A Phase-Solution Annealing Strategy to Control the Cobalt

Article pubs.acs.org/JPCC

A Phase-Solution Annealing Strategy to Control the Cobalt Nanocrystal Anisotropy: Structural and Magnetic Investigations Zhijie Yang,† Manon Cavalier,† Michael Walls,‡ Pierre Bonville,§ Isabelle Lisiecki,†,* and Marie-Paule Pileni†,* †

Laboratoire des Matériaux Mésoscopiques et Nanométriques, UMR CNRS 7070, Université P. et M. Curie, Bât F, 4 Place Jussieu, 75005 Paris, France ‡ Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, Bât. 510, 91405 Orsay Cedex, France § CEA, Centre de Saclay, DSM/IRAMIS/Service de Physique de l’Etat Condensé, 91191 Gif-sur-Yvette, France ABSTRACT: Here, we report a phase-solution annealinginduced structural transition of 7 nm-Co nanocrystals from the fcc polycrystalline phase to the hcp single-crystalline phase. For any annealing temperature, contrary to what was down in our previous paper (Langmuir 2011, 27, 5014), the same solvent (octyl ether) is used preventing any change in adsorbates related to various solvents on the nanocrystal surface. A careful transmission electron microscopy study, combined with the electron diffraction, confirms the nanocrystal recrystallization mechanism. The annealing process results in neither coalescence nor oxidation. The converted nanocrystals can be easily manipulated and due to their low size dispersion self-organize on an amorphous-carbon-coated grid. Magnetic property investigations, keeping the same nanocrystal environment, show that the structural transition is accompanied by a significant increase in both the blocking temperature (to a near room-temperature value) and the coercivity.

I. INTRODUCTION The organization of identical passivated nanometer-scale magnetic particles into micrometer-scale ordered arrays shows considerable potential in regard to applications such as ultrahigh density recording.1,2 However, with decreasing particle size, we come into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle.3−6 To overcome this problem, the use of hard magnetic nanomaterials is required. Because of its high magnetic moment per nanocrystal, (higher than FePt) hcpCo is a good candidate. Up to now, and despite intensive research into the growth of hcp-Co nanocrystals, most of the experiments have produced nanocrystals characterized by anisotropic shapes such as disks,7 or rods.8,9 Spherical cobalt nanocrystals can be synthesized with three different crystal structures, ε,7,10−15 face-centered cubic, (fcc),16−18 and a mixture of fcc-hcp16 either by thermally decomposing dicobalt octacarbonyl or reducing cobalt salts at room and high temperature. ε-Co is the most frequently obtained phase. Synthesis of hcp-phase cobalt nanoparticles is not as yet satisfactory, as many fcc stacking faults form. Most of the colloidal chemical approaches used to synthesize the various Co crystal structures involve either injection of reagents into hot surfactant solution followed by aging at high temperature, or the mixing of reagents at a low temperature, with or without slow heating, under controlled conditions. The maximum temperature using such an approach is around 200 °C. However, it is likely that the formation of the pure © 2012 American Chemical Society

hexagonal phase requires higher temperatures. One strategy for obtaining the magnetically hard hcp phase is to recrystallize the magnetically soft Co nanocrystals through a postsynthesis thermal treatment. The process is not trivial, as in many systems (e.g., FePt, CoPt, and Co),15,19−22 it leads to an undesirable nanocrystal coalescence and/or oxidation. In 2007, we reported the first example of the dry-phase heating induced synthesis of long-range 2D and 3D supracrystals made with hcp Co nanocrystal building blocks.23,24 In 2011, we initiated the solution-phase annealing for converting the particle structure from Co polycrystal to Co-hcp single-crystalline phase.25 The nanocrystals were dispersed in various solvents differing by their boiling points. In order to get rid of a possible solvent effect on the nanomaterial annealing process, we propose in this paper to exploit this strategy but with the use of a unique solvent, i.e., octyl ether. In this way, we show that the nanoparticles with tunable crystallographic structure can be freely manipulated and, once deposited on an amorphous carbon substrate, self-organize into 2D and thin 3D regular structures. Because of the use of a unique solvent, we clearly evidence the key role of the van der Waals interactions in the self-organization process. In the first part of this paper, we give the structural investigation of the different crystallinities obtained under different conditions of temperature and we Received: April 3, 2012 Revised: June 14, 2012 Published: June 25, 2012 15723

dx.doi.org/10.1021/jp303182n | J. Phys. Chem. C 2012, 116, 15723−15730

The Journal of Physical Chemistry C

Article

σ = {[∑ ni(Di − D)2 ]/[n − 1]}1/2

also study the effect of aging time. In the second part, we follow the effect of Co crystallinity on the magnetic behavior. A significant increase in both the blocking temperature and the coercivity is evidenced when Co polycrystals convert to hcp single crystals.

Here n corresponds to the number of measured particles and D to the average diameter.

III. RESULTS AND DISCUSSION III.1. Annealing Process for 7 nm fcc Co Polycrystals. The resulting particles are annealed at a temperature ranging between 190 and 250 °C. The solvent used is octyl ether, whose boiling point is 286 °C. A 2 mL sample of the 10−2 M colloidal solution resulting from the synthesis is first evaporated to remove the original solvent (hexane). Then, the powder is redispersed in 5 mL of octyl ether. The annealing treatment takes place in a refluxing bath. This is a four-necked flask allowing a nitrogen flux, the control of the solution temperature, the injection and the withdrawing of the solution with a syringe in order to avoid any oxidation of cobalt and the refluxing of the solvent during the heating step. The solution is heated to the required temperature with a heating rate of 10 °C per minute up to 140 °C then more slowly with a heating rate of 2 °C per minute. Once the temperature is reached, either the solution is cooled down for few minutes in the flask or it is maintained at the temperature for a given aging time and then cooled down. The cooling down occurs under stirring. Then, using the syringe, the sample is removed from the flask and placed inside a glovebox under an N2 flux. 0.8 mL of 5 × 10−3 M of dodecanoic acid is added in order to avoid aggregation of the nanocrystals. The solution is left overnight and then subjected to cleaning cycles via suspension in ethanol and centrifugation twice to ensure a complete removal of octyl ether and dodecanoic acid in excess. Finally, the nanocrystals are dispersed in 0.5 mL of hexane. The annealing treatment and all the other steps occur under an N2 flux. We stress that in order to study the exact influence of the aging process at 220 °C, a first sample milliliter) is taken as soon as the required temperature is reached while the rest of the colloidal solution is aged. In this way, the same populations of Co nanocrystals undergo the two annealing treatments and their behaviors can be rigorously compared. III.2. Structural Investigation. III.2. 1. As-Synthesized Co Nanocrystals. The cobalt nanocrystals in the native state are characterized by a mean diameter and size polydispersity of 7.2 nm and 10% respectively and are stabilized by a coating of dodecanoic acid chains.16 The TEM image (Figure 1a) of a sample obtained by deposition of two droplets (20 μL) of the colloidal solution ([nanocrystals] = 10−2 mol L−1) on TEM grids covered with amorphous carbon shows that nanocrystals 2D long-range self-organize into a hexagonal compact network. The corresponding electron diffraction pattern (Figure 1b) shows two diffuse diffraction rings at 2.04 and 1.24 Å (±0.04 Å) which could be indexed as the (111) and (220) spacings of fcc cobalt. The HRTEM pattern reveals the existence of crystallized domains less than 1 nm in size (Figure 2a). This clearly indicates that these native nanocrystals are very poorly crystallized. We call them small domain nanocrystals. In the first series of experiments, the samples are heated to the desired temperature without any aging, whereas in the second series, an aging process is involved. III.2.2. Recrystallization of Co Nanomaterial Driven by the Solution-Phase Heating Process (without Aging). When the heat treatment takes place below 190 °C, we do not observe any structural change. After heating to 190 °C, the diffraction pattern remains unchanged compared to the native state

II. EXPERIMENTAL SECTION II.1. Chemical. All materials were used without further purification: cobalt acetate, dodecanoic acid, sodium borohydride and octyl ether are from Aldrich, isooctane and hexane from Fluka, sodium di(ethylhexyl) sulfosuccinate (NaAOT) from Sigma. The synthesis of cobalt(II) bis(2-ethylhexyl) sulfosuccinate, (Co(AOT)2) was described previously.26 II.2. Apparatus. Conventional transmission electron microscopy was performed using a JEOL 1011 microscope at 100 kV. High-resolution transmission electron microscopy was performed using a JEOL 2010 microscope at 200 kV and using a Nion Ultrastem 100 scanning transmission electron microscope operating at 100 kV. The Nion Ultrastem 100 microscope is equipped with a spherical aberration corrector, which enables a probe-size of under 0.1 nm to be obtained, although the spatial resolution here was less than this due to an alignment problem. Magnetic measurements were carried out using a Cryogenics Ltd. S600 SQUID magnetometer. The zero field cooled (ZFC) magnetization versus temperature measurements were carried out by cooling the sample from 300 to 5 K in zero field then applying a field of 20 Oe and measuring the magnetization while the sample was heated from 5 to 300 K. The isothermal magnetization measurements were performed at 5 K after zero field cooling. All magnetic measurements were carried out with the field applied parallel to the substrate. II.3. Synthesis. The synthesis and characterization of Co polycrystals coated with dodecanoic acid have been described in a previous paper:16 reverse micelles of 5 × 10−2 M Co(AOT)2 form an isotropic phase. The amount of water added in solution is fixed to reach a water concentration defined as w = [H2O]/[AOT] = 32. Sodium borohydride, NaBH4, added to the micellar solution reduces the cobalt ions. The sodium borohydride content is defined as R = [NaBH4]/ [Co(AOT)2] = 8. Immediately after NaBH4 addition, the micellar solution color changes from pink to black, indicating the formation of colloidal Co nanocrystals. The nanocrystals are coated and then extracted from the surfactant. The coating process is as follows: 0.2 M lauric acid, C11H23COOH, is added to the solution containing nanocrystals, surfactants, water, and isooctane, inducing a chemical bond between the oxygen of dodecanoic acid and the Co atoms located at the interface. The coated Co nanocrystals are then washed and centrifuged several times with ethanol to remove all the AOT surfactant and the black powder obtained is dispersed in hexane. In order to eliminate the largest nanocrystals formed, the solution is again centrifuged and only the upper part containing the smallest sized nanocrystals is collected. At the end of the synthesis, ∼7 nm cobalt polycrystals coated with dodecanoic acid with a ∼10% size distribution are produced. The entire synthesis is carried out in an N2 glovebox using deoxygenated solvents to prevent particle oxidation. II.4. Histograms. The histograms of the nanocrystals are obtained by measuring the diameter Di of at least 500 nanocrystals deposited on a grid coated with amorphous carbon. The standard deviation, σ, is calculated from the experimentally determined distribution using the formula: 15724



Article

crystallized and composed of hcp crystallized domains having different orientations. Rarely and only for the smaller nanocrystals, ∼5 nm, we can observe single-crystals with regular lattice planes characterized by a 1.91 Å spacing corresponding to the hcp structure in the [002] zone axis orientation (Figure 2e). Heating to 220 °C gives rise to the appearance of 6 rings characterized by 2.23, 2.1, 1.99, 1.29, 1.20, and 1.10 Å distances corresponding respectively to the (100), (002), (101), (110), (103), and (112) planes of the Co hcp phase (Figure 1f). However, both the internal and external triplets are composed of discrete (but still somewhat diffuse) rings having a low intensity. This diffraction pattern indicates that a major structural transition is occurring. Indeed, the native small domain polycrystals no longer constitute the main population (Figure 3b). They have partly disappeared in favor of a significant proportion of larger hcp domain particles and, to a lesser extent, hcp single-crystals (Figure 3b). In this latter case, the diameter can be of the order of the average value, i.e. 7 nm. Figure 2g illustrates such a regular single-crystal with two sets of lattice planes characterized by a 1.91 Å spacing corresponding to the hcp structure in the [002] zone axis orientation. Some of these single-crystals reveal stacking faults and other defects (Figure 2f). Increasing the temperature to 250 °C and aging 1 h at this temperature makes the diffraction pattern subtly change (Figure 1h). First, both the external and internal diffuse rings become sharper and more intense compared to those observed for Ta = 220 °C (Figure 1f), second, two additional rings appear, which are characterized by 1.49 and 1.07 Å distances, corresponding to the (102) and (201) planes of the hcp Co phase. Both the appearance of these first and second order reflections and the fact that the two triplet rings become increasingly sharp indicate clearly that the coherence length of the hcp nanocrystals has increased. Additionally, the absence of a diffusive signature in the region of the internal triplet attests to the absence of the fcc polycrystalline phase which would appear to predominate before annealing. This is supported by the HRTEM images showing a majority population of hcp single-crystals (Figure 3c), some of which are characterized by twins, stacking faults and other defects at moderate or quite high densities (Figure 2f). It is noteworthy that despite the heat treatment, the Co nanocrystals keep their integrity, as neither aggregation nor coalescence nor oxidation is detected. Additionally, irrespective of the heating temperature, once deposited on amorphous carbon grid they self-organize into a 2D hexagonal network (Figure 1, parts c, e, and g). The average diameter and size distribution of these annealed nanocrystals remains unchanged compared to the native state (Table 1). As shown in Figure 4, the deposition of a slightly higher quantity of nanocrystals annealed at 220 °C and deposited on an amorphous carbon coated grid gives rise to thin 3D self-organizations composed of a few layers of nanocrystals. The heating process thus does not destroy the ability of the nanocrystals to form self-organizations either in 2D or 3D. III.2.3. Aging Effect. Let us now consider the second experimental series, which is similar to the first one, i.e., at the same temperatures and in the same solvent, except that once the temperature reached, nanocrystals are maintained in the hot solution for various aging times before the cooling step. Below 190 °C, the aging of nanocrystals proves ineffective; they keep the same polycrystalline structure as the nanocrystals in their native state. Above 190 °C, the aging favors their structural transition to the cobalt hexagonal phase. Parts a−d of Figure 5

Figure 1. TEM images of ∼7 nm cobalt nanocrystals 2D self-organized in a compact hexagonal network: native (a), solution-phase annealed without aging at 190 °C (c), 220 °C (e), and aging at 250 °C for 60 min (g); Panels b, d, f, and h are the corresponding electron diffraction (ED) patterns made over the regions shown in the TEM images. The scale bar for parts a, c, e and g is 100 nm. The internal triplet rings are referred to as [(100), (101), (002)] and external triplet rings [(110), (103), (112)].

(Figure 1d), but the HRTEM images reveal a subtle but real changes. In coexistence with the native poorly crystallized nanocrystals, which constitute the main population (Figure 3a), we observe nanoparticles composed of slightly larger domains. When these domains are large enough, ∼3 nm, the planes can be characterized by a 2.00 Å spacing consistent with the hcp phase (Figure 2b). In almost the same proportion, particles that we call very large domain polycrystals reveal two domains differing by their crystalline orientation: on one side a part showing clear lattice fringes and the other side no visible lattice planes (Figure 2, parts c and d). A structural investigation made by imaging the sample at various tilts with respect to the electron beam, reveals that such nanocrystals are in fact fully 15725



Article

Figure 2. Schematic illustration with high-resolution TEM (HRTEM) images of the evolution process of Co nanocrystals: (a) small domain Co nanocrystals, (b) hcp Co with large domains, (c and d) very large domains, (e) small hcp single-crystals, (f) hcp single-crystals with stacking faults, and (g) hcp single crystals; The scale bar is 3 nm.

show the diffraction patterns of nanocrystals heated to 220 °C, without aging and aged for 30, 90 and 180 min, respectively. The two triplet rings typical of the Co-hcp phase, which are diffuse before aging, progressively transform into fairly sharp and intense rings. This clearly indicates the disappearance of the initial cubic phase in favor of the hcp phase whose coherence length increases with the annealing time. The last statement is further confirmed by the gain of the additional (102) hcp ring observed for tag = 90 min. Up to at least 180 min, the Co nanocrystals remain highly stable against coalescence and oxidation processes. In this set of experiments, the mean diameter of the Co nanocrystals decreases gradually from 7.2 nm (native state) to 6.5 nm after heating to 220 °C for 180 min (Figure 6a). This feature may be partly explained by the structural transition from the disordered native structure to the more densely ordered hcp structure especially to the hcp single-crystalline structure. This seems to be supported by the behavior observed during the aging of the nanocrystals at 190 °C for 30 and 180 min, which is also accompanied by a decrease in the mean diameter from 7.2 to 6.8 nm. However, in the experiments conducted at Ta = 250 °C, we clearly observe a 2 step behavior (Figure 6a). First, the heating without aging leads to a size decrease from 7.2 to 6.6 nm, then the aging favors a progressive increase in size up to 7.1 nm at tag =60 min. The first step is related to the structural transition from the native polycrystals to the hcp single-crystals (Figure 3c) while the second one is attributed to Ostwald ripening. This thermodynamically driven spontaneous process that favors the shrinking of the smaller nanocrystals while larger one’s grow is consistent with the decrease in the size distribution that accompanies the increase in the average size (Table 1). Parts b and c of Figure 6 display the various interparticle distances and center-to-center particle distances for the various conditions of

annealing temperatures and aging. It appears that the mean interparticle distance decreases when the mean particle diameter increases, leaving almost invariant the center-to-center particle distance, as clearly established in Figure 6c. This finding indicates that the key interactions involved in the formation of these 2D self-organizations are of type van der Waals. III.2.4. Discussion on the Structural Investigation. From these results, it can be concluded that the two annealing strategies we propose successfully promote the structural transformation of Co nanocrystals from what are fcc polycrystals to hcp single-crystals. The thermal treatment parameters are not trivial, as many experiments have led to an undesirable nanocrystal coalescence and/or oxidation. However, in the present study, nanocrystals remain highly stable even after heating to 250 °C for 60 min, due to the high thermal stability of the dodecanoic acid coating. The structural conversion of the nanocrystals starts at a temperature of around 190 °C, below which, no aging is effective. A population mostly composed of hcp Co nanocrystals (multidomain, very large domain polycrystals and single crystals) can form either by aging the sample above 190 °C or its heating to above 220 °C. Finally, the complete conversion toward hcp single-crystals is achieved either by the longer aging processes or by heating to a temperature of 250 °C. The dry-phase annealing of similar Co nanocrystals has been described previously.24,27 2D and 3D self-organizations of Co nanocrystals were placed in a furnace at various temperatures (250, 300, and 350 °C) for 15 min in a nitrogen atmosphere. The three major differences between the two approaches are the annealing time, the fact that in one case the sample is gradually brought to the temperature while in the other the heating is abrupt, and the environment of the nanocrystals (in a solvent or not). In both cases, the structural conversion of Co 15726



Article

Figure 4. TEM image of a thin 3D superlattice of ∼7 nm cobalt nanocrystals solution-phase annealed and aged at 220 °C deposited on an amorphous-carbon-coated copper grid. The areas marked 1, 2, and 3 are related to monolayer, bilayer, and multilayers, respectively.

Figure 3. Histograms of the statistical analysis of HRTEM images of Co nanocrystals with solution-phase annealing, without aging at (a) 190 (b) and 220 °C and (c) with aging at 250 °C for 60 min.

Figure 5. Electron diffraction patterns of ∼7 nm cobalt nanocrystals solution-phase annealed at 220 °C with various aging times (a) 0 min, (b) 30 min, (c) 90 min, and (d) 180 min; Tint and Text marked on the ED pattern refer to the internal triplet rings [(100), (101), (002)] and external triplet rings [(110), (103), (112)] respectively.

Table 1. Structural and Magnetic Parameters for the Native Samples and after Solution-Phase Annealing, As Found from the TEM Images, the ZFC Magnetization Curves, and the Hysteresisa native Co NCs nanoparticle diameter (nm) (±0.2 nm) size distribution (%) interparticle distance (nm) (±0.2 nm) center to center particle distance (nm)(± 0.4 nm) Tb (K) Hc (Oe) a

Ta (190 °C), tag (0 min)

Ta (220 °C), tag (0 min)

Ta (250 °C), tag (60 min)

7.2

7.0

7.1

7.1

10 2.5

9 3.0

9 3.1

8 2.7

9.7

10.1

10.0

9.8

98 1000

167 300

187 1850

257 1800

Equally important is the flexibility of the annealing approaches we propose here. They allow the free manipulation of the asannealed nanocrystals, which is an important goal in modern materials chemistry. Once deposited on amorphous carbon substrate, these nanocrystals self-organize into long-range assemblies. From this comprehensive structural study, we can propose a mechanism of particle recrystallization from the initial fcc polycrystalline to the hcp single-crystalline phase. At the early stage of the transition, the native “small domains” whose sizes are less than 1 nm, initiate a size and structural transformation from the fcc to the hcp phase. The resulting “large-domain” particles are composed of crystals whose size is initially too small (∼2 nm) for the phase to be identified. On reaching a critical size (∼3 nm), they can be defined as hcp. Then, domain growth continues giving rise to the “very large domain” particles (∼3−5 nm), which, if not well oriented with regard to the electron beam, do not display any lattice planes. Finally, at a more advanced stage, the defects present in these polycrystals tend to disappear to give rise to hcp single-crystals.

Tb, blocking temperature and Hc coercivity.

particles to hcp single-crystals occurs, but the required temperatures are significantly lower in the solution than in the dry-phase, the structural conversion starting at around 190 °C in the first case compared with 250 °C in the second. The annealing in solution, being slower, allows more precise structural control than is possible in the dry-phase experiments. 15727



Article

III.3.1. Low Field Magnetization Measurements: Blocking Temperature. Parts a and b of Figure 7 show the Zero field-

Figure 7. (a) Zero field cooled (ZFC) magnetization versus temperature curves measured at 20 Oe for the native (black) and solution-phase annealing, without aging at 190 (blue) and 220 °C (red), and with aging at 250 °C for 60 min (purple); (b) ZFC M versus T/Tmax curves for the native (black) and solution-phase annealing, without aging at 190 (blue) and 220 °C (red) and with aging at 250 °C for 60 min (purple).

Figure 6. Plots of various samples with different solution-phase annealing conditions: (a) particle diameters, (b) interparticle distances, and (c) center-to-center particle distances.

cooled (ZFC) magnetization versus temperature curves for 3D organizations of Co nanocrystals in the native state and after heating to 190 °C (tag= 0), 220 °C, (tag= 0), and 250 °C (tag = 60 min). We stress that these samples were characterized by the same average particle size, i.e., 7 nm (±0.2 nm) and almost the same size distribution (∼9%, Table 1). In Figure 7b, the curves are normalized to the magnetization of the ZFC peak. For the ZFC curves, the sample has been cooled in zero field and, at 5 K, there is no alignment of the moments and the magnetization is close to zero. As the temperature is increased, the spins become progressively “unblocked”, aligning themselves toward the field direction, and the magnetization increases until it reaches a maximum at what is defined as the “blocking temperature”, TB. Above TB, the behavior is superparamagnetic; that is, the thermal energy increases to such an extent that the increased dynamic rotation of spins prevents alignment in the field direction, and the magnetization decreases with increasing temperature. The native sample is characterized by a TB of 98 K, which is in good agreement with the value found previously for a similar

III.3. Magnetic Investigation. As already mentioned, the magnetic investigation is performed on Co nanocrystals dispersed in the same solvent, submitted or not to an annealing process and then deposited on an amorphous-carbon-coated grid. Here, we focus on changes in the blocking temperature and in the high field response as a function of nanocrystallinity, so we recorded only zero field cooled (ZFC) magnetization curves. Because drops are deposited on the substrate, it is impossible to deduce, in our experimental conditions and with the equipment available in our laboratory, the exact value of the deposited mass of nanocrystals. Consequently, the exact value of the saturation magnetization is not known. This is why we plot only the reduced magnetization value. In such annealing processes, no traces of oxygen are detected, neither from structural studies nor from the ZFC curve, which allows oxide to be detected by a feature around 8 K. Consequently exchange bias effects can be excluded for nanocrystals coated with dodecanoic acid. 15728



Article

Figure 8. Magnetization versus field curves at 5K of ∼7 nm cobalt nanocrystals: native (a), solution-phase annealing, without aging at (b) 190 °C (c) and 220 °C, and (d) with aging at 250 °C for 60 min. (M/Ms = f(H)). The insets show the low field part of the hysteresis loops in an extended scale.

assembly.28 This value is higher than that obtained for a dilute system of Co nanocrystals of a similar size,27 revealing the existence of strong dipolar interactions between the nanocrystals. We do not observe any low-temperature peak (∼8 K) in the ZFC curve, which indicates the absence of severe oxidation (more than 1 nm in thickness) of metallic Co.29,30 We cannot exclude the formation of a thin shell of CoO on the nanocrystals.31,32 However, this is detected neither by HRTEM, nor by electron diffraction and does not affect the magnetic properties. The solution-phase heating of Co nanocrystals to Ta = 190, 220, and 250 °C, induces a drastic increase in TB from 98 K to 167, 187, and 257 K respectively (Figure 7a and Table 1). This behavior is explained by the structural transition of the metallic nanocrystals. Depending on the material, the magnetic anisotropy energy of a system composed of metallic nanocrystals can strongly depend on the particle structure. Indeed, in the literature, for similar particle size, the anisotropy of cobalt metal in the fcc structure is reported as 2.7 × 106 erg/cm3,33 or 1.35 × 106 erg/cm3,27 whereas it can reach 4.7 × 106 erg/cm3 for the hcp structure.34 Here, the 2.6-fold increase in TB after the heating to 250 °C is consistent with the increase in the anisotropy found after the fcc-hcp transition. As mentioned below, the crystallographic structure of the native Co polycrystals is not well-defined due to the small domain sizes. However, the electron diffraction study does not exclude an fcc structure that is furthermore the expected structure when using the chemical salt reduction approach at room temperature. So, heating to increasing temperatures up to 250 °C progressively converts the native polycrystals from most probably fcc to hcp, inducing an increase in their magnetic anisotropy, which in turn gives an increase in the energy barriers and consequently in TB. After the annealing at 190 °C, the ZFC peak normalized to TB (ZFCnorm) is broadened with respect to the native sample (Figure 7b). The presence of larger nanocrystals or sintered nanocrystals could explain this behavior, however, the TEM study does not reveal any such size or morphological changes.

Actually, at this stage, the population is composed of around 50% of the native phase and 50% of the hcp phase, which leads to a distribution of the anisotropy in the sample and then to a distribution of the barrier energies.24 After annealing at higher temperature, 220 and 250 °C, a progressive narrowing of the ZFCnorm peak is observed, indicating the progress of the structural transition to the hcp phase. At 250 °C, the native width is almost recovered but not totally, which is in good agreement with the structural study revealing very few “large domain” nanocrystals whose structure could be still that of the native nanocrystals. These low-field magnetization studies show clearly that a transition from soft to hard magnetic cobalt nanocrystals is obtained. III.3.2. High Field Magnetization Measurements. High-field magnetic studies have also been conducted. Figure 8 shows the magnetization as a function of field measured at 3 K for the samples in the native state and after heating to 190 °C (tag= 0), 220 °C (tag= 0), and 250 °C (tag = 60 min). The saturation of the magnetization is reached at around 1 T in all the cases except for the sample heated at 220 °C which saturates from 0.5T. The major change between the native sample and the annealed ones is observed in the value of the coercive field HC, which significantly increases from 1000 Oe to ≈1800 Oe for Ta = 220 and 250 °C respectively. This result is explained by the increase in the anisotropy energy density Ka of the system induced by the structural transition of the Co nanocrystals. This behavior markedly differs from that obtained for the same nanocrystals when dry-annealed, which do not exhibit such a variation.24 The coercivity is a complex parameter that depends on the balance between anisotropy and Zeeman energy. In the case of uniaxial symmetry, Hc ≈ Ka/MS. For the case of dry annealing, it is likely that the increase in the anisotropy constant due to the structural transition is compensated by the increase in the saturation magnetization. Whatever the annealing process (dry or in solution), the structural transition from polycrystals to hcp single crystals is achieved, and is 15729



Article

(6) Schwalom, D. D.; von Molnar, S. Physical Properties of Nanometer-Scale Magnets. In Nanotechnology; Timp, G. L., Ed.; Springer: New York, 1999; Chapter 12, p 437. (7) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Appl. Phys. Lett. 2001, 78, 2187−2189. (8) Soumare, Y.; Garcia, C.; Maurer, T.; Chaboussant, G.; Ott, F.; Fievet, F.; Piquemal, J. Y.; Viau, G. Adv. Funct. Mater. 2009, 19, 1971− 1977. (9) Dumestre, Y.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove, M. J.; Renaud, P.; Zurcher, P. Angew. Chem. 2002, 114, 4462−4465. (10) Dinega, D. P.; Bawendi, M. G. Angew. Chem., Int. Ed. 1999, 38, 1788−1791. (11) Leslie-Pelecky, D. L.; Bonder, M.; Martin, T.; Kirk Patrick, E. M.; Liu, Y.; Zhang, X. Q.; Kim, S. H.; Rieke, R. D. Chem. Mater. 1998, 10, 3732−3736. (12) Respaud, M.; Broto, J. M.; Racoto, H.; Fert, A. R.; Thomas, L.; Barbara, B.; Verelst, M.; Snoeck, E.; Lecante, P.; Mosset, A.; Osuna, J.; Ould Ely, T.; Amiens, C.; Chaudret, B. Phys. Rev. B 1998, 57, 2925− 2935. (13) Yang, H. T.; Shen, C. M.; Wang, Y. G.; Su, Y. K.; Yang, T. Z.; Gao, H. J. Nanotechnology 2004, 15, 70. (14) Chaudret, B. C. R. Physique 2005, 6, 117. (15) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325−4330. (16) Murray, C. B.; Sun, S.; Gasschler, W.; Doyle, H.; Betley, T. A.; Gagan, C. R. IBM J. Res. Dev 2001, 45, 47−56. (17) Legrand, J.; Petit, C.; Bazin, D.; Pileni, M. P. Appl. Surf. Sci. 2000, 164, 193−199. (18) Lisiecki, I.; Pileni, M. P. Langmuir 2003, 19, 9486−9489. (19) Zeng, H.; Sun, S.; Sandstrom, R. L.; Murray, C. B. J. Magn. Magn. Mater. 2003, 266, 227−232. (20) Yu, A. C. C.; Mizuno, M.; Sasaki, Y.; Kondo, H.; Hiraga, K. Appl. Phys. Lett. 2002, 81, 3768−3771. (21) Yang, H. T.; Shen, C. M.; Su, Y. K.; Yang, T. Z.; Gao, H. J.; Wang, Y. G. Appl. Phys. Lett. 2003, 82, 4729−4731. (22) Nie, X.; Jiang, J. C.; Meletis, E. I.; Tung, L. D.; Spinu, L. J. Appl. Phys. 2003, 93, 4750−4755. (23) Lisiecki, I.; Salzemann, C.; Parker, D.; Albouy, P. A.; Pileni, M. P. J. Phys. Chem. C 2007, 111, 12625−12631. (24) Parker, D.; Lisiecki, I.; Salzemann, C.; Pileni, M. P. J. Phys. Chem. C 2007, 111, 12632−12638. (25) Cavalier, M.; Walls, M.; Lisiecki, I.; Pileni, M. P. Langmuir 2011, 27, 5014−5020. (26) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620−2625. (27) Petit, C.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 15309−15316. (28) Lisiecki, I.; Parker, D.; Salzemann, C.; Pileni, M. P. Chem. Mater. 2007, 19, 4030−4036. (29) Bao, Y.; Beerman, M.; Pakhomov, A. B.; Krishnan, K. M. J. Phys. Chem. B 2005, 109, 7220−7222. (30) Tracy, J. B.; Weiss, D. N.; Dinega, D. P.; Bawendi, M. G. Phys. Rev. B 2005, 72, 064404. (31) Lisiecki, I.; Walls, M.; Parker, D.; Pileni, M. P. Langmuir 2008, 24, 4295−4299. (32) Lisiecki, I.; Turner, S.; Bals, S.; Pileni, M. P.; Van Tendeloo, G. Chem. Mater. 2009, 21, 2335−2338. (33) Chen, J. P.; Sorensen, C. M.; Klabunde, J. K.; Hadjipanayis, G. C. Phys. Rev. B 1995, 51, 11527−11532. (34) Luis, F.; Torres, J. M.; Garcia, L. M.; Bartolomé, J.; Stankiewicz, J.; Petroff, F.; Fettar, F.; Maurice, J. L.; Vaurés, A. Phys. Rev. B 2002, 65, 094409. (35) Paulus, P. M.; Bönnemann, H.; van der Kraan, A. M.; Luis, F.; Sinzig, J.; de Jongh, L. T. Eur. Phys. J. D 1999, 9, 501−504. (36) Van Leeuwen, D. A.; van Ruitenbeek, J. M.; de Jongh, L. J.; Ceriotti, A.; Pacchioni, G.; Häberlin, O. D.; Rösch, N. Phys. Rev. Lett. 1994, 73, 1432−1435. (37) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565−568.

expected to give rise to a similar increase in the anisotropy of the system. However, it is well-known that surface effects play an important role in the magnetic properties. Organic ligands can decrease the magnetic moment of the metal atoms located at the surface of the nanocrystals.35−37 It has been proposed that surface-bonded ligands lead to the quenching of the surface magnetic moments, resulting in the reduction of magnetization.36 In our experiments, the annealed nanocrystals are coated with native dodecanoic acid molecules that covalently bind to the metallic surface (see sectionIII.1). Conversely, during the dry annealing, the C12 coating underwent the heating treatment and it is likely that it no longer acts as efficiently on the metallic nanocrystals. We speculate that the MS increase resulting from the solution-phase heating is weaker than that for dry annealing due to the strong ligand/metallic surface interactions, HC thus increases almost linearly with Ka. Very surprisingly, the coercivity of the sample annealed at 190 °C is found to be 300 Oe. This quite unexpected very low value remains, up to now, to be elucidated. In fact, it is as if the crystallized domain size was smaller compared to that of the native nanoparticles, i.e., around 1 nm, a behavior that could be related to the crystallographic transition that is occurring at this stage.

■

CONCLUSION By a solution-phase annealing, we have induced a structural transition of 7 nm Co nanocrystals from a polycrystalline phase to an hcp single-crystalline phase. Importantly, the thermal treatment does not damage the nanocrystals; i.e., neither oxidation nor coalescence of the nanomaterial occurs. Because of their stability, these uniform nanocrystals 2D self-organize into a hexagonal network. A careful structural investigation has allowed us to derive a mechanism for the recrystallization of the cobalt nanocrystals. After annealing at higher temperatures, we see a drastic increase in both blocking temperature and coercive field arising from the higher anisotropy and crystallinity associated with the pure hcp structure.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: (I.L.) [email protected]; (M.-P.P.) marie-paule. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the European Union for supporting this work project “Growth and supra-organization of transition and noble metal nanoclusters” contract number, NMP4-CT-2004001594 and the French MET and atom-probe network METSA. The MPP and ZJY research leading to these results has received funding from Advanced Grant of the European Research Council under Grant 267129.

■

REFERENCES

(1) Special issue on “Nanoscale Materials” Acc. Chem. Res. 1999, 32. (2) Lambeth, D. N.; Velu, E. M. T.; Bellesis, G. H.; Lee, L. L.; Laughlin, D. E. J. Appl. Phys. 1996, 79, 4496−4501. (3) Weller, D.; Moser, A. IEEE Trans. Magn. 1999, 35, 4423−4439. (4) Thompson, D. A.; Best, J. S. IBM J. Res. Dev. 2000, 44, 311−322. (5) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogués, J. Nature 2003, 423, 850−853. 15730


A Phase-Solution Annealing Strategy to Control the Cobalt

Recommend Documents