# Modeling Magnetic Anisotropy of Single Chain Magnets in $|d/J| \geq 1$ Regime

## TL;DR

This paper provides theoretical analysis of single molecule magnets with strong on-site anisotropy, revealing state crossings, the breakdown of total spin conservation, and methods to approximate magnetic anisotropy constants in such systems.

## Contribution

It introduces a theoretical framework for understanding magnetic anisotropy in spin chains with large on-site anisotropy comparable to exchange interactions, including effective Hamiltonian construction.

## Key findings

- Large on-site anisotropy causes state crossings within spin manifolds.
- Total spin conservation breaks down at large anisotropy values.
- Effective spin Hamiltonians can approximate anisotropy constants for low-lying states.

## Abstract

Single molecule magnets (SMMs) with single-ion anisotropies $\mathbf d$, comparable to exchange interactions J, between spins have recently been synthesized. In this paper, we provide theoretical insights into the magnetism of such systems. We study spin chains with site spins, s=1, 3/2 and 2 and on-site anisotropy $\mathbf d$ comparable to the exchange constants between the spins. We find that large $\mathbf d$ leads to crossing of the states with different $M_S$ values in the same spin manifold of the $\mathbf d = 0$ limit. For very large $\mathbf d$'s we also find that the $M_S$ states of the higher energy spin states descend below the $M_S$ states of the ground state spin manifold. Total spin in this limit is no longer conserved and describing the molecular anisotropy by the constants $D_M$ and $E_M$ is not possible. However, the total spin of the low-lying large $M_S$ states is very nearly an integer and using this spin value it is possible to construct an effective spin Hamiltonian and compute the molecular magnetic anisotropy constants $D_M$ and $E_M$. We report effect of finite sizes, rotations of site anisotropies and chain dimerization on the effective anisotropy of the spin chains.