Surface Segregation in Chromium-Doped NiCr Alloy Nanoparticles

Apr 9, 2015 - Mahindra Ecole Centrale, Survey Number 62/1A, Bahadurpally Jeedimetla, Hyderabad 500043, Telangana, India ... In situ annealing measurem...
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Surface Segregation in Chromium-Doped NiCr Alloy Nanoparticles and Its Effect on Their Magnetic Behavior Murtaza Bohra,*,†,‡ Panagiotis Grammatikopoulos,† Rosa E. Diaz,† Vidyadhar Singh,† Junlei Zhao,§ Jean-François Bobo,∥ Antti Kuronen,§ Flyura Djurabekova,§ Kai Nordlund,§ and Mukhles Sowwan*,†,⊥ †

Nanoparticles by Design Unit, Okinawa Institute of Science and Technology (OIST) Graduate University, 1919-1 Onna-Son, Okinawa 904-0495, Japan ‡ Mahindra Ecole Centrale, Survey Number 62/1A, Bahadurpally Jeedimetla, Hyderabad 500043, Telangana, India § Department of Physics and Helsinki Institute of Physics, University of Helsinki, P. O. Box 43, FIN-00014 Helsinki, Finland ∥ Centre d’Elaboration de Materiaux et d’Etudes Structurales (CEMES), 29 rue Jeanne Marvig, 31055 Cedex 4 Toulouse , France ⊥ Nanotechnology Research Laboratory, Al-Quds University, East Jerusalem, P. O. Box 51000, Palestine S Supporting Information *

ABSTRACT: Surface segregation designates the phenomenon of variation in chemical composition between the surface and the bulk of an alloy, which can have a beneficial or detrimental effect on its physical and chemical properties. This is even more pronounced in nanoalloys, i.e., alloy systems comprised of nanoparticles, with significant surface-to-volume ratios. In this case study we demonstrate the element-specific Cr segregation in Ni-rich NiCr alloy nanoparticles and nanogranular films grown by gas-phase synthesis methods. In situ annealing measurements (300−800 K), performed under vacuum using aberration-corrected environmental transmission electron microscopy (E-TEM), and vibrating sample magnetometry (VSM) revealed progressive Cr segregation with annealing temperature and subsequent complete transformation into core− satellite structures at 700 K. Simultaneously, atomistic computer simulations (molecular dynamics (MD) and Metropolis Monte Carlo (MMC)) elucidated the resultant structures, explaining the driving force behind segregation energetically. Most importantly, we emphasize the significant effects of Cr segregation on magnetic properties, namely, (i) the highly nonsaturated M−H loops (below the Néel temperature of antiferromagnetic Cr) with reduced coercivities and (ii) the uncompensated high Curie temperatures, TC, compared to the NiCr bulk, which approach bulk Ni values upon annealing. Both are clear evidence that the distribution of Cr in the nearest-neighbor shells of Ni atoms differs from that of the bulk NiCr alloy, reconfirming our structural findings. with a view to identify optimized chemical structures.8−10 However, despite these efforts, it is often very difficult to predict the resultant structure of a realistic nanoalloy under changing external conditions, and, most importantly, to associate it with its consequent physical properties. This becomes particularly vital in view of recent advances in gas-phase synthesis techniques,11−13 which have facilitated the production of bimetallic alloy nanoparticles with tunable size distribution, shape, and concentration. Their growth involves fast kinetics and nonequilibrium processes which often result in metastable phases, a fact that on one hand offers opportunities for designing novel structures with unique properties,14 but on the other hand hampers the stability with damaging effects.15 In this work, we present gas-phase synthesized Ni-rich NiCr alloy nanoparticles and nanogranular thin films and their structural correlation with magnetic properties. Such systems

1. INTRODUCTION Surface segregation is the preferential enrichment of a material constituent of a multicomponent system at a free surface.1 Especially in nanoalloy systems, where surface-to-volume ratio is high, surface segregation is of vital importance since it can affect most physical and chemical properties.2 However, a complete quantitative description is lacking, mainly because of the scarcity of reliable experimental data, which are very sensitive to environmental conditions, and due to the multitude and complexity of possible nanoalloy structures, which do not allow simple models to work in all cases.2−4 Resorting to bulk phase diagrams does not help either, as they are often inadequate to describe phenomena governed by a complex interplay of thermodynamic and kinetic parameters,4 such as core−shell or −satellite formation, especially considering the significant fraction of surface atoms with electronic structures different than in the bulk.5 A number of theoretical studies have been reported over past years, in an attempt either to construct nanophase diagrams6,7 or to apply global optimization methods © XXXX American Chemical Society

Received: November 19, 2014

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0.1 ppm and moisture < 1.2 ppm) and characterized by atomic force microscopy (AFM) to determine coverage. In order to prevent oxidation, HCD NiCr samples were fully capped by a G-varnish (GE7031) epoxy capping (60−80 nm)21 for low-temperature magnetic measurements of 5−400 K, and a high-temperature cement capping for high-temperature measurements of 300−800 K, inside the glovebox, immediately after the deposition. Ultrathin carbon film and silicon nitride (Si3N4) membrane TEM grids were used as substrates for TEM and STEM analysis of LCD NiCr samples, using a Cs-corrected environmental TEM (FEI Titan G2 80−300 kV) operating at 300 kV. The in situ heating studies were performed in scanning TEM (STEM) mode, using a single-tilt heating holder (Gatan). Moreover, energy dispersive X-ray (EDX) analysis was done in order to confirm the average composition of the nanoalloy, and electron energy loss spectroscopy (EELS) elemental mapping was performed to elucidate the structural changes of the nanoalloy after the annealing experiments. X-ray photoelectron spectroscopy (XPS) measurements were performed in a separate ultrahigh-vacuum (UHV) chamber with a base pressure of 2 × 10−9 mbar, using a Kratos AXIS Ultra DLD photoelectron spectrometer with a Al Kα anode. Magnetic properties were measured using a vibrating sample magnetometer (VSM) attached to the physical property measurement system (PPMS), after correcting for the diamagnetic contribution of the Si substrate used for sample deposition. M−H loops were measured at 5 and 300 K. For zero field cooled (ZFC) magnetization, the sample was initially cooled to 5 K in zero fields and then magnetizations were measured in the presence of a fixed field upon heating. Subsequently, in the same field, the field cooled (FC) magnetization was recorded during cooling. The δM curves were measured at 10 K, following the procedures reported in ref 8. The high-temperature experiments (300−700 K) were performed under vacuum, whereas a He gas flow was used for the low-temperature (5− 400 K) ones. Computer Simulation. For the determination of the structure and potential elemental segregation, both atomistic MD and MMC methods were utilized. A number of NiCr nanoparticles were created, their sizes ranging between 3 and 9 nm. To resemble experimental compositions, all nanoparticle sizes contained ∼5 at. % Cr. Since the particles contained a much higher concentration of Ni, the fcc Ni structure was chosen for the initial configurations, with Cr atoms taking random substitutional positions. The initial configurations were cut from bulk structures in near-spherical forms, obtaining 14-hedron and 26-hedron shapes. The interatomic potential (IAP) utilized was the recent embedded-atom method (EAM) potential by Bonny et al.,22 which was fitted to reproduce, among other properties, the Nimatrix fcc structure and lattice constants, defect migration energies, and cross-potential vacancy-solute migration barriers, and was validated by thermal annealing experiments. Molecular Dynamics Simulation. For the MD annealing studies, we used the PARCAS parallel, classical MD code.23 The main principles of the MD algorithm are presented in refs 24 and 25. A number of NiCr nanoparticles were created, 3, 5, and 7 nm in diameter, containing 1289, 5979, and 16295 atoms, respectively. We examined the nanoparticles’ behavior after thermal aging up to 200 ns at temperatures ranging between 300 and 1500 K, using the Berendsen thermostat.26 The temperature range was extended to higher values than the experimental ones to counterbalance the relatively short simulation times, as discussed in Results and Discussion. For the analysis of the segregation of the nanoparticles, we calculated the average short-range order (SRO) parameter, as defined by Cowley,27 for each atom. According to this definition, if an alloy consists of two atomic species, A and B, present in proportions mA and mB, the SRO parameter for the atomic site with coordinates x, y, and z, with respect to a given B atom, is

usually demonstrate superior physical properties for numerous industrial applications, ranging from biomedical sensing and high catalytic conversion to self-assembly based magnetic memory devices and magnetic fluids.16−18 Their Curie temperature (TC) and saturation magnetization (MS) drop sharply with increasing Cr concentration and attain zero values within low Cr content of 13 at. %,19 marking a departure from the Slater−Pauling linear curve of the Fe1−xCrx (TC and MS = 0 at x = 80%) system.16−19 This feature is very attractive and provides an avenue to tune the Curie temperature of NiCr alloy nanoparticles to a few degrees above the human body temperature (40−60 °C) by varying the critical concentration of Cr (4−6 at. %) for self-controlled magnetic hyperthermia for cancer treatment applications.17,18 However, we will show that Cr segregation can deteriorate the magnetic properties of such alloy nanoparticles, thus minimizing their application potential. Recent studies in similar FeCr alloy nanoparticles have shown that, besides inducing superparamagnetic dominating properties (due to the high surface-to-volume ratio), structural anomalies further affect the magnetic properties (e.g., MS, TC, and coercivity), especially causing magnetic interaction reversal from dipolar to exchange, and producing tight-waisted M−H loops with increasing σFeCr phase in FeCr core−shells.16 Frequently surface segregation is associated with oxidation, which determines the final structure of the alloy nanoparticles complicating the picture even more; for example, an in situ transmission electron microscopy (TEM) oxidation study under O2/Ar (25/75 ratio) on NiCr nanoclusters showed Cr preferential oxidation at room temperature. Progressive high-temperature oxidation led to a unique structure of Cr-rich oxide embedded into NiO, sandwiched near the inner wall of the hollow particles.20 However, it should be noted that even when oxidation is avoided (as in the case discussed here), Cr segregation can still occur as a result of thermal treatment. To this day, low-temperature magnetic studies and structural correlation with TC of these NiCr alloy nanoparticles have not been reported. Therefore, it was imperative to investigate Nirich NiCr alloy nanoparticles and their structural and magnetic properties under such conditions, especially considering that the mixing of atoms is expected to be much faster at the nanoscale level compared to that in the bulk.14,16 The time scale of such processes is within the range and resolution capabilities of atomistic modeling methods; therefore we coupled our experimental studies with computational investigations in order to analyze and explain the resultant structures and their properties.

2. EXPERIMENTAL AND THEORETICAL METHODOLOGY Experimental Method. NiCr alloy nanoparticles were prepared using a modified magnetron-sputtering system (Mantis Deposition Ltd. UK),21 as schematically illustrated in Supporting Information Figure S1. A Ni95Cr5 alloy target was used in order to get nanoparticles with well-defined compositions. Since the magnetic properties of the deposits were expected to depend on the density of the nanoparticles, the deposition time was adjusted at around 3 min for low-coverage deposits (LCDs, i.e. monodispersed nanoparticles) and around 60 min for high-coverage deposits (HCDs, i.e. interconnected nanoparticles), respectively. Subsequently, in order to perform a comparative study, NiCr nanogranular thin films were also grown using a dc magnetronsputtering gun located inside the main deposition chamber. After substrate landing, nanoparticle-loaded Si (100) substrates were loadlock transferred to an inert gas (N2) glovebox (maintained at oxygen
0) and p (>3/2) are semiempirical fit parameters and β is the critical exponent of the order parameter. Fitting data are summarized in Table 1. The TC of as-grown alloy Table 1. Parameters (s, p, β, and TC) Used in Fitting of Equation 2 to the Normalized Magnetization vs Temperature Curves of Ferromagnetic Ordered Samples sample

s

p

β

TC (K)

Ni95Cr5 bulk target Ni reference16 NiCr thin film NiCr alloy nanoparticles

0.15 0.15 0.55 0.67

1.7 2.5 3.55 2.56

0.46 0.33 0.48 0.55

341 ± 1 631 496 ± 9 474 ± 11

nanoparticles was determined around 474 K, well above both the Tblock (105 K) and TC (351 K) of the bulk Ni95Cr5 values. A similar feature can also be observed for NiCr nanogranular thin films, with an even higher TC (496 K). Once more, both findings are evidence of at least partial compositional changes (i.e., Cr segregation) occurring during the gas-phase synthesis of NiCr alloy nanoparticles and nanogranular films, even in the as-grown state, with respect to the parent bulk alloy. The critical exponent β for as-grown NiCr alloy nanoparticles is almost similar to that for the thin film and Ni95Cr5 bulk value of ∼1/2, a theoretical mean-field value for the long-range interacting ferromagnetic system. An increase in the shape parameter s (0.67), compared to that of the bulk system (0.15), can be ascribed to the competing exchange interactions between Ni−Cr and Cr-segregate domains. In order to understand the effect of annealing on as-grown NiCr alloy nanoparticles, M−T curves were measured at high temperatures (300 K ≤ T ≤ 700 K) under a vacuum better than 10−6 mbar at fixed field of 1 kOe, as shown in Figure 3b for the full heating−cooling cycle. The maximum temperature was 700 K, and the heating/cooling rate 12 K/min. Marked differences in behavior of the M−T curves can be observed for the heating and cooling stages. Upon heating, a smooth, almost linear, slow decrease of the magnetization can be observed in the broad temperature interval between 300 and 600 K, showcasing a typical behavior of elemental segregation of nanoalloys which lack a well-defined composition. As derived from eq 2, an alloy of a fixed composition would have had a steeper M−T curve, resembling more the broken red line curve shown in Figure 3b, with an average Curie temperature ( TC) of 470 K. However, with increasing temperature, gradual growth of progressively richer Ni segregates possessing a significantly higher TC than TC offsets the curve, causing magnetization to retain a high value even above TC. After heating to 700 K, during the cooling stage, the TC of the nanoalloys is found to be roughly equal to that of pure Ni

Figure 3. (a) Normalized magnetization curves vs temperature (5 K ≤ T ≤ 400 K) (under FC condition, H = 1 kOe) for as-grown NiCr alloy nanoparticles. For comparison, NiCr thin film and Ni95Cr5 bulk data are also presented in the same panel. Nonzero magnetization values of NiCr alloy nanoparticles and film above Ni95Cr5 bulk TC (351 K) demonstrate partial Cr segregation in the gas-phase synthesis of the NiCr nanoalloys, even at the as-grown state. Extrapolated TC values deduced from the empirical formula M(T) = M(0)[1 − s(T/TC)3/2 − (1 − s)(T/TC)p]β are in the range of 471−496 K. (b) Magnetization vs temperature curves (300 K ≤ T ≤ 700 K) for as-grown NiCr alloy nanoparticles during heating and cooling cycle. The almost linear M− T curve in the heating cycle indicates progressive Cr segregation. Magnetization vanishes around 600 K, well above the predicted TC values for as-grown nanoparticles of fixed composition (red-dot fitted line). After heating up to 700 K, a complete Cr segregation can be observed along the cooling cycle, which is a typical M−T behavior of bulk Ni, with a TC value of 635 K (red solid fitted line).

strong intercluster magnetic exchange interactions dominate over superparamagnetism. However, it is evident that as-grown nanoparticles retain their magnetization even at temperatures well above the TC value of the bulk Ni95Cr5 target (351 K). In an ideal noninteracting system, magnetization would be expected to drop to zero around Tblock (∼105 K). Even at the presence of strong intercluster interactions, magnetization should not persist beyond the bulk Ni95Cr5 TC value. Since even a small doping of Cr, which is antiferromagnetic, would greatly affect the ferromagnetic matrix of Ni, this unexpected result clearly indicates that the SRO of the nanoparticles (i.e., the distribution of Cr atoms in the nearest-neighbor shells of Ni E

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Chemistry of Materials (635 K), as shown by the red line curve of Figure 3b, a fit of eq 2 with parameter values s = 0.15 and β = 0.8. More importantly, the magnetization curve during the cooling stage stands above that of the heating stage, indicating that heating up to 700 K was evidently sufficient, not just for recrystallization of the NiCr alloy nanoparticles but, mainly, for the complete segregation of Cr. The magnetic properties of the annealed sample can, therefore, be attributed solely to Ni. Structural Analysis of Annealed Nanoparticles. To make a connection between structural changes and this unusual magnetic behavior, in situ TEM annealing experiments were performed under vacuum (∼10−6 Torr) at 550 °C. EDX analysis shows that the average composition pre- and postannealing remained unaffected, at Ni ∼ 95% and Cr ∼ 5%. Morphology and compositional changes of the alloy nanoparticles were recorded before (Figure 4a,c−e) and after annealing (Figure 4b,f−h). The average size of the nanoparticles increased from 5 to 11 nm due to coalescence of neighboring particles, as shown inside the red rectangles in Figure 4a,b. Most of the as-grown alloy nanoparticles show a Ni core and a Ni−Cr mixed satellite shell (Figure 4d,e). In

contrast, annealed nanoparticles show a Ni core and a Cr satellite shell only (Figure 4g,h). The observed surface oxidations (Figure 4c,f) are present only because of air exposure of the sample during transfer to the TEM chamber. Thus, even in nanoparticles of increased size due to coalescence, it is the rearrangement of the Cr atoms on the surface which is responsible for such drastic changes in magnetic behavior, reflected in uncompensated TC values and highly nonsaturated M−H loops compared to Ni95Cr5 bulk, as mentioned earlier. The presence of Cr satellites is further confirmed by the fact that there is no exchange shift in the FC M−H loops (Supporting Information Figure S4) below the Néel temperature of the antiferromagnetic Cr (or Cr2O3, if any) shells. This means that no complete Cr shell exists; otherwise a ∼1−2 nm thick antiferromagnetic shell would be enough to produce some exchange effect. Simulations of Nanoparticle Annealing and Structural Analysis. In parallel with our experimental analysis, atomistic MD simulations were employed in order to elucidate the segregation mechanism upon annealing. Figure 5 depicts exemplary initial and final configurations of a 3 nm Ni95Cr5 nanoparticle, before and after 200 ns of thermal aging at 600 K. The first noteworthy feature is that the nanoparticle changes shape toward that of a truncated octahedron (TO) with lowindex facets (Figure 5a). In doing so, it minimizes its potential energy, removing many of its dangling bonds. The atoms in Figure 5a are colored irrespectively of their species, but according to their coordination number; the darker the color of an atom, the less it is bonded to its neighboring atoms. It is clear that the final configuration has acquired a much lighter color, with many surface atoms becoming yellow, i.e., increasing their coordination number toward values expected for lowindex facets of fcc structures.34 Despite the shape change, there is no structural change in the nanoparticle, as shown in Figure 5b, where the atoms are depicted smaller in comparison to Figure 5a so that a view through the body of the nanoparticle is possible. In this figure, the color of the atoms represents the crystal structure. In agreement with our aforementioned experimental results, all atoms retained their original fcc character, depicted in green. The surface atoms are evaluated as not belonging to any system, due to their dangling bonds. Their percentage remained practically unchanged after annealing. Most importantly, in Figure 5c (which is identical to Figure 5a, except now Ni and Cr atoms are represented by red and blue spheres, respectively) some indication of partial Cr precipitation is evident. Due to the anisotropy of the newly formed near-TO structure, nonuniform segregation occurred, with Cr atoms preferentially clustering on vertex and edge sites, as previously shown for other bimetallic systems.36,37 As confirmed by Figure 5b, due to their small numbers, the Cr atoms did not reorganize themselves in a bcc arrangement, but rather remained epitaxially aligned to the fcc Ni matrix underneath them. To explain the lack of low-index fcc Cr surfaces, we calculated surface energies for both species and found that Cr low-index fcc surfaces would be energetically expensive (by ∼0.2−0.4 eV/atom, depending on facet orientation). For a more detailed analysis on nonuniform segregation, the interested reader can refer to refs 38 and 39. In all MD simulations, Cr segregation was investigated through the evolution of the SRO parameters of the nanoparticles. Indeed, SROs with initial zero values for all five nearest-neighbor shells slowly obtained positive values. An

Figure 4. Representative STEM images of NiCr alloy nanoparticles taken before (a) and after annealing (b). These images show the size evolution of the NiCr alloy nanoparticles due to coalescence during annealing at 550 °C for 120 min in vacuum. EELS elemental maps for O K edge, Ni L3,2 edge, and Cr L3,2 edges of representative nanoparticles taken before (c−e) and after (f−h) annealing at 550 °C in vacuum show the as-grown nanoparticles having both a Ni-rich core with a Ni−Cr−O shell and the annealed nanoparticles with a Ni core and a Cr−O shell. F

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Figure 5. Structural analysis of a 3 nm Ni95Cr5 nanoparticle after annealing at 600 K. Shape change and low-index faceting (a), no crystallographic phase transformation (b), and partial precipitation of Cr atoms (c) were observed. Visualization was performed using Ovito2.35

Figure 6. Evolution of the SRO parameter and the total potential energy of a Ni95Cr5 nanoparticle, after 200 ns at 1200 K. Insets depict initial and final configurations: Ni and Cr atoms are depicted in red and blue, respectively. Pronounced yet partial Cr segregation and precipitation are driven by potential energy minimization and evident through SRO increase.

example of such evolution for a 7 nm Ni95Cr5 nanoparticle after annealing at 800 K for ∼30 ns is shown in Supporting Information Figure S5. Despite clearly increasing tendencies of the SROs, however, their rates were too slow to allow for full segregation within time scales attainable with MD at temperatures directly comparable to the experimental ones. To overcome this limitation, we investigated Cr segregation at elevated temperatures. The validity of such a treatment was based on SRO results at low temperatures and validated with MMC, as will be shown later.

The SRO parameter of a 3 nm Ni95Cr5 nanoparticle at 1200 K is plotted in Figure 6, along with the total potential energy of the nanoparticle. For clarity, the SRO of only one nearestneighbor shell is depicted; the rest showed exactly identical behavior. The insets depict the initial and final configurations. More pronounced Cr segregation is evident than in the 600 K case of Figure 5, as more thermal energy was available for the Cr atoms to overcome diffusion barriers and reach the surface, where they could cluster together. Due to the elevated temperature, faceting is not clear in this case; however, G

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potential energy of a 7 nm Ni95Cr5 nanoparticle as an example. As shown in Figure 7b, the nanoparticles were initially constructed with the Cr atoms randomly distributed. The simulation was set to calculate the potential energy of the nanoparticles at each step and switch the type of two Ni and Cr atoms every 20 steps. The final state, which shows clear Ni/Cr phase segregation, is depicted in Figure 7d, in agreement with our experimental findings shown in Figure 4. A striking feature is that very early on in this process Cr atoms were first exchanged to the surface and formed a core− shell like structure (see Figure 7c) before forming a single cluster. This caused a fast dramatic reduction in the potential energy, as shown in Figure 7a. To confirm this, we calculated the energy gain by a single Cr atom surfacing on various lowindex facets of Ni matrices reproduced by the interatomic potential in use. At all cases, the potential energy of the system dropped between ∼0.3 and 0.5 eV. Hence, it can be deduced that it is energetically favorable for the Cr atoms to stay on the surface. This is graphically demonstrated in Figure 8b, where a potential energy map of the nanoparticle pre- and postsegregation is shown (for comparison, Figure 8a contains an elemental map of the system). One can detect the position of the Cr satellite in the final configuration just by observing the cyan atoms. However, despite Cr atoms having higher potential energies than the bulk Ni atoms (dark blue), these energies decreased from their original values, depicted mainly in yellow in Figure 8b. This potential energy minimization can be associated with the different crystallographic systems Ni and Cr belong to (i.e., the number of bonds each atoms wants to create), and is the main driving force behind the segregation. Considering that both atomic species involved in this study are roughly equal in size, no substantial strain fields usually accompanying binary systems with atoms of significant radii differences were expected. Indeed, we calculated the bond lengths of both species pre- and post-segregation and found only infinitesimal changes, which could hardly justify preferential segregation due to nanoparticle internal strain. Nevertheless, local stress calculations40 performed for both species showed that internal stress was actually released upon

structural analysis showed that the nanoparticle retained its fcc crystalline structure. Since a shape conversion did not occur, potential energy minimization is solely associated with Cr segregation, as can be assessed by the complementarity of the Epot and SRO graphs. However, even at 1200 K, full Cr segregation proved to be beyond the capability of MD, since, typically, phase segregation as a result of diffusion has a time scale of microseconds. Higher temperature simulations ended up melting the nanoparticles and were, thus, discarded. Therefore, to prolong the simulation time scale, we utilized MMC. Running MMC allows one to optimize the structure in order to sample the free energy ground state at a given temperature more efficiently than in MD. We simulated Ni95Cr5 nanoparticles 4, 7, and 9 nm in diameter at 600 K. Within this range, their behavior was found to be independent of size; Figure 7a shows the evolution of the

Figure 7. (a) Evolution of potential energy of a 7 nm Ni95Cr5 nanoparticle: inner box, potential energy at very beginning of simulation; outer box, potential energy over whole simulation. Positions with labels correspond to three states indicated by nanoparticle snapshots: (b) initial state, Cr atoms (blue) randomly distributed in Ni (red) matrix; (c) intermediate state, Cr atoms migrated onto the surface after 4 × 103 steps; (d) final state, NiCr nanoparticles fully segregated into Cr and Ni phases.

Figure 8. (a) 7 nm Ni95Cr5 nanoparticle before and after Cr segregation. (b) Potential energy map of same particle: Cr atoms, initially identified as high-energy yellow spheres, clustered on the surface reducing their potential energy and, thus, appear cyan in the final configuration. (c) Stress tensor map of same particle. A comparison of the stress tensor values of diluted and surface-precipitated Cr atoms clearly illustrates that Cr segregation also led to a substantial reduction of stress. H

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Cr segregation, and could have acted as a secondary driving force behind the segregation. For the nanoparticle depicted in Figures 7 and 8, this is analytically shown in Supporting Information Figure S6, which shows the distribution of stress tensor RR, pointing to the center of the nanoparticle, for Ni and Cr atoms separately, for the initial and final configurations of the MMC simulation run. Clearly, whereas the stress tensor distribution of Ni atoms remained essentially unaffected by the segregation process, that of Cr atoms shifted toward lower values, with its center repositioning to zero, indicating stress release. This is also graphically illustrated in Figure 8c, where a color mapping of the stress tensor RR, shown for Cr atoms alone, clearly indicates a marked reduction of stress after Cr surface segregation and precipitation.

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S6 showing a schematic diagram of the nanoparticles deposition system, size histograms, AFM images, FC− ZFC loops, and MD and MC simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS In this study, we investigated the close correlation between structural and magnetic properties of technologically important Ni-rich NiCr alloy nanoparticles. More specifically, we demonstrated the occurrence of Cr surface segregation and precipitation into Cr satellites in gas phase synthesized NiCr alloy nanoparticles and nanogranular films in the as-grown state, as well as upon annealing, both experimentally and theoretically. We performed a number of magnetic measurements on our samples, both as-grown and after annealing, such as low- (5−400 K) and high-temperature (300−700 K) magnetization (M−T) measurements along with magnetic interaction analysis, and detected a deterioration of their magnetic properties. Two possible explanations were put forward, with Cr surface segregation actually providing a satisfactory description to the experimental findings. Direct in situ TEM and EDX analysis confirmed this phenomenon. Classical MD and MMC simulations were utilized to clarify the segregation mechanism. MD provided a valid shape and structural description of the nanoalloys and, through SRO analysis, confirmed the tendency of segregation. A Ni-core/Crsatellite configuration was identified through MMC as a lowenergy and -strain state of the system, and the segregation was assessed as mainly energetically driven, since it resulted in overall potential energy minimization.

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AUTHOR INFORMATION

Corresponding Authors

*(M.B.) E-mail: [email protected]. *(M.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the Okinawa Institute of Science and Technology Graduate University. We are also grateful to the Finnish IT Centre for Science CSC and the Finnish Grid Infrastructure (FGI) for grants of computer time. I

DOI: 10.1021/acs.chemmater.5b00837 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.5b00837 Chem. Mater. XXXX, XXX, XXX−XXX