A Prerequisite for Purely Organic Molecule-Based Ferrimagnetics:  Breakdown of Simple Classical Pictures

Abstract

Spin state energies were calculated for an antiferromagnetic linear chain composed of a doublet (S ) 1 / 2 ) radical and a biradical with a triplet (S ) 1) ground state by numerical diagonalization of a finite-size Heisenberg-Dirac spin Hamiltonian. A ferrimagnetic-like ground state with a spin value of S ) N/2 (N stands for the number of repeating units) appears if the intermolecular antiferromagnetic exchange interactions are uniform between all the S ) 1 / 2 sites. This finding is consistent with a simple picture of differing and neighboring spin angular moments S's ordered in sublattices in an antiparallel manner. On the other hand, quasi-degenerate low-lying states appear as the spatial symmetry of the intermolecular antiferromagnetic interactions is lowered. The quasi-degeneracy destabilizes the ferrimagnetic-like ground state. This instability is inherent in multicentered antiferromagnetic interactions between open-shell organic molecular systems, suggesting a prerequisite for long-range magnetic order in purely organic molecule-based ferrimagnetics. Such prerequisite has never been reported until now. The results of the calculations are invoked to explain the susceptibility and the spin-spin relaxation time measured for the first model system for purely organic ferrimagnetics, a nitronyl nitroxide-based molecular crystal composed of an S ) 1 / 2 and S ) 1 molecules (

Low-Temperature Properties of Quasi-One-Dimensional Molecule-Based Ferromagnets

“…While most of thus-farsynthesized ferrimagnets are inorganic-organic hybrid materials, there may be genuine organic ferrimagnets, where the variety and flexibility of the crystal structure, as well as sufficiently small magnetic anisotropy, may enable us to investigate quantum mixed-spin phenomena extensively and essentially. An example approach [49,50] consists of synthesizing an alternating chain of a monoradical and a biradical with a triplet ground state. However, the low-temperature properties of the obtained compounds deviate from the ferrimagnetic behavior suggesting the formation of local singlet clusters.…”

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

“…Thus, the molecular design serving for the syntheses of the organic ferrimagnets is a challenging issue in materials science. We have examined the possible occurrence of the ferrimagnetic spin alignments in organic molecular assemblages both by experiments and by theoretical calculations in quantum terms [24][25][26][27][28][29][30][31][32].…”

Benzoic acid and phenol derivatives of nitronyl nitroxide biradical as building blocks of organic molecule-based ferrimagnets

“…The structural features described above suggest that the magnetic behavior of 1 is approximated as an assembly of the isolated dimers. The dimer consisting of four electron spins is described by a model Hamiltonian of a four-spin cluster cam= -2J1 (S 1 •S2 + S3 •S4) -2J2(S,•S3 + S2 S4) (4) where J, is the intramolecular exchange interaction and J 2 is the intermolecular exchange interaction. The Hamiltonian Eq.…”

“…The model compound exhibited no phase transition to ferrimagnetic long-range ordered state. In our previous papers [4,5], we reported the continuous wave (cw) and pulsed electron spin resonance (ESR) spectra of the model compound and numerical calculations of a spin Hamiltonian for the S = 1/2 and S = 1 alternating chain. We proposed a formation of singlet (S = 0) pairs between the monoradical and the biradical molecules as a possible mechanism for suppressing ferrimagnetic spin alignment.…”