Spin Hamiltonians and Theoretical Models

Soos, Z. G.

doi:10.1007/978-94-009-9067-8_8

The Physics and Chemistry of Low Dimensional Solids 1980

DOI: 10.1007/978-94-009-9067-8_8

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Spin Hamiltonians and Theoretical Models

Z. G. Soos

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1984

1994

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Cited by 3 publications

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Organische und metallorganische molekulare magnetische Materialien: Designer‐Magnete

1994

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Der Entwurf neuartiger magnetischer Materialien auf molekularer Basis, die für eine Vielzahl von Anwendungen von Interesse sind, ist immer noch eine Kunst. Um ein makroskopisches ferro‐oder ferrimagnetisches Verhalten zu erzielen, ist es erforderlich‐unter Zugrundelegung von Modellen zur Spin‐Spin‐Kopplung‐, Materialien mit ganz bestimmten Primär‐, Sekundär‐und Tertiärstrukturen aufzubauen. Kooperative Phänomene zeigen beispielsweise einige metallorganische Festkörper, die aus linearen Ketten aus Metallocen‐Donoren D und Cyankohlenwasserstoff‐Acceptoren A nach…︁D.+A.+D.+A.+…︁ aufgebaut sind.

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Organische und metallorganische molekulare magnetische Materialien: Designer‐Magnete

1994

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Organic and Organometallic Molecular Magnetic Materials—Designer Magnets

Miller

Epstein

1994

Angew. Chem. Int. Ed. Engl.

1,351

579

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Magnets composed of molecular species or polymers and prepared by relatively low‐temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular‐species‐based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low‐frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin‐containing moieties, and dipole—dipole, through‐space interactions) which enable the design of new molecular‐based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro‐ or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular‐based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …︁D•+ A•− D•+ A•−…︁, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]•+ [TCNE]•− (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean‐field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one‐dimensional ferrimagnetic materials based on metalloporphins is discussed.

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Diagrammatic valence‐bond theory for finite model Hamiltonians

Ramasesha

Soos

1984

Int J of Quantum Chemistry

103

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Valence bond (VB) diagrams form a complete basis for model Hamiltonians that conserve total spin, S, and have one valence state, +p, per site. Hubbard and Pariser-Parr-Pople (PPP) models illustrate ionic problems, with zero, one, or two electrons in each 4 , while isotropic Heisenberg models illustrate spin problems, with only purely covalent V B diagrams. The difficulty of nonorthogonal V B diagrams is by-passed by exploiting the finite dimensionality of the complete basis and working with unsymmetric sparse matrices. We introduce efficient bit manipulations for generating, storing, and handling V B diagrams as integers and describe a new coordinate relaxation method for the ground and lowest excited states of unsymmetric sparse matrices. Antiferromagnetic spin-; Heisenberg rings and chains of N S 2 0 spins, or 2N spin functions, are solved in C, symmetry as illustrative examples. The lowest S = 1 and 0 excitations are related to domain walls, or spin solitons, and studied for alternations correiponding to polyacetylene. V B diagrams with arbitrary S and nonneighbor interactions are constructed for both spin and ionic problems, thus extending diagrammatic VB theory to other topologies.

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Spin Hamiltonians and Theoretical Models

Cited by 3 publications

References 58 publications

Organische und metallorganische molekulare magnetische Materialien: Designer‐Magnete

Organische und metallorganische molekulare magnetische Materialien: Designer‐Magnete

Organic and Organometallic Molecular Magnetic Materials—Designer Magnets

Diagrammatic valence‐bond theory for finite model Hamiltonians

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