A single measurement of a sample's magnetisation is relatively easy to obtain, especially with modern technology. Often it is simply a case of loading the sample into the magnetometer in the correct manner and performing a single measurement.
This value is, however, the sum total of the sample, any substrate or backing and the sample mount. A sample substrate can produce a substantial contribution to the sample total. The samples studied in this thesis using magnetometry have a single type of substrate, silicon, substantially greater in volume and mass than the sample itself.
Fortunately the substrate is diamagnetic (see Section 3.2.1). Under zero applied field this means it has no effect on the measurement of magnetisation. Under applied fields its contribution is linear and temperature independent. The diamagnetic contribution can be calculated from a knowledge of the volume and properties of the substrate and substracted as a constant linear term to produce the signal from the sample alone.
The diamagnetic background can also be seen clearly at high fields where the sample has reached saturation: the sample saturates but the linear background from the substrate continues to increase with field. The gradient of this background can be recorded and subtracted from the readings if the substrate properties are not known accurately.
Dr John Bland, 15/03/2003