Detection of magnetic nanoparticles using simple AMR sensors in Wheatstone bridge

Wheatstone bridges incorporating a serially connected ensemble of simple AMR elements of Ni80Fe20 film were produced, targeting an application of a pinned magnetic field along the sensing magnetoresistor length. For the optimal dimension, the magnetoresistive element with length l = 4 mm, width w = 150 μm and thickness t = 5 nm still shows a rather modest AMR ratio (of about 0.85% only). However, the resulting bridge exhibits a sensitivity as large as 2.15 mV/Oe. This is according to a standard sensitivity of 1.80 mV/V/Oe and a detection limit better than 10−6 emu, which is almost doubled with respect to that in the typical commercial AMR devices and is comparable with Permalloy based PHE sensor. This is suitable to detect the superparamagnetic fluid of 50 nm-Fe3O4-chitosan.