a New Magnetic Effect for Strain Sensing

High sensitivity magnetooptical ellipsometry has been used for measuring the evolution of the coercive force in Ni films subjected to an externally applied stress in various geometries. A strong effect of mechanical stress on the coercive force has been experimentally observed. For a stress parallel to the magnetising field a strong dependence on film thickness is observed and in general the relationship between coercive force variation and stress is complex. The most interesting situation occurs for a film thickness of 20 nm. In this case in fact the above relationship is linear over a large stress range, from compressive (positive values) to tensile (negative values) and for a maximum value of stress close to the elastic limit of bulk Ni. This effect has been understood by modelling the magnetic medium as a collection of domains with random orientation of the easy magnetisation axis. Here we discuss the possibility to exploit this effect for making stress sensors. In order to realise real sensors based on this principle a micromachined magnetic circuit must be realised coupled to the electronic circuitry suitable for performing magnetic measurements.