Modeling Molecular Magnets with Large Exchange and On-Site Anisotropies

TL;DR

This paper investigates how exchange and on-site anisotropies influence the magnetic properties of molecular magnets, revealing their combined effects on low-energy spectra and the limitations of effective Hamiltonian descriptions.

Contribution

It provides a detailed analysis of the interplay between exchange and on-site anisotropies, especially for large systems, highlighting when simple models fail.

Findings

Axial anisotropy parameter D_M is additive for small anisotropies.

Intruder states appear in low-energy spectra at higher anisotropy levels.

Synergy between exchange and on-site anisotropies affects large systems.

Abstract

Spins in molecular magnets can experience both anisotropic exchange interactions and on-site magnetic anisotropy. In this paper we study the effect of exchange anisotropy on the molecular magnetic anisotropy both with and without on-site anisotropy. When both the anisotropies are small, we find that the axial anisotropy parameter $D_M$ in the effective spin Hamiltonian is the sum of the individual contributions due to exchange and on-site anisotropies. We find that even for axial anisotropy of about $15\%$, the low energy spectrum does not correspond to a single parent spin manifold but has intruders states arising from other parent spin. In this case, the low energy spectrum can not be described by an effective Hamiltonian spanning the parent spin space. We study the magnetic susceptibility, specific heat as a function of temperature and magnetization as a function of applied field to characterize the system in this limit. We find that there is synergy between the two anisotropies, particularly for large systems with higher site spins.