Spin-spin correlation function and magnetic susceptibility of quantum ferrimagnetic spin chains as model for organic molecule-based ferrimagnetics

Spin-spin correlation function and temperature dependence of magnetic susce ptibility were calculated for the finite-size Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic Heisenberg chain. The S = 1 site in the chain is c omposed of two S = 1/2 spins coupled by finite ferromagnetic interaction. T his is the simplest model Hamiltonian for organic molecule-based ferrimagne tics consisting of two kinds of molecules with different spin quantum numbe rs. The Hamiltonian possesses a magnetic degree of freedom in the S = 1 sit e and a multicentered nature of the intermolecular interactions, both of wh ich are essential features of molecular magnetics. The low-temperature limi t of susceptibility calculated by a quantum Monte Carlo simulation method w as found to obey the Curie law of S = N/2 N(1-1/2) (N: the number of repeat ing units), indicating that the ground state of the chain is ferrimagnetic. A quasi-exponential decay, however, was found in the groundstate correlati on function calculated from the numerical diagonalization of the Hamiltonia n matrix, which implies that the ground state has no long-range order at th e critical point of zero-temperature.