Hybrid organicinorganic nanocomposite magnets are novel materials in which [625528]
ABSTRACT
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 hy
brid 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 syn
thesis allowing in both cases a wide range of structural changes. We propose that
for the quasi-one-dimensional manganeseporphyrin-based magnets the mechanism
ii
for the magnetic ordering is, likely, the interplay between the strong intrachain ex
change 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).y(o-DCB), and (MnTPP)(TCNE).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, Cu2(OH)3(CmH2m+1COO), 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.
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