Micrometer-sized highly sensitive strain sensors are presented. The sensors are based on magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layers. The influence of mechanical strain upon the free layer is explained by a model taking into account the total free energy of the sensing layer. Those MTJ devices prepared in situ with magnetostrictive Fe50Co50 layers exhibit a tunneling magnetoresistance (TMR) ratio of 48%. The changes in strain Δε on the order of 0.4 parts per thousand (‰) result in resistance changes of 24%, which in turn leads to gauge factors [(ΔR/R)/Δε] on the order of 600, whereas gauge factors of 2–4 are typical for metal based, and 40–180 for piezoresistive semiconductor strain gauges.

We have applied magnetic force microscopy ͑MFM͒ with an in situ electromagnet to study the switching of the magnetization of submicrometer Co dots fabricated by means of electron-beam lithography. By using the MFM tip as a local-field source, the magnetization of individual single-domain Co dots could be reversed. Micromagnetic simulations show that the switching process is induced by the stray field of the MFM tip. Furthermore, the external field that is necessary to support switching of the dot depends on the tip-dot separation.

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We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross-tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross-tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross-tie wall.

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