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

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 (dipole-dipole 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 (Hz) in alternating directions. This switching behaviour has no known analogy at the macroscopic level, and may represent a uniquely nanoscale phenomenon. [ABSTRACT FROM AUTHOR]