Sensing magnetic flux of Langmuir-Blodgett films of a molecular magnetic system using superconducting films and nano-SQUID devices

We report a study on the response of superconducitng micro-tracks and quantum interference devices (SQUIDs) to a proximal SMM film. As a test case, Langmuir-Blodgett $Mn_{12}$-ac SMM films have been grown on 2 $\mu$m wide Nb tracks and Nb nano-SQUIDs to observe the proximity effect of magnetic moment and magnetization tunneling, respectively. The superconducting critical temperature of thin Nb tracks (thinner than the coherence length of Nb) were found to decrease by the magnetic moment of $Mn_{12}$-ac SMM. Following the thermally activated flux flow (TAFF) model, we found an increase in the vortex unbinding energy of the SMM coated Nb tracks, near critical temperature. More importantly, the random alignment of moments of the $Mn_{12}$-ac molecules at low fields seemed to have the enhancing effect on vortex unbinding energy rather than the saturated state of $Mn_{12}$-ac molecules at high fields. In the fully superconducting state, on the other hand, the vortex pinning effects were found to be more effective in the saturated state of the $Mn_{12}$-ac molecules, as seen from magnetoresistance and field dependent critical current measurements. In a separate experiment, a Langmuir-Blodgett film of SMM was grown on a nano-SQUID to look for local changes in magnetization arising from magnetizatin tunnelling phenomenon in SMMs. Upon magnetizing the SMM (deposited on SQUIDs) at 2 K along the plane of the film and allowing it to relax, we found occasional jumps in the underlying SQUID voltage, unlike bare nano-SQUIDs, which did not show any such jumps over several hours. Therefore, we believe that the jumps in the SQUID voltage are the signatures of random tunneling of magnetization in the SMM layer.