Magnetic cooling at single molecule level: a spectroscopic investigation

Magnetic cooling relies on large entropy variation of magnetic systems under the application of an external magnetic field. While giant magnetocaloric effect (MCE) in intermetallic compounds is related to the interplay between long range magnetic and lattice order, molecular nanomagnets have recently shown superior cooling performances at cryogenic temperatures. The molecular cage Fe14(bta)6 was one of the first examples on which enhanced MCE was experimentally observed in bulk samples [1] followed by a dozens of other cases in the recent years. Analysis of the low temperature thermodynamic properties of these molecular compounds shows that a large part of the entropy variation is due to the magnetic degeneracy of the ground molecular state and therefore high cooling power is expected at single molecule level, an interesting feature that can be exploited for applications down to the nanoscale. Yet, the deposition of large molecular cages on surfaces might be an non-innocent process since the interaction with the surface may provoke some drastic chemical changes or structural distortions that may alter the magnetic features and therefore the functionalities of the molecule. For this reason we checked if the functionality of potential molecular coolers is preserved when molecules are deposited on a substrate. Here we report an investigation on the Fe14(bta)6 molecular nanomagnet to demonstrate that large MCE is a property held at single molecule level [2]. To this end we have characterized well isolated Fe14(bta)6 molecules deposited by liquid phase on a gold surface by a combined analysis carried out by STM, XPS, XAS and XMCD to independently measure how the chemical, electronic and magnetic features of the isolated molecules are modified by the interaction with the surface. The relevant point is that the MCE is directly observed in our experiments at a single molecule level. This demonstrates, for the first time, that an important contribution to magnetic refrigeration is an intrinsic molecular property and it opens the possibility of scaling cooling devices down to a molecular level with no need of a cooperative behavior.