Organic Molecule-based Ferrimagnets Research Articles

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 (chiT) 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.

Magnetic Properties of a Nitronyl Nitroxide Triradical as a Model for Single-Component Molecule-Based Ferrimagnets

Two kinds of organic triradicals consisting of a nitronyl nitroxide biradical with S =1 and a monoradical with S = 1/2 were synthesized as model molecules for single-component organic ferrimagnets. The intramolecular exchange interactions of the triradicals were examined from magnetic susceptibility. The results indicate that the triradicals have potential as building blocks for single-component organic molecule-based ferrimagnets.

Building Blocks for Organic Heterospin, Heteromolecular Complexes as Models for Organic Molecule-Based Ferrimagnets

Magnetic properties and crystal structure of a pyridine-substituted iminonitroxide biradical, 3,5-bis(4,4,5,5,-tetramethyl-4,5-dihydro-1H-imidazol-2-yl-1-oxyl)pyridine (1), were examined. The molecular ground state of1 was found to be triplet (S=1) with a singlet-triplet energy gap of 2J/kB=7.0K. The triplet biradical serves as a building block for moleculebased magnets of S>1/2 based on intermolecular noncovalent bonds.

Benzyl-phenyl ether derivatives of nitronyl nitroxide triradicals as a model for single-component organic molecule-based ferrimagnetics

As a novel molecular design for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of “single-component organic molecule-based ferrimagnetics”. In this study, we have designed and synthesized a triradical p - trisNN ( 1) as a building block for single-component ferrimagnetics, in which a m-phenylene-based π-biradical and a π-monoradical are connected in a benzyl-phenyl ether framework. The triradical retains the magnetic degrees of freedom for both S = 1 and S = 1/2 spins in the single molecule. X-ray crystallographic analyses show that 1 has a head-to-tail packing with an approximately isosceles triangular geometry, which is favorable for a ferrimagnetic spin alignment. From the analysis of crystal and magnetic susceptibility measurements, the triradical is found to form exchange-coupled systems composed of six S = 1/2 spins in the repeating unit of the crystalline solid state.

Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan–Wigner transformation. The ferrimagnetic chain includes an S = 1 biradical and an S = 1 / 2 monoradical with antiferromagnetic alternating interactions, and the S = 1 site in the chain is composed of two S = 1 / 2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S = 1 biradical is negative, while that of the S = 1 / 2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.

Building blocks for organic heterospin, heteromolecular complexes as models for organic molecule-based ferrimagnets

Building blocks for organic heterospin, heteromolecular complexes as models for organic molecule-based ferrimagnets

Thermodynamics of a quasi-one-dimensional spin system for molecule-based ferrimagnets

A physical picture of electron spin alignments in organic molecule-based ferrimagnets is given from numerical calculations of magnetic specific heat ( C) and magnetic susceptibility ( χ) as functions of temperature and static magnetic field ( B) in terms of an Ising Hamiltonian for an alternating spin chain. The double-peak structure of specific heat appears for different parameter ratios and different magnetic field B, indicating that one peak originates from the ferromagnetic nature and the other from the antiferromagnetic nature. Meanwhile, we study successively the influence of intermolecular and intramolecular interaction on the magnetic susceptibility, showing that the ferromagnetic spin alignment in the alternating molecular chains of biradicals and monoradicals is equivalent to the ferromagnetic alignment of the effective S=1/2 spins. Our results are consistent with those of the Quantum Monte Carlo simulations and the exact diagonalization method and in qualitative agreement with the experimental ones.

TMRG studies on spin alignment in molecule-based ferrimagnetics

A physical picture of spin alignment in organic molecule-based ferrimagnets is presented from studying the thermal effective magnetic moment of the sublattice by use of the transfer matrix renormalization group. We conclude that the classical antiparallel spin alignment is not the most stable state. The three-spin system tends to parallel alignment when the exchange interaction between the biradical and the monoradical molecules is much weaker than that within the biradical, which can result in the decrease of the effective magnetic moment upon lowering the temperature. More importantly, we give the theoretical evidence that even the antiparallel spin alignment in the biradical–monoradical alternating chain does not necessarily lead to ferrimagnetic spin ordering due to the formation of the spin singlet pairs, which suppresses the ferrimagnetic spin alignment.

Exchange Interaction in Covalently Bonded Biradical−Monoradical Composite Molecules

As a novel molecular designing for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of "single-component ferrimagnetics". When a pi-biradical with an S = 1 ground state and a pi-monoradical with S = (1)/(2) are united by sigma-bonds, the pi-conjugation between the biradical and the monoradical moieties should be truncated in the resultant triradical. This gives magnetic degrees of freedom for both S = 1 and (1)/(2) in the single molecule, serving as a building block for organic molecule-based ferrimagnets under favorable conditions (single-component ferrimagnetics). We have designed and synthesized a triradical, 3-(1'-oxyl-3'-oxido-4',4',5',5'-tetramethylimidazolin-2-yl)benzoic acid 2,4-bis(1' '-oxyl-3' '-oxido-4' ',4' ',5' ',5' '-tetramethylimidazolin-2-yl)phenyl ester (4), as a model compound for the novel approach to genuinely organic ferrimagnets. In the triradical 4, a m-phenylene-bis(nitronyl nitroxide) biradical with a triplet (S = 1) ground state is united with a phenyl nitronyl nitroxide monoradical (S = (1)/(2)) by an ester coupler. Solution-phase ESR spectra from 4 exhibited a complex hyperfine splitting due to (14)N and (1)H nuclei. The analysis of the hyperfine structure based on perturbation calculations has revealed that the exchange interaction within the biradical moiety is much larger than those between the biradical and the monoradical moieties and the magnetic degrees of freedom for both S = 1 and (1)/(2) are retained in 4. An X-ray crystal structure analysis showed that the triradical molecules are arranged in a one-dimensional molecular chain in the crystal. The magnetic susceptibility in a crystalline solid state is consistent with the crystal structure.

A stable organic triradical with truncated π-conjugation as a model for single-component organic molecule-based ferrimagnetics

As a novel molecular design for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of ‘single-component ferrimagnetics.’ When a π-biradical with an S=1 ground state and a π-monoradical with S=1/2 are united by σ-bonds, the π-conjugation between the biradical and the monoradical moieties should be truncated in the resultant triradical. This gives magnetic degrees of freedom for both S=1 and S=1/2 in the single molecule, serving as a building block for organic molecular ferrimagnets. We have designed and synthesized a triradical, 2,2,6,6-tetramethyl-pipelidine-1-N-oxyl-4-carboxylic acid 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenyl ester (2) as a model compound for single-component ferrimagnetics. Solution-phase ESR spectra from 2 are explained by a perturbation treatment assuming that the exchange interaction within the biradical moiety is much larger than those between the biradical and the monoradical moieties, which is suitable for single-component ferrimagnetics. From susceptibility measurements for a cyclohexane-substituted biradical, cyclohexane carboxylic acid 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl) phenyl ester (4) as a biradical analogue of 2, it is shown that the intramolecular ferromagnetic interaction has been found to be unaffected by the chemical modification for anchoring the monoradical moiety.

Mechanism for spin alignments in organic molecule-based ferrimagnetics

A mechanism for spin alignments in molecule-based ferrimagnets is proposed in terms of a Heisenberg spin Hamiltonian and molecular orbital calculations for a supramolecule composed of an S = 1 biradical and an S = 1/2 radical. Spin polarization in the supramolecule with an effective S = 1/2 spin is found to give ferromagnetic interactions between the supramolecules, leading to the ferrimagnetic ground state.

Organic Molecule-Based Ferrimagnetism as Studied by Numerical Calculations of a Model Hamiltonian

A physical picture of spin alignments in organic molecule-based ferrimagnets is deduced from quantum Monte Carlo (QMC) simulations of magnetic susceptibility. It is found from the QMC calculations on the basis of a Heisenberg spin Hamiltonian that the ferrimagnetic spin alignment in S =1 and S =1/2 alternating molecular chains is equivalent to a ferromagnetic alignment of effective S =1/2 spins appearing in the unit cells of the heteromolecular chains. A spin polarization effect affording the effective ferromagnetic interactions is demonstrated in terms of a Heitler-London approximation.

Organic Molecule-Based Ferrimagnetism as Studied by Numerical Calculations of a Model Hamiltonian

A physical picture of spin alignments in organic molecule-based ferrimagnets is deduced from quantum Monte Carlo (QMC) simulations of magnetic susceptibility. It is found from the QMC calculations on the basis of a Heisenberg spin Hamiltonian that the ferrimagnetic spin alignment in S =1 and S =1/2 alternating molecular chains is equivalent to a ferromagnetic alignment of effective S =1/2 spins appearing in the unit cells of the heteromolecular chains. A spin polarization effect affording the effective ferromagnetic interactions is demonstrated in terms of a Heitler-London approximation.

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

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.

Single-component ferrimagnetics: triplet–doublet composite molecules as constituents of purely organic molecule-based ferrimagnets

Three kinds of organic triradicals consisting of a nitronyl nitroxide biradical with S=1 and nitroxide or nitronyl nitroxide monoradicals with S=1/2 were synthesized by the esterification of corresponding phenol and carboxylic acid derivatives. These triradicals are model molecules for single-component organic ferrimagnetics. It was found from the solution ESR spectra of the triradicals that the time-averaged intramolecular exchange interactions between the S=1 and S=1/2 moieties were much weaker than that within the S=1 moiety and their magnitudes were in the order of 10 mK. These results indicate that the triradicals have the potential to be building blocks of single-component organic molecule-based ferrimagnetics.

Magnetic properties of phenol-substituted nitronyl nitroxide biradicals as building blocks of organic salt ferrimagnets

A practical difficulty in constructing organic molecule-based ferrimagnetics is co-crystallization of two distinct molecules with different spin quantum numbers. Coulombic energy between cationic and anionic molecules in organic salts is a promising driving force for co-crystallization. We investigated magnetic properties of four kinds of phenoxide-substituted nitronyl nitroxide biradical salts; a potassium and a sodium salt of 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenol ( 1a and 1b), and a potassium and a sodium salt of 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenol ( 2a and 2b). For 1a and 2a, it was found from the analysis of magnetic susceptibility in the solid state and ESR spectra in diluted frozen solutions that the ground-state spin multiplicities changed from triplet ( S=1) into singlet ( S=0) on ionization. For 1b and 2b, magnetic susceptibility in the solid state suggested the weakening of their intramolecular ferromagnetic interaction or the conversion of the ground-state spin multiplicities into singlet.

Molecular design of organic ferrimagnets

Two kinds of molecular design for genuinely organic molecule-based ferrimagnetics are reported. (I) The coulombic interaction between an anion and a cation is utilised for co-crystallizing organic radical molecules with different spin quantum numbers. This is referred to organic salt ferrimagnetics. A phenol derivative of nitronyl nitroxide n-biradical with S=1 has been synthesised as an anionic component of the salt and the electronic spin structure characterized. (II) The phenolic n-biradical with S=1 and a carboxyl-substituted n-radicals with S=1/2 have been esterified to give model compounds for single-component ferrimagnetics. Potentialities of the model compounds as building blocks of molecule-based ferrimagnetics have been examined by susceptibility and ESR measurements.

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

A crystal-engineering approach to organic ferrimagnets is reported. Coulombic energy between an anionic biradical withS = 1 and a cationic monoradical withS = 1/2 can be utilized as a driving force of cocrystallization of open-shell molecules with different spin quantum numbers, leading to organic salt ferrimagnets. In this study, 3,5-substituted phenol and benzoic acid derivatives of nitronyl nitroxide biradicals were synthesized as an ionizableS = 1 component of organic salt ferrimagnets. The molecular ground states of the biradicals in the neutral state were examined by continuous wave electron spin resonance (ESR) spectroscopy and static paramagnetic susceptibility measurements in the solid state. The molecular ground state of the phenol derivative was found to be triplet (S = 1) with the singlet-triplet energy gap of ΔE/kB ≈ 25 K, indicating that the biradical can be a building block of organic salt ferrimagnetics. The benzoic acid derivative was found to have a singlet (S = 0) ground state (ΔE/kB≧ −5 K), exemplifying thatmeta-(3,5)-linkage of unpaired electrons in π-aromatic rings does not necessarily give a triplet ground state for heteroatomic-substituted π conjugation. The molecular ground states of the biradicals determined in the ESR experiments were confirmed by the susceptibility in the solid state.

Spin−Spin Correlation Function and Magnetic Susceptibility of Quantum Ferrimagnetic Spin Chains as Models for Organic Molecule-Based Ferrimagnetics

Spin−spin correlation function and temperature dependence of magnetic susceptibility were calculated for the finite-size Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic Heisenberg chain. The ...

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

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 (Izuoka, A.; Fukuda, M.; Kumai, R.; Itakura, M.; Hikami, S.; Sugawara, T. J. Am. Chem. Soc. 1994, 116, 2609−2610.)