Magnetically Enhanced Mechanical Stability and Super‐Size Effects in Self‐Assembled Superstructures of Nanocubes

Abstract

Artificial materials from the self‐assembly of magnetic nanoparticles exhibit extraordinary collective properties; however, to date, the contribution of nanoscale magnetism to the mechanical properties of this class of materials is overlooked. Here, through a combination of Monte Carlo simulations and experimental magnetic measurements, this contribution is shown to be important in self‐assembled superstructures of magnetite nanocubes. By simulating the relaxation of interacting macrospins in the superstructure systems, the relationship between nanoscale magnetism, nanoparticle arrangement, superstructure size, and mechanical stability is established. For all considered systems, a significant enhancement in cohesive energy per nanocube (up to 45%), and thus in mechanical stability, is uncovered from the consideration of magnetism. Magnetic measurements fully support the simulations and confirm the strongly interacting character of the nanocube assembly. The studies also reveal a novel super‐size effect, whereby mechanically destabilization occurs through a decrease in cohesive energy per nanocube as the overall size (number of particles) of the system decreases. The discovery of this effect opens up new possibilities in size‐controlled tuning of superstructure properties, thus contributing to the design of next‐generation self‐assembled materials with simultaneous enhancement of magnetic and mechanical properties.

Reconfigurable Mechanical Anisotropy in Self‐Assembled Magnetic Superstructures

“…[27] This is consistent with previous findings in identical (but translationally static) systems. [13] In the cobalt ferrite case, however, the macrospins do not undergo any of the above relaxation processes and remain vertically aligned owing to the strong MA, and, crucially, to the fact that their initial magnetization already lies along one of the Figure 2. Spin relaxation and interaction energies in in-field self-assembled iron oxide and cobalt ferrite superstructures of different aspect ratios (A = {2/5, 5/5, 8/5}).…”

“…To develop a conceptual understanding of the effect of magnetic anisotropy on mechanical properties, we compare the two material systems on equal grounds by only considering the more stable simple cubic arrangement for iron oxide as based on previous reports. [13,26] During self-assembly, the macrospins of the NCs in a given solution will align in the direction of the (sufficiently strong) applied magnetic field and remain aligned after the assembly process until the field is switched off. To gain deeper insight into how the NC macrospins interact in the model systems after self-assembly, we use an MC method based on a model reported in a previous study (Metropolis-Hastings algorithm in the NVT ensemble).…”

“…Moreover, in both material systems, a super-size effect is evident, implying on average a mechanical destabilization as the superstructure systems become smaller in size, consistent with our previous study. [13] Further analysis of the variation in cohesive energy after taking magnetism into account in the two different systems reveals the influence of magnetic anisotropy on the mechanical stability. The percentage increase (%increase) in cohesive energy is calculated relative to the equivalent nonmagnetic system of iron oxide and cobalt ferrite (i.e., where the simulation is rerun after magnetism has been removed (details in Figure S8, Supporting Information), enabling a comparison of the two material systems on equal footings) and plotted in Figure 3c,d, respectively.…”

“…Very recently, we showed (for the first time) how nanoscale magnetic interactions can mechanically stabilize self‐assembled magnetic nanocomposite superstructures (based on oleic acid‐coated superparamagnetic magnetite nanocubes), leading to a significant enhancement of mechanical properties (increase in the cohesive energy per nanoparticle up to 45%). [ 13 ] To date, there is no rigorous fundamental understanding of how intrinsic magnetic properties of materials (e.g., magnetic anisotropy and coercivity) will affect the mechanical properties of superstructures. To maintain stable collective properties over time, it is imperative to develop new materials design rules on how the mechanical properties may be optimized.…”

New insights into size-controlled reproducible synthesis of anisotropic Fe3O4 nanoparticles: the importance of the reaction environment

“…18,19 Such magnetic superstructures exhibit collective magnetic and magnetically enhanced mechanical properties which can be tailored by the shape of the NPs as well as the morphology of self-assembled superstructures. 20 In the last two decades, various approaches based on the microemulsion, 21 co-precipitation, 22 hydrothermal, 23 and thermal decomposition 10,[24][25][26][27] methods have been researched for the synthesis of anisotropic Fe 3 O 4 NPs. Among these, thermal decomposition of iron oleate in the presence of oleic acid and/or sodium oleate and octadecene has been used to synthesize Fe 3 O 4 NPs in different shapes (cubic, octapods, octahedra, plates, and rods), 11,18,28 resulting in good control over the shape and size (3-25 nm).…”