Molecular refrigerators
Experiments performed at DEIMOS beamline
by a team of researchers of the Italian
National Research Councils (CNR), the
University of Manchester and French CNRS
found that a single molecule may well work
as magnetic refrigerator on its own.
This is possible by exploiting the magnetic
properties of special class of molecules,
i.e. paramagnetic organometallic compounds
that show a particular behavior, known
as magnetocaloric effect (MCE), which
makes them excellent refrigerants
at cryogenic temperatures. “besides
the curiosity”- explains the researcher-
“this opens the way to miniaturize devices
–such as highly sensitive detectors of
electromagnetic waves- on one chip where
a thin layer of such a molecule can be used
to cool down the whole chip.” Their results
are published in Advanced Materials.
Research in nanotechnology aims
at scaling down the size of systems
and devices while maintaining interesting
performances down to few nanometers,
that is approx. 10000 times smaller than
a human hair. Along this line, researchers
are currently investigating how small
a refrigerator can be.
The magnetocaloric effect occurs
by performing a particular thermodynamic
cycle, called adiabatic demagnetization,
which exploits the variation of entropy
due to the application and subsequent
adiabatic removal of an external magnetic
field that induces a temperature decrease.
The magnetocaloric effect is quantified
by the change in entropy per unit
mass: although this effect occurs
in many magnetic materials, only
in a few cases the entropy change
is large enough to make magnetic
materials usable as refrigerants
in practice. Molecular nanomagnets,
to exhibit a strong magnetocaloric effect,
must be characterized by a ground state
with high spin and low entropy, should
have a low magnetic anisotropy, which
facilitates the polarization in the presence
of a magnetic field, these facts give rise
to an excess of entropy resulting from
the presence of low-lying excited spin
states. Finally, to optimize cooling
performances in working conditions,
the molecular mass should be small
for a given change in magnetic entropy.
In the experiments performed at SOLEIL,
a sub-monolayer of molecular iron
clusters, namely Fe
14
(bta)
6
, have been
deposited on substrates and its magnetic
cycle has been studied by using polarized
X-ray radiation. The substrates used
were gold and graphite (HOPG, Highly
Ordered Pyrolytic Graphite). The samples
were produced from liquid solutions with
dichloromethane (DCM) as solvents.
For the analysis other surface techniques
have been employed, including
STM, XPS, AFM, FTIR. However,
the use of synchrotron radiation allowed,
for the first time, to directly observe
huge magnetocaloric effect at the level
of a single molecule and to establish
that cooperative effects, like long range
order which is dominant in conventional
magnetic refrigerants, play only a minor
role in this case.
Introduction
The magnetocaloric effect
A layer of molecular iron clusters
PHYSICS AND CHEMISTRY OF CONDENSED MATTER, EARTH SCIENCES
102
SOLEIL
HIGHLIGHTS
2013
