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

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 spin chains, each of six identical spins with nearest neighbour anisotropic exchange interactions. Each spin is also anisotropic with an axial anisotropy. The site spin on these short chains take values 1, 3/2 or 2. Using eigenstates of these short chains as the states of Markov chain, we carry out a kMC simulation starting with an initial state in which all chains are completely spin polarized and assembled so as to experience ferromagnetic spindipolar interaction with each other. From these simulations we obtain the relaxation time τ 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 the small size as well as large spin on the molecules.