them, but a trend in stoichiometry of EDS data is observed (Figure 3c). It can be seen that as the temperature is increased, more phosphine is included into the particles. This is caused from the phosphine ligand doping into the Fe shell. The corresponding magnetization hysteresis (M-H) loops after the reductive annealing are presented in Figure 3c, showing that as the in-pot annealing is increased, the Ms drops until almost complete destruction of the nanocomposite phase at 290 °C. While the 190 °C in-pot annealing exhibits the lowest phosphine concentration, it has a lower Ms than the 220 °C particles. Lower Ms of composite is attributed to the incomplete decomposition of Fe(CO)5 at 190 °C, supported by the Fe to Pd stoichiometry is lower than the other synthesis and the supernatant during clean-up of these particles displays a yellow tint, corresponding to the Fe(CO)5. Further optimization via different ligand removal steps after synthesis is now being pursued. With this data, it is concluded that 220 °C is the optimal in-pot annealing to reduce the amount of phosphine into the final particles, and studies of Fe:Pd ratio effect on the final magnetic nanocomposite need to be examined.


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The FePd/Fe2O3 core/shell particles provide a starting foundation for reductively annealing into a L10-ordered FePd/α-Fe exchanged coupled nanocomposite. To study the effect of shell thickness on the final nanocomposite’s magnetic properties, each of the different Fe:Pd molar ratio particles were reductively annealed. The annealing temperature (500 °C) was chosen as it has been shown to produce the most optimized L10-FePd phase by many different studies.6, 17, 19, 22

If the annealing temperature is increased higher, the formation of a disordered FCC FePd

becomes the competitive phase. Figure 4 presents the XRD of final annealed nanocomposites with varying Fe:Pd molar ratios. All spectra display complete elimination of Fe2O3 diffraction after the reductive annealing in 10% H2/90% N2. Both α-Fe and L10 peaks of FePd appear after this treatment, confirming that indeed the proper phases for exchange coupling are being formed. As the iron ratio increase the (110) peak of α-Fe grows indicating the increase of the phase in the final nanocomposite. The increase of the final α-Fe phase corresponds well with the EDS data of the pre-annealed particles presented in Figure 3a. Utilizing the Scherrer’s equation and the (111) FePd and the (110) α-Fe peaks in Figure 4 (2:1 Fe:Pd sample), the average domain sizes were solved for. The L10-FePd phase has an average size of 60 nm and the α-Fe phase has an average size of 76 nm. These sizes are much larger than the original core/shell particles, which were ~7 nm, indicating that the nanocomposites post-annealed need to be examined. To investigate final phase domain sizes and phase interfaces of the final L10-FePd/α-Fe, TEM imaging was employed. Figure 5a shows a low resolution TEM image of the final nanocomposite. Large domains appear after annealing, which are encapsulated in a carbon matrix. The domains can be roughly differentiated from the tint of the particles, with the FePd being darker due to the heavier Pd element. Elemental mapping of a section of the image is displayed to the right of Figure 5a. Within this elemental mapping, there are different domains


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with only Fe and domains with both Fe and Pd. It is also important to note that the P signal is localized on the domains that show higher Fe content, confirming that the P is doping into the iron phase. The domains are indeed much larger than the starting core/shell structure particles. Measuring the size of the domains from the TEM images give an average of 58 nm and 70 nm for the L10-FePd and α-Fe domain sizes respectively. These match well with the average domain sizes solved for by the Scherrer’s equation above. Having such large domains after annealing indicates that the starting particle phases must move and coalesce during high temperature annealing. Figure 5b displays HRTEM image of a typical section with the two different phases. Even with the coalescence occurring, it can be seen that there is still intimate contact of the two phases. The phase identity can be confirmed from the lattice, with Fe showing 0.20 nm corresponding to the (110) plane and 038 nm which matches the (001) plane of L10-FePd. Elemental mapping of the same section is displayed to the right, certifying that both phases exist and are in intimate contact, suitable for exchange coupling to occur in the final nanocomposites. To evaluate the extent of exchange coupling, magnetic measurements were performed. Room temperature M-H loops of the annealed particles with different Fe:Pd ratios are displayed in Figure 6a. All the hysteresis loops produced were smooth and display no kinks, indicating effective exchange coupling in all of the composites. It is seen that there is trends in both the Hc and Ms of the different composites better displayed in the graph in Figure 4b. As the Fe increases, the Ms of the particles increases, which can be correlated to an increase of the soft α-Fe phase being generated. Opposite to the Ms, the Hc decreases as higher amounts of Fe are used. This can be attributed to the destruction of the L10 ordering in the hard phase most likely due to excess Fe doping into the L10 FePd phase. A similar phenomenon is seen in previous L10FePd/α-Fe system studying stoichiometry of the final composites.19 The composite displaying the


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highest area under the hysteresis loop is the 2:1 Fe:Pd ratio. It displays a balance between an Ms of 91 emu/g and an Hc of 2.2 kOe. TEM image of these particles after the annealing process are shown in Figure S3 of Supporting Information, confirming intimate contact with the final two magnetic phases. With this it is concluded that the magnetic properties of the final composite can be tuned by simple adjustment of the Fe:Pd ratio and are able to effectively exchange couple. In conclusion, a versatile one-pot interdiffusion method is presented to create FePd or FePd/Fe2O3 core/shell nanoparticles through controlling the ratio between palladium and Fe precursors. It is seen that the Pd cores size can be controlled via ligand ratio from 2-7 nm and the final single FePd particles can be controlled from 4-11 nm. Upon increasing the amount of Fe molar ratio, a Fe2O3 can be successfully deposited and shell thickness controlled. These FePd/Fe2O3 particles can be reductively annealed to form L10-FePd/ alpha-Fe nanocomposites with tuneable Hc from 0.7 kOe to 2.4 kOe and Ms from 42 to 140 emu/g. Finally, it is seen that while the phosphine ligand is necessary for control of the synthesis, it becomes a detriment to the final magnetic properties if more is doped into the particles via higher reaction temperatures. This information should help further improve nanoparticle synthesis understanding and optimization for magnetic composites. Utilizing this synthetic method also shows promise for use on other bimetallic systems, and possibly shape control from via a unique core shape.


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Supporting Information. Experimental detail, TEM images of Pd cores with no TBP, FePd/Fe2O3 particles with different in-pot annealing temperatures and FeNi/Fe2O3 core/shell particles. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest.

Acknowledgement S.R. thanks the financial support from the National Science Foundation under Award No. NSFCMMI-1332658 for material synthesis and assembly, and the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) under contract No. 11/CJ000/09/03 for magnetic property characterization. A.K. would like to acknowledge the Microscopy and Analytical Imaging Laboratory (MAI) lab at KU for help in TEM characterization and also Dr. Victor Day for help with XRD characterization.


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REFERENCES 1. Sakuma, N.; Ohshima, T.; Shoji, T.; Suzuki, Y.; Sato, R.; Wachi, A.; Kato, A.; Kawai, Y.; Manabe, A.; Teranishi, T. ACS Nano 2011, 5, 2806-2814. 2. Coey, J. M. D. Solid State Commun. 1997, 102, 101-105. 3. Estrader, M.; López-Ortega, A.; Estradé, S.; Golosovsky, I. V.; Salazar-Alvarez, G.; Vasilakaki, M.; Trohidou, K. N.; Varela, M.; Stanley, D. C.; Sinko, M.; Pechan, M. J.; Keavney, D. J.; Peiró, F.; Suriñach, S.; Baró, M. D.; Nogués, J. Nature Commun. 2013, 4, 2960 (1-8). 4. Jones, N. Nature 2011, 472, 22-23. 5. Rui, X.; Shield, J. E.; Sun, Z.; Xu, Y.; Sellmyer, D. J. Appl. Phys. Lett. 2006, 89, 122509122509-3. 6. Teranishi, T.; Wachi, A.; Kanehara, M.; Shoji, T.; Sakuma, N.; Nakaya, M. J. Am. Chem. Soc. 2008, 130, 4210-4211. 7. Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 11480-11485. 8. Tzitzios, V.; Niarchos, D.; Gjoka, M.; Boukos, N.; Petridis, D. J. Am. Chem. Soc. 2005, 127, 13756. 9. Pellegrino, T.; Fiore, A.; Carlino, E.; Giannini, C.; Cozzoli, D. P.; Ciccarella, G.; Respaud, M.; Palmirotta, L.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2006, 128, 6690. 10. Yu, Y.; Yang, W.; Sun, X.; Zhu, W.; Li, X. Z.; Sellmyer, D. J.; Sun, S. Nano Lett. 2014, 13, 4915-4979. 11. Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. 12. Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. Nano Lett. 2003, 4, 187-190. 13. Sun, S. H. Adv. Mater. 2006, 18, 393-403. 14. Poudyal, N.; Chaubey, G. S.; Rong, C.; Liu, J. P. J. Appl. Phys. 2009, 105, 07A749. 15. Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395-398. 16. Wang, C.; Peng, S.; Lacroix, L.-M.; Sun, S. Nano Res. 2009, 2, 380-385. 17. Hou, Y.; Kondoh, H.; Kogure, T.; Ohta, T. Chem. Mater. 2004, 16, 5149-5152. 18. Weller, D.; Moser, A.; Folks, L.; Best, M. E.; Wen, L.; Toney, M. F.; Schwickert, M.; Thiele, J. U.; Doerner, M. F. IEEE Trans. 2000, 36, 10-15. 19. Yu, Y.; Sun, K.; Tian, Y.; Li, X. Z.; Kramer, M. J.; Sellmyer, D. J.; Shield, J. E.; Sun, S. Nano Lett. 2013, 13, 4975-4979. 20. Carenco, S.; Boissière, C.; Nicole, L.; Sanchez, C.; Le Floch, P.; Mézailles, N. Chem. Mater. 2010, 22, 1340-1349. 21. Park, J.; Koo, B.; Yoon, K. Y.; Hwang, Y.; Kang, M.; Park, J.-G.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 8433-8440. 22. Kang, S.; Jia, Z.; Nikles, D. E.; Harrell, J. W. J. Appl. Phys. 2004, 95, 6744-6746.


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List of Figure Captions

Figure 1. TEM images of Pd core nanoparticles with tributylphosphine (TBP):Pd molar ratio a) 0.5:1, b) 1:1, c) 2:1, and d) 4:1 and the corresponding FePd particles created after Fe addition. e) Graph of the Pd core and final FePd diameter dependent on TBP:Pd molar ratio. All scale bars are 10 nm.

Figure 2. TEM images of final FePd and FePd/Fe2O3 particles with Fe:Pd molar ratio of a) 1:1, b) 2:1, c) 4:1 and d) 6:1. e) Graph of Fe2O3 shell thickness dependent on Fe:Pd molar ratio. f) XRD of Pd cores, FePd particles and FePd/Fe2O3 core/shell particles. All scale bars 10 nm. Figure 3. a) EDS of different Fe:Pd ratios particles. b) particles formed at 290 °C inpot annealing (scale bar 10 nm). C) M-H loops of reductively annealed nanocomposites dependent on inpot annealing temperature.

Figure 4. XRD of nanocomposites annealed at 500 °C for 5 hours with varying Fe:Pd molar ratios. Figure 5. a) TEM image of annealed L10-FePd/Fe nanocomposite after annealing at 500 °C (scale bar 50 nm). Elemental mapping of the black square region is presented to the right. b) HRTEM image of a typical interface of L10-FePd/Fe post-annealing (scale bar 5 nm). Elemental mapping of (b) is displayed to the right. Figure 6. a) M-H loops of reductively annealed nanocomposites dependent on Fe:Pd ratio. b) The Ms and Hc of final nanocomposites dependent on Fe:Pd molar ratio.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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