Calixarene-stabilised cobalt nanoparticle rings: Self-assembly and collective magnetic properties

Abstract

Calixarenes can be used to promote the self-assembly of thermoremanent cobalt nanoparticles into bracelet-like rings below 100 nm in diameter. These kinetically stable assemblies are regulated by the equilibrium between enthalpic gain (dipoledipole and long-range van der Waals interactions) and entropic loss, analogous to the thermodynamic balance of forces governing supramolecular self-assembly. Examination of the Co nanoparticle rings by electron holography (an electron microscopy technique for imaging in-plane magnetic induction) reveals the existence of chiral flux closure (FC) domains at room temperature, comprising a 'racemic' mixture of clockwise and anticlockwise states. Furthermore, these FC polarisations can be reversed by applying out-of-plane magnetic pulses (H z ) in alternating directions. This switching behaviour has no known analogy at the macroscopic level, and may represent a uniquely nanoscale phenomenon.

Self-assembly and flux closure studies of magnetic nanoparticle rings

“…7bd). 20,31 The in-plane magnetization of the MNP rings was characterized at 298 K by off-axis EH, which revealed a continuous loop of magnetic induction with a net CW polarization (Fig. 7e and f).…”

“…8a-f). 31 A racemic distribution of CW and CCW states is assumed, given the absence of a polarization bias. The magnetic flux loop is continuous despite structural defects in the MNPs, or deviations from radial symmetry in the self-assembled rings.…”

“…22 The retention of polarization by the MNP rings is surprisingly robust: the FC states are not switched after applying +H z fields as high as 2 Tesla, although minor perturbations in magnetic flux can be observed. 31 However, roughly 90% of the bracelet-like MNP rings experience a switch in FC polarization after exposure to magnetic pulses in the opposite (ÀH z ) direction. 34 The threshold for the H z -induced FC switching is determined by the collective magnetization (coercivity) of the MNP ring, which depends more on magnetocrystalline anisotropy and the dipolar coupling between NPs than on ring size.…”

Fabrication and magnetic properties of hierarchical nickel microwires with nanothorns

“…It has been proven that bracelet-like cobalt rings by the self-assembly of dipole-dipole and long range van der Waals interactions, reveals the existence of chiral flux closure (FC), which represents a unique nanoscale phenomenon [12]. Xie et al reported that alpha-Fe nanowires were synthesized via a simple, mild reduction process, and exhibited excellent magnetic behavior [13].…”

Organization and controlled coupling between soft and hard magnetic nanocrystals using mesoporous materials

“…43 While cubic (fcc) FePt nanocrystals show superparamagnetic behavior similar to pure metal nanocrystals, FePt nanocrystals in the face centered tetragonal (fct) structure possess an unusually high uniaxial magnetocrystalline anisotropy (K u ∼ 5 × 10 7 J/cm 3 ), 43 which results in hard (ferromagnetic behavior) in nanocrystals as small as 4 nm in size. 44 A variety of methods are available to create order in nanocrystal arrays, [45][46][47][48][49][50][51][52][53]82,83 but, here, we chose to arrange nanocrystals in the pores of a mesoporous silica host, created using block-copolymer templating. 54 Many phases of polymer-templated silica can be synthesized, [55][56][57][58] with pore sizes ranging from 10 to 200 Å.…”