Beyond Entropy: Magnetic Forces Induce Formation of Quasicrystalline Structure in Binary Nanocrystal Superlattices

Here, it is shown that binary superlattices of Co/Ag nanocrystals with the same size, surface coating, differing by their type of crystallinity are governed by Co–Co magnetic interactions. By using 9 nm amorphous-phase Co nanocrystals and 4 nm polycrystalline Ag nanocrystals at 25 °C, triangle-shaped NaCl-type binary nanocrystal superlattices are produced driven by the entropic force, maximizing the packing density. By contrast, using ferromagnetic 9 nm single domain (<i>hcp</i>) Co nanocrystals instead of amorphous-phase Co, dodecagonal quasicrystalline order is obtained, together with less-packed phases such as the CoAg₁₃ (NaZn₁₃-type), CoAg (AuCu-type), and CoAg₃ (AuCu₃-type) structures. On increasing temperature to 65 °C, 9 nm <i>hcp</i> Co nanocrystals become superparamagnetic, and the system yields the CoAg₃ (AuCu₃-type) and CoAg₂ (AlB₂-type) structures, as observed with 9 nm amorphous Co nanocrystals. Furthermore, by decreasing the Co nanocrystal size from 9 to 7 nm, stable AlB₂-type binary nanocrystal superlattices are produced, which remain independent of the crystallinity of Co nanocrystals with the superparamagnetic state

Influence of Protected Annealing on the Magnetic Properties of γ‑Fe₂O₃ Nanoparticles

It is usually considered that nanoparticles synthesized by low-temperature routes present structural disorder, from extended defects to local rearrangements (e.g., vacancy ordering or inversion in spinel ferrites), that may severely impact their magnetic properties. In the present work, we have investigated the influence of postsynthesis thermal treatments on 7-nm-sized γ-Fe₂O₃ nanoparticles prepared by room temperature coprecipitation of ferric and ferrous salts in alkaline medium, followed by the dispersion of the preformed particles in a sol–gel silica binder. Such protected annealing in a refractory matrix prevents coalescence and growth, thus preserving the mean size and size distribution of the pristine particles. Structural characterizations show that heat treatments up to 1000 °C turned the raw grains into well-crystallized particles without transformation into hematite. This strategy thus allows accounting for the influence of structural rearrangements on magnetic properties at fixed particle size. For such 7 nm particles, postsynthesis heat treatments were found to mainly influence the shell of misaligned spins at the surface

Dispersion of Hydrophobic Co Supracrystal in Aqueous Solution

Assembly of nanoparticles into supracrystals provides a class of materials with interesting optical and magnetic properties. However, supracrystals are mostly obtained from hydrophobic particles and therefore cannot be manipulated in aqueous systems, limiting their range of applications. Here, we show that hydrophobic-shaped supracrystals self-assembled from 8.2 nm cobalt nanoparticles can be dispersed in water by coating the supracrystals with lipid vesicles. A careful characterization of these composite objects provides insights into their structure at different length scales. This composite, suspended in water, retains the crystalline structure and paramagnetic properties of the starting material, which can be moved with an applied magnetic field