A Molecular Quantum Description of Spin Alignments in Molecule-Based Ferrimagnets: Numerical Calculations of Thermodynamic Properties

Abstract

A physical picture of electron spin alignments in organic molecule-based ferrimagnets is given from numerical calculations of magnetic specific heat (C), magnetic entropy (Smag), and magnetic susceptibility (χ) as functions of temperature (T) and static magnetic field (B) in terms of a Heisenberg Hamiltonian for an alternating spin chain. The numerical results are compared with those for atom-based ferrimagnets. One of the two kinds of spin sites in the chain represents an organic molecule with two S = 1/2 spins, which are coupled to give a ground-state triplet (S = 1) biradical molecule. The biradical molecule is coupled with adjacent S = 1/2 monoradicals by the intermolecular antiferromagnetic interactions. When the strength of the intermolecular antiferromagnetic interactions is dimerized along the chain, three peaks in the C vs T curve appear. Two of the three peaks shift to higher and lower temperatures with increasing magnetic field, B, indicating that one originates in the ferromagnetic nature and the other in the antiferromagnetic nature, respectively. The magnetic entropy, Smag, exhibits 3-fold stepwise drops as T is lowered. One of the drops with the stationary value of Smag = kB ln 2 corresponds to generation of an effective S = 1/2 spin in the unit cell of the chain as a result of the coupling of adjacent S = 1 and S = 1/2 spins. With the aid of quantum Monte Carlo simulations of magnetic susceptibility, χ, the ferrimagnetic spin alignment in the alternating molecular chains of biradicals and monoradicals is shown to be equivalent to the ferromagnetic alignment of the effective S = 1/2 spins. A spin polarization effect affording the effective ferromagnetic interactions between the effective S = 1/2 spins is demonstrated in terms of a simple Heisenberg model.