Spontaneous and externally driven quantum spin fluctuations of 3d and 4d single atoms adsorbed on graphene

TL;DR

This study investigates quantum spin fluctuations of 3d and 4d transition metal atoms on graphene, revealing negligible effects for most 3d elements but significant, sometimes stabilizing, effects for 4d atoms, with implications for nanoscale magnetic memory.

Contribution

It provides a first-principles analysis of zero-point spin-fluctuations on graphene, highlighting their contrasting effects on 3d and 4d transition metal adatoms and their influence on magnetic stability.

Findings

Quantum fluctuations are negligible for most 3d atoms.

4d atoms exhibit significant quantum spin fluctuations.

Fluctuations can switch the magnetic easy axis, notably for Tc.

Abstract

At the heart of current information nanotechnology lies the search for ideal platforms hosting the smallest possible magnets, i.e. single atoms with magnetic moments pointing out-of-plane, as requested in a binary-type of memory. For this purpose, a 2D material such as graphene would be 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 detrimental for the magnetic stability: the zero-point spin-fluctuations modifying the effective energy landscape perceived by the local spin moments of 3$d$ and 4$d$ transition metal atoms deposited on a free standing graphene. 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 of the 3$d$ elements, in strong contrast to the 4$d$ atoms. Surprisingly, we find that such fluctuations can 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 impurity states settling in some cases the magnetocrystalline anisotropy energy --- the quantity that defines the energy barrier protecting the magnetic moments and, consequently, the spin-excitation behavior detectable with inelastic scanning tunneling spectroscopy. A detailed analysis is provided on the impact of electron-hole excitations, damping and lifetime of the spin-excitations on the dynamical behavior of the adsorbed magnetic moments on graphene.