SEMPA Measurements of Magnetic Nanostructure in Magnetic Sensors

Measurement and control of magnetic nanostructure is essential for the development of low frequency, room temperature magnetic sensors with high sensitivity and low noise. In this study, we use scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic nanostructure in single film anisotropic magnetoresistance sensors. The sensors are fabricated using patterned NiFe microstructures that are designed so that the shape anisotropy biases the magnetization relative to the current. This study focused on 'zig-zag'shaped sensors which are composed of multiple repeats of overlapped rectangular unit cells, aligned at 45 degrees from the direction of replication (the easy- or long-axis of the device). SEMPA images show geometric and size dependent magnetic structures, such as vortices that may be responsible for measured magnetoresistance instabilities. SEMPA measurements also provide a direct, quantitative comparison to micromagnetic calculations. This comparison has been made for remanent sates after applying magnetic fields in various directions. In the optimized structures the measured magnetization along the axis of the device oscillates by approximately +/- 25 degrees from the direction of the current flow. For the magnetic simulations, the angle of modulation is somewhat larger at +/- 30 degrees. Detailed comparisons will be presented.