Article pubs.acs.org/cm
Phase Transformation-Driven Surface Reconstruction of FeNi Nanostructures Maogang Gong and Shenqiang Ren* Department of Mechanical Engineering, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *
ABSTRACT: Numerous attempts have been made to search for large magnetic anisotropy and coercivity in all ferrous metal materials, but our capability to realize metastable tetragonal ferrous nanostructures is still limited. Here we present a rational epitaxial core/ shell design to stabilize tetragonal iron−nickel (FeNi) nanostructures to reveal substantial coercivity and anisotropy. Phase transformation of gold−copper (AuCu) cores induced a surface stress that efficiently triggers a tetragonal reconstruction of FeNi shell within a critical thickness range. A tunable magnetic performance is dictated by the chemical composition and thickness of FeNi shell, as well as a plasmon effect from the AuCu core. Rapid progress in epitaxial core/shell growth and phase transformation opens up exciting opportunities to stabilize unusual nanostructures and develop high performance multifunctional nanomaterials.
■
INTRODUCTION Today’s advanced permanent magnets are predominantly based on rare-earth (RE) elements, such as Nd2Fe14B- or SmCo5based supermagnets.1−8 In light of unsustainable RE supply and ever-growing demand for RE alternatives in energy-critical technologies, there is a strong motivation to develop rare-earthfree ferrous magnetic materials. In this context, tetrataenite derived from metastable tetragonal iron−nickel (FeNi) alloys attract intense attention due to its large magnetocrystalline anisotropy (Ku) of 1.3 × 107 erg/cm3 and an estimated maximum energy product of 40 MGOe at room temperature.9,10 However, its low order−disorder transition temperature (593 K) inhibits tetragonal or L10-ordered FeNi phase due to its sluggish kinetics (1 atomic jump per 10 000 years at or below an equilibrium temperature of 573 K, taking up to one billion years to complete ordering).11,12 Therefore, it is very challenging to synthesize and stabilize tetragonal FeNi nanoalloys in regular laboratory settings.13−15 The tendency to reduce the surface energy of nanocrystals, particularly under phase transformation, could drive structural change within a few surface atomic layers.16−19 This potentially emerges as a promising strategy to tetragonally reconstruct surface atomic layers of standard magnetic cubic alloys, leading to an increased magnetocrystalline anisotropy. To reach this goal, a rational design of epitaxial core/shell nanostructures is necessary to enable tetragonal distortion of an outer FeNi shell through phase transformation of the nonmagnetic core. Here we developed a facile heteroepitaxial core/shell growth method to control the synthesis of FeNi shell-based nanocrystals. The selection of lattice-matched core materials is critical to efficiently transfer the strain energy through epitaxially grown interfaces. A survey of several lattice-matched metal cores with the FeNi shell is shown in Table S1 of Supporting Information. It is seen that the Bain lattice correspondence makes the gold− © XXXX American Chemical Society
copper (AuCu) alloy a prime candidate for templating the FeNi shell. More importantly, AuCu also exhibits a well-defined L10 tetragonal phase transformation under thermal annealing, and this transformation temperature window contains the FeNi order−disorder transition.20−22 Thermal annealing-induced phase transformation of the AuCu core from the disordered face-centered cubic (fcc) to an ordered face-centered tetragonal structure (L10 or AuCu I) allows for a critical thickness of the FeNi cubic shell to transform into a body-centered tetragonal structure. This transformation occurs due to the compressive stress caused by tensile surface-stress components, directly induced from the L10 ordering of the AuCu core. An optimum coercivity (1010.2 Oe) of L10-AuCu/FeNi nanocrystals shows more than eight times increase in comparison to the FeNi without the AuCu core (more details are shown in Figure S1 of Supporting Information). Different annealing temperatures within and outside of the L10 ordering thermal window presents supporting evidence for the importance of the proper phase transformation to induce tetragonal distortion.
■
METHODS
Materials. Gold(III) chloride trihydrate (>99.9% trace metals basis), nickel(II) acetylacetonate (Ni(acac)2 95%), copper(II) chloride (anhydrous, ≥99.995% trace metals basis), 1,2-hexadecanediol, 1,2dichlorobenzene (1,2-DCB), oleylamine, cobalt carbonyl (Co2(Co)8), iron pentacarbonyl (Fe(Co)5 99.9+% trace metals basis), and hexane were all purchased from Sigma-Aldrich. All of the reaction chemicals and solvents were used as received without purification. Methods. AuCu/FeNi Core/Shell with Fe/Ni = 46:54 Nanocrystal Synthesis. The AuCu/FeNi core/shell nanomagnets were synthesized as follows. HAuCl4·3H2O (0.25 mM), CuCl2 (0.25 mM), Received: September 22, 2015 Revised: November 3, 2015
A
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. (a) Schematic images of the growth of AuCu/FeNi (core/shell) nanocrystals. The shape control is introduced by the ancillary complex ions (Ni2+) during the growth process. (b−e) Transmission electron microscopy (TEM) and elemental mapping images of shape-controlled AuCu core nanostructures under different Ni2+ ion concentrations, 0, 0.129, 0.194, and 0.258 mM, respectively. The scale bar is 50 nm.
intermediates. Without the Ni2+ addition, the AuCu nanocrystals tend to grow into branched structures (BNSs, R1 of Figure 1a and Figure 1b) from high-energy twinning planes of AuCu seed nanoparticles. Adding a small amount of Ni2+ ions retards the growth of branches exhibiting a relatively small aspect ratio (R2 of Figure 1a, Figure 1c, and Figure 1d).23 Under high concentration of Ni2+ ions (R3 of Figure 1a, and Figure 1e), isotropic growth results in spherical nanoparticle structures (SNPs) due to a complete capping of Ni ions onto the AuCu surface. Elemental mapping and quantitative analysis (Figure 1 and Table S2 in Supporting Information) confirms the existence of the Ni element on the AuCu core surface, where the amount of Ni controls the morphology of the AuCu core. The composition of the crystals was also examined with XRD spectra. As shown in Figure S2 in Supporting Information, the XRD peaks of nanocrystals can be indexed as a fcc structure, each of which locates in between that of pure fcc Au (ICDD PDF no. 04-0784) and pure fcc Cu (ICDD PDF no. 04-0836), and no peaks related to AuCuNi alloys. The XRD analysis supports the AuCu alloy-absorbed Ni elemental structure, consistent with elemental mapping results. Both spherical and branched AuCu nanocrystals are utilized to template the FeNi shell growth. The average thickness of the FeNi shell could be tuned at 3.5, 5.1, and 7.3 nm by modifying the reaction time to 20, 30, and 40 min, respectively (Figure 2a,
and Ni(acac)2 (0.129 mM) were mixed with 10 mL of oleylamine in a three-neck flask and heated to 448 K for 3 h under argon atmosphere with rapid magnetic stirring. The heating mantel was removed, and the AuCu/Ni solution was cooled to 353 K, after which Fe(CO)5 (6 mM) and Ni(acac)2 (0.25 mM) were injected simultaneously. The Fe(CO)5 was used neat, and Ni(acac)2 was dissolved in 2 mL of oleylamine under argon atmosphere and stirred at 353 K. The mixture of AuCuNi−FeNi reaction solution was kept at 483 K for 20 min under argon atmosphere with magnetic stirring. The as-prepared colloidal solution was then cooled to room temperature and crashed out using hexane for magnetic and characterization testing. Structural and Property Characterization. Transmission electron microscope (TEM) and high resolution TEM (HRTEM) images were obtained on a field emission FEI Tecnai F20 XT with an accelerating voltage of 200 kV. Room temperature X-ray diffraction (XRD) characterization was performed using monochromatic Cu−Kα radiation (λ = 1.54178 Å). Magnetic properties of all samples were measured at room temperature by a MircoSense EV7 high sensitivity vibrating sample magnetometer (VSM) with fields up to 18 kOe. The mass contribution of the AuCu core to the magnetization of core/shell samples shown in the work was deduced.
■
RESULTS AND DISCUSSION
As schematically shown in Figure 1a, a rational control over the morphology of AuCu core is enabled by a face-selective electrostatic adsorption of ancillary nickel ions (Ni2+) to competitively limit the adsorption of reactive Au−Cu B
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 2. AuCu/FeNi (core/shell) nanostructures with a controlled shell thickness. (a, b) TEM images of AuCu/FeNi spherical structure with a shell thickness of 3.5 and 5.1 nm, respectively. (c) HRTEM image of one single AuCu/FeNi nanoparticle. (d) Scanning TEM (STEM) and elemental mapping images of a AuCu/FeNi nanoparticle. (e, f) TEM images of AuCu/FeNi branched nanostructure with a shell thickness of 4.1 and 5.8 nm. (g) The magnified TEM image of a AuCu/FeNi branched nanostructure. (h) STEM and elemental mapping images of AuCu/FeNi branched nanostructures.
Figure 3. (a) The XRD patterns of AuCu/FeNi (core/shell) nanocrystals before and after 653 K annealing. The black and red labeled peaks represent the diffraction peaks of the AuCu and FeNi phases. (b) The HRTEM image of an L10-AuCu/tetragonal FeNi nanocrystal annealed at 653 K. (c) Heteroepitaxial interfacial structure of AuCu/FeNi (core/shell) systems: before (left) and after (right) 653 K annealing. The bicolor spheres indicate the structure disorder for the elemental locations.
2b, and S3 in Supporting Information). Figure 2c displays the high-resolution transmission electron microscopy (HRTEM) image of one single AuCu/FeNi (core/shell) SNP. A lattice spacing of 0.381 nm from the AuCu core matches the (100) lattice constant of fcc-AuCu. The interplanar distance of 0.362 nm from the FeNi shell corresponds to the (100) surface plane of the fcc-FeNi shell. The stoichiometry of the FeNi shell could be controlled by the amount of iron and nickel precursors used
during the reaction (full details are shown in Table S3 of Supporting Information). Meanwhile, the average thickness of the FeNi shell could be tuned from 4.1 to 16.6 nm when the AuCu BNSs are selected as the templating core (Figure 2e. Figure 2f, and Figure S4 in Supporting Information). The elemental mapping analysis and scanning TEM (STEM) images of AuCu/FeNi (core/shell) SNPs and BNSs confirm C
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 4. (a, b) Magnetic-hysteresis (M-H) loops of L10-AuCu/FeNi (core/shell) spherical and branched nanocrystals at different shell thicknesses for the same stoichiometry (Fe:Ni = 46:54) and annealing temperature 653 K, and the inset images show the corresponding magnified M-H loops. (c) The summary of the FeNi stoichiometry and shell thickness-dependent coercivity.
Figure 5. (a) The M-H loops of AuCu/FeNi (core/shell) BNSs with the stoichiometry of Fe46Ni54 under different annealing temperatures, and the inset images show the corresponding magnified M-H loops. (b) The summary of annealing temperature-dependent coercivity of AuCu/FeNi BNSs. (c) The c/a ratio of the FeNi shell under different annealing temperatures. (d) Photocontrolled magnetic characteristics of AuCu/FeNi (core/shell) BNSs with light intensity-dependent on−off cycles under a bias magnetic field (1010 Oe). D
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
loops of AuCu/FeNi BNSs with an average shell thickness of 4.1 nm for different annealing temperatures. Figure 5b summarizes the annealing temperature-dependent coercivity of AuCu/FeNi BNSs, where an optimized thermal window is developed, matching well with the L10 ordering phase transformation of the AuCu core. It should be noted that when the annealing temperature is raised above 673 K, the coercivity begins to rapidly decrease and reach 334.5 Oe at 773 K. The coercivity decrease of the FeNi shell is primarily due to the AuCu structural change from ordered L10 to a disordered fcc, as the annealing temperature is above the order−disorder transition of AuCu (673 K).27 This behavior further confirms that the enhanced coercivity of AuCu/FeNi (core/shell) nanocrystals is a consequence of the AuCu L10 phase transformation-induced tetragonal distortion of the FeNi shell. To correlate the tetragonal effect with the magnetic coercivity, the c/a ratio (tetragonality) of the FeNi shell nanostructures has been investigated (inset of Figure 5b), where the tetragonality of FeNi evolves along with the phase transition of the AuCu core at different annealing temperatures. The c/a ratio of the FeNi shell reaches the optimum value of 1.06 at 653 K, which is consistent with the temperaturedependent coercivity of the FeNi shell. To further verify the interfacial effect on magnetic characteristics of the FeNi shell, the plasmon-induced hotspot of the AuCu core is utilized to control and rearrange the local interfacial structure and electron transfer between the AuCu core and FeNi shell (plasmonic resonance of the AuCu core is shown in Figure S8, Supporting Information). Figure 5c shows the light-controlled magnetization of the FeNi shell under a bias magnetic field of 1010 Oe (Hc), on which the structural tunability and magnetization control of the FeNi shell are attributed to the surface plasmon effect from the AuCu core under light illumination. This could yield a new class of light-controlled functional material systems that combine magnetism, electric, and photonic order parameters.
the FeNi shell coating onto the AuCu core with a controlled thickness and stoichiometry (Figure 2d and 2h). Phase transformation of AuCu from fcc to L10 tetragonal structure is initiated by thermal annealing at a selected temperature window, where the (002) and (201) peaks of Xray diffraction (XRD) pattern signify the L10 ordering of AuCu core after thermal annealing (Figure 3a).16,20,24 Meanwhile, the XRD analysis after thermal annealing indicates the structural change from (100) fcc-FeNi to (110) bct-FeNi, due to the lattice contraction and expansion of the AuCu core along the different crystallographic orientations.25 The TEM and elemental images of AuCu/FeNi annealed at 653 K are shown in Figure S5, Supporting Information. The HRTEM and corresponding FFT pattern indicate the epitaxy relationship between the AuCu core and FeNi shell (Figure S6, Supporting Information). Figure 3b shows the HRTEM image of L10AuCu/tetragonal-FeNi BNSs after thermal annealing at 653 K. The core domain exhibits a lattice spacing of 0.184 nm, corresponding to the (200) plane of L10 AuCu, while the outer shell exhibits an interplanar spacing of 0.188 nm matched with (110) tetragonal FeNi. The HRTEM and inverse-FFT pattern show the epitaxial growth of the FeNi shell on the AuCu core (Figure S7, Supporting Information). The schematic interfacial structure of heteroepitaxially grown AuCu/FeNi nanocrystals is shown in Figure 3c before and after phase transformation, respectively. The calculated lattice mismatch between L10AuCu/tetragonal FeNi and L10-AuCu/bcc FeNi are 2.17% and 9.67% and further confirms the preferable L10-AuCu/bct FeNi interfacial structure after thermal annealing-induced phase transformation (detailed calculations are presented in Supporting Information). After phase transformation, the thickness and stoichiometry of the tetragonal FeNi shell dictates its magnetic performance. The magnetic hysteresis (M-H) loops of annealed nanocrystals are shown for both thickness-dependent L10-AuCu/tetragonalFeNi SNPs and BNSs (Figure 4a and 4b). The optimum coercivity (Hc) of 772.3 Oe is achieved at an average shell thickness of 3.5 nm for AuCu/FeNi SNPs (Figure 4a), while a coercivity of 1010.2 Oe is shown in AuCu/FeNi BNSs with an average shell thickness of 4.1 nm (Figure 4b). The thickness and stoichiometry-dependent coercivity of L10-AuCu/tetragonal-FeNi BNSs are shown in Figure 4c, where the coercivity decreases from 1010.2 to 441.8 Oe as the FeNi shell thickness increases from 4.1 to 16.6 nm. This drop could be attributed to the FeNi thickness being above the critical shell thickness, where the tetragonal distortion is gradually weakened due to the strain energy decrease of outer atomic layers as the shell thickness increases. This critical thickness, tc, is determined by the elastic constant and Bain strain misfit m.16,26 In the L10AuCu/tetragonal-FeNi system, the calculated critical thickness tc is about 2.3 nm (detailed calculations are shown in Supporting Information). If the shell exceeds this estimated critical thickness, the outer atomic layers of the FeNi shell favor the bcc structure and result in a reduced Hc, which corresponds with the experimental results. By changing the stoichiometry of the FeNi shell at an average shell thickness of ∼4.1 nm, the highest Hc is achieved at Fe46Ni54, consistent with the reported equiatomic tetragonal FeNi structure.10,15 It is known that the temperature of thermal annealing plays an important role in controlling the order−disorder transition of the AuCu phase. This is critical because L10 ordering of the AuCu core controls the extent and degree of tetragonal distortion within the FeNi shell. Figure 5a shows the M-H
■
CONCLUSION In summary, a facile core/shell design is reported for the heteroepitaxial growth of AuCu/FeNi nanostructures, which exhibit high Hc (1010.2 Oe) and Ms (122.0 emu/g). The unique magnetic performance of the FeNi shell is realized by a tetragonal distortion, which is induced by L10 phase transformation of the AuCu core. The epitaxial interface between the FeNi shell and AuCu core allowed for efficient transfer of the strain energy to trigger and stabilize the tetragonal FeNi nanostructures. Stoichiometry and shell thickness of the FeNi shell, as well as the annealing temperature of the AuCu core, played a key role in magnetic performance of the core/shell nanocrystals. In addition, the plasmonic AuCu core also enables the tunable magnetization of the FeNi shell under light illumination. This work provides a strategy to fabricate and stabilize metastable functional nanomaterials through heteroepitaxial growth and phase transformation.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03736. Lattice mismatch, M-H loops of FeNi and AuCu/FeNi core/shell structures, TEM images of AuCu/FeNi core/ E
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
■
Oshima, M.; Watanabe, Y.; Taniguchi, M. Novel Magnetic Domain Structure in Iron Meteorite Induced by the Presence of L10-FeNi. Appl. Phys. Express 2010, 3, 013001. (14) Kotsugi, M.; Maruyama, H.; Ishimatsu, N.; Kawamura, N.; Suzuki, M.; Mizumaki, M.; Osaka, K.; Matsumoto, T.; Ohkochi, T.; Ohtsuki, T. Structural, magnetic and electronic state characterization of L1 0-type ordered FeNi alloy extracted from a natural meteorite. J. Phys.: Condens. Matter 2014, 26, 064206. (15) Lewis, L. H.; Mubarok, A.; Poirier, E.; Bordeaux, N.; Manchanda, P.; Kashyap, A.; Skomski, R.; Goldstein, J.; Pinkerton, F. E.; Mishra, R. K.; Kubic, R. C.; Barmak, K. Inspired by nature: Investigating tetrataenite for permanent magnet applications. J. Phys.: Condens. Matter 2014, 26, 064213. (16) Gong, M.; Kirkeminde, A.; Wuttig, M.; Ren, S. Q. Phase Transformation-Induced Tetragonal FeCo Nanostructures. Nano Lett. 2014, 14, 6493−6498. (17) Xu, J.; Lee, C. S.; Tang, Y. B.; Chen, X.; Chen, Z. H.; Zhang, W. J.; Lee, S. T.; Zhang, W. X.; Yang, Z. H. Large-Scale Synthesis and Phase Transformation of CuSe, CuInSe2, and CuInSe2-CuInS2 CoreShell Nanowire Bundles. ACS Nano 2010, 4, 1845−1850. (18) Wu, J. M.; Huang, H. J.; Lin, Y. H. Thermally pressure-induced partial structural phase transitions in core-shell InSb-SiO2 nanoballs/ microballs: Characterization, size and interface effect. Nanotechnology 2014, 25, 395705. (19) Li, M.; Chen, X.; Guan, J.; Wang, J.; Liang, C. Thermally induced phase transition and magnetic properties of Fe-FeSi2 with core-shell structure. Phys. Status Solidi A 2013, 210, 2710−2715. (20) Anraku, T.; Sakaihara, I.; Hoshikawa, T.; Taniwaki, M. Phase transitions and termal expansion behavior in AuCu alloy. Mater. Trans. 2009, 50, 683−688. (21) Yamauchi, M.; et al. Hydrogen-induced structural transformation of AuCu nanoalloys probed by synchrotron X-ray diffraction techniques. Nanoscale 2014, 6, 4067−4071. (22) Zhu, Y.; Cai, J. W. Low-temperature ordering of FePt thin films by a thin AuCu underlayer. Appl. Phys. Lett. 2005, 87, 032504. (23) Joo, J.; Chow, B. Y.; Prakash, M.; Boyden, E. S.; Jacobson, J. M. Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis. Nat. Mater. 2011, 10, 596−601. (24) Sra, A. K.; Schaak, R. E. Synthesis of Atomically Ordered AuCu and AuCu3 Nanocrystals. J. Am. Chem. Soc. 2004, 126, 6667−6672. (25) Cayron, C. One-step model of the face-centred-cubic to bodycentred-cubic martensitic transformation. Acta Crystallogr., Sect. A: Found. Crystallogr. 2013, 69, 498−509. (26) King, D. A.; Woodruff, D. P. The Chemical Physics of Solid Surfaces; Elsevier Science B. V.: The Netherlands, 1997; Vol. 8, pp 153−155. (27) Anraku, T.; Sakaihara, I.; Hoshikawa, T.; Taniwaki, M. Phase Transitions and Thermal Expansion Behavior in AuCu Alloy. Mater. Trans. 2009, 50, 683−688.
shell structures at different reaction times, and the relationship between stoichiometry and injection chemical ratio (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
M.G. carried out the experiments and prepared the manuscript. S.R. supervised the project and edited the manuscript. All authors have given approval to the final version of the manuscript. Funding
S.R. acknowledges financial support from the National Science Foundation (NSF) under Career Award no. NSF-DMR1551948. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS The authors thank Dr. Alec Kirkeminde for the XRD measurement. REFERENCES
(1) Schultz, L.; Wecker, J.; Hellstern, E. Formation and properties of NdFeB prepared by mechanical alloying and solid-state reaction. J. Appl. Phys. 1987, 61, 3583−3585. (2) Brown, D.; Ma, B. M.; Chen, Z. M. Developments in the processing and properties of NdFeb-type permanent magnets. J. Magn. Magn. Mater. 2002, 248, 432−440. (3) Vial, F.; Joly, F.; Nevalainen, E.; Sagawa, M.; Hiraga, K.; Park, K. T. Improvement of coercivity of sintered NdFeB permanent magnets by heat treatment. J. Magn. Magn. Mater. 2002, 242, 1329−1334. (4) Ma, B. M.; Herchenroeder, J. W.; Smith, B.; Suda, M.; Brown, D. N.; Chen, Z. Recent development in bonded NdFeB magnets. J. Magn. Magn. Mater. 2002, 239, 418−412. (5) Yu, L. Q.; Wen, Y. H.; Yan, M. Effects of Dy and Nb on the magnetic properties and corrosion resistance of sintered NdFeB. J. Magn. Magn. Mater. 2004, 283, 353−356. (6) Chakka, V. M.; Altuncevahir, B.; Jin, Z. Q.; Li, Y.; Liu, J. P. Magnetic nanoparticles produced by surfactant-assisted ball milling. J. Appl. Phys. 2006, 99, 08E912. (7) Hou, Y. L.; Xu, Z. C.; Peng, S.; Rong, C. B.; Liu, J. P.; Sun, S. H. A Facile Synthesis of SmCo5 Magnets from Core/Shell Co/Sm2O3 Nanoparticles. Adv. Mater. 2007, 19, 3349−3352. (8) Cui, W. B.; Takahashi, Y. K.; Hono, K. Nd(2)Fe(14)B/FeCo anisotropic nanocomposite films with a large maximum energy product. Adv. Mater. 2012, 24, 6530−6535. (9) Kojima, T.; Ogiwara, M.; Mizuguchi, M.; Kotsugi, M.; Koganezawa, T.; Ohtsuki, T.; Tashiro, T. Y.; Takanashi, K. Fe-Ni composition dependence of magnetic anisotropy in artificially fabricated L10-ordered FeNi films. J. Phys.: Condens. Matter 2014, 26, 064207. (10) Néel, L.; Pauleve, J.; Pauthenet, R.; Laugier, J.; Dautreppe, D. Magnetic Properties of an Iron−Nickel Single Crystal Ordered by Neutron Bombardment. J. Appl. Phys. 1964, 35, 873−876. (11) Larsen, L.; Roy-Poulsen, H.; Roy-Poulsen, N.; Vistisen, L.; Knudsen, J. Order-Disorder Transitions in Iron-Nickel (50%-50%) Alloys from Iron Meteorites as Studied by Mössbauer Spectroscopy. Phys. Rev. Lett. 1982, 48, 1054−1056. (12) Scorzelli, R. B. A study of phase stability in invar Fe-Ni alloys obtained by non-conventional methods. Hyperfine Interact. 1997, 110, 143−150. (13) Kotsugi, M.; Mitsumata, C.; Maruyama, H.; Wakita, T.; Taniuchi, T.; Ono, K.; Suzuki, M.; Kawamura, N.; Ishimatsu, N.; F
DOI: 10.1021/acs.chemmater.5b03736 Chem. Mater. XXXX, XXX, XXX−XXX
