High sensitivity torsion magnetometers for oligatomic films

For thin film studies in magnetic surface and size effects, magnetometers of extreme sensitivity are required, capable to detect a small part of one monolayer. They may be realized as torsion magnetometers using the torque on an anisotropic film in a homogeneous field. A complete analysis of the method is given for ferromagnetic films, both saturated and unsaturated, and for paramagnetic (superparamagnetic) films. Two existing instruments are described, both working in a contact gas, from 4 to 600 K. The first one works in the oscillation mode, using the period of oscillation of the oscillating film as a measure of the magnetic directional moment; this instrument is capable to detect 2·10⁻¹⁸ Wb·m (2·10⁻⁹ G·cm³). The second one works preferentially in the static mode, as a loop tracer, in which the magnetic torque is compensated by a quadrant electrometer; this second instrument detects 6·10⁻¹⁸ Wb·m in the static mode; with a reduced sensitivity (2·10⁻¹⁶ Wb·m), it works also in the oscillation mode. The relative merits of both instruments are discussed.