Study of magnetization relaxation in molecular spin clusters using an innovative kinetic Monte Carlo method

TL;DR

This study uses an innovative kinetic Monte Carlo method to model magnetization relaxation and blocking temperature in molecular spin clusters, revealing how various anisotropies and interactions influence energy barriers.

Contribution

The paper introduces a novel kinetic Monte Carlo simulation approach to analyze magnetization relaxation in large assemblies of molecular spin clusters, accounting for anisotropic interactions and dipolar effects.

Findings

Relaxation times exhibit non-Arrhenius behavior with weak on-site interactions.

Energy barriers increase with on-site anisotropy, exchange anisotropy, and dipolar interaction strength.

Barrier saturation occurs at high on-site anisotropy and larger site spins.

Abstract

Modeling blocking temperature in molecular magnets has been a long standing problem in the field of molecular magnetism. We investigate this problem using a kinetic Monte Carlo (kMC) approach on an assembly of 100,000 short molecular magnetic chains (SMMCs), each of six identical spins with nearest neighbour anisotropic ferromagnetic exchange interactions. Each spin is also anisotropic with an uniaxial anisotropy. The site spin on these SMMCs take values $1$, $3/2$ or $2$. Using eigenstates of these SMMCs as the states of Markov chain, we carry out a kMC simulation starting with an initial state in which all SMMCs are completely spin polarized and assembled on a one-dimensional lattice so as to experience ferromagnetic spin-dipolar interaction with each other. From these simulations we obtain the relaxation time $\tau_r$ as a function of temperature and the associated blocking temperature. We study this for different exchange anisotropy, on-site anisotropy and strength of dipolar interactions. The magnetization relaxation times show non-Arrhenius behaviour for weak on-site interactions. The energy barrier to magnetization relaxation increases with increase in on-site anisotropy, exchange anisotropy and strength of spin dipolar interactions; more strongly on the last parameter. In all cases the barrier saturates at large on-site anisotropy. The barrier also increases with site spin. The large barrier observed in rare-earth single ion magnets can be attributed to large dipolar interactions due to short intermolecular distances, owing to their small size and large spin of the rare earth ion in the molecule.