The torque that is exerted by a magnetic field on a magnetic moment may lead to a change of the atomic conformation in a molecule. This effect has been demonstrated recently for a single Ho magnetic moment in the endohedral Fullerene HoLu₂@C₈₀ [1].
From the statistical model that describes the equilibrium orientation of a magnetic moment in an external magnetic field with the ratio between the Zeeman energy and the thermal energy k_BT, for a given saturation magnetisation a freezing temperature may be determined. Below this freezing temperature the conformation does not change. For an applied field of 7 T and a cooling rate of 5 mK/s the freezing temperature is 55 K. For fields below 7 T the extra magnetisation due to the molecular orientation depends on the cooling rate, and quadratically on the external magnetic field during cool down. From the corresponding kinetics of the freezing temperature vs. cooling rate data we may infer an activation energy and the corresponding attempt frequency for the endohedral rotation. Since the magnetisation measurements are accurate, variations in the percent range of this activation energy can be detected for different endohedral units. With this molecular dynamics potentials may be tested and steric effects of the endohedral unit can be inferred. On a longer perspective this effect, mimicking the smallest compass, also opens avenues to orient endohedral units on surfaces by help of magnetic fields.

[1] Switching Molecular Conformation with the Torque on a Single Magnetic Moment, Kostanyan et al. Phys. Rev. Lett. accepted for publication.