Magnetic superlattices and their nanoscale phase transition effects

Abstract

The systematic assembly of nanoscale constituents into highly ordered superlattices is of significant interest because of the potential of their multifunctionalities and the discovery of new collective properties. However, successful observations of such superlattice-associated nanoscale phenomena are still elusive. Here, we present magnetic superlattices of Co and Fe3O4 nanoparticles with multidimensional symmetry of either AB (NaCl) or AB2 (AlB2). The discovery of significant enhancement (Ϸ25 times) of ferrimagnetism is further revealed by forming previously undescribed superlattices of magnetically soft-hard Fe3O4@CoFe2O4 through the confined geometrical effect of thermally driven intrasuperlattice phase transition between the nanoparticulate components.binary superlattices ͉ magnetic nanoparticle E arly investigations of nanoparticle assembly have focused on the fabrication of ordered monomeric nanoparticle superlattices with specific packing symmetry (e.g., fcc, hcp) (1-4), and the novel properties arising from these assemblies have been elucidated, such as metal-insulator transitions (2) and band-gap engineering (3). Researchers have further explored ordered binary superlattice systems composed of nanoparticle components that are isostructural with binary crystals of atoms such as NaCl (5), AlB 2 (6-8), and NaZn 13 (8). Ordered two-dimensional (2D) arrays of gold nanoparticles with bimodal size distribution have been reported (6), and further studies have included binary assemblies of gold and silver nanoparticles (6) and of FePt and Fe 3 O 4 in which the interesting magnetic phenomenon of exchange-coupled magnetism (9) has been observed. These methods have been extended to three dimensions as demonstrated in the binary assembly of PbSe and ␥-Fe 2 O 3 nanoparticles (8, 10). Although multimodal superlattices have the potential for the development of advanced nanoarchitectural systems with enhanced properties or multifunctional capabilities, studies on their fabrication and the nanoscale phenomena arising from the assembly are still in its early stages. Here, we present threedimensionally (3D) ordered binary magnetic superlattices constructed through the coassembly of Fe 3 O 4 and Co nanoparticles. By elucidating the effects of relative particle size ratio and stoichiometry ratio on the formation of superlattices, we have developed methods to control the assembled geometry starting from a separated phase and progressing to simple mixed assemblies and highly ordered AB-and AB 2 -type binary superlattices. These binary magnetic nanoparticle superlattice systems exhibit a phase transformation to a previously undescribed superlattice of core-shell type Fe 3 O 4 @CoFe 2 O 4 through thermally driven nanoscale diffusion and redox chemical-reaction processes between symmetrically adjacent nanoparticle components. Results and DiscussionWhen hard spheres of two different sizes are mixed together, these spheres tend to assemble into a new structure with maximum packing density (11-13). Two crucial parameters determine ...

Nanoparticle Superlattices: The Roles of Soft Ligands

“…This can be directly related to the length of the soft capping ligands, which plays a critical role in resulting superlattice properties by controlling interparticle spacing. In ensembles of magnetic nanoparticles, the capping ligand influences the packing density, packing order, and lattice structure of the assemblies, leading to interesting magnetic properties such as increase in blocking temperature or change in shape of hysteresis loop 176, 177, 178, 179, 180, 181. For instance, in a system involving FePt nanoparticle superlattices, the exchange of ligand from oleic acid and oleyl amine to hexanoic acid or hexylamine led to a change in interparticle spacing from ≈4 to ≈1 nm, and a transition change in superlattice structure from hexagonal to cubic packing 182.…”

“…Reproduced with permission 10, 11, 12, 18, 20, 21, 79, 120, 133, 135, 138, 144, 171, 172, 173, 174, 175, 176, 177. Copyright 1998, 2001, 2006, 2007, 2009, 2010, 2011, 2013, Nature Publishing Group.…”

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

“…When analyzing the magnetic properties observed experimentally for diverse hard-soft core/shell nanoparticles a spread of different behaviors can be found [91][92][93][94][95][98][99][100][102][103][104]106,107,110,111,114,115,[121][122][123][125][126][127][128][129][130][131][132]136,140,141,143,175,. First, it should be pointed out that given that we are dealing with nanoparticles the core and shell sizes are rather small (i.e., usually smaller than 2 H ), thus, most of the systems exhibit smooth hysteresis loops typical of strongly exchange coupled hard-soft counterparts.…”

Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

“…Single-and multicomponent superlattices composed of spherical NCs are increasingly studied and a rich family of structures is now accessible (4,5), where the electronic and magnetic interactions between the constituents gives rise to new cooperative properties (6,7). New synthetic approaches are yielding nonspherical NCs with physical properties unobtainable by simply tuning the size of the spheres (8)(9)(10)(11), providing an even broader array of nanoscale building blocks.…”