Magnetic properties of Cr$_8$ and V$_8$ molecular rings from ab initio calculations

TL;DR

This study uses density functional theory to calculate exchange interactions in Cr8 and V8 molecular rings, revealing contrasting magnetic ground states and providing insights into their electronic structure and magnetic behavior.

Contribution

It introduces a DFT-based method to determine exchange parameters in ring-shaped molecular magnets, highlighting differences in magnetic states despite structural similarities.

Findings

Cr8 exhibits antiferromagnetic ground state

V8 exhibits ferromagnetic ground state

Second neighbor couplings are significant in V8

Abstract

Molecular nanomagnets are systems with a vast phenomenology and are very promising for a variety of technological applications, most notably spintronics and quantum information. Their low-energy spectrum and magnetic properties can be modeled using effective spin Hamiltonians, once the exchange coupling parameters between the localized magnetic moments are determined. In this work we employ density functional theory (DFT) to compute the exchange parameters between the atomic spins for two representative ring-shaped molecules containing eight transition-metal magnetic ions: Cr$_8$ and V$_8$. Considering a set of properly chosen spin configurations and mapping their DFT energies on the corresponding expressions from a Heisenberg Hamiltonian, we compute the exchange couplings between magnetic ions which are first, second and further neighbors on the rings. In spite of their chemical and structural similarities the two systems exhibit very different ground states: antiferromagnetic for Cr$_8$, ferromagnetic for V$_8$, which also features non-negligible couplings between second nearest neighbors. A rationalization of these results is proposed that is based on a multi-band Hubbard model with less-than-half filled shells on magnetic ions.