Chemistry Reference
In-Depth Information
the segregated layered arrangement does not necessarily imply that the electronic
structures are equivalent.
Organic magnets
Under the general termmolecule-based magnets we include solids exhibiting spon-
taneous magnetic ordering (i.e., for
B
0) below a critical temperature and con-
sisting of molecular units (building blocks) bound together by ionic, covalent or
van der Waals interactions. When such materials possess unpaired electron spins
residing in
p
orbitals they are called organic molecule-based magnets or simply
organic magnets. Organic magnets possessing only electron spins residing in
p
or-
bitals are called purely organic (metal-free) magnets. Note that classical inorganic
magnets are based on unpaired spins localized in metal
d
or
f
orbitals. In this sec-
tion we mention the most representative reported molecule-based magnets. Some
recommended general references on this subject are Kahn, 1993; Gatteschi, 1994;
Miller & Epstein, 1994; Day, 2002; Blundell & Pratt, 2004 and the series initiated
by Miller & Drillon, 2001.
At
=
this point one can argue whether single-molecule magnets,
individual
molecules with high spin values (
S
10), should be considered as molecule-
based magnets. Such molecules are chemically synthesized and obtained in the
form of single crystals and their properties derived from this aggregated state. The
molecules are intentionally designed with a metallic core (e.g., Mn) surrounded
by inert molecules, strongly reducing magnetic intermolecular interactions. In this
sense, such solids are built fromweakly interactingmolecules, and cannot be consid-
ered as molecule-based magnets, because no collective effects are found (no spon-
taneous magnetic ordering); their properties are related to the individual molecules
and not to the solid. Magnetic clusters such as Mn
12
O
12
(CH
3
COO)
16
(H
2
O)
4
com-
pounds, known as Mn
12
clusters, exhibit a high-spin ground-state value (
S
≥
=
10)
because eight of the manganese ions are in the
+
3 oxidation state (
S
=
2, up) and
four are in the
2, down). These clusters undergo a very slow re-
laxation of the magnetization below the so-called blocking temperature (
+
4 state (
S
=
3
/
4 K).
This is different from paramagnetism, where the magnetization vanishes with the
applied field (Sessoli
et al.
, 1993).
Let us briefly explore MOMs exhibiting transitions to a ferromagnetic state. The
first genuine examples of bulk ferromagnetism in molecule-based crystals were
reported for transition-metal cyanide complexes of the Prussian Blue type (Holden
et al.
, 1956). Prussian Blue, Fe
+
4
[Fe
+
2
(CN)
6
]
3
∼
·
14H
2
O, is itself a ferromagnet with
5Kand it builds a 3Dcubic network of alternating Fe
+
2
and Fe
+
3
bridged by
CNgroups. PrussianBlue is the first example ofmixed-valence compounds. Follow-
ing the definition of molecule-based magnets given above, Prussian Blue is clearly
T
C
5
.
