Magnetic Anisotropy of Isolated Cobalt Nanoplatelets

TL;DR

This study models the magnetic anisotropy of two-monolayer thick cobalt nanoplatelets using a tight-binding approach, revealing how lattice truncation and exchange interactions influence magnetic easy directions.

Contribution

It introduces a microscopic tight-binding model to analyze magnetic anisotropy in cobalt nanoplatelets, considering different lattice truncations and exchange interactions.

Findings

Higher coordination in (111) truncation favors perpendicular easy axis.

The intra-atomic exchange integral J critically influences anisotropy.

Larger J values lead to perpendicular easy axes and increased anisotropy energy.

Abstract

Motivated in part by experiments performed by M.H. Pan et al. (nanoletters, v.5, p 83, 2005), we have undertaken a theoretical study of the the magnetic properties of two-monolayer thick Co nanoplatelets with an equilateral triangular shape. The analysis is carried out using a microscopic Slater-Koster tight-binding model with atomic exchange and spin-orbit interactions designed to realistically capture the salient magnetic features of large nanoclusters containing up to 350 atoms. Two different truncations of the FCC lattice are studied, in which the nanoplatelet surface is aligned parallel to the FCC (111) and (001)crystal planes respectively. We find that the higher coordination number in the (111) truncated crystal is more likely to reproduce the perpendicular easy direction found in experiment. Qualitatively, the most important parameter governing the anisotropy of the model is found to be the value of the intra-atomic exchange integral J. If we set the value of J near the bulk value in order to reproduce the experimentally observed magnitude of the magnetic moments, we find both quasi-easy-planes and perpendicular easy directions. At larger values of J we find that the easy-axis of magnetization is perpendicular to the surface, and the value of the magnetic anisotropy energy per atom is larger. The possible role of hybridization with substrate surface states in the experimental systems is discussed.