Theoretical studies on effective spin interactions, spin alignments and macroscopic spin tunneling in polynuclear manganese and related complexes andtheir mesoscopic clusters

Theoretical efforts to investigate molecular magnetic materials are reviewe d mainly from the viewpoint of our interest. Ab initio calculations of effe ctive exchange interactions between spins are performed for H-H, W-I-Ie-H a nd simplified models of binuclear manganese and related complexes by using the spin unrestricted Hartree-Fock (UHF) and spin-polarized density functio nal (DFT), and UHF plus DFT hybrid methods. The scope and limitation of the se broken-symmetry approaches are briefly discussed in relation to several computational schemes of effective exchange integrals (J(ab)). The calculat ed J(ab) values for the three systems are summarized for comparison of the computational methods. The natural orbitals (UNO or DNO) of the UHF and DFT solutions for magnetic clusters are determined by diagonalizing their firs t-order density matrices. They are used for MO-theoretical interpretation o f superexchange interactions. The effective spin Hamiltonians such as the H eisenberg model are constructed for polynuclear complexes assuming the calc ulated and experimental effective exchange integrals. The macroscopic quant um tunneling (MQT) and coherence (MQC) of spins in the manganese oxide clus ters are analyzed using the Heisenberg model, and the tunneling rate of spi ns is calculated by the coherent state path integral method. The topologica l rules for MQT and MQC are derived from this analysis. The path integral f ormulations are extended to tunneling probabilities for clusters of cluster s and spin lattices with mesoscopic size. The resulting ideas are also appl ied to the molecular design of mesoscopic clusters of clusters in intermedi ate and strong correlation regimes. The active control of spins are finally discussed from the viewpoint of functionalities in molecular and biologica l materials, and technological applications of mesoscopic molecular magnets to quantum computing. (C) 2000 Elsevier Science S.A. All rights reserved.