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., Cr-8, Cr7Zn, and Cr7Ni, 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.