Giant magnetic anisotropy at nanoscale: overcoming the superparamagnetic limit

TL;DR

This paper investigates giant magnetic anisotropy in nanoscale gold and palladium particles, attributing it to surface-induced orbital moments and spin-orbit interactions, enabling permanent magnetism at high temperatures.

Contribution

It reveals that surface orbital moments driven by spin-orbit coupling are key to understanding giant magnetic anisotropy in nanoscale gold films, surpassing traditional superparamagnetic limits.

Findings

Giant magnetic anisotropy observed in nanoscale gold and palladium.

Surface orbital moments induce effective magnetic fields around 10^3 T.

Permanent magnetism persists up to room temperature in small nanoparticles.

Abstract

It has been recently observed for palladium and gold nanoparticles, that the magnetic moment at constant applied field does not change with temperature over the range comprised between 5 and 300 K. These samples with size smaller than 2.5 nm exhibit remanence up to room temperature. The permanent magnetism for so small samples up to so high temperatures has been explained as due to blocking of local magnetic moment by giant magnetic anisotropies. In this report we show, by analysing the anisotropy of thiol capped gold films, that the orbital momentum induced at the surface conduction electrons is crucial to understand the observed giant anisotropy. The orbital motion is driven by localised charge and/or spin through spin orbit interaction, that reaches extremely high values at the surfaces. The induced orbital moment gives rise to an effective field of the order of 103 T that is responsible of the giant anisotropy.