A combined first-principles and thermodynamic approach to M-Nitronyl Nitroxide (M=Co, Mn) spin helices

TL;DR

This study combines first-principles calculations and thermodynamic analysis to understand the magnetic properties of Co- and Mn-based Nitronyl Nitroxide helices, revealing complex interactions and anisotropies.

Contribution

It introduces a detailed microscopic model and spin Hamiltonian for these molecular magnetic helices, advancing understanding of their magnetic behavior.

Findings

Strong metal-radical exchange coupling (~44-48 meV)

Mn-Mn antiferromagnetic interactions (~6 meV)

Non-collinear anisotropies in Co-sites

Abstract

The properties of two molecular-based magnetic helices, composed of 3$d$ metal Co and Mn ions bridged by Nitronyl Nitroxide radicals, are investigated by density functional calculations. Their peculiar and distinctive magnetic behavior is here elucidated by a thorough description of their magnetic, electronic, and anisotropy properties. Metal ions are antiferromagnetically coupled with the radicals, leading to a ferrimagnetically ordered ground state. A strong metal-radical exchange coupling is found, about 44 meV and 48 meV for Co- and Mn-helices, respectively. The latter have also relevant next-nearest-neighbor Mn-Mn antiferromagnetic interactions (of $\sim$ 6 meV). Co-sites are characterized by non-collinear uniaxial anisotropies, whereas Mn-sites are rather isotropic. A key result pertains to the Co-helix: the microscopic picture resulting from density-functional calculations allows us to propose a spin Hamiltonian of increased complexity with respect to the commonly employed Ising Hamiltonian, suitable for the study of finite-temperature behavior, and that seems to clarify the puzzling scenario of multiple characteristic energy scales observed in experiments.