Spontaneous and Externally Driven Quantum Spin Fluctuations of 3d and 4d Single Atoms Adsorbed on Graphene

Published: 2019
arXiv.org (Cornell University)

Abstract

At the heart of current information nanotechnology lies the search for ideal platforms hostingthe smallest possible magnets, i.e. single atoms with magnetic moments pointing out-of-plane, asrequested in a binary-type of memory. For this purpose, a 2D material such as graphene wouldbe an ideal substrate thanks to its intrinsic low electron and phonon densities, as well as its 6-fold symmetry. Here we investigate, from first-principles, a fundamental mechanism detrimentalfor the magnetic stability: the zero-point spin-fluctuations modifying the effective energy landscapeperceived by the local spin moments of 3d and 4d transition metal atoms deposited on a free standinggraphene. Utilizing time-dependent density functional theory and by virtue of the fluctuation-dissipation theorem, these spontaneous quantum fluctuations are found to be negligible for most ofthe 3d elements, in strong contrast to the 4d atoms. Surprisingly, we find that such fluctuationscan promote the magnetic stability by switching the easy direction of the magnetic moment of Tc from being initially in-plane to out-of-plane. The adatom-graphene complex gives rise to impuritystates settling in some cases the magnetocrystalline anisotropy energy — the quantity that definesthe energy barrier protecting the magnetic moments and, consequently, the spin-excitation behaviordetectable with inelastic scanning tunneling spectroscopy. A detailed analysis is provided on theimpact of electron-hole excitations, damping and lifetime of the spin-excitations on the dynamicalbehavior of the adsorbed magnetic moments on graphene