Magnetic ordering in hybrid organic/inorganic nanocomposites: Magnets by design.

摘要

Hybrid organic/inorganic nanocomposite magnets are novel materials in which electrons in molecular orbitals, consisting of superpositions of p and even s atomic orbitals, play a crucial role in the magnetic ordering. The interest in molecular magnets is twofold. The flexibility in synthesis brings the possibility of creating new materials that are experimental realizations for "exotic" theoretical models. It is, therefore, possible to study the unusual effects of randomness and competing interactions, of spin- and lattice-dimensionality crossovers, of quantum fluctuations, etc. The interest in molecular magnetism also comes from the range of practical applications for the room-temperature molecule-based magnets, which have some properties similar to ferrites.; Based on extensive experimental magnetic studies of two distinct families of hybrid organic/inorganic compounds, this dissertation addresses three main issues: the mechanism for the unusual magnetic ordering phenomena that occur in these novel systems, the correlation between their structure and their magnetic behavior, and new strategies for the design of novel magnetic nanocomposites.; The two different families of compounds discussed in this dissertation were chosen due to the possibility of magnetic lattice engineering, small variations in the synthesis allowing in both cases a wide range of structural changes. We propose that for the quasi-one-dimensional manganeseporphyrin-based magnets the mechanism for the magnetic ordering is, likely, the interplay between the strong intrachain exchange interactions, the weak interchain exchange and/or dipolar interactions, and the single-ion anisotropy at the manganese site. Based on studies of (MnOEP)(HCBD), (MnTPP)(TCNE){dollar}cdot{dollar}y(o-DCB), and (MnTPP)(TCNE){dollar}cdot{dollar}z(o-Xy) (where OEP = octa-ethylporphyrin, TPP = meso-tetraphenylporphyrin, HCBD = hexacyanobutadiene, TCNE = tetracyanoethylene, o-DCB = ortho-dichlorobenzene, and o-Xy = o-xylene), we show that various spin- and lattice-dimensionality crossovers take place.; For the second family of magnets, Cu{dollar}sb2{dollar}(OH){dollar}sb3{dollar}(C{dollar}sb{lcub}m{rcub}{dollar}H{dollar}sb{lcub}2m+1{rcub}{dollar}COO), m = 7, 9 and 11, we propose that these systems are experimental realizations of triangular quantum Heisenberg antiferromagnets with additional Dzyalosinskii-Moriya interaction. The interplay of Heisenberg and Dzyalosinskii-Moriya exchanges leads to an unusual state in which canted antiferromagnetism and spin glass-like behavior appear to coexist.