Structure and Magnetic Properties of FeCo Clusters: Carbon

Article pubs.acs.org/JPCC

Structure and Magnetic Properties of FeCo Clusters: Carbon Environment and Annealing Effects Ghassan Khadra,† Alexandre Tamion, Florent Tournus, Olivier Boisron, Clément Albin, and Véronique Dupuis* Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon F-69622 Villeurbanne cedex, France ABSTRACT: In this article, we study the thermally activated chemical order transition in FeCo nanoalloys prepared by low energy clusters beam deposition (LECBD). Due to the low Z-contrast (ΔZ = 1) between Fe and Co atoms, a particular strategy based on extended X-ray absorption edge fine structure (EXAFS) experiments and simulations at both Fe and Co K-edges has been developed as well as accurate magnetic characterizations on as-prepared and annealed FeCo clusters embedded in a carbon matrix. We show that upon annealing crystallographic order and magnetic properties are improved, but we are not able to detect without ambiguity the B2 (CsCl-type) chemical order as expected in bulk-like phase, due to competition between stable iron carbide formation and cobalt−carbon demixing in bimetallic nanoparticles.

I. INTRODUCTION From recording media to medical applications, there is constant need for the miniaturization of magnetic materials.1 Whether it is to increase the areal density of hard disk drives or to have functionalized biocompatible magnetic resonance imaging (MRI) contrast agents, the study of nanoalloys and hybrid nanoparticles with adjustable properties has witnessed staggering amounts of research and publications.2 Nevertheless, simply scaling down the size of magnetic materials from the bulk to nanoparticles has brought up critical physical limitations. Indeed, while bulk materials have magnetic anisotropic energies (MAE) that are much larger than the thermal energy (kBT), for a nanoparticle the thermal energy can be sufficient to readily invert the magnetic moment direction. Thus, at room temperature, rapid thermal fluctuations can lead the nanoparticles to continually switch magnetization direction; this is named the superparamagnetic regime. For magnetic storage applications, to block these ultimate storage “bits” in one direction of magnetization, their energy barrier KeffV (where Keff is the effective magnetic anisotropy constant and V is the nanoparticle volume) must be increased. However, increasing the anisotropy of nanoparticles would require large writing fields, i.e., the magnetic field necessary to switch the magnetization of the particle from one direction to another. Nevertheless, the switching field (Hsw) is proportional to the ratio of the anisotropy to the magnetization (Hsw ∝ Keff/Ms). So, it is possible to minimize the switching field by using materials with a high Keff value provided they have a large saturation magnetization Ms. According to the Slater−Pauling graph3 for magnetic alloys, bulk chemically ordered B2 FeCo alloy has the largest recorded Ms, but remains a soft magnetic material with a low Keff. Structural distortion of FeCo in B2 phase is theoretically © XXXX American Chemical Society

expected to lead to giant MAE while conserving large saturation magnetic moment, as required for future recording media.4 In thin films, one approach can be to use an epitaxial growth on various substrates where an enhanced MAE was predicted by taking into account some chemical disorder in FeCo alloys.5,6 Recently, an alternative way to stabilize the expected tetragonal distortion in FeCo alloys was theoretically predicted, by alloying it with carbon.7 Besides, cobalt and iron−carbon interactions play an important role in various industrial activities such as liquid fuel production by the Fischer− Tropsch process8 and carbon nanotube synthesis.9 In particular for the growth of carbon nanotubes forests, Hardeman et al. claimed that bimetallic nanocatalyst systems can lead to synergistic effects. They observed how the absence of a stable carbide promotes an effective carbon diffusion through the metal particles providing much higher activity for FeCo catalysts compared to Fe catalysts where iron carbides are more favorable. This article focuses on the experimental study of magnetic nanoparticles and carbon based nanohybrid systems, prepared from 3 nm (median diameter) preformed FeCo clusters embedded in a carbon matrix. Ultra-high vacuum (UHV) annealing at 500 °C was performed in view to promote B2 chemical order, as already evidenced in FeRh clusters assemblies from high-resolution transmission electron microscopy (HRTEM) where the electronic contrast was high enough Special Issue: ISSPIC XVIII: International Symposium on Small Particles and Inorganic Clusters 2016 Received: October 24, 2016 Revised: January 5, 2017

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DOI: 10.1021/acs.jpcc.6b10715 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C to distinguish the (001) Fe and Rh planes.10 However, in this present case, due to the low atomic number difference (ΔZ = 1) between Fe and Co atoms, it is a challenge to prove experimentally that FeCo nanoalloys are chemically ordered after annealing, even though the CsCl-B2 phase is the stable phase for equimolar alloys. Thus, systematic EXAFS simulations were performed at both environments before and after annealing of FeCo clusters assemblies diluted in carbon matrix. A particular attention has been given to carbon reactivity in such bimetallic nanoparticles and revealed a competition between stable iron carbide formation and cobalt−carbon demixing upon annealing. In addition, the magnetic properties of these FeCo nanoalloys has been studied from accurate simulations of several magnetometry measurements. After thermal treatment, both crystallographic order, saturation magnetization, and MAE improve but not in the same proportion to what is expected for the tetragonal FeCo alloy.7

the nanoparticles was quantitatively investigated during the image treatment process. The ratio of the two ellipses axis (minor and major) was used to estimate the sphericity and has been found to be around 1.28 ± 0.1 compared to 1 for an ideal sphere. HRTEM observations on as-prepared samples did not reveal any clear crystallographic structure (see Figure 1c). The annealed FeCo samples are better crystallized (see Figure 1d,e) but because of the low contrast density between Fe and Co, a clear chemically ordered B2 (CsCl-type) structure cannot be distinguished from random body-centered cubic A2 (Wtype) structure. In addition, by using energy dispersive X-ray (EDX) analysis and Rutherford BackScattering spectroscopy (RBS), the analysis accuracy revealed that no significant contamination with oxygen can be depicted in the film and that an average composition of 47% Fe and 53% Co was obtained for asprepared sample and of 49% Fe and 51% Co after annealing.13 Thus, local order has been studied from X-ray absorption spectroscopy (XAS) measurements at both Co- and Fe−K edges (respectively at 7709 and 7112 eV). These experiments were performed at room temperature at the CRG-BM30bFAME beamline of the ESRF14 on two FeCo thick samples in 2D configuration with alternating layers of amorphous carbon and of FeCo nanoparticles (respectively with 2 and 0.8 nm equivalent thickness and a total of eight layers). The samples were both capped with amorphous carbon. One was annealed under UHV conditions at a temperature of 500 °C for 2 h, while the other was left as-prepared. First of all, the X-ray absorption near edge spectroscopy (XANES) spectra in the Figure 2, do not show any

II. SAMPLE SYNTHESIS FeCo clusters are preformed in the gas phase thanks to a laser vaporization source working in the low energy clusters beam deposition (LECBD) regime. Briefly, a YAG laser (λ = 532 nm, pulse duration 8 ns, frequency ≤ 30 Hz) is used to vaporize a mixed equiatomic FeCo target rod, and a continuous flow of inert gas (He, 30 mbar) is injected to rapidly cool the generated plasma and to nucleate clusters submitted to a supersonic expansion under vacuum. The nanoparticles are deposited in an UHV deposition chamber.11,12 The matrix is evaporated with an electron gun working under UHV conditions (base pressure of 5 × 10−10 mBar). Clusters and atomic carbon matrix beams have been simultaneously codeposited on 45°-tilted substrate in front of both independently arriving beams. Monocrystalline commercial silicon substrates were used for the cluster assemblies’ thick-samples. III. STRUCTURE First of all, structural characterization was performed using transmission electron microscopy (TEM) on FeCo clusters deposited on an amorphous carbon coated grid (then protected by a carbon thin film) (see Figure 1a,b). We obtained FeCo nanoparticles with a median diameter of around 3.2 nm and a log-normal size dispersion of around 0.45. The morphology of

Figure 2. XANES signal for as-prepared (in blue) and annealed (in red) FeCo clusters assemblies embedded in carbon matrix at Fe (×) and Co (○) K edge.

characteristic features of the ones for transition metal oxide. Apart from a reduction in the amplitude of the oscillations, all the spectra are more similar to that of bcc Fe than to fcc Co reference, which is expected when a bcc FeCo alloy is formed,15 and has been especially observed with the superposition of XANES at both edges after annealing. For a qualitative analysis, using the Athena software16 we extracted the Fourier transform (FT) of the EXAFS oscillations, which corresponds to the noncorrected radial distribution, respectively, in the iron and cobalt environment (see Figure 3). At the Fe−K edge, we can see two peaks clearly identified as iron carbide signature (especially at 1.5 Å).17

Figure 1. Size histogram (a), TEM observations (b), and HRTEM image (c) of as-prepared FeCo clusters. HRTEM image obtained on annealed FeCo cluster (d) along with the corresponding FFT (e). B


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The Journal of Physical Chemistry C

Table 1. Values Obtained for the Best Fits of EXAFS Oscillations for the As-Prepared and Annealed FeCo Sample at Both Fe and Co K-Edges FeCo

edge

path

NN

σ2 (Å2)

R (Å)

as-prepared

Fe−K

Fe−Fe Fe−Co Fe−C Co−Co Co−Fe Co−C Fe−Fe Fe−Co Fe−C Co−Co Co−Fe Co−C

1.5 1.5 2 1.7 1.7 0.6 4.4 5.9 0.7 4.1 5.4 0.7

0.0077 0.0075 0.059 0.0074 0.0076 0.0059 0.013 0.0151 0.059 0.0278 0.0126 0.0059

2.52 2.41 2.25 2.45 2.41 2.19 2.78 2.47 1.99 2.74 2.47 2.19

Co−K

Figure 3. Fourier transform of the EXAFS signal for as-prepared (in blue) and annealed (in red) FeCo cluster assemblies embedded in carbon matrix at Fe (left) and Co (right) K edge.

annealed

Moreover, modifications of the FT shape and relative intensities after thermal treatment both indicate a change in the local chemical environment and a decrease of structural disorder. At the Co−K edge, we observe the decrease after annealing of a shoulder-like structure before the main metallic peak (at 2 Å) due to carbon neighbors. This evolution is likely due to the demixing of cobalt and carbon atoms upon annealing previously observed in pure Co nanoparticles embedded in a carbon matrix.18 Notice that the first peaks in χ(R) (the Fourier transform (FT) of the EXAFS oscillations) correspond to the contribution of the nearest-neighbors (NN). To go a step further, a quantitative analysis has been done from the simulation of the inverse Fourier transform [FT−1 χ(R)] filtered around 1−3 Å, using the Artemis software. 13 Comparison between experimental k2χ(k) signal and best fits are presented in Figure 4, before and after annealing at both

Fe−K

Co−K

heteroatomic distance at both edges, then (dFe−Co = dCo−Fe) is used. In the other hand, both homoatomic distances, dFe−Fe and dCo−Co, are fitted at their respective edge. In this condition, the best fit obtained on as-prepared FeCo nanoparticles displayed some differences between intermetallic distances mainly in that the one for Co−Co pair is smaller than the Fe−Fe one (dFe−Co < dCo−Co < dFe−Fe, namely, 2.41 < 2.45 < 2.52 Å) with a 0.01 Å error bar. This can be compared to bcc30 and fcc bulk-Fe references (respectively equal to 2.485 and 2.54 Å). Moreover, the low metallic NN number obtained in Table 1 for our as-prepared FeCo clusters is to relate to the poor crystallographic order in our small NPs as seen in HRTEM, along with sample inhomogeneity detected by EXAFS technique.16 One can also mention that the coordination for Fe−C is three time larger than the one for Co−C environment before annealing. So, reversely to the authors of ref 7, our experimental EXAFS results are in favor with the fact that the preferred position of C atoms is nearby Fe atoms, and so the dFe−Fe distance is dilated compared to dCo−Co in the cluster. It must be remembered that from steel structure, a well-known metastable martensite iron carbide phase in a body-centered tetragonal (bct) structure (with an equivalent dilated dFe−Fe value) can be obtained from rapid quenching of fcc austenite structure where up to 3.5 at. % of interstitial carbon can be dissolved instead of only 0.1 at. % in the bcc ferrite structure. Upon annealing, the number of metallic NNs increases to up to 5.9 and 4.4 at Fe edge (respectively 5.4 and 4.1 at Co edge) compared to less than a total of four metallic NN at both edges in the case of the as-prepared nanoparticles. This enhancement of NN numbers up to 10 is in direct correlation with the crystal coordination expected for well-crystallized 3 nm-nanoparticles.19 Moreover, there is less carbide as the coordination number for Fe−C decreases down to the Co−C NN value, corresponding to the C atoms NN proportion at the cluster interface.18 To compare with bulk-B2 alloy presented in the Scheme 1, the fit of EXAFS oscillations for the annealed samples has been possible on both edges (Co and Fe) with similar values, allowing us to propose a model based on a “distorted” chemically ordered B2-like FeCo. Quantitatively, the ratio between hetero- and homoatomic NN numbers presented in the Table 1 has been found equal to 1.34 at the Fe edge and to 1.32 at the Co edge, both very close to 8/6 = 1.333 as expected for a chemically ordered CsCl-B2 phase structure.

Figure 4. Experimental (dots) and simulated (continuous) EXAFS curves in FeCo sample before (left) and after annealing (right) at both K-edges (Fe in top and Co in bottom).

edges. To obtain a high quality fit, a contribution of the neighboring carbon atoms of the matrix has been added. The evolution of the contribution of Fe−C (respectively Co−C) interatomic pair is presented in Table 1 alongside the metallic distances to the absorber. As the effective scattering is very similar for surrounding Fe and Co atoms, it is very difficult to distinguish among Fe−Fe, Fe−Co, and Co−Co pairs. So it was necessary to use a specific strategy to simulate EXAFS oscillations. In one hand, a common value is expected for the C


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The Journal of Physical Chemistry C Scheme 1. Model for Bulk-B2 FeCo Alloy

The obtained distance dFe−Co= R1 = 2.47 Å approaches the value for bulk FeCo with R1 = 2.484 Å,20 while the homoatomic distances dFe−Fe= 2.78 Å and dCo−Co= 2.74 Å are far from R2 = 2.868 Å expected for a perfect B2 phase. Thus, both in Fe and Co environment, the ratio τ = R1/R2, respectively, equal to 0.89 and 0.90 for nanoparticles (τNP) is significantly larger than √3/ 2 ∼ 0.87 expected for the bulk (τbulk). It can be seen as a kind of distortion with τNP/τbulk = 1.02 and 1.04, respectively, for Fe and Co environment. In the next paragraph, we present how such a complex structure and chemical affinity affects the intrinsic magnetic properties of as-prepared and annealed FeCo clusters diluted in carbon matrix. Nevertheless, to reach the intrinsic magnetic properties of our clusters, it is important to study sufficiently diluted samples to minimize the dipolar interactions between the nanoparticles and to avoid possible coalescences upon annealing.

Figure 5. (a) ZFC/FC and m(H) experimental data (dots) for asprepared FeCo clusters along with their best fits (lines). (b) IRM experimental data with the corresponding biaxial contribution simulation. (c) IRM/DcD curves with Δm. (d) Hysteresis loop at 2 K with simulation.

IV. MAGNETISM Magnetic measurements were performed using superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS 5 XL). In view to minimize direct and indirect interactions between nanoparticles that prevent unambiguous interpretation of magnetization data,21,22 we prepared a diluted magnetic cluster sample with concentration of nanoparticle to matrix lower than 1 vol % as confirmed by RBS measurements. Such diluted sample has been codeposited with the matrix at the same time using an amorphous carbon crucible in the e-beam evaporator. Then, this sample has been measured as-prepared using the SQUID after which it has been annealed at 500 °C for 2 h and were measured again. First of all, we have performed magnetization m(H) measurements as a function of the magnetic field at different temperatures. All curves were corrected for diamagnetic response of the silicon substrate contribution. At high temperature, the an-hysteretic m(H) magnetization curves show a typical superparamagnetic Langevin-like form (see inset of the Figures 5a and 6a). In addition, low field susceptibility measurements were performed as a function of temperature by using the zero field cooled/field cooled (ZFC/FC) protocols (as presented in Figures 5a and 6a).22 In such a case, the particles’ switching has a thermal origin as the measurements are done under a weak external field (5 mT). Thus, the path chosen by the magnetization to switch can be considered independent from the direction of the applied field. These curves were fitted, together with the high temperature m(H) (measured at least five times higher temperature than the Tmax of ZFC peak) using an accurate “triple fit” method23 (see results presented in Table 2). We have also systematically verified that the magnetic interactions between the clusters are negligible, before and after

Figure 6. (a) ZFC/FC and m(H) experimental data (dots) for annealed FeCo clusters along with their best fits (lines). (b) IRM experimental data with the corresponding biaxial contribution simulation. (c) IRM/DcD curves with Δm. (d) Hysteresis loop at 2 K with simulation; the dashed line is the as-prepared experimental data.

annealing, by performing magnetic remanence measurements. We used the isothermal remanent magnetization (IRM) and direct current demagnetization (DcD) protocol to estimate the nature of magnetic interactions via the parameter Δm = DcD(H) − [mR − 2IRM(H)] with mR simply being the remanent moment in the hysteretic m(H) curve at 2 K.24 Without interaction, the Δm parameter is equal to 0 whatever the applied magnetic field, whereas the presence of magnetizing or demagnetizing interactions leads respectively to Δm > 0 and Δm < 0. As presented in the Figures 5c and 6c, the Δm parameter has been found to be negligible. In the IRM curves and hysteresis loops performed at 2 K, the particles’ switching is due to the externally applied field. In order to simulate such curves (as in Figures 5 and 6b,d), it was necessary to include a biaxial contribution where the anisotropy function of a magnetic nanoparticle can be expressed as G(θ , φ) = K1mz 2 + K 2my 2 D


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Table 2. Maximum of the ZFC (Tmax), Coercive Field (μ0HC), and the Deduced Parameters from the Fit of the SQUID Measurements for FeCo Nanoparticles Embedded in C Matrix As-Prepared and after Annealing as-prepared annealed

Tmax (K)

μ0Hc (mT)

Dmag (nm) [ωmag]

Ms (kA·m−1)

|K1|(kJ·m3) [ωk]

|K2/K1|

10 23

20 34

3.2 ± 0.2 [0 32 ± 0.02] 3.4 ± 0.2 [0.27 ± b.02]

900 ± 100 1220 ± 100

44 ± 10 [0.35 ± 0.0s] 125 ± 10 [0.25 ± 0.05]

1.2 ± 0.4 1.2 ± 0.4

Here z is the easy axis, y the hard axis, and K1 < 0 < K2. K1 and K2 represent the second order anisotropy constants, and mz and my being the normalized magnetization projection on the easy axis and hard axis, respectively. Finally θ and φ represent the magnetization angles in a spherical basis. Notice that it has been necessary to introduce a Gaussian K1 anisotropy distribution of relative width ωk to take into account the various alloying configurations.25 The results and the corresponding fits for the as-prepared and annealed diluted FeCo sample are presented in Figures 5 and 6 along with the fits’ results in Table 2. The fits give initial magnetic diameter and dispersion (Dmag = 3.2 nm, ωmag = 0.32) in agreement with the TEM values. After annealing the median diameter marginally increases with a narrowing of the size dispersion (Dmag = 3.4 nm, ωmag = 0.27). We noticed that the maximum temperature Tmax in the ZFC curves and the coercive field Hc increase significantly upon annealing. Indeed in Figure 6b, the IRM curve for annealed sample is slightly shifted to the right suggesting a larger K1 value but also saturates at a significantly higher field value indicating an increase in the global magnetic moment, in agreement with the m(H) loop intensity at 300 K (directly sensitive to Ms). Nevertheless, it should be noted that in all cases the triple-fit, as well as IRM and hysteresis loop simulations at 2 K were adjusted with a saturation magnetization lower than 1910 kA·m−1 as expected for the bulk-like FeCo alloy.26 Thus, the saturation magnetization value was found equal to Ms = 900 kA.m−1 for the as-prepared sample that increased to 1220 kA·m−1 upon annealing due to a carbon pollution neighboring decrease and a better crystallization as evidenced from EXAFS measurements. The magnetic anisotropy K1 displays almost a 3-fold increase after annealing with a lower anisotropy dispersion ω k decreasing from 0.35 to 0.25 that could indicate a better chemical ordering of the bimetallic clusters.24 However, the anisotropy value is one order of magnitude lower than what is expected from theoretical predictions4 and comparable with that of the anisotropy of pure Co nanoparticles, meaning that at nanoscale, the MAE is weakly dependent on the cluster nature.17 As for the biaxial contribution, almost no change in K2/K1 is observed after annealing.

That has led us to develop a particular strategy based on EXAFS experiments and simultaneous simulations with two different dFe−Fe and dCo−Co distances but identical heteroatomic distance at both Fe and Co edges. This allowed us to suggest that a preferred intercalation of C atoms nearby Fe atoms introduces important disorder in as-prepared FeCo nanoparticles embedded in C-matrix. Upon annealing at 500 °C, crystallographic and chemical order has been promoted, but we are not able to detect without ambiguity the B2 stable order because the backscattering amplitudes are extremely similar for Fe and Co and probably complicated in our bimetallic clusters by antagonist effects as carbide formation for Fe and carbon demixing for Co atoms after thermal treatment in carbon environment.9,17 Nevertheless, such results have been further corroborated from the magnetic findings using SQUID magnetometry measurements and simulations where the magnetic anisotropy constant K1 and the saturation magnetization Ms have been found to increase upon annealing in agreement with better structural order and carbon depollution. So in one hand, we obtain a larger anisotropy constant K1 useful to block the macrospin, almost triple after annealing. In the other hand, the switching field (Hsw ∝ K1/Ms), which needs to be minimized for applications, is only increased by a factor two. To go a step further, it would be essential to study massselected FeCo nanoparticles in carbon environment in order to verify if carbon solubility effects are size dependent as recently predicted in phase diagrams for Ni−C nanoparticles.28 Moreover, complementary techniques well-adapted to lack of strong chemical contrast would be necessary to relate finite size-effects with magnetic and chemical order transitions. Notice that recently atomic resolution Z contrast images have been obtained on Co/Ni multilayers29 by using advanced aberration-corrected scanning transmission electron microscope (STEM) and high angle annular dark field (HAADF). In our case, such STEM-HAADF coupled with electron energy loss spectroscopy (EELS) could be performed to determine B2 order transition upon annealing on size-selected FeCo NPs in carbon matrix.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

V. CONCLUSION Because in the bulk phase, near equiatomic composition, FeCo alloy is expected to present a huge Ms in the CsCl-like B2 chemically ordered phase, we have studied such bimetallic magnets at nanosize, from FeCo clusters prepared by LECBD embedded in carbon matrix and postannealed in UHV at high temperature. While, due to the low Z-contrast between Fe and Co atoms, a neutron diffraction measurement where the nuclear scattering factors for iron and cobalt differ considerably is generally preferred to determine the degree of long-range order in FeCo alloy,27 it is incompatible with the low quantities of matter in our samples.

ORCID

Véronique Dupuis: 0000-0001-6225-6608 Present Address †

Interfaces, Confinement, Matériaux et Nanostructures, UMR7374 Université d’Orléans-CNRS, 45071 Orléans cedex, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to O. Proux for his help during EXAFS experiments on the French BM30b-FAME beamlines at ESRF. E


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(17) Davis, B. H.; Occelli, M. L. Advances in Fischer−Tropsch Synthesis, Catalysts, and Catalysis; CRC Press: Boca Raton, 2009. (18) Tamion, A.; Hillenkamp, M.; Hillion, A.; Tournus, F.; TuaillonCombes, J.; Boisron, O.; Zafeiratos, S.; Dupuis, V. J. Appl. Phys. 2011, 110, 063904. (19) Dupuis, V.; Jamet, M.; Tuaillon-Combes, J.; Favre, L.; Stanescu, S.; Treilleux, M.; Bernstein, E.; Melinon, P. Pure and Mixed Magnetic Clusters Assembled Nanostructures. Rec. Rec. Dev. Magn. Magn. Mater. 2003, 1, 101−121. (20) Diaz-Ortiz, A.; Drautz, R.; Fahnle, M.; Dosch, H.; Sanchez, J. M. Structure and Magnetism in bcc-Based Iron-Cobalt Alloys. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 224208−15. (21) Hillion, A.; Tamion, A.; Tournus, F.; Flament, J. B.; Hillenkamp, M.; Bonet, E.; Dupuis, V. Magnetic Interactions Effects on Magnetic Measurements for Nanoparticle Assemblies. IEEE Trans. Magn. 2011, 47, 3154−3156. (22) Oyarzun, S.; Tavares de Sa, A. D.; Tuaillon-Combes, J.; Tamion, A.; Hillion, A.; Boisron, O.; Mosset, A.; Pellarin, M.; Dupuis, V.; Hillenkamp, M. Giant Magnetoresistance in Cluster-Assembled Nanostructures: on the Influence of Inter-particle Interactions. J. Nanopart. Res. 2013, 15, 1928−8. (23) Tamion, A.; Hillenkamp, M.; Tournus, F.; Bonet, E.; Dupuis, V. Accurate Determination of the Magnetic Anisotropy in ClusterAssembled Nanostructures. Appl. Phys. Lett. 2009, 95, 062503−3. (24) Hillion, A.; Tamion, A.; Tournus, F.; Gaier, O.; Bonet, E.; Albin, C.; Dupuis, V. Advanced Magnetic Anisotropy Determination Through Isothermal Remanent Magnetization of Nanoparticles. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 094419−6. (25) Tournus, F.; Blanc, N.; Tamion, A.; Hillenkamp, M.; Dupuis, V. Dispersion of Magnetic Anisotropy in Size-Selected CoPt Clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 220405. (26) Briones-Leon, A.; Ayala, P.; Liu, X.; Yanagi, K.; Weschke, E.; Eisterer, M.; Jiang, H.; Kataura, H.; Pichler, T.; Shiozawa, H. Orbital and Spin Magnetic Moments of Transforming One-Dimensional Iron Inside Metallic and Semiconducting Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 195435−7. (27) Smith, A. W.; Rawlings, R. D. Physica status solidi (a) 1976, 34, 117−123. (28) Magnin, Y.; Zappelli, A.; Amara, H.; Ducastelle, F.; Bichara, C. Size Dependent Phase Diagrams of Nickel-Carbon Nanoparticles. Phys. Rev. Lett. 2015, 115, 205502−5. (29) Gottwald, M.; Andrieu, S.; Gimbert, F.; Shipton, E.; Calmels, L.; Magen, C.; Snoeck, E.; Liberati, M.; Hauet, T.; Arenholz, E.; et al. Co/ Ni(111) superlattices studied by microscopy, x-ray absorption, and ab initio calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 014425−15. (30) Notice that in bcc-bulk case, a model with 14 neighbors on two shells for the absorber is generally used to reproduce the first peak in the FT of EXAFS oscillation, where eight metallic NN at R1 and six metallic NN at R2 are expected with a τ = R1/R2 ratio equal to √3/2 ∼ 0.866. Moreover, in a chemically disordered structure (A2 phase), there is a 50% chance to have a Fe or Co atom as nearest neighbor (NN).

All the cluster samples were prepared at the PLYRA platform, while SQUID measurements were performed at the CML platform. Support is acknowledged from both GDR CNRS 3182 and COST-STSM-MP0903 on Nanoalloys.

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REFERENCES

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Structure and Magnetic Properties of FeCo Clusters: Carbon

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