Suppression of electronic friction on Nb films in the superconducting state.

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient Γ is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Γ is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory, meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. Γ is found to be proportional to d-1 and V2 in the metallic state, whereas Γ∼d-4 and Γ∼V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

Magnetic properties of nanomagnetic and biomagnetic systems analyzed using cantilever magnetometry.

Magnetic properties of nanomagnetic and biomagnetic systems are investigated using cantilever magnetometry. In the presence of a magnetic field, magnetic films or particles deposited at the free end of a cantilever give rise to a torque on the mechanical sensor, which leads to frequency shifts depending on the applied magnetic field. From the frequency response, the magnetic properties of a magnetic sample are obtained. The magnetic field dependences of paramagnetic and ferromagnetic thin films and particles are measured in a temperature range of 5-320 K at a pressure below 10(-6) mbar. We present magnetic properties of the ferromagnetic materials Fe, Co and Ni at room temperature and also for the rare earth elements Gd, Dy and Tb at various temperatures. In addition, the magnetic moments of magnetotactic bacteria are measured under vacuum conditions at room temperature. Cantilever magnetometry is a highly sensitive tool for characterizing systems with small magnetic moments. By reducing the cantilever dimensions the sensitivity can be increased by an order of magnitude.