Topology and spin dynamics in magnetic molecules

We investigate the role of topology and distortions in the quantum dynamics of magnetic molecules, using a cyclic spin system as reference. We consider three variants of an antiferromagnetic molecular ring, i.e., ${\mathrm{Cr}}_{8}$, ${\mathrm{Cr}}_{7}\mathrm{Zn}$, and ${\mathrm{Cr}}_{7}\mathrm{Ni}$, characterized by low-lying states with different total spin $S$. We theoretically and experimentally study the low-temperature behavior of the magnetic torque as a function of the applied magnetic field. Near level crossings, this observable selectively probes quantum fluctuations of the total spin (``$S$ mixing'') induced by lowering of the ideal ring symmetry. We show that while a typical distortion of a model molecular structure is very ineffective in opening new $S$ mixing channels, the spin topology is a major ingredient to control the degree of $S$ mixing. This conclusion is further substantiated by low-temperature heat capacity measurements.