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The detection capability of magnetic beads and nanoparticles by tunneling magnetoresistance sensors is analyzed in a finite element framework. The limitations for single particle detection and the determination of the particle position are investigated. It will be shown how varying the geometrical sensor design may readily be employed to adjust the setup to a specific measurement task. Especially, we show up strategies increasing the sensitivity by introducing magnetically soft areas. Further, number sensitive detections are discussed and the influence of dipolar particle coupling on the measured signal is calculated.