Magnetic Nanorods: Genesis, Self-Organization and Applications

Abstract

Magnetic-field-assisted self-assembly of magnetic dipole moment carrying iron nanoparticles is shown to result in the formation of magnetic and mechanically stiff nanoscale rods. The cooperative behavior of an ensemble of such rods and bundles thereof exhibits self-organized pattern formation on different length scales. Pattern formation on large length scales reveals great similarity with physical systems undergoing spinodal decomposition. Possible applications for dipolar magnetic nanorods in the field of perpendicular storage media are highlighted. We discuss an aerosol-synthesis-route allowing to prepare ferrofluids (FF) with shape-anisotropic particles constituting the magnetic phase immersed in the nonmagnetic carrier fluid. These so-called nanorod FF unveil a two orders of magnitude increase of viscosity enforced by an applied field of 10mT even at shear rates larger than 10-2s. This raises prospects for applications in microfluidics and MEMS.

Magnetic, Multilayered Nanotubes of Low Aspect Ratios for Liquid Suspensions

“…[ 54 , 55 ] It is remarkable that even at a magnetic volume fraction of only 0.02 vol% a significant magnetoviscosity could be measured, comparable to the values obtained for a magnetic nanorod suspension, as reported by Birringer et al in 2008. [ 27 ] comparison to superparamagnetism, in which thermal energy causes a rapid change in orientation of the magnetic moments within a particle, the majority of the magnetic moments within our nanotubes is fi xed in direction, but the tubular particle itself is able to move and rotate. Thereby, the magnetization direction imposed by an external magnetic fi eld is lost in the zero-fi eld state.…”

“…Additional viscosity measurements performed at a constant applied magnetic fi eld (45 mT) exhibited a nonlinear decrease in the magnetoviscosity as a function of the frequency-an example of the shear-thinning effect. [ 54 , 55 ] It is remarkable that even at a magnetic volume fraction of only 0.02 vol% a significant magnetoviscosity could be measured, comparable to the values obtained for a magnetic nanorod suspension, as reported by Birringer et al in 2008. [ 27 ] comparison to superparamagnetism, in which thermal energy causes a rapid change in orientation of the magnetic moments within a particle, the majority of the magnetic moments within our nanotubes is fi xed in direction, but the tubular particle itself is able to move and rotate.…”

“…Replacement of the spherical nanoparticles of conventional ferrofl uids by rigid elongated nanorods may circumvent this viscosity drop at higher shear rates. [ 27 ] The colloidal behavior of elongated solid nanowires suspended in a carrier liquid as well as their individual behavior has already been investigated. [ 28 , 29 ] On the one hand, liquid suspensions of magnetic particles have found technological application in magnetic seals, [ 30 ] lithography masks, [ 31 ] and damping systems.…”

Translational and rotational dynamics in suspensions of magnetic nanorods

“…Indeed, magnetic nanorods can nowadays be synthesized from a variety of materials, [6][7][8][9] and experiments of ensembles of such particles already suggest that they display special material properties when compared to those of ferrouids with spherical particles. Examples are enhancements of the magnetoviscous effect, 10 magnetic birefringence 11 and the thermal conductivity. 12 However, a thorough, fundamentally oriented understanding of these features from a microscopic perspective is still missing.…”

Ferrofluids: Synthetic Strategies, Stabilization, Physicochemical Features, Characterization, and Applications

“…However, this problem can be reduced by replacing loosely bound MNPs with stiff magnetic nanotubes or nanorods. [240] Considerable progress in understanding the interdependence of the magnetoviscous effect and thermal properties of magnetic nanofluids is being made, which can help in the enhancement of nanofluid thermal conductivity for heat transfer and cooling applications. [241] The exact mechanism of heat transfer is poorly understood.…”