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Chemical Synthesis and Characterization of γ‑Co2NiGa Nanoparticles with a Very High Curie Temperature Changhai Wang,† Aleksandr A. Levin,† Lucia Nasi,‡ Simone Fabbrici,‡,§ Jinfeng Qian,† Carlos E. Viol Barbosa,† Siham Ouardi,† Julie Karel,† Franca Albertini,‡ Borrmann Horst,† Gerhard H. Fecher,† and Claudia Felser*,† †

Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany Institute of Materials for Electronics and Magnetism, IMEM-CNR, I-43124 Parma, Italy § MIST E-R Laboratory, Via Gobetti 101, I-40129 Bologna, Italy ‡

S Supporting Information *

ABSTRACT: We report the first study on the chemical preparation, structural characterization, and magnetic properties of Co2NiGa nanoparticles (NPs) of a single γ-phase (γCo2NiGa). The γ-Co2NiGa NPs were prepared by impregnation in colloidal silica followed by high temperature annealing under hydrogen atmosphere. The crystal structure of tetragonal γ-Co2NiGa was confirmed by selected area electron and X-ray diffraction studies. Energy-dispersive X-ray spectroscopy mapping and X-ray absorption near edge structure data provided evidence for the successful preparation of the intermetallic NPs and the absence of Co, Ni, Co−Ni, or metalcontaining impurity phases. Extended X-ray absorption fine structure spectroscopy data confirmed the formation of the γCo2NiGa phase by examining the atomic environments surrounding Co, Ni, and Ga. The Co2NiGa NPs are ferromagnetic with a high saturation magnetization, which is consistent with the theoretical model. γ-Co2NiGa NPs exhibit a very high Curie temperature (≈1139 K), which make them promising candidates for high temperature magnetically activated nanoscale devices.

1. INTRODUCTION

The synthesis of single-phase Co2NiGa compounds is challenging. This is reflected by the fact that most of the Co2NiGa alloys investigated to date are dual-phase. The successful preparation of single β-phase Co2NiGa alloys requires precise controls of composition and heat treatment conditions6,8,12−16 and suitable synthetic methods.17−19 In particular, suitable synthetic methods such as the bottom-up chemical preparations need to be developed. The synthesis, structure, and magnetic properties of single γ-phase Co2NiGa, however, have not been extensively investigated. To our knowledge, only one study has reported the synthesis, crystallographic, and magnetic properties of single γ-phase Co2NiGa.20 In that investigation, nanoscale single γ-phase Co2NiGa compounds were obtained by ball-milling and appropriate annealing. The crystal structure of the γ-phase was identified as the tetragonal phase (space group P4/mmm), instead of the face-centered cubic (fcc) A1 structure generally reported in the literature.9,10,17,20−23 Two factors might be accounted for to explain the discrepancy in defining the crystal structure of the γ-phase Co2NiGa. First, single γ-phase Co2NiGa samples were not successfully prepared prior to the

Ferromagnetic shape memory alloys (FSMAs) represent novel materials systems with great potential for future smart materials and device applications.1 Among all Heusler compound-based FSMAs, Ni2MnGa has been the most intensively studied, showing promising performance for magnetic actuation.2 However, the disadvantages of Ni2MnGa compounds such as poor machining workability,3,4 low shape memory transformation, and Curie temperatures5,6 hinder their practical applications. Since the early 2000s, Co2NiGa Heusler compounds have been extensively investigated as potential alternatives to Ni2MnGa,7,8 especially for high temperature smart device applications.7,9 These Co2NiGa compounds frequently exhibit dual-phase structures that consist of a parent (β) phase and a second (γ) phase. The β-phase has a B2 ordered cubic structure and exhibits the martensitic transformation and therefore the shape memory properties. The γphase is not martensite-transformable and in general does not contribute to the shape memory performance. On the other hand, the presence of the γ-phase modifies the ductility and workability of Co2NiGa compounds.10,11 Furthermore, the dual-phase Co2NiGa compounds exhibit high saturation magnetization and high Curie temperatures,5,6,9 which might be attributed to the γ -phase. © XXXX American Chemical Society

Received: June 13, 2015 Revised: October 8, 2015

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DOI: 10.1021/acs.chemmater.5b02227 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials work of Dai et al.20 Second, previous experimental investigations on dual-phase Co2NiGa were mostly concentrated on their phase equilibrium, martensitic transformations, and properties. Therefore, less effort has been made to perform the structure refinement analysis of the XRD data of the γphase. Most of the presented XRD data lack professional Rietveld analysis.9,10,17,20−23 Recently, interest in the development of nanoscale shape memory materials has been stimulated by the trend toward downsizing of devices based on the shape memory effect.22−28 Controlling particle size represents an important avenue through which the phase transition and magnetic properties of shape memory materials can be tailored. The understanding of the structural and magnetic properties of Co2NiGa nanoparticles (NPs) is extremely important in terms of both fundamental science and practical applications. There are, however, only a few studies on Co2NiGa Heusler shape memory NPs.29,30 Size effect on the shape memory effect presents an important issue in nanoscale shape memory alloys. With decreasing particle size, more and more atoms are located at the surfaces. Therefore, the surface effect comes into play and is anticipated to play a decisive role in preventing martensite transformation.29 Other important issues for Heusler shape memory NPs include whether there is a critical size for martensitic transformation29 and the role of shape memory treatment.30 In this work, we develop the first bottomup approach for preparing the γ-Co2NiGa NPs using colloidal silica as a template. We found that the crystal structure of the γCo2NiGa NPs was consistent with that for bulk γ-Co2NiGa compounds. The element-specific short-range order of the nanoparticles was investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy, which confirmed the Xray diffraction (XRD) results. The chemically synthesized γCo2NiGa NPs demonstrate robust ferromagnetism with a very high saturation magnetization and a high Curie temperature.

measurements of the powder sample mixed with a Si 640c powder standard (National Institute of Standards and Technology, Gatihersburg, MD, USA). The XRD measurements of the powder samples were carried out using a low background Si (119) substrate. The contribution of the substrate was corrected by subtracting the XRD pattern of the substrate measured with high counting statistics. According to profile-type criteria (the ratio of the full width at halfmaximum (fwhm) and integral breadths (Bint)), the obtained fwhm/ Bint ratio was 0.69(2) (averaged for the observed reflections).32 Therefore, the observed XRD reflections were attributed to a pseudoVoigt profile type.32 An appropriate correction for instrumental broadening according to the profile type33 was carried out. The absence of a strain contribution to the XRD line broadening was confirmed by the Williamson−Hall plot technique modified for the pseudo-Voigt reflections.34 The average size of the γ-Co2NiGa was derived from the Scherrer’s equation using the fwhm of the reflections (corrected for instrumental broadening) and the Scherrer coefficient, CScherrer = 0.94. All steps of the reflection profile broadening analysis were carried out by means of the program SIZECR.35 The Rietveld fits of the recorded XRD patterns were performed by means of the Rietveld program WinCSD36 using a structural model of γ-Co2NiGa (Inorganic Crystal Structure Database (ICSD) No. 15778820) and the weighting scheme wi = 1/yi, where yi is the recorded intensity at step i. To exclude the contribution of the amorphous silica phase, the 2θ range was cut starting from 23°. The amorphous phase contribution at higher 2θ angles was modeled by a background function. The overall temperature factors Biso overall of the atoms were refined (Biso overall = 1.3(1) Å2). The estimated standard deviations (esds) of the refined structural parameters (e.g., unit cell parameters, Biso overall, and so on), which were underestimated due to serial correlations, were corrected by means of Berrar’s formalism37 implemented in WinCSD. The corresponding coefficient, mesd = 3.71, for the esd corrections and agreement factors were checked with the program, Riet_esd.38 2.3. X-ray Absorption Fine Structure Analysis. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were performed at the XAFS beamline of Elettra Sincrotrone Trieste (Elettra, Trieste, Italy) and at the beamline 17C1 of the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan). The γ-Co2NiGa/silica nanopowders were pressed into pellets. The Co, Ni, and Ga K-edge spectra of the γ-Co2NiGa NPs were collected using the transmission mode. The XANES and EXAFS data analyses were performed using the IFEFFIT program package.39,40 The EXAFS spectra χ(k) were extracted using the AUTOBK program.41 The ATOMS program42 was used to prepare the structural input for FEFF643 and IFEFFIT. 2.4. Chemical Analysis. The chemical composition of the γCo2NiGa NPs and the metal weight fractions were determined via inductively coupled plasma optical emission spectrometer (ICP-OES, VISTA, Varian Inc.). The composition of γ-Co2NiGa NPs was determined as Co2.00Ni1.05Ga1.13 with a surplus of Ga. The weight fraction of Co−Ni−Ga was 6.20 wt %, which was determined by a summation of the mass of Co−Ni−Ga divided by the overall mass of the Co−Ni−Ga/silica composite. 2.5. Transmission Electron Microscopy. A Tecnai 10 transmission electron microscope (TEM, FEI, Eindhoven, The Netherlands) equipped with a LaB6 source at 100 kV acceleration voltage was used for the investigation of particle morphology and size distribution. Images were recorded with a F224HD 2k slow-scan CCD camera (Tietz Video and Image Processing Systems, Gauting, Germany). High resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) were carried out with a JEOL 2200FS field emission gun TEM operating at 200 kV. Images were recorded with a Gatan Ultrascan CCD. Quantitative EDS analysis was performed applying the Cliff−Lorimer method to the thin specimen. The silica-supported γ-Co2NiGa NPs were etched by NaOH to remove the silica and to enable a clear observation of the individual NPs. The etching was performed by mixing the Co2NiGa/ silica nanopowder (10 mg) in NaOH (2.7 mL, 4 M) and aging for 24

2. EXPERIMENTAL SECTION 2.1. Chemicals and Synthesis. All chemicals were purchased from Sigma-Aldrich or Alfa Aesar and used as received. The silica support was a commercially available colloidal silica (Ludox-LS). The precursors for the Co2NiGa NPs were CoCl2·6H2O (99.9%), Ni(NO3)2·6H2O (99.999%), and Ga(NO3)3·xH2O (99.9%). The value of x in Ga(NO3)3·xH2O was set to 8 for calculations based on the literature.31 In a typical preparation of γ-Co2NiGa NPs, CoCl2· 6H2O (0.117 g, 0.4912 mmol), Ni(NO3)2·6H2O (0.075 g, 0.2579 mmol), and Ga(NO3)3·xH2O (0.116 g, 0.2900 mmol) were dispersed in methanol (46 mL) and sonicated for 10 min. Subsequently, colloidal silica (4 mL) was added to the precursor solution and the suspension was sonicated for another 20 min. The methanol was removed using a rotary evaporator, and the resultant solid was dried completely at 353 K for approximately 8 h. The resultant powder (200 mg) was then heated under H2 atmosphere at 5 K·min−1 rate, to 1123 K, and held at this temperature for 2 h. After preparation, the samples were contained in glass vials and stored under vacuum condition to prevent oxidation and hydrolization. 2.2. X-ray Diffraction. The crystal structure of the γ-Co2NiGa NPs was investigated by the powder XRD technique. An X’Pert PRO diffractometer (PANalytical B.V., Almelo, The Netherlands) designed in the Bragg−Brentano geometry and supplied with a solid state X’Celerator linear detector (measuring window of 1 deg/step) was used. Cu Kα1 (λ = 1.540598 Å) radiation monochromatized by a primary Ge (111) Johansson-type monochromator was utilized. To minimize any preferred orientation effects, the XRD sample was prepared in an ultrasonic ethanol bath, dried at a substrate, and rotated around the sample holder axis during the measurements. A Δ2θzero shift correction of the XRD pattern was obtained by additional XRD B


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Chemistry of Materials h. Free-standing γ-Co2NiGa NPs were collected by repeated centrifugation and washing until the supernatant became neutral (five times). Suspensions of the NaOH-etched NPs were used for sample preparation. Several drops of a suspension were loaded on a carbon-coated copper grid and transferred to the microscope after being completely dried. The average particle size was evaluated by counting more than 100 individual particles. 2.6. Magnetic Measurements. The field-dependent isothermal magnetization curves were measured at 2 and 300 K in fields up to 7 T in order to investigate the magnetic properties of the γ-Co2NiGa NPs. These measurements were performed using the commercial Quantum Design MPMS-XL-7 and MPMS-3 magnetometers (Quantum Design, Inc., San Diego, CA, USA). In all of these magnetic measurements, silica-supported samples were used, considering the diamagnetic nature of the silica in the measured temperature range.44 We adopted a simple slope subtraction procedure to remove the contribution of the diamagnetic silica. The actual magnetization of the γ-Co2NiGa NPs was calculated based on the weight fraction of Co−Ni−Ga obtained by chemical analysis. It was assumed that all of the metallic precursor salts were completely reduced and converted to Co−Ni−Ga intermetallic compounds. The γ-Co2NiGa NPs were fixed in quartz with molten paraffin wax to avoid unwanted sample movement during the measurements. The Curie temperature of the γ-Co2NiGa NPs was determined by thermomagnetic analysis (TMA), which entails measurements of the temperature dependence of the initial susceptibility. The experiments were performed by means of a home-built alternating current (AC) susceptometer at a frequency of 500 Hz, over a wide temperature range (up to 1273 K) under an applied field of 1 mT. The typical heating rate was 3−5 K·min−1. The γ-Co2NiGa/silica nanopowders were pressed into pellets to reduce their volume.

Figure 1. Schematic illustration of the chemical synthesis of γCo2NiGa nanoparticles using colloidal silica.

3. RESULTS AND DISCUSSION 3.1. Chemical Synthesis. The formation of γ-Co2NiGa nanoparticles might involve several steps including loading the metal precursors within colloidal silica solution, solvent removal, drying, and high temperature reduction. After solvent removal, the metal loaded silica opals are condensed and the silica particles formed pores of specific dimension and morphology. The metal precursors accommodated in such interparticle voids are treated by high temperature annealing under H2 atmosphere to form nanoparticles. Free-standing γCo2NiGa nanoparticles can be obtained by removing the silica supports using NaOH etching. A schematic illustration of the chemical synthesis of γ-Co2NiGa nanoparticles using colloidal silica is shown in Figure 1. 3.2. Transmission Electron Microscopy. Figure 2 shows a representative TEM micrograph and size distribution plot of the γ-Co2NiGa NPs. The average particle size is ∼80 nm with a reasonable dispersion. It is worth noting that some smaller NPs (approximately 5−25 nm) are also observed. This is consistent with Co2FeGa Heusler NPs synthesized by a similar approach, where a bimodal particle size distribution was observed.45 Silica residues can be observed in Figure 2a due to incomplete etching by NaOH. The crystal structure of single Co2NiGa NPs was investigated by SAED and fast Fourier transformation (FFT) of the HRTEM images. Both the techniques showed the same results, that is, the γ-phase nature of the nanoparticles. The nanoparticles were crystalline with the tetragonal γCo2NiGa structure (space group P4/mmm); an example is shown in Figure 3a. As revealed in Figure 3b, the EDS composition mapping confirms the formation of homogeneous Co−Ni−Ga NPs with an average composition of Co (58 at. %)−Ni (26 at. %)−Ga (16 at. %). A detailed EDS investigation highlighted variations in the composition in the range of Co (53−61 at. %)−Ni (24−28 at. %)−Ga (14−19 at. %). This

Figure 2. TEM micrograph (a) and particle size distribution (b) of the γ-Co2NiGa nanoparticles.

compositional range is different from that obtained by chemical analysis: Co (50 at. %)−Ni (26 at. %)−Ga (28 at. %). The discrepancy might be due to a variation in the elemental sensitivity of the different analytical methods and the variables introduced by sample etching. Furthermore, the statistically significant EDS-derived compositional analysis allows us to exclude the presence of Co, Ni, Co−Ni, or other impurity phases (e.g., metallic silicides), either as clusters or as individual nanoparticles. C


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Chemistry of Materials

Figure 4. Crystal structure of the γ-Co2NiGa nanoparticles: (a) experimental (Iobsd) and Rietveld-simulated (Icalcd) XRD patterns; (b) Williamson−Hall plot (Kstrain = 4). The reflection Miller indices hkl in panel a follow those for the tetragonal crystalline γ-phase of bulk Co2NiGa. The central-angle 2θ positions of all possible Bragg reflections according to the refined unit cell parameters of the γCo2NiGa phase are indicated by bars. The difference curve (Iobsd − Icalcd) is shown at the bottom in panel a. The solid line in panel b is the result of a linear fit y = 0.0058 − 5.9425 × 10−7x, resulting in the relation s2 = −5.9(2.9) × 10−7; i.e., s = 0 in frames of two esds.

Figure 3. HRTEM micrograph (a) of one γ-Co2NiGa nanoparticle in the (110) zone axis with the FFT in the inset and the EDS composition maps (b) of the γ-Co2NiGa nanoparticles.

3.3. Crystal Structure. Figure 4a presents the experimental XRD pattern of the γ-Co2NiGa NPs together with the Rietveld refinement fitting. The XRD pattern exhibits an amorphous halo due to amorphous silica and XRD reflections of the crystalline γ-Co2NiGa phase. The unit cell parameters of the crystalline phase were derived using Rietveld fitting. According to the phase analysis and Rietveld fitting, a single γ-Co2NiGa phase can be identified. The γ-phase has a slightly distorted tetragonal structure (space group no. 123, P/4mmm, a = 3.5800 Å, c = 3.5800 Å, and ICSD no. 15778820) with refined lattice parameters of a = 3.5844(4) Å and c = 3.5753(8) Å. The presence and identification of the γ-phase is confirmed by the good quality of the Rietveld fitting of the XRD patterns. This is reflected in the small values of the fitting agreement factors (weighted profile factor Rwp = 0.79% and the corrected-tobackground weighted profile factor cRwp = 11.63%) and the structural Bragg factor RB (2.94%). The crystal structure of the γ-phase is consistent with the structure of the bulk γ-Co2NiGa compounds.20 As discussed in the Introduction, it is even not likely to differentiate the tetragonal from the fcc without Rietveld analysis, due to a slight tetragonal distortion and close lattice constants of the two structures. The tetragonal distortion from cubic Heusler compounds is important for modifying their magnetic and electronic properties.46,47 As an example, for Mn2-based Heusler compounds, the tetragonal distortion from fcc might correspond to the Jahn−Teller-type electronic instabilities compounds that are required for materials with applications in spin-transfer torque (STT)-based logic devices and spin-torque oscillators (STO).47 In accordance with the literature,20 the crystal structure of the γ-phase has been identified as a tetragonal phase (P4/ mmm) with Pt2FeCu as the prototype material. The weak

superimposed fingerprint reflections with Miller indices hkl = 100/001, 110/011, and 211/112, etc., which are indicative of Ni−Ga ordering, are not observed, suggesting Ni−Ga site disorder in the γ-Co2NiGa NPs (see Figure S3b of the Supporting Information (SI)). As shown in Figure 4b, the absence of a strain contribution to the reflection profile broadening is confirmed by the observation of a slightly negative slope (close to zero in frames of two esds) of the calculated Williamson−Hall plot for reflections of the pseudoVoigt type.37 Taking into account the absence of a strain contribution to the XRD reflection profile broadening, the calculated crystallite size is 27(4) nm. This indicates that the nanoparticles are composed of roughly three nanocrystallites. 3.4. X-ray Near Edge Structure. The short-range-order structure of the γ-Co2NiGa NPs was probed by the XAFS technique aiming to confirm the single γ-Co2NiGa phase and to validate the success of this new chemical preparation of γCo2NiGa NPs. The valence states of the metallic components were investigated by XANES spectroscopy. Figure 5 shows the XANES spectra of the γ-Co2NiGa NPs at the K-edges of Co, Ni, and Ga. As shown in Figure 5a,b, similar to the reference Co and Ni foils, edge jumps and oscillations typical of metallic XANES patterns are observed at the Co and Ni edges. The derived Co and Ni absorption edge positions of the nanoparticles are almost identical to those of the references within experimental error. This verifies the metallic nature of the γ-Co2NiGa NPs. Therefore, the presence of significant quantities of Co/Ni oxides can be excluded, which is consistent D


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with the EDS results. The absorption edge energies are much lower than those for the precursor metal ions. This provides evidence that the Co- and Ni-containing precursor salts were completely reduced to the metallic states during high temperature annealing under H2 atmosphere. As shown in Figure 5c, at the Ga edge, the oscillation and resonance features of the nanoparticles are distinctively different from those for the Ga oxide and nitrate. This indicates a different atomic environment for the Ga absorbers in the γ-Co2NiGa NPs and verifies the reduction of the Ga ions. The edge position of the nanoparticles is 10371.8 eV. This value is significantly lower than those for Ga2O3 (10380.3 eV) and Ga(NO3)3 (10380.8 eV) but is slightly larger than that for the zerovalent Ga (10367 eV).48 The errors in the absorption edge position data shown in the insets of Figure 5 were estimated by independent measurements conducted under the same conditions at Elettra and NSRRC. 3.5. Extended X-ray Absorption Fine Structure. Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful technique for determining the structural ordering in Co-based Heusler compounds and nanoparticles.49−51 In this work, the EXAFS fitting and analysis were conducted to unambiguously verify the single-phase structure of the γ-Co2NiGa NPs. The EXAFS fittings were performed assuming the structural model derived from the Rietveld analysis. The EXAFS fitting was carried out for the first coordination shell in the intermetallic distance R-range of 1−3 Å. The fitted k-ranges were 3.3−14.4 Å−1 for Ni/Ga and 3.3− 11.9 Å−1 for Co (due to the limited EXAFS range), respectively. The multiple k-weighting was utilized in order to achieve similar resolutions for comparison. The fitted structural parameters determined from the analyses of the EXAFS signals are compiled in Table 1. The Fourier transforms of the spectra at the K-edges of Co, Ni, and Ga, together with the fits, are shown in Figure 6. At the Co edge, a high degree of structure matching between the theoretical model and experimental data is achieved (see Figure 6b). The general agreement of the data and the model is supported by the extremely small RF-factor value, RF = 0.001. The fitted values of the energy shift (E0) and bond length (R) are physically reasonable. The passive electron reduction factors (S02) are between 0.8 and 0.9. The thermal mean square variation in the bond length (Δσ2) for different paths are 0.002−0.017. According to the calculated structure model, the first coordination shell surrounding Co is composed of two closely located subshells. The first comprises the nearest neighbors (four Ni atoms and four Ga atoms) with equal

Figure 5. XANES data for the γ-Co2NiGa nanoparticles at the absorption edges of Co (a), Ni (b), and Ga (c). The data for reference and metallic precursors are also plotted for comparison. The insets are the absorption edge positions for the measured samples and references, which were determined by the first inflection point of the corresponding absorption curves.

Table 1. Fitting Parameters and Structural Parameters of the γ-Co2NiGa Nanoparticles Determined from the Analyses of EXAFS Signals at the K-Edges of Co, Ni, and Gaa edge

coordination

Rcalcd (Å)

R (Å)

Δσ2 (Å2)

S02

ΔE0 (eV)

RF

Co

Ni × 4 Ga × 4 Co × 4 Co × 8 Ga × 4 Co × 8 Ni × 4

2.531 2.531 2.535 2.531 2.535 2.531 2.535

2.47(3) 2.57(3) 2.39(3) 2.51(1) 2.50(6) 2.56(2) 2.51 (1)

0.002(1) 0.003(1) 0.017(1) 0.007(6) 0.005(2) 0.015 (2) 0.005(1)

0.90 0.80 0.80 1.03 1.00 0.80 0.80

6.73 6.73 −6.73 7.55 −7.55 7.62 −7.62

0.001

Ni Ga

0.003 0.009

a

The calculated coordination shells and interatomic distances are also presented. Rcalcd refers to the bond length obtained by theoretical calculations with the Rietveld software WinCsd. R refers to the bond length obtained in EXAFS fitting, and Δσ2 is the fitted thermal mean square variation in the bond length. S02 refers to the passive electron reduction factor. ΔE0 refers to the energy shift. RF refers to the quality of fittings. Figures in parentheses denote uncertainty in the last digit. The fits are in the R-range of 1.0−3.0 Å. E


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Figure 6. EXAFS data (a, c, and e) and magnitude of FT spectra and fits (b, d, and f) of EXAFS spectra of the γ-Co2NiGa nanoparticles at the K-edges of Co, Ni, and Ga, respectively. The fitted R-space ranges are indicated by the solid lines in panels b and d−f). The spectra shown in panels b, d, and f are not corrected for phase shifts. Data: circles.

Figure 7. Magnetic properties of the γ-Co2NiGa nanoparticles: (a) hysteresis curves at 2 and 300 K; (b) thermomagnetic curve in the range of 300−1255 K. The inset in panel a shows an enlarged view of the M−H curve at low field. The Tc is indicated by an arrow in panel b.

interatomic distances of 2.531 Å. The other consists of four Co atoms that are slightly further away at 2.535 Å. The quality of the fits using the structure model of fcc Co is also very good (RF = 0.001), but with a slightly smaller S02 of 0.646. For fcc Co, the first coordination shell comprises 12 Co atoms with Co−Co distances of 2.506 Å. Therefore, the Co EXAFS alone is insufficient to verify the structure of γ-Co2NiGa NPs. We then examined the EXAFS signals at the Ni and Ga edges. As shown in Figure 6d, the Fourier transform (FT) spectra at the Ni K-edge are similar to that of the Co edge due to the similarities in the backscattering amplitudes and phase shifts of Co and Ni. For γ-Co2NiGa, the nearest neighbors around Ni are eight Co atoms, and the next nearest neighbors are four Ga atoms with the same interatomic distances as for the case of the Co edge. As described in Figure 6f, the EXAFS probe of the atomic environment of Ga, with a reasonably good profile fitting in the first coordination shell, provides additional evidence for the formation of a well-ordered intermetallic Co2NiGa phase. The RF values for the EXAFS fittings at the Ni and Ga edges are also small: 0.003 and 0.009, respectively. The high degree of structural matching between the theoretical model and the measured EXAFS signals is also supported by the correlation of the interatomic distances obtained at multiple edges. For example, the lengths of the Co−Ga bonds derived at the Co and Ga edges are almost identical. A similar observation is found for the Ni−Ga bonds. In short, the formation of a wellordered γ-Co2NiGa phase is confirmed by analyzing the EXAFS spectra at the K-edges of Co, Ni, and Ga. 3.6. Magnetic Properties. Figure 7a shows the fielddependent magnetization (M−H) curves of the γ-Co2NiGa NPs measured at 2 and 300 K. At both temperatures, the nanoparticles are ferromagnetic. The saturation magnetizations (Ms) are 60.72 and 55.03 A·m2/kg at 2 and 300 K, respectively.

The observed Ms values are slightly lower than the theoretical value (67.8 A·m2/kg or 2.99 μB at 5 K)20 for perfectly ordered Pt2FeCu-type bulk γ-Co2NiGa. This is partly attributed to Ni− Ga disorder as revealed by the XRD analysis. In addition, reduced surface magnetization could also play a role. As shown in the inset of Figure 7a, the measured μoHc are 29.9 and 22.8 mT at 2 and 300 K, respectively. Compared to the bulk material, reduced Ms and enhanced coercivity (μoHc) were observed for the ferromagnetic Heusler nanoparticles.52−54 Therefore, within experimental error, the M−H data for the γCo2NiGa NPs provide evidence of the formation of the γCo2NiGa phase. The Curie temperature (Tc) of the γ-Co2NiGa NPs was evaluated by temperature-dependent AC susceptibility measurements with an applied AC field of 1 mT. As shown in Figure 7b, a significant drop in magnetic susceptibility with an inflection point around 1139 K is observed, which can be attributed to a ferromagnetic−paramagnetic transition. The observed Tc is much higher than that reported for the γ-phase of dual-phase Co50Ni20Ga30 ribbons (≈950 K).9 To our knowledge, this is the highest Tc among all known Heusler compounds. Before this work, the highest reported Tc was 1100 K for Co2FeSi Heusler compounds.55 The highly stable ferromagnetism of the γ-Co2NiGa NPs, coupled with their high Ms, makes them ideal candidates for nanoscale magnetically activated devices operating at high temperature. Also note that there is a broad susceptibility drop centered around 800 K, which probably can be ascribed to another structural or magnetic phase transition. As discussed earlier, from the room temperature (RT) XRD, the γ-Co2NiGa NPs exhibit a singlephase structure. Therefore, a phase transition is probably F


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4. CONCLUSION We report the first chemical preparation, structural, and magnetic properties of 80 nm γ-Co2NiGa nanoparticles. The synthesis is facile, low cost, and easily scalable. The SAED and Rieltveld fitting of the measured XRD pattern confirmed the same tetragonal structure as that for bulk γ-Co2NiGa. XANES and EXAFS data revealed the successful preparation of impurity-free γ-Co2NiGa nanoparticles. The γ-Co2NiGa nanoparticles exhibited a high saturation magnetization and a very high Curie temperature. High temperature XRD data reveal the high thermostability of γ-Co2NiGa nanoparticles and exclude the presence of a phase transition at elevated temperature. Therefore, besides the role of enhancing ductility, the γ-phase also contributes significantly to the magnetic properties of Co2NiGa compounds. Furthermore, γ-Co2NiGa nanoparticles could be interesting candidates for nanoscale magnetic devices working under high operating temperature conditions.

induced by heating during the TMA measurement and might also be associated with the thermal stability of the γ-Co2NiGa phase. We have performed additional temperature-dependent XRD measurements from RT up to 1173 K to check the possible phase transition and high temperature stability of the tetragonal γ-Co2NiGa nanoparticles. As shown in Figure 8, the tetragonal

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02227. Description of the experimental setup and results of the temperature-dependent XRD measurements (PDF)

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Figure 8. XRD patterns of the γ-Co2NiGa nanoparticles measured in the temperature range of 298−1173 K. The upper 298 K data were collected after cooling. The Miller indices hkl of γ-Co2NiGa reflections observed are shown. The XRD patterns are shifted vertically for better visualization. The vertical dashed lines are guides for the eyes.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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structure of γ-Co2NiGa nanoparticles was unchanged even after heating to 1173 K and cooling to RT. This unambiguously reveals the absence of a crystalline phase transition of γCo2NiGa NPs in this temperature range. Therefore, the increase in magnetization after 900 K and the high Curie temperature in Figure 7b are unambiguously due to the tetragonal γ-Co2NiGa phase. It was also found that the heating and cooling procedures also resulted in interesting variations in the Ni/Ga ordering, unit cell parameters, and crystallite size of the tetragonal γ-Co2NiGa NPs (see the detailed discussions in the SI), which will be the focus of following investigations. It is worth noting that fcc Co has the highest known Curie temperature of around 1360 K, and Co50Ni20 alloys also have high Curie temperatures of about 1270 K.10 Therefore, the possible formation of Co-rich phases such as Co and Co50Ni20 in the γ-Co2NiGa NPs must be considered. This possibility, or lack thereof, was evaluated by means of the EDS, XRD, and XANES analyses, as described previously. According to ICSD, the lattice parameters of the fcc Co and Co−Ni cubic phases are distinctively smaller than those obtained by the Rietveld refinement analysis for the γ-Co2NiGa nanoparticles. In addition, the absence of a hexagonal ε-phase indicates the absence of the Co50Ni20 phase.9,56 The XANES spectra of the γCo2NiGa nanoparticles (see Figure 4a,b) are distinct from those for the Co and Ni references. This excludes the presence of Co or binary Co−Ni phases in the γ-Co2NiGa NPs. EXAFS analyses at the K-edges of Co, Ni, and Ga also provide evidence for the absence of Co and Co−Ni phases and confirm the presence of the ternary γ-Co2NiGa.

ACKNOWLEDGMENTS We acknowledge financial supports by the German Research Foundation (DFG) under the Project of TP 2.3-A in research unit FOR 1464 "ASPIMATT" and the ERC Advanced Grant (291472 Idea Heusler). We are also grateful to Dr. Guido Kreiner (MPI-CPfS) and Dr. Peter Höhn (MPI-CPfS) for sample preparation and annealing experiments; Prof. Andreas Hütten (Department of Physics, Bielefeld University) for stimulating discussions; Dr. Reiner Ramlau and Ms. Uta Köhler (MPI-CPfS) for TEM support; Dr. Gudrun Auffermann (MPICPfS) for the chemical analysis; Mr. Ralf Koban and Dr. Walter Schnelle for kind help with sample preparation and magnetic measurements (MPMS); and Dr. Luca Olivi (Elletra Sincrotrone Trieste) for stimulating discussion and kind help with the XAFS experiments. The XAFS measurements were performed at the Eletrra Sincrotrone Trieste (Trieste, Italy) under the approval of Proposal No. 20140471 and at the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan) under the approval of Proposal No. 2013-2027-4.

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