Correlation effects and orbital magnetism of Co clusters

TL;DR

This paper compares various theoretical methods for calculating the orbital magnetic moments of Co clusters, highlighting that only the LDA+DMFT approach accurately reproduces experimental results, emphasizing the role of dynamic correlations.

Contribution

It evaluates different computational methods for orbital magnetism in Co clusters and demonstrates the necessity of LDA+DMFT for accurate predictions.

Findings

Conventional DFT underestimates orbital moments.

LDA+DMFT accurately reproduces experimental orbital moments.

Dynamic correlations are crucial for magnetic properties in nano-sized magnets.

Abstract

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nano-size regime.