Emergence of New Collective Properties of Cobalt Nanocrystals

It has been shown that by annealing three dimensional (3D) disordered and fcc supracrystal assemblies of 7.5 nm Co nanocrystals, a structural transiti...
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12632

J. Phys. Chem. C 2007, 111, 12632-12638

Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals: II, Magnetic Investigation Dinah Parker, Isabelle Lisiecki, Caroline Salzemann, and Marie-Paule Pileni* Laboratoire des Mate´ riaux Me´ soscopiques et Nanome´ triques, UMR CNRS 7070, UniVersite´ Pierre et Marie Curie, Baˆ t F, 4 Place Jussieu, 75005 Paris, France ReceiVed: March 6, 2007; In Final Form: June 18, 2007

It has been shown that by annealing three dimensional (3D) disordered and fcc supracrystal assemblies of 7.5 nm Co nanocrystals, a structural transition of the nanocrystals from a poorly crystallized fcc structure to a pure hcp phase occurs. The annealing process does not result in either oxidation or coalescence of the nanocrystals. Not only is the system highly stable, but also the thermal treatment actually improves the mesoscopic structural order in the fcc supracrystals. In the native state, an effect of the mesoscopic order on the magnetic properties of the 3D assemblies is observed which we attribute to the difference in distribution of dipolar interaction energies. In this paper, we extend this study to the same systems after annealing in order to investigate the effect of increased anisotropy and crystallinity on the magnetic behavior. We find a significant increase in both blocking temperature (to a near room-temperature value) and saturation magnetization. Surprisingly, the effects of mesoscopic order are no longer observed.

I. Introduction It has been found that nanocrystals with a low size distribution can self-organize spontaneously into three dimensional (3D) arrays with a regular, ordered structure known as supracrystals.1-6 This new generation of materials offers an intermediate system between nanocrystals and the bulk phase. Over the last two years, new collective properties have emerged which arise from the periodic arrangement of nanocrystals.2,7-10 In our lab, it was discovered that vibrational coherence in Raman scattering is observed when Ag nanocrystals are ordered in an fcc supracrystal; this behavior is absent in a disordered 3D assembly.11 It was also shown that enhanced mechanical stability of columns made of cobalt nanocrystals arises from the fcc suprastructure.12 On a similar note, Zaitseva et al. showed a change in photoluminescence properties depending on the order of CdSe nanoparticles in a 3D assembly.10 It has been found that 3D nanocrystal assemblies show different magnetic properties compared with isolated or two dimensional (2D) arrangements because of the increase in interparticle dipolar interaction energies.13-16 We have recently shown, to our knowledge, for the first time, that significant differences in the magnetic properties of 3D Co nanocrystal assemblies are found for ordered and disordered systems.17 This observation was made possible by our ability to prepare either disordered or fcc long-range ordered 3D assemblies from the same nanocrystals.18 We found a significant narrowing of the zero field cooled (ZFC) magnetization versus temperature peak and an increased coercivity (Hc) due to the nanocrystal order. The Co nanocrystals used to form these 3D assemblies have a poorly crystallized (nearly amorphous) fcc structure. It has been shown that annealing Co nanocrystals at temperatures up to 300 °C induces a structural transition to a well-crystallized hcp structure.19,20 In a paper detailing the structural properties of the 3D Co assemblies, we have shown for the first time that * Corresponding author.

it is possible to maintain the fcc supracrystal structure during annealing at temperatures up to 350 °C.21 This therefore enables us to control the crystallinity of our Co 3D assemblies on both the atomic and the mesoscopic scales. A transition from the poorly crystallized fcc structure to an hcp structure has been shown to lead to a drastic increase in the blocking temperature (TB) and saturation magnetization (MS) of Co nanocrystals.20,22 Here, we show that we are able to extend this phenomenon to nanocrystals arranged in 3D to give a near room-temperature magnetic transition. In order to continue our study on the effect of structural order that was carried out on native 3D assemblies,17 we report here the effects of mesoscopic order on the annealed 3D assemblies. II. Experimental Section II.1. Samples. The synthesis of 7.5 mm Co nanocrystals and the preparation of ordered and disordered 3D assemblies is described elsewhere.18,21,23 The 3D assemblies were annealed in nitrogen filled quartz ampoules for 15 min in a furnace at 250, 300, and 350 °C. As the magnetic behavior of the 3D assemblies is influenced by the size and size distribution of the Co nanocrystals, disordered and ordered assemblies for direct comparison are prepared at the same time and from the same batch of nanocrystals. The magnetization curves are measured for each sample before and after annealing. The native samples are found to be relatively stable in air (for at least 2 weeks) whereas the annealed samples are more prone to oxidation. To prevent oxidation, the annealed samples were stored under nitrogen where possible, and the absence of an 8 K peak in the zero field cooled magnetization versus temperature curve confirms that the samples were free of significant oxidation. II.2. Apparatus. Transmission electron microscopy (TEM) was performed using a JEOL 1011 microscope. Grazing incidence small-angle X-ray scattering (GISAXS) measurements were carried out using a rotating anode generator

10.1021/jp071821u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

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TABLE 1: Structural and Magnetic Parameters Extracted from the GISAXS Patterns, the ZFC Magnetization Curves, and the Hysteresis Measurementsa native

350 °C

annealing T

ordered

disordered

ordered

disordered

Dc-c (nm) Di-p (nm) Mr /Ms Ms nat/Ms ann Hc (Oe)

10.5 ( 0.1 3.0 ( 0.5 0.53 ( 0.2

11.5 ( 0.1 4 0.0 ( 0.5 0.54 ( 0.2

900 ( 50

600 ( 50

9.7 ( 0.1 2.2 ( 0.5 0.51 ( 0.2 0.63 ( 0.2 900 ( 50

11.2 ( 0.1 3.7 ( 0.5 0.52 ( 0.2 0.78 ( 0.2 800 ( 50

a

Dc-c: center-to-center nanocrystal distance; Di-p: border-to-border distance of nanocrystals considering a nanoparticle size of 7.5 nm; Mr/ Ms: ratio of remanent-to-saturation magnetization; Ms nat/Ms ann: ratio of native Ms to annealed Ms; Hc: coercive field.

operated with a small-size focus (copper anode; focus size 0.2 mm × 0.2 mm; 50 kV, 30 mA). The optics consisted of two parabolic multilayer graded mirrors in KB geometry providing a parallel monochromatic beam. The sample was mounted on a rotation stage, and the diffraction patterns were recorded on photostimulable imaging plates. Vacuum pipes were inserted between the sample and the imaging plate to reduce air scattering. A single GISAXS measurement probes a section several micrometers wide, from one edge to the other of the substrate. 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 field cooled (FC) measurements were performed in the same manner with the difference that the field was applied before cooling. The magnetization versus field measurements were performed at 5 K after zero field cooling. All magnetic measurements were carried out with the field applied parallel to the substrate. III. Sample Characterization A detailed structural analysis of the Co nanocrystal 3D assemblies used for this study is given in ref 21. Here, we give a brief description summarizing the relevant details. The key structural parameters are given in Table 1. Co nanocrystals are characterized by a mean diameter and size polydispersity of 7.5 nm and 9.4% respectively and are stabilized by a coating of dodecanoic acid chains (Figure 1). These nanocrystals, once deposited on a highly orientated pyrolitic graphite (HOPG) coated TEM grid, spontaneously form a 2D hexagonal compact network. Electron diffraction (inset, Figure 4A) shows that the native nanocrystals have a poorly crystallized fcc structure (nearly amorphous). The annealing process induces a phase transformation. After annealing at 250 °C, the structure is largely hcp with some remaining fcc (see inset, Figure 4B). After annealing at 300 °C, pure hcp Co is formed (see inset, Figure 4C), the coherence length of which increases when the annealing is performed at 350 °C to give monocrystals (see inset, Figure 4D). The average diameter of the annealed nanocrystals is decreased (although unchanged within error) compared with the native nanocrystals. However this decrease in diameter is difficult to quantify as the HOPG substrate deteriorates during annealing resulting in a loss of contrast in the TEM image. This feature may be explained by the structural transition from the disordered fcc structure to the more dense hcp structure. The supracrystals deposited on an HOPG substrate form aggregates with a thickness and surface area that can reach 5

Figure 1. (A) Size histogram corresponding to the Co nanocrystals shown in Figure 1B. (B) TEM image of cobalt nanocrystals ordered in a compact hexagonal monolayer. (C) SEM images of the Co supracrystal sample before annealing, (D) after annealing at 250 °C, (E) after annealing at 300 °C, and (F) after annealing at 350 °C.

Figure 2. Diffractogram of supracrystals of 7.5 nm Co nanocrystals (A) native, annealed at (B) 250, (C) 300, and (D) 350 °C. Inset: corresponding GISAXS patterns.

µm (equivalent to approximately 500 Co nanocrystal layers) and 1 mm2, respectively (see Figure 1C). The GISAXS study indicates a long-range fcc structure which remains stable during the annealing processes at 250, 300, and 350 °C (Figure 2). The center-to-center distance (Dc-c) between the nanocrystals progressively decreases with increasing annealing temperature

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Parker et al. IV. Results

Figure 3. SEM images of a disordered 3D assembly of 7.5 nm Co nanocrystals (A) in the native state and (B) annealed at 350 °C. Inset: the corresponding GISAXS patterns.

Figure 4. FC (full lines) and ZFC (dashed lines) M vs T curves of supracrystalline samples (A) native, and (B) annealed at 250, (C) 300, and (D) 350 °C. Inset: the corresponding electron diffraction patterns.

(Tann). After annealing at 350 °C, the Dc-c decreases by 0.8 nm to give a value of 2.2 nm. This observation could be explained by the nanocrystal size decrease. In parallel, it is also possible that the Dc-c decrease is the direct consequence of a decrease in the interparticle distance. Because of the difficulty in determining the nanocrystal size after annealing, we are not able to judge the relative role played by these two parameters. In addition to these changes in Dc-c, we also observe a progressive improvement in the suprastructure ordering. After annealing, some cracks develop in the supracrystal sample, which become wider with increasing annealing temperature (see Figure 1D-F). The disordered 3D assemblies deposited on an HOPG substrate form thin aggregates of less than 500 nm2 consisting of a maximum of approximately 50 Co nanocrystal layers (Figure 3A). The GISAXS study of the native sample shows a broad, diffuse ring typical of the absence of any long-range crystalline order (see inset, Figure 3A). We deduce a Dc-c of 4.0 nm, which is larger than what was found for the fcc supracrystals. After annealing at 350 °C, there is no change in the film morphology compared to the native sample (Figure 3A,B). The GISAXS study shows no change in mesoscopic structural order as a diffuse ring is always observed (inset, Figure 3B). However, we detect a slight decrease in Dc-c of 0.3 nm.

IV.1. Magnetization vs Temperature Measurements. Figure 4 shows the FC and ZFC magnetization versus temperature curves for supracrystals of Co nanocrystals in the native state (A) and after annealing at 250, 300, and 350 °C (B-D). As the sample has been cooled in zero field, there is no net alignment of the spins at 5 K, and hence the magnetization is close to zero. As the temperature is increased, the spins become progressively “unblocked”, aligning toward the field direction, and the magnetization increases until it reaches a maximum which we define here as the blocking temperature, TB. Above TB, the behavior is paramagnetic; that is, the thermal energy increases to such an extent that the increased dynamic rotation of the spins prevents alignment in the field direction, and the magnetization decreases with increasing temperature. In the FC curve, the magnetization remains nearly constant from 5 K to TB. Above TB, the behavior is paramagnetic, and the magnetization decreases with increasing temperature in line with the ZFC curve. The native 3D cobalt nanocrystal assemblies, ordered and disordered, are both characterized by a TB of around 100 K and a flat FC curve at temperatures below TB. This is indicative of strong dipolar interactions between the nanocrystals, and it has been shown for various similar interacting nanocrystal systems that this is an indication of spin glass-like behavior.24,25 The absence of a low-temperature peak in the ZFC curve indicates that the Co nanocrystals are not severely oxidized. Advanced oxdidation (>1 nm for Co particles of approximately the same size as ours) has been shown to give rise to a ZFC peak at 8 K.26,27 We cannot rule out the possibility of a thin shell of CoO on our particles; however, this is not detectable by TEM and does not seem to affect the magnetic properties. In a previous publication,17 we demonstrated that an ordered 3D assembly shows different magnetic behavior to a disordered 3D assembly made with the same nanocrystals (i.e., of the same size and size distribution). The energy barriers of the Co nanocrystals (Eb) are dependent on the anisotropy energy, Ea ) kaV, and the dipole interaction energy, Edd; the width of the ZFC peak is an indication of the distribution of Eb. We observe a narrower ZFC peak for the ordered sample, which arises from a narrower distribution of barrier energies compared with the disordered sample. A difference in the size distribution of the nanocrystals can be eliminated; we attribute this behavior to the difference in structural environment of the Co nanocrystals in the two samples. As dipolar interactions are strongly directionally dependent, the overall behavior is expected to be sensitive to the geometrical arrangement of the nanocrystals in the 3D assembly. In the ordered sample, within a single fcc domain, we can consider that the dipolar interaction energies (and hence Eb) are very similar for all nanocrystals. In the disordered sample, there are many small domains characterized by different degrees of order, and therefore the distribution of energy barriers in this sample as a whole is much greater than in the ordered sample. This phenomenon, despite being subtle, was found to be highly reproducible and is suggestive of an intrinsic collective behavior in fcc supracrystal domains. Annealing the ordered sample induces a progressive increase in TB which reaches 280 K when Tann ) 350 °C. This behavior is explained by the progressive crystallographic transition of the Co nanocrystals from a nearly amorphous fcc structure to a monocrystalline hcp structure. This leads to an increase in the anisotropy of the nanocrystals, which in turn gives an increase in the energy barriers and hence in TB.

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Figure 5. (A) ZFC M vs T/TB curves for supracrystalline samples, native (full line) and annealed at 250 (dashed line), 300 (dotted line), and 350 °C (dot-dashed line); (B) TB versus annealing temperature for supracrystalline (full circles) and disordered (crosses) samples; (C) definition of ∆(T/TB) parameter; (D) ∆(T/TB) vs annealing temperature for supracrystalline (full circles) and disordered (crosses) samples.

The annealing at 300 °C was repeated three times with supracrystals made from different batches of nanocrystals. In Figure 5B, TB is plotted versus Tann. We also include in this plot the data from the five samples in their native state for comparison (note that some of the points are superimposed). In the case of annealing at 300 °C, we see a variation of TB (up to 30 K) from sample to sample which was also observed in the native state. As we discussed in more detail in ref 17, changes in TB of this order of magnitude can arise from very small changes in the average size of the Co nanocrystals (smaller than the error of the size determination by TEM (0.5 nm)), which occur from one synthesis to another. Despite these variations in TB, the general trend of a systematic increase in TB with Tann is still clear. Figure 5A shows the ZFC curves of the supracrystal samples in the native and annealed states, normalized to TB (hereafter referred to as ZFCnorm). For Tann ) 250 °C, we see an enlargement of the ZFCnorm peak below TB compared with the native sample. In order to quantify this, we have defined a parameter ∆(T/TB) (see Figure 5) which is the distance in T/TB between T/TB ) 1 and the ZFC curve, measured at the half maximum. Figure 5D shows a plot of ∆(T/TB) versus Tann. After annealing at 250 °C, we observe that the ZFCnorm peak is broadened with respect to the native sample. We attribute this to the fact that crystallographic conversion is not complete, and this is coherent with the electron diffraction study which shows a high proportion of hcp order coexisting with the native structure (poorly crystallized fcc). This leads to a distribution of ka in the sample and hence a distribution of Eb. Focusing on the ordered sample, after annealing at higher temperatures (300 and 350 °C), we observe a progressive narrowing of the ZFCnorm peak until the native width is recovered. This result is again in agreement with the electron diffraction study, which shows that after annealing at 350 °C the conversion to a monodomain hcp structure is complete. The fact that we find the same peak width for the native and annealed sample further confirms the absence of nanocrystal coalescence in the sample, as found from the structural investigation. Coalescence during annealing would lead to a significant increase in nanocrystal size distribution, which would in turn give a broadening of the ZFCnorm peak. Staying with the ordered sample, we will now consider the changes in the structural parameters observed after annealing.

As found from the GISAXS study, annealing induces a progressive decrease in the nanocrystal center-to-center distance which reaches a maximum of 0.8 nm after annealing at 350 °C, indicating a smaller interparticle distance after annealing. As dipolar interactions are stronger at shorter distances, we expect that this decrease in interparticle distance has increased the interparticle dipolar interactions in the system. However, we cannot separate the effect of this increase in TB from the effect of the increased nanocrystal anisotropy. In order to do this, it would be necessary to fully redisperse in a solvent the whole population of nanocrystals in the annealed supracrystal and measure the TB of the dilute dispersion. This would give the TB of the nanocrystals in the absence of dipolar interactions, and from this we could estimate the relative contributions of anisotropy and dipolar interaction in the annealed supracrystal. At this stage, however, full redispersion of the 350 °C annealed supracrystal cannot be achieved, probably because of a partial loss of the organic coating during annealing (see ref 21). Figure 6 shows the FC and ZFC magnetization versus temperature curves for disordered 3D assemblies of Co nanocrystals in the native state (A) and annealed at 250 and 350 °C (Figure 6B,C, respectively); the ZFCnorm curves are shown in Figure 6D. Here, we see similar dependence of TB on Tann as for the supracrystal samples. The behavior of the ∆(T/TB) parameter (shown in Figure 5D) is also similar to that of the supracrystal samples with the exception of the native sample. As already reported in a previous publication concerning the native assemblies,18 the ZFCnorm curves are consistently narrower for the ordered sample compared with the disordered sample (see Figure 9 A). After annealing at 250 °C, ∆(T/TB) increases because of the incomplete structural transition of the nanocrystals, as was observed for the supracrystal sample. After annealing at 350 °C, the ZFCnorm peak width is not restored to the native value, as observed for the supracrystal, but is significantly narrower. The ZFCnorm peak of the annealed (350 °C) disordered sample now superimposes the ZFCnorm peak of the supracrystal sample. The origins of this effect are discussed below. In Figure 4, for all samples, we see that there is a slight decrease in the field cooled magnetization with temperature below TB. After annealing there is an increase in the gradient of the FC curves compared with the native sample for all values

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Figure 6. FC (full line) and ZFC (dashed line) M vs T curves for disordered samples (A) native and (B) annealed at 250 and (C) 350 °C; (D) ZFC M vs T/TB curves for disordered samples, native (full line) and annealed at 250 (dashed line) and 350 °C (dotted line).

Figure 7. Normalized FC M vs T/TB curves for the native supracrystal sample (full line) and supracrystal samples after annealing at 250 (dotdashed line), 300 (dotted line) and 350 °C (dashed line). Note that there are three examples given for the supracrystals annealed at 300 °C.

of Tann. This is seen clearly in Figure 7 where we have plotted the FC curves for values of T/TB < 0.5 of the native and annealed samples (the magnetization is normalized to 1 at TB ) 0.5). We have included all three curves corresponding to the three supracrystal samples annealed at 300 °C (see above). The increase in gradient with Tann appears to be a general trend; however, as there is large dispersion in gradient of the samples annealed at 300 °C, this cannot be confirmed without further investigation. This feature of a slight decrease in the FC curve below TB is well-known for atomic spin glasses28 and has also been observed previously in other strongly interacting magnetic nanocrystal systems.24,29 For noninteracting magnetic nanocrystal systems, it has been found both experimentally and through calculations that MFC will increase below TB until a plateau is reached at a temperature, Tsat, which is relative to the smallest volume nanocrystal in the size distribution.24,30 When interparticle interactions are introduced into the system, the increase in MFC below TB is suppressed, and for strongly interacting systems, this can lead to a nearly constant MFC below TB. Calculations performed by Chantrell et al. on the FC and ZFC curves of strongly interacting magnetic nanocrystals

Parker et al. showed that the MFC actually decreases with temperature below TB giving rise to a peak in the FC curve;31 this has also been observed experimentally in strongly interacting granular films.32 In both the disordered and the supracrystal 3D assemblies reported here, a systematic enhancement of the decrease in MFC below TB was observed after annealing. This is coherent with an increase in the dipolar interaction energies due to the enhanced magnetic moment of the nanocrystals and the decrease in interparticle distance. It must be noted that the rate of cooling of the sample through TB before starting the FC measurement can influence the behavior of MFC. However, in order to have a significant effect, the cooling rate would need to differ by at least a factor of 10 between measurements.24 In our experiments, the cooling rate was relatively constant, and therefore, we do not think that the cooling rate can be responsible for the observed effects. IV.2. Magnetic Hysteresis Measurements. The high field behavior of the 3D assemblies has also been studied. Figure 8 shows the magnetization as a function of field for a disordered and an ordered assembly of Co nanocrystals in the native state and annealed at 350 °C. Figure 8A shows the hysteresis curves for the native samples (already reported in ref 17). We observe that the Hc of the ordered sample is increased with respect to that of the disordered sample (900 and 600 Oe, respectively; see Table 1). This is attributed to a more collective behavior in the supracrystal sample arising from the ordered fcc structure which inhibits the flipping of the superspins. Additionally, the ordered sample appears to saturate at higher fields than the disordered sample. Figure 8C shows the hysteresis curves for the ordered sample before and after annealing at 350 °C; the results for the disordered sample are shown in Figure 8D. In both cases, there is an increase in Ms after annealing. This was also observed for Co nanocrystals in powder form and 2D locally organized assemblies after annealing at 300 °C.20 We attribute this increase in Ms to the crystallographic transition of the nanocrystals from a poorly crystallized fcc structure in the native state to a monodomain hcp structure after annealing. This transition from a multidomain to a monodomain structure leads to the enhancement in Ms.20 The increase in Ms is greater for the supracrystal compared with the disordered sample (Ms nat/Ms ann ) 0.63 and 0.78, respectively). The reason for this is not clear, and it is probable that it could simply arise from a slight loss of mass during the manipulation of the sample before and after annealing. For the ordered sample, no change in Hc is observed after annealing (see Table 1), whereas for the disordered sample, Hc increases from 600 to 800 Oe after annealing. Figure 8B shows the hysteresis curves of the ordered and disordered samples after annealing at 350 °C. We see that, contrary to that in the native state, the two hysteresis curves show similar behavior with an Hc of 800 and 900 Oe for the disordered and ordered samples respectively. This behavior is coherent with that observed from the low field measurements. In Figure 9, we show the ZFC magnetization versus temperature curves which correspond to the magnetization versus field measurements shown in Figure 8. We see that in the native state the supracrystal and disordered samples show differences in both high and low field measurements (i.e., change in ZFC peak width and in Hc), whereas after annealing, the disordered sample behaves as the supracrystal. Before annealing, the ordered and disordered samples have an Mr/Ms of 0.53 and 0.54, respectively (see Table 1). After annealing, these values decrease slightly for both samples to 0.51 for the ordered sample and 0.52 for the disordered sample.

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Figure 8. M vs H curves of (A) native disordered (dashed line) and supracrystalline (full line) samples; (B) disordered (dashed line) and supracrystalline (full line) samples annealed at 350 °C; (C) supracrystalline sample, native (dashed line) and annealed at 350 °C (full line); (D) disordered sample, native (dashed line) and annealed at 350 °C (full line). Inset: magnification of the low field regions.

Figure 9. ZFC M vs T/TB curves of (A) native disordered (dashed line) and supracrystalline (full line) samples; (B) disordered (dashed line) and supracrystalline (full line) samples annealed at 350 °C; (C) supracrystalline sample, native (dashed line) and annealed at 350 °C (full line); (D) disordered sample, native (dashed line) and annealed at 350 °C (full line).

These changes in Mr/Ms are however very small and may be considered to be within the limits of experimental error. Figure 9 shows the ZFCnorm curves that correspond to the M versus H curves shown in Figure 8. In the native state (Figure 9A), the ZFCnorm peak of the disordered sample is significantly enlarged compared with that of the ordered sample (as mentioned above). After annealing, the width of the ZFCnorm peak of the ordered sample remains the same (Figure 9B), whereas that of the disordered sample significantly decreases (Figure 9D). We have previously considered the possibility that the difference in Hc observed between the native supracrystal and disordered samples could simply be due to a difference in nanocrystal packing density which could in turn influence the interparticle interactions. This theory is not consistent with the results found after annealing. The structural investigation showed

that there was a decrease in nanocrystal center-to-center distance (and hence an increase in packing density) after annealing of 0.8 nm for the supracrystal sample; however, no change in Hc is observed. For the disordered sample, a smaller decrease in Dc-c was observed after annealing (0.3 nm), and yet the Hc was significantly increased. We therefore conclude that the differences in Hc observed before annealing must arise from the nanocrystal ordering. The similarity of the magnetic behavior of the disordered and fcc supracrystal assemblies after annealing also confirms that the differences in behavior observed in the native state are not related to the sample morphology (thickness and homogeneity). The disordered sample is considerably thinner than the supracrystal sample (by nearly a factor of 10) and with a more uniform surface. As there is no significant change in film morphology after annealing, we can reject the possibility of the

12638 J. Phys. Chem. C, Vol. 111, No. 34, 2007 film thickness playing a role in the magnetic behavior of either the native or the annealed samples. As described above, the differences in magnetic behavior observed in the native systems which arise from mesoscopic order are no longer observed after annealing at 350 °C. The similarity observed in the hysteresis behavior of the supracrystal and disordered samples after annealing is consistent with the results of the low field magnetization versus temperature experiments as is clearly illustrated in Figures 8 and 9. We will first consider this feature from a structural point of view. After annealing at 350 °C, the GISAXS pattern shows no change in long-range structural order for the disordered sample. However, it is possible that a local, short-range improvement in the fcc order could have taken place, and this would be coherent with the slight decrease in Dc-c after annealing which was detected by GISAXS. This improvement in order at short-range would lead to a more regular geometric environment of the nanocrystals and, as for the supracrystal, lead to a decreased distribution of interaction energies and hence the observed narrowing of the ZFCnorm peak. In fact, it has been shown that short-range order, undetectable by GISAXS, can lead to vibrational coherence in Raman scattering in assemblies of silver nanocrystals.7 However, in the absence of conclusive evidence of a structural improvement in our system, this explanation for the observed changes in magnetic properties after annealing cannot be confirmed. Another explanation for the observed behavior comes from the increase in single particle anisotropy after annealing. Previous investigations suggest that the transition from an amorphous/fcc to a pure hcp cobalt nanocrystal leads to a more than threefold increase in the anisotropy constant, ka.20 In a paper by Krechrakos et al., a qualitative study is carried out using Monte Carlo simulations into the magnetic properties of a system of interacting single domain particles.33 It was concluded that the magnetic properties of the system are controlled by the interplay of ka and the dipolar interaction strength. As the study is purely qualitative, it is not possible for us to relate our system exactly to the calculations; however, we can imagine that the increase in ka after annealing could lead to the dipolar interactions no longer being the dominant influence on the magnetic behavior of the 3D assemblies. This would explain why, after annealing, the effects of order related to the interparticle interactions are no longer observed. In light of this, it is slightly surprising that the ZFCnorm peak width and Hc of the supracrystal sample remain unchanged after annealing. Further insight could be gained into this phenomenon by studying samples formed from Co nanocrystals annealed in powder form and subsequently arranged in either supracrystal or disordered 3D assemblies. V. Conclusion We have shown that annealing supracrystals of Co nanocrystals at 350 °C leads to an increase in the nanocrystal anisotropy and saturation magnetization with no damage to the fcc superlattice. The enhanced magnetic properties are a result of the crystallographic transition to pure hcp nanocrystals. Surprisingly, the effects of mesoscopic nanocrystal order on the magnetic properties found in the native systems (see ref 17) are no longer observed after annealing. We propose that this is due to the increased nanocrystal anisotropy after annealing

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