Graphene lattices with embedded transition-metal atoms and tunable magnetic anisotropy energy: Implications for spintronic devices

Abstract

Doping of the graphene lattice with transition-metal atoms resulting in a high magnetic anisotropy energy (MAE) is an important goal of materials research owing to its potential application in spintronics. In this article, using spin-polarized density functional theory including spin-orbit coupling, we examined the magnetic properties of graphene with vacancy defects, both bare and nitrogen-decorated, and doped by Cr, Mn, and Fe transition-metal single atom (TM-SA) and two different TM atoms simultaneously. The adsorption of a second TM atom on an already embedded TM atom, i.e., the formation of upright TM dimers, was also considered. It is found that the graphene-mediated coupling between TM dopants can significantly increase MAE compared to that of SA impurities. While the MAE of TM-SA did not exceed 2 meV, it was enhanced to -23 meV for Cr and Fe simultaneously embedded into two separated double-vacancy (DV) defects and to a remarkably high value of 119.7 meV for two upright Fe-Mn dimers bound to two separate DVs, considerably exceeding the sum for individual TM-SAs. The latter MAE corresponds to a blocking temperature of 34 K assuming a relaxation time of 10 years. The origin of the enhanced MAE is discussed in relation to the spin excitations at the Fermi level and changes in d-derived states accompanying the rotation of the magnetization between in-plane and out-of-plane directions. We demonstrate that the presence of partially occupied degenerate states at the Fermi level favors its formation. The stability of the systems is also discussed. The computational findings are supplemented by an atomic-resolution characterization of an incidental Mn impurity bonded to four carbon atoms, whose localized spin matches expectations as measured using core-level electron energy-loss spectroscopy. Conducting TM-doped graphene with robust magnetic features offers prospects for the design of graphene-based spintronic devices.