Organic Polymer Ferromagnets Research Articles

A Monte Carlo study of the thermodynamic properties of a quasi-one-dimensional organic polymer ferromagnet

The thermodynamic properties of a quasi-one-dimensional organic ferromagnet at different temperatures and in different applied magnetic fields have been investigated by means of the Heisenberg model combined with the Monte Carlo method. The results indicate that a peak in the magnetic susceptibility is obtained at low temperatures. Furthermore, the effect of dimerization on the magnetic properties has also been studied. We find that the dimerization suppresses the magnetization in this model. The ferromagnetic couplings between the side-free radicals stabilize the ferromagnetism and increase the apparent Curie temperature.

Specific Heat and Charge Density of Quasi-One-Dimensional Interchain Coupling Organic Ferromagnets

On the basis of a generalized SSH model, an organic polymer ferromagnet theory is proposed at the finite temperature in the self-consistent mean field approximation, and the specific heat and charge density of the quasione-dimensional interchain coupling organic ferromagnets are presented. We find that an obvious feature is to present itself the round peak for the specific heat with the temperature. This indicates unambiguously the presence of the phase transition in the system. The transition temperature plays down with increasing of the interchain coupling t2 or decreasing of the electron repulsion u. The curves of charge density with the temperature debase monotonously. This result illustrates that the higher the temperature is, the more electrons are excited.

Density matrix renormalization group studies on the stability of quasi-1-D undoped and doped polymer ferromagnets

We study the ferromagnetic ground state of the theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets by using the density matrix renormalization group method. We find that the chain–radical interaction, the on-site electron–electron repulsion, and the electron–phonon coupling play important roles in the stability of the ferromagnetic state, whether in doped systems or not. We are convinced that when the system is doped by one electron, a polaron appears, diminishing the stability of the high-spin state.PACS No.: 75.10.Jm

Charge density and spin density in interchain coupled organic polymer ferromagnet with non-nearest neighbour hopping interactions

Considering the non-nearest neighbour hopping interactions, we investigate the distribution of the charge density (CD) and spin density (SD) in two neighbouring π-conjugated organic polymer ferromagnetic chains coupled through interchain electron transfer terms. The results show that the non-nearest neighbour hopping interactions lead to new features of the distribution of the CD and SD with respect to different sites. The second-nearest neighbour hopping can induce a charge density wave (CDW) along the main chain while the third-nearest neighbour hopping plays an opposite effect, and meanwhile, the second-nearest neighbour hopping makes the SD transfer mainly along the main chain while the third-nearest neighbour hopping makes that transfer mainly from the odd sites on the main chain to the side radicals. In addition, the effects of the interchain coupling on the distribution of the CD and SD are also discussed.

Random phase approximation of the spin-wave excitation for quasi-one-dimensional magnetic polymer

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the ground state and low-lying magnetic excitation are studied. Within Hartree-Fock approximation, the ground state of the system is shown to be a stable ferromagnetic state due to the electron-electron correlation and topological structure of the system. The random-phase approximation is employed to explore the magnon excitation and the excitation spectrum is obtained, including an acoustic mode and four optical modes. It is found that the acoustic mode possesses the characteristic of the ferromagnetic magnon.

Study of the magnon excitation in a quasi-one-dimensional organic polymer ferromagnet

The ground state and magnon excitation of a theoretical model proposed for a quasi-one-dimensional π-conjugated organic polymer ferromagnet are studied. Within the mean-field approximation, considering Hubberd repulsion and the hopping of the π electrons on the main chain and the unpaired electrons on the side radicals, the ground state of the system turns out to be a high-spin ferromagnetic state. The magnon excitation is investigated by the random-phase approximation (RPA), and a magnon excitation spectrum that includes an acoustic mode is obtained. The results of the calculations show that the Hubberd repulsion of the unpaired electrons on the side radicals as well as the hopping interaction between the π electrons on the main chain and the unpaired electrons on the side radicals make an impact on the acoustic mode which holds the property of the ferromagnetic magnon. The large Hubberd repulsion and hopping interaction are favourable to the stability of the ferromagnetic ground state of the system.

Four-fold structural distortion in quasi-ID organic ferromagnets

Based on a model for quasi-one-dimensional organic polymer ferromagnets, the Peierls distortion in a conjugated carbon chain with side radicals, which contains unpaired electrons, is investigated. When the hopping mediated interaction between the main chain and the side radicals is greater than a critical value, there exists a four-fold structural distortion instead of a Peierls dimerization in the main chain bonded to NR=N/4 side radicals ( N is the main chain length). In addition, the spin and charge density wave in the system have been studied.

Solitons in quasi-one-dimensional π-conjugated organic ferromagnets

Abstract Quasi-one-dimensional π-conjugated organic polymer ferromagnets are studied self-consistently. It is shown that due to the electron-phonon interaction and the Hubbard energy, there exist domain wall solitons describing the displacement of lattices along the main chain, and spin solitons describing the localization of spin at side radicals. The spin soliton corresponds to the highly degenerate band in the middle of the energy spectra, which gives the main contribution in the high-spin state of the system.

Molecular Designs for Polyionic Organic Ferromagnetics in Terms of Band Structure Calculation/ Crystal Orbital Approach; Exploitation of Topological Super Pseudo-Degeneracy of π-Bco's and π-Abco's with Hetero-Atomic Perturbation

One of the highlighted conceptual advances in organic magnetics and molecule-based magnetism has been that the use of topological-symmetry requirement in the π-electron network of “open-shell homoatomic neutral hydrocarbon systems” gives rise to the unlimited number of the degeneracy in non-bonding molecular orbitals (coined as topological degeneracy). This paper deals with the π-topological version of an approach to purely organic polyionic polymer ferromagnets, emphasizing that molecular design exploits the topological super pseudo-degeneracy of π-bonding and π-antibonding crystal orbitals (π-BCOs and π-ABCOs; it-bands) appearing close to zero energy in heteroatomic π-conjugated organic systems, for polycationic and polyanionic organic polymer ferromagnets, respectively. The appearance of the BCOs and ABCOs close to zero energy as well as their pseudo-degeneracy is due to heteroatomic perturbation. Their super degeneracy arises from the topological nature of the π-conjugated electron network of the elaborate molecular design. Oxidation and reduction states of polymers designed according to the topological approach are expected to undergo the additive operation of dynamic spin polarization, leading to high-spin ground states for polycationic and polyanionic model polymers, respectively. One-dimensional and two-dimensional star-burst model oligomers with heteroatoms in π-conjugation were designed as prototypical examples. Also, description of ferromagnetic or antiferromagnetic spin alignment depending on the topological symmetry in π-conjugation is given in terms of dynamic spin polarization.

Syntheses of Model Oligomers for Polyionic Organic Ferromagnetics and their High-Spin Characterization by FT Pulsed Esr/Electron Spin Transient Nutation (Estn) Spectroscopy as a Novel Experimental Technique

This paper deals with syntheses and unequivocal spin identification of prototypical model oligomers for the π-topological version of an approach to purely organic polyionic polymer ferromagnets, emphasizing that molecular design exploits the topological super pseudo-degeneracy of π-bonding and π-antibonding molecular orbitals (π-HOMOs and π-LUMOs) appearing close to zero energy in heteroatomic π-conjugated organic systems, for polycationic and polyanionic organic polymeric high spins, respectively. The oxidation and reduction states of the oligomers designed according to the topological approach are expected to undergo dynamic spin polarization, leading to high-spin ground states for both polycationic and polyanionic model oligomers. One-dimensional and two-dimensional star-burst polycationic model oligomers with heteroatoms participating in π-conjugation were designed and synthesized. Their electronic high-spin ground states have been identified by means of Two-Dimensional Electron Spin Transient Nutation (2D-ESTN) spectroscopy based on an FT pulsed ESR technique to which recently we have exerted ourselves in order to obtain technical advances in molecular spin science underlying the research field of molecule-based magntics. In this study, the polyketone-based polyanionic high-spin species have been generated also by a dry process, i.e., electron doping by γ-irradiation in organic glasses.

Competition Between the SDW and CDW in Quasi-One-Dimensional Organic Polymer Ferromagnet**The project supported by National Natural Science Foundation of China

The next-nearest-neighbor hopping interactions of -electrons in quasi-one-dimensional organic polymer ferromagnet are considered by Peierls–Hubbard model, and a set of self-consistent equations are established to optimize the system. The competition between the SDW and CDW states, which is determined by the interplay between the electron-electron correlation and the next-nearest-neighbor hopping interaction, is studied. At the CDW state, the SDW along the main chain will be tuned by the CDW. Consequently the ferromagnetic state, in which all the spins of the unpaired electrons at side free-radicals are arranged parallelly, will be no longer a stable ground state of the system.

The effect of interchain coupling on energy bands and ferromagnetic order in quasi-one-dimensional organic polymer ferromagnets

Considering the interchain interaction as an interchain electron transfer, a theoretical model is developed to describe quasi-one-dimensional organic ferromagnets. We also take into account the interaction between the itinerant electrons and the distortion of the lattice, the Hubbard electron - electron repulsion, and the antiferromagnetic correlation between the electrons and the nonbonding electrons at the site radicals. By employing the mean-field approximation, four energy bands which exhibit a three-dimensional anisotropic aspect are obtained. When the interchain coupling increases, the energy gap decreases. In the case of small dimerization order parameter, due to the interchain coupling, the band gap may disappear and the conjugated electrons in the main chain show ferromagnetic order in the ground state. The interchain coupling destabilizes the ferromagnetic ground state of the system.

Molecular design for polyionic organic ferromagnets and syntheses of model oligomers; Exploitation of topological superdegeneracy of π-BCO's and π-ABCO's, and high spin characterization by FT pulsed ESR/ESTN spectroscopy as a novel experimental technique

The conceptual proposals of organic magnetism/molecule-based magnetism in terms of non-charged open-shell units were made as early as 1960's. One of the highlighted advances has been that the use of topological-symmetry requirement in the electron network of it-conjugated homoatomic neutral hydrocarbon systems gives rise to the unlimited number of the degeneracy in non-bonding molecular orbitals (coined as topological degeneracy). This paper deals with the π-topological version of an approach to purely organic polyionic polymer ferromagnets, emphasizing that molecular design exploits the topological pseudo superdegeneracy of π-bonding and -antibonding crystal orbitals (π-BCOs and π-ABCOs; π-bands) appearing close to zero energy in heteroatomic π-conjugated organic systems, for pofycationic and poryanionic organic polymer ferromagnets, respectively. The appearance of the BCOs and ABCOs close to zero energy as well as their pseudo degeneracy is due to heteroperturbation. Their superdegeneracy arises from the topological nature of the electron network of π-conjugation. Oxidation and reduction states of polymers designed according to the topological approach are expected to undergo dynamic spin polarization, leading to high-spin ground states for porycationic and poryanionic model porymers, respectively. One-dimensional and two-dimensional star-burst model oligomers with heteroatoms in π-conjugation were designed and synthesized Their electronic high-spin ground states have been identified by means of Electron Spin Transient Nutation (ESTN) spectroscopy based on FT pulsed ESR technique to which recently we have exerted ourselves in order to obtain technical advances in molecular spin science underlying the research field of molecule-based magnetism. Most of the poryionic high-spin species have been generated by wet processes, i.e., chemical reaction in solution. In this study, the poryketone-based poryanionic high-spin species have been generated also by a dry process, i.e., electron attachment via γ-irradiation in organic glasses.

Spin-Wave Excitations and Energy Gap in Quasi-One-Dimensional Organic Polymer Ferromagnets11The project supported by National Natural Science Foundation of China

Based on a theory model proposed for organic ferromagnets, the spin configuration and spin-wave excitations in quasi-one-dimensional organic polymer ferromagnets are studied. The results show that: 1) Owing to the unpaired electrons at side free-radicals, the degeneracy of the spin-wave will be removed, the lower branch of the spin-wave excitations will split off into two branches, and it shows ferromagnetic characters; 2) The anisotropic interactions between the unpaired electrons at side free-radicals will make an energy gap appear between the ground state and the lowest excited state.

Studies on the next-nearest neighbor hopping interactions of π-electrons in quasi-one-dimensional organic ferromagnetic model

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the next-nearest neighbor hopping interactions of π-electrons are considered. Allowing for full lattice relaxation, a set of self-consistent equations is established to study the system. The spin-density-wave (SDW) and the possible ferromagnetic ground state of the system are investigated in detail. It is found that the next-nearest neighbor hopping interactions will make the SDW stronger and consequently make the ferromagnetic state more stable as compared with the nonmagnetic reference state.

Effects of Electron‐Electron Interaction on the Ferromagnetic State in a Quasi‐One‐Dimensional Organic Polymer Ferromagnet

AbstractBased on a theoretical model proposed for quasi‐one‐dimensional organic polymer ferromagnets, the effects of electron‐electron interaction on the ferromagnetic state are studied. Allowing for full lattice relaxation, the system is optimized by a set of self‐consistent iterative equations. The results of the calculation indicate that larger on‐site electron‐electron repulsive interactions will make the ferromagnetic ground state of the system more stable. It is also found that the inter‐site electron‐electron interaction cause a charge density distribution along the main chain and consequently weakens the stability of the ferromagnetic ground state.

Studies on the ferromagnetic state in quasi-one-dimensional organic polymer ferromagnets with competition between electron-electron interaction and electron-phonon coupling

The possible ferromagnetic ground state of a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets is investigated in detail. Within the self-consistent-field Hartree-Fock approximation, allowing for full lattice relaxation, the system is studied self-consistently. We find a critical interaction, which is determined by the competition between the electron-electron interaction and the electron-phonon coupling. When electron-electron interaction is larger than the critical value, the ground state of the system is a stable ferromagnetic state. It is also found that in this stable ferromagnetic state the π-electrons SDW will become very strong, and almost zero dimerization happens for the main chain.

Quasi-One-Dimensional Organic Polymer Ferromagnet Based on Polaron

A new kind of organic polymer ferromagnet based on polaron is proposed. In this kind of polymer, there are no organic free-radicals, which are rather reactive and unstable, instead spins are provided by polarons. Allowing for full lattice relaxation, the system is studied self-consistently by Peierls-Hubbard model. The formation energies of polarons are calculated. The possible ferromagnetism in polymers of this type is demonstrated.

Effects of electron-electron interaction and electron-phonon coupling on the spin-density-wave of quasi-one-dimensional organic polymer ferromagnets

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnet poly-BIPO, the effects of electron-electron interaction and electron-phonon coupling on the spin-density-wave (SDW) state of poly-BIPO system are studied. It is found that the SDW state is the stable ground state of the system, in which the system shows ferromagnetic order. And the larger electron-electron interaction and the smaller electron-phonon coupling will make the SDW state more stable than the nonmagnetic state.

The CDW transition in quasi-one-dimensional organic polymer ferromagnets: effects of next-nearest neighbor hopping interactions

The next-nearest neighbor hopping interactions of pi-electrons in quasi-one-dimensional organic polymer ferromagnets are considered. Within the mean-field theory and allowing for full lattice relaxation, a set of self-consistent equations is established to study the system. It is found that with increasing of the next-nearest neighbor interaction a charge-density-wave (CDW) transition will happen. At the CDW state, a strong charge density distribution along the main chain will appear, and the spin density-wave (SDW) along the main chian will be tuned by the CDW. Consequently the ferromagnetic state, in which all the spins of the unpaired electrons at side free-radicals are arranged parallely, will be no longer the stable ground state of the system.