Teraflop 73 - Novembre - cesca

During the last years a new interdisciplinary
field has appeared,
focusing its interest
on nanoscale objects. The size of the
systems involved must be of the order
of nanometers, that corresponds
to the medium-sized molecules.
Thus, usual molecules with biological
interest are too large and bulk materials
widely employed for their physical
properties are also excluded.
Nevertheless, the limits of the size
are in some cases not strictly fulfilled
and some studies with antibodies
and viruses are, for instance, also
considered in the field of Nanotechnology.
The main goal in this area is
to be able to manufacture molecules
with properties analogous to those of
objects that we employ commonly.
Hence new terms such as nanotubes,
nanoengines, molecular electronics,
molecular machines or nanomagnets
among others are now employed
commonly by biologists, chemists
and physicists.
Magnetism is one of the fundamental
properties of matter that is related
to the presence of unpaired
electrons in the atoms, generally
quantified at the microscopic scale
with a magnitude called spin. In the
field of Magnetism, during decades
the studies using molecular entities
were basically aimed at the replacement
of many metal and ceramic materials
by lightweight molecule-based
materials. Such metallic and ceramic
materials are widely employed in
electricity installations and electric
motors, telecommunication devices, as
well as for information storage in computers
among many other important
common uses. However, at the beginning
of the 90s, it was discovered that
a single molecule containing transition
metal atoms can behave as a magnet.
Are Single-Molecule Magnets
Potentially Nanomagnets?
Eliseo Ruiz Sabín
Departament de Química Inorgànica
Universitat de Barcelona
The first system studied was the complex
known as Mn 12 (see Figure 1).
The main difference of such system
with the usual magnetic behavior previously
known for transition metal
complexes is that it shows a slow relaxation
of the magnetization. In other
words, the molecule is able to keep
the spin oriented for months at low
temperature after being exposed to a
magnetic field. In contrast to the most
common behavior is that the spin of
the molecules disappears suddenly
when the molecule is retired of the
magnetic field. Since these experiments
with the Mn 12 complex, other
transition metal complexes that present
a similar behavior have been
found. It is worth mentioning the Fe 8
complex that has been also widely
studied. These molecules are now often
called “single-molecule magnets”
(SMMs) and they can be considered
as potential nanomagnets.
A second important property of
SMMs is the presence of quantum tunneling
that facilitates the relaxation of
the magnetization. This effect is interesting
in the field of quantum computing
where information can be handled
taking advantage of quantum effects
but it should be suppressed to have an
“ideal” magnet. In the presence of a
magnetic field, the spin of the molecule
adopts the direction imposed by
the field (see 1 and 2 in Figure 2).
However, if the system returns to zerofield
conditions it can be described
again as two degenerate minima corresponding
to the two opposite directions
of the spin. In such conditions, the
relaxation of the spin can be thermally
induced jumping the barrier from one
minimum to the other or simply
through tunneling between thermally
activated states (see 3 in Figure 2).
Hence there is a crucial parameter that
controls mainly all these processes
that is the height of the barrier, which
depends of the magnetic anisotropy
and the total spin of the molecule.
In order to increase the singlemolecule
magnet character, the goal
is to make molecules with high total
spin and simultaneous large magnetic
anisotropy. The total spin of the molecule
is equal to the sum or the difference
of the local spins of the paramagnetic
centers, depending on the
nature of the interaction between
them. Thus, if the coupling is ferromagnetic,
the total spin is the sum of
the local spins, while if the interaction
is antiferromagnetic (or ferrimagnetic
when the local spins are different) the
total spin is the difference between local
spins. The ideal picture to obtain
large total spin values would be to
have ferromagnetic interactions, unfortunately
antiferromagnetic (or ferrimagnetic)
couplings are the most
common behaviors. For that reason,
chemists are trying to synthesize molecules
with a larger number of paramagnetic
transition metals with ferromagnetic
couplings as well as ferrimagnetically-coupled
systems with
transition metals with different local
spins to obtain the largest possible total
spin. Up to now, two complexes
with S=33/2 and 39/2 are the largest
values for well-characterized compounds,
corresponding to Fe 19 and
Mn 9W6 complexes, respectively, whilst
the most studied Mn 12 and Fe8 complexes
have S=10. From the experimental
point of view it is practically
impossible to predict a priori the total
spin of this kind of molecules. In such
a way, obtaining better SMMs is a
serendipitous task, synthesizing many
new complexes to obtain systems with
improved magnetic properties. For
that reason, theoretical methods are