L10 Ordering of Ultrasmall FePt Nanoparticles Revealed by TEM In

Article pubs.acs.org/JPCC

L10 Ordering of Ultrasmall FePt Nanoparticles Revealed by TEM In Situ Annealing Michael̈ Delalande,†,‡ Maxime J.-F. Guinel,*,§ Lawrence F. Allard,∥ Anastasia Delattre,†,‡ Rémy Le Bris,† Yves Samson,‡ Pascale Bayle-Guillemaud,⊥ and Peter Reiss*,† †

INAC/SPrAM (UMR 5819 CEA-CNRS-UJF)/LEMOH, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble, France INAC/SP2M/NM, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble, France § Departments of Physics and Chemistry, Faculty of Natural Sciences, University of Puerto Rico, P.O. Box 70377, San Juan, Puerto Rico 00936-8377 ∥ Materials Science and Tech. Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ INAC/SP2M/LEMMA, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble, France ‡

ABSTRACT: The transformation of ≤4 nm equiatomic FePt nanoparticles from the disordered cubic A1 to the ordered tetragonal L10 phase was studied by means of high-resolution transmission electron microscopy coupled with in situ heating experiments. In accordance with ex situ annealing experiments, a transition temperature of around 500 °C was determined. Diffusion is enhanced at surfaces and plays a dominant role in the ordering process. Hence, the ordering of the crystallographic structure starts at the surface of the nanoparticles and propagates toward their center, resulting in complete ordering within some minutes, for temperatures above 600 °C. Unlike the generally assumed lower limit of ordering (3.5 nm), we demonstrate that ultrasmall (less than 3 nm) FePt nanoparticles can also be fully transformed to the L10 phase. The well-controlled and precise stoichiometry (Fe50 at.%Pt50 at.%) and the homogeneous composition of these particles both play a major role in their successful phase transformation.

1. INTRODUCTION Higher data transfer speeds and lower energy consumption drive the thrust to develop the next-generation data storage media with tremendously higher recording densities. Selfassembled monolayers of FePt nanoparticles (NPs) in their L10 ordered phase are promising candidates for these new devices with recording densities capable of exceeding 1 Tbit/in.2.1 In the L10 ordered phase, Fe and Pt atoms alternate in layers along the c direction of the tetragonal unit cell. This structure results in a very high uniaxial magnetocrystalline anisotropy along their c direction (7 × 106 J·m−3), large enough to thermally stabilize the magnetization of NPs as small as 3.5 nm. Therefore, assembled in an array within discrete storage media and oriented in the right direction, each of these particles could, in principle, be used to store a single data bit. Triggered by the seminal work of Sun and co-workers,2 the chemical synthesis of FePt NPs has been extensively studied for the past few years.3 Unfortunately, the synthesis yields chemically disordered FePt NPs in the face-centered cubic A1 phase. Thermal annealing is, therefore, generally applied to obtain the high-anisotropy L10 phase. The A1 to L10 phase transformation requires NPs with a composition close to Fe50 at.%Pt50 at.% because it is only possible for FexPt100−x NPs with x in the range of 40−68. In contrast, Fe-depleted particles (from Fe20Pt80 to Fe40Pt60) will generate the ordered L12 phase, characterized by stacks of pure Pt © 2012 American Chemical Society

atomic planes and mixed Fe−Pt atomic planes, which show poor magnetic properties. Our previous study has shown that NPs prepared with oleic acid and oleylamine as surfactants generally display an Fe-depleted core with an Fe-rich shell.4 In contrast, replacing oleylamine with pentadecanenitrile, which binds more strongly to Pt atoms and thus reduces its tendency for nucleation, leads to FePt NPs with a homogeneous radial composition.5 This study focuses on near-equiatomic FePt NPs synthesized using pentadecanenitrile as the stabilizer for Pt instead of oleylamine.5 The NPs display a very well controlled size and stoichiometry, homogeneous internal structure, and radial composition. Straightforward annealing of FePt NPs in powder form, typically carried out at more than 600 °C, induces the decomposition of the surface ligands, coalescence, and sintering of the NPs.6−8 These effects are further enhanced by the higher magnetic moment of the particles once the transformation has occurred. To get a better insight into the ordering mechanism, we studied the L10 ordering process of FePt NPs using in situ annealing experiments in the (scanning) transmission electron microscope (STEM or TEM). Similar studies have been carried out so far only on Fe/Pt multilayers9 Received: January 2, 2012 Revised: February 29, 2012 Published: March 6, 2012 6866

dx.doi.org/10.1021/jp300037r | J. Phys. Chem. C 2012, 116, 6866−6872

The Journal of Physical Chemistry C

Article

and FePt NPs grown by e-beam evaporation,10 for which the formation of L10-FePt nuclei starts by the diffusion of Fe into the Pt layers. Finally, we investigated the possibility of achieving the A1−L10 phase transisiton in sub-3-nm FePt NPs, which approach the lower size limit for chemical ordering determined for epitaxially grown particles.11

3. RESULTS AND DISCUSSION Figure 1 shows a bright-field HRTEM image of the assynthesized NPs; they are in the A1 phase before annealing.

2. EXPERIMENTAL SECTION 2.1. Synthesis. FePt NPs with a mean size of 4 nm, a size distribution of approximately 10%, and close to equimolar stoichiometry were synthesized following our previously reported procedure using pentadecanenitrile and oleic acid as stabilizing ligands.5 2.2. Characterization. The size of the NPs and the overall lattice parameters were obtained at room temperature by powder X-ray diffraction (XRD) in the reflection geometry mode (Philips X′PERT) using the Co Kα radiation (1.789 Å). To identify the suitable parameters for the in situ experiments, ex situ annealing treatments for 1 h under vacuum (∼10−6 mbar) at various temperatures were carried out on a thin layer of FePt NPs deposited on a silicon substrate. The samples were prepared by casting the NPs, dispersed in toluene, on a silicon wafer by letting the solvent evaporate. The lattice parameters, crystallite size, and long-range chemical order were calculated by means of Rietveld refinement (Fullprof software). The elemental composition was determined using X-ray energydispersive spectrometry (EDS) in scanning electron microscopes (SEM) and TEMs. The magnetic properties of the NPs were measured using a superconducting quantum interference device (Quantum Design DC). The ordering process was investigated in situ using two different high-resolution imaging modes. Aberration-corrected high-angle annular dark-field (HAADF) images were recorded using a JEOL JEM-2200FS STEM-TEM instrument equipped with a CEOS corrector on the illuminating lenses. Nanoparticle samples for these experiments were heated in situ using the Aduro heating system manufactured by Protochips, Inc. (Raleigh, NC). The Aduro technology allows for heating and cooling at rates reaching 106 °C/s. Similar experiments (high-resolution BF images) were performed using a JEOL JEM-3010 HRTEM and a JEOL specimen holder allowing heating up to 800 °C. The Aduro design is different and allows for a better control of the temperature and the stability of the sample at high temperature (i.e., limited drift). For each experiment, a drop of the diluted FePt NP suspension was deposited onto a disposable device (for the Aduro system) or onto a 30 nm thick carbon film supported on a copper grid (for the JEOL holder). This carbon film had to be thick enough to resist the high temperature and drift, which resulted in lower image quality. Further analyses were done using an FEI-Titan microscope equipped with a probe corrector and an EDS detector. The chemical ordering of the NPs was followed in situ, usually characterized by the appearance of the {001} ordering contrast that is only visible if the particles are oriented along specific zone axes. Indeed, chemical ordering (26Fe and 78Pt) gives rise to stacks of Fe and Pt planes with less and more contrast in STEM mode, respectively. Particle coalescence may occur during in situ annealing experiments and was carefully controlled by employing very diluted suspensions. The additional contribution from the electron-beam irradiation was not considered because sample exposure was kept at a strict minimum during annealing.

Figure 1. Bright-field HRTEM image of as-synthesized FePt NPs. Their average diameter is 4.0 nm.

They exhibit a mean size of 4.0 nm and a size distribution of 10%. EDS measurements on numerous zones of the sample show that the NPs have a nearly stoichiometric composition (Fe50 at.%Pt50 at.%). This is in accordance with the results obtained from XRD (a = 3.87 Å) and Végard’s law.12 The results are summarized in Table 1. Table 1. Composition and Structural Characteristics of the As-Prepared FePt NPsa global composition (EDS) at. % Fe

average diameter (nm) and dispersion (%)

crystallite size from XRD (nm)

lattice constant (Å)

49 ± 5

4.0 10%

3.7 ± 0.1

3.8674 ± 0.0016

a

Diameter obtained from TEM images; crystallite size and cell parameters determined from Rietveld refinement of the diffractograms.

The ex situ annealing experiments conducted at 650 °C resulted in the complete phase transformation of the NPs, as evidenced by XRD and magnetic data. The presence of superstructure diffraction peaks in the diffractograms and the opened hysteresis loop observed at room temperature are shown in Figures 2 and 3, respectively. This behavior is consistent with the large anisotropy of the ferromagnetic L10 phase. The obtained results are summarized in Table 2. Ex situ annealing experiments were also carried out at lower temperatures. The recorded X-ray diffractograms for 1 h annealing performed at 400, 450, and 500 °C are reported in Figure 4. The phase transformation starts at temperatures around 450 °C. At this temperature, the size determined with the Scherrer formula from the superstructure peak widths is smaller than that from the fundamental peaks (respectively, 2.8 and 5 nm), suggesting that only a fraction of the NPs have been ordered. Also, the narrowing of the initial XRD peaks indicates 6867



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Figure 2. X-ray diffractograms of the FePt NPs recorded before and after ex situ annealing at 650 °C for 1 h (red line: Rietveld refinement). The appearance of superstructure diffraction peaks (∗) and the splitting of the diffraction peaks (200), (220), and (311) demonstrate the presence of the L10 phase of lower symmetry.

Figure 4. X-ray diffractograms of as-synthesized and ex situ annealed FePt NPs at 400, 450, and 500 °C (annealing time: 1 h). The appearance of superstructure diffraction peaks (∗) indicates the start of the phase transformation at temperatures as low as 450 °C.

between the FePt NPs during the heat treatment.13,14 Figure 5 shows a HAADF-STEM image of an array of FePt NPs

Figure 3. Room-temperature magnetic hysteresis loops of FePt NPs before and after ex situ annealing at 650 °C for 1 h. While the assynthesized NPs are superparamagnetic, they exhibit an opened hysteresis loop after annealing at 650 °C, consistent with the large anisotropy ferromagnetic L10 phase.

Table 2. Properties of the FePt NPs after Ex Situ Annealing at 650°C for 1 h lattice constant (Å)

crystallite size (nm)

order parameter S

coercive field at 300 K/kOe

a= 3.8525 ± 0.0012 c= 3.7245 ± 0.0018

6.3 ± 0.4

0.95 ± 0.05

7.6

Figure 5. HAADF-STEM image of ex situ annealed FePt NPs.

annealed ex situ at 700 °C for 8 h in such a salt matrix. After treatment with cysteine, the NPs form stable colloidal solutions with no detected sign of agglomeration. The image shows no sintering effects, and some particles display the L10 phase characteristic contrast between their successive Fe and Pt atomic planes. On the basis of these ex situ studies, all in situ heating experiments were conducted above 450 °C. In a typical experiment using the JEOL JEM-3010 HRTEM and the JEOL sample holder, the following heating cycle was applied: quickly heating to 550 °C, remaining at 550 °C for 50 min, increasing to 600 °C, remaining at 600 °C for several minutes, and eventually increasing to 650 °C. Note that the slow response of

that sintering occurs. Such coalescence of the NPs is of course highly detrimental for their potential application in data storage media as it results in a larger mean size and a broad size and shape distribution. On the other hand, sintering can be avoided by increasing the distance between the NPs. This can be achieved, for example, by dispersing them on the surface of micronic salt (NaCl) particles, providing physical separation 6868



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Figure 6. Series of TEM images of individual FePt NPs recorded during in situ heating (JEOL 3010). Images (a)−(c) were recorded at a temperature of 550 °C, after 7, 17, and 35 min, respectively. The {001} planes of the L10 phase start to appear on the surface of the particles at the onset of the ordering before propagating to the entire volume. Image (d) was recorded after 5 min at 600 °C (after prior treatment at 550 °C), the annealing resulting in near total ordering of the particle, but with persistent antiphase boundary. After a temperature increase to 650 °C and maintaining for 20 min, fully ordered NPs are obtained (e) (image taken after cooling down to room temperature).

Figure 7. HAADF-STEM images obtained on a JEOL JEM-2200FS microscope. (a) Partially ordered FePt NP after 24 min heating at 400 °C, 1 min at 500 °C, and 55 min at 400 °C. (b) Fully ordered NP.

this holder does not allow for fast heating and cooling rates and that there are comparably large instabilities and drifts. As can be seen from Figure 6, after only a few minutes at 550 °C, the {001} planes characteristic of the L10 phase start to appear at

the surface of some NPs, which are oriented along an appropriate zone axis. The onset of the reordering is illustrated in Figure 6a−c with images recorded after 7, 17, and 35 min, respectively. The ordering starts at the surfaces and propagates 6869



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Figure 8. HAADF-STEM image of a twinned NP after heating at 650 °C for 80 min (a) and of a NP showing multiple defects recorded after heating at 400 °C for 24 min, 500 °C for 1 min, 400 °C for 82 min, and 500 °C for 35 min (b).

7b shows a HAADF-STEM image recorded at room temperature of a fully ordered NP that was kept at 800 °C for 20 min, where successive Fe and Pt atomic planes are clearly visible. The cell parameters were measured to be a = 3.85 Å and c = 3.71 Å (L10). These values are in accordance with powder Xray diffraction data (Table 2). The presence of crystallographic defects reduces the degree of chemical order and, therefore, the magnetic anisotropy. Among the NPs suitably oriented for HR imaging, only a very small fraction of defective particles was observed. An example is shown in Figure 8a, where a HAADF-STEM image of a twinned NP was recorded after in situ heating to 650 °C for 80 min. A grain boundary splits the particle in half, but both parts are fully ordered. In very rare cases, NPs can present a combination of several defects, such as the one illustrated in Figure 8b; this particle was subjected to the following cycle: 400 °C for 24 min, 500 °C for 1 min, 400 °C for 82 min, and 500 °C for 35 min. The upper portion of the NP ordered to the L10 phase, while the lower part may be either in the A1 or in the L10 phase, and several grain boundaries can be identified. Again, the ordering appears to start from the surface. It is difficult to quantitatively assess the proportion of chemically ordered NPs using high-resolution imaging because the orientation of the particle examined, its morphology, and the possible existence of defects are critical when determining order.18 Only a limited set of low index zone axes can be used to unambiguously determine whether the NPs are in the L10 or the A1 phase. In the former case, they will display the characteristic alternating layers of Fe and Pt atoms. A large number of particles must, therefore, be examined to make quantitative assessments. Finally, we report on ultrasmall FePt NPs, with sizes reaching below 3 nm. In fact, there exists a critical size for the L10 phase transformation to be triggered. Miyazaki et al. showed, using electron diffraction on FePt NPs epitaxially grown on MgO, that, for sizes below 1.5−2.0 nm, no ordering occurs.11 Furthermore, the order parameter was shown to drop sharply at a particle size below 3.0 nm and decreases to zero for NPs smaller than 2 nm in diameter. Rong et al. reported a significantly lower order parameter (0.62) and magnetocrystal-

toward the center of the particles. The proportion of ordered NPs naturally increases with the annealing time, though some NPs were observed to remain only partially ordered after 50 min at 550 °C. Fully ordered NPs were observed at 600 °C, as shown in Figure 6d. We noticed that structural defects, such as antiphase boundaries, can be present in very small NPs (≈3 nm) even after annealing at 600 °C. Total chemical order was achieved after annealing at higher temperatures, such as in the NP shown in Figure 6e, which was held at 650 °C for 20 min. Changing imaging conditions, such as the focusing of the image, does not affect our observations. In bulk materials, phase transformations are generally initiated by the formation of small nuclei of the new phase at crystalline defects, such as grain boundaries. In a single-crystal NP, the surface is the main source of such defects. Moreover, the mobility of atoms at or near the NP surface is greatly enhanced thanks to their liquidlike properties, which, therefore, facilitates diffusion and formation of L10 nuclei.15 We also point out that in no case has an outermost layer of pure Pt (“Pt-skin”) been observed in our experiments. Its formation has been reported in studies of the catalytic activity of first-row transition-metal (e.g., Ni, Co, Fe) alloys of platinum used for the oxygen reduction reaction in fuel cells.16,17 In particular, Malheiro et al. obtained 3−4 nm FePt NPs with a Pt-skin after annealing 2 nm particles at 550 °C under hydrogen.17 However, these NPs exhibit a stoichiometry of 75:25 (Pt/Fe), which could explain their increased tendency for the formation of a Pt surface layer as compared to the NPs in our study. A second set of in situ annealing experiments was performed using the JEOL JEM-2200FS microscope with an Aduro heating system (in operation at ORNL). In the HAADF-STEM mode, direct evidence of chemical ordering can be obtained from the difference in intensity, or Z-contrast (26Fe and 78Pt), between two successive planes of atoms along the ⟨001⟩ direction. Figure 7a shows a HAADF-STEM image recorded at 400 °C, of a partially ordered NP that was successively subjected to heating for 24 min at 400 °C, then 1 min at 500 °C, and finally 55 min at 400 °C. The lower right portion of the particle does not show ordering. This observation is in perfect agreement with the results discussed above (Figure 6). Figure 6870



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Figure 9. HAADF-STEM images of three fully ordered

L10 Ordering of Ultrasmall FePt Nanoparticles Revealed by TEM In

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