Magnetic properties and quantum phase transitions of purely organic molecule-based ferrimagnets based on Green’s function theory

Magnetic properties of a Heisenberg diamondlike spin chain model for purely organic molecule-based ferrimagnets are investigated by means of the many-body Green’s function method within random phase approximation. The molecule-based ferrimagnet is composed of $S=1$ biradical and $S=1∕2$ monoradical molecules alternating with intermolecular antiferromagnetic (AF) interactions, and the $S=1$ site is composed of two $S=1∕2$ spins by a finite intramolecular ferromagnetic $(F)$ interaction. The numerical results reveal that occurrence of ferrimagnetic spin alignments along the chain is determined by the intermolecular AF interactions. Owing to the very small intermolecular AF interactions, the curves of the product of magnetic susceptibility and temperature $(χT)$ against temperature display as a round peak at low temperatures, and the ferrimagnetic phase transition could only be detected at ultralow temperatures in practical organic compounds. Temperature- and magnetic-field-induced magnetic phase transitions are discussed, which are consistent with the experimental findings. The lower spatial symmetry of intermolecular interactions makes it easy to form spin pairs with a singlet $(S=0)$ ground state along the chain and to reduce Curie temperature. The formations of molecular dimers and trimers along the chain have contributions to bring about $F$ alignments with effective $S=1∕2$ magnetic supramolecules and to enhance Curie temperature. In addition, the experimental data of the magnetic susceptibility measurements for a molecule-based ferrimagnet are also fairly compared with our theoretical results.