8 Magnetic Self-Assembling of Spherical Co Nanoparticles …
205
8.6.1.4 Collective Magnetic Properties in Fcc Supercrystals of Hcp-Co
Single Crystals
The aim is here to investigate the effect of fcc supercrystals anisotropy on the superspin glass behavior and the possibility for these systems to undergo a transition
toward a dipolar SFM state. For this aim, 3D fcc supercrystals of Co polycrystals are
submitted to the same thermal treatment used to anneal the 2D ordered arrays, at 250,
300 and 350 °C [23]. SEM study (Fig. 8.15a, c, e, g) indicates that heating process
induces the formation of cracks in the supercrystalline film. The beginning of the
formation of cracks is observed at 250 °C (Fig. 8.15c). With increasing the temperature, the process is progressively intensifying to give rise, at 350 °C, to isolated
domains (Fig. 8.15g). GISAXS study clearly indicates that, whatever the heating
temperature is, the fcc superstructure is maintained but not only. We observe both
(1) a progressive decrease of the width at half maximum (δ1/2) of the first-order (111)
Bragg peak and (2) an increase in the second-order reflection intensity (Fig. 8.15b,
d, f, h and Table 8.5). This clearly indicates an increase in the coherence length
of the fcc supercrystals. Besides, the interparticle distance between NPs progressively decreases from 3 to 2.2 nm (Table 8.5). This result shows that we are able to
control both the crystalline structure of Co NPs and their 3D superstructure. Hence,
heating the fcc supercrystals at 350 °C allows the formation of long-range ordered
fcc supercrystals composed of hcp-Co single crystals.
The thermal treatment performed on the fcc supercrystals induces drastic changes
in the magnetic behavior [11]. As shown in Fig. 8.16a–d, the blocking temperature, T B , progressively increases from 112 K for the native sample to 280 K for
the sample annealed at 350 °C. This behavior is mainly explained by the crystallographic transition from almost amorphous NPs to hcp-Co single crystals. After the
annealing at 250 °C, the normalized ZFC peak to the T B (Fig. 8.16e) is broadened
compared to the native sample. At this temperature, the crystallographic transition
is not complete as evidenced by electron diffraction study, leading to a distribution
of the anisotropy in the sample and then to a distribution of the barrier energies.
Annealing performed at higher temperature, 300 and 350 °C, induces a progressive
narrowing of the ZFC norm peak until the native width is recovered. This behavior is
attributed to the crystallographic transition that is complete at 350 °C.
The AC susceptibility as a function of temperature and frequency shows usual
“critical slowing down” behavior indicating the existence of collective states below
T c in all fcc supercrystals [79]. Using the Vogel-Fulcher model for weakly interacting
systems, the particle anisotropy energy E a and the effective temperature (related to the
interaction energy between NPs) are extracted. As a general trend, the “interactionto-anisotropy” energy ratio becomes larger with heating of the sample, i.e., with both
improving Co nanocrystallinity and decreasing the interparticle gap. Such a behavior
is favorable for the formation of a dipolar SFM state.
205
8.6.1.4 Collective Magnetic Properties in Fcc Supercrystals of Hcp-Co
Single Crystals
The aim is here to investigate the effect of fcc supercrystals anisotropy on the superspin glass behavior and the possibility for these systems to undergo a transition
toward a dipolar SFM state. For this aim, 3D fcc supercrystals of Co polycrystals are
submitted to the same thermal treatment used to anneal the 2D ordered arrays, at 250,
300 and 350 °C [23]. SEM study (Fig. 8.15a, c, e, g) indicates that heating process
induces the formation of cracks in the supercrystalline film. The beginning of the
formation of cracks is observed at 250 °C (Fig. 8.15c). With increasing the temperature, the process is progressively intensifying to give rise, at 350 °C, to isolated
domains (Fig. 8.15g). GISAXS study clearly indicates that, whatever the heating
temperature is, the fcc superstructure is maintained but not only. We observe both
(1) a progressive decrease of the width at half maximum (δ1/2) of the first-order (111)
Bragg peak and (2) an increase in the second-order reflection intensity (Fig. 8.15b,
d, f, h and Table 8.5). This clearly indicates an increase in the coherence length
of the fcc supercrystals. Besides, the interparticle distance between NPs progressively decreases from 3 to 2.2 nm (Table 8.5). This result shows that we are able to
control both the crystalline structure of Co NPs and their 3D superstructure. Hence,
heating the fcc supercrystals at 350 °C allows the formation of long-range ordered
fcc supercrystals composed of hcp-Co single crystals.
The thermal treatment performed on the fcc supercrystals induces drastic changes
in the magnetic behavior [11]. As shown in Fig. 8.16a–d, the blocking temperature, T B , progressively increases from 112 K for the native sample to 280 K for
the sample annealed at 350 °C. This behavior is mainly explained by the crystallographic transition from almost amorphous NPs to hcp-Co single crystals. After the
annealing at 250 °C, the normalized ZFC peak to the T B (Fig. 8.16e) is broadened
compared to the native sample. At this temperature, the crystallographic transition
is not complete as evidenced by electron diffraction study, leading to a distribution
of the anisotropy in the sample and then to a distribution of the barrier energies.
Annealing performed at higher temperature, 300 and 350 °C, induces a progressive
narrowing of the ZFC norm peak until the native width is recovered. This behavior is
attributed to the crystallographic transition that is complete at 350 °C.
The AC susceptibility as a function of temperature and frequency shows usual
“critical slowing down” behavior indicating the existence of collective states below
T c in all fcc supercrystals [79]. Using the Vogel-Fulcher model for weakly interacting
systems, the particle anisotropy energy E a and the effective temperature (related to the
interaction energy between NPs) are extracted. As a general trend, the “interactionto-anisotropy” energy ratio becomes larger with heating of the sample, i.e., with both
improving Co nanocrystallinity and decreasing the interparticle gap. Such a behavior
is favorable for the formation of a dipolar SFM state.