Molecular magnet goes ultracool

Posted by: Tarun Kumar
Researchers have succeeded in cooling a
molecular magnet to below 1K , the first time this
has been achieved with a nanomagnet. The
finding is important because it demonstrates
experimentally that such low temperatures are
achievable, opening the possibility of novel
refrigeration systems. The work has also shed
light on aspects of the nanomagnet’s quantum
behaviour.
Certain molecular systems exhibit unusual and
exotic magnetic behaviour, driven by quantum
mechanics, which might one day be harnessed for
applications such as quantum computing. One
phenomenon of interest is the magnetocaloric
effect (MCE), whereby when an applied external
magnetic field is removed from a magnetic
material, there is a reduction in temperature.
Eric McInnes, of the University of Manchester in
the UK, and colleagues prepared a molecular
cluster containing seven gadolinium centres, with
six forming a hexagon and one sitting in the
centre. ‘Each gadolinium ion is magnetic and in
this structure we expect the weak interaction of
these magnetic ions to produce a large MCE,’
McInnes says.
When the team carried out the MCE experiments
with samples of the crystals, the temperature fell
to around 0.2K, the first time a molecular
nanomagnet has achieved sub-Kelvin cooling. The
researchers noted that as the magnetic field was
withdrawn, the drop in temperature was not
linear, rather it was ‘bumpy’, which provides an
insight into the quantum state of the system.
Each gadolinium centre sits at the corner of a
triangle; the electrons in these positions want to
pair their spin with their neighbours at the other
corners of the triangle; but because there are
three of them, one at each corner, they cannot all
align simultaneously. This is termed ‘spin
frustration’.
‘As the demagnetisation proceeds, the spin
frustration leads to a metastable spin state in a
certain magnetic field range, which causes a
temporary rise in temperature, before the system
continues to cool on decreasing the magnetic
field,’ McInnes says. ‘We set out to exploit the
spin physics to achieve refrigeration in this
magnetic molecule, and we succeeded in that. But
in fact the cooling behaviour of the system
revealed new insights into the spin physics, and
we discovered more about the quantum magnetic
properties of this system than we knew before we
started.’
Commenting on the work, Euan Brechin , of the
University of Edinburgh in the UK, says: ‘What this
demonstrates is that certain types of polymetallic
molecules are realistic candidates for employment
in a Carnot cycle [the thermodynamic process
used to create prolonged refrigeration], and that
they offer more freedom to design the cycles
according to specific needs.’
REFERENCES
J W Sharples et al , Nat. Commun ., 2014, DOI:
10.1038/ncomms6321