Shape Anisotropy in Magneto-Impedance NiFe-Based Microsensors

MI sensors are based on the impedance variations due to changes of the skin depth when applying an external magnetic field. They present high sensitivity but relative spatial resolution. Thin film technology is an interesting way to reduce size and realize arrays of microsensors.

The present work reports the fabrication and characterization of microsensors based in the MI effect. The sensors consisted in a multilayered structure of two ferromagnetic layers with a copper layer in between. Permalloy has been chosen as magnetic material due to both its magnetic properties and the possibility of obtaining thick layers of this material by means of electrodeposition. Silicon substrates and UV thick photolithography have been used to perform the micromolding of the sensors. Electrodeposition conditions have been optimized in previous works [1]. A post annealing process under magnetic field has been applied in order to study its influence on the sensibility of the sensors. The size of the obtained sensors was from 1 mm x 100 µm to 5 mm x 250 µm. The thickness of the copper and NiFe layers was ranging from 2 to 10 µm.

The optimal excitation frequency leads to a matching between the film thickness and the skin depth, what has been experimentally validated: from 0.8 MHz for 5 µm thick NiFe films to 3 MHz for 2.5 µm thick NiFe films. The sensors have been characterized at these frequencies as a function of the magnetic field and the best results have been obtained for sensors annealed at 300°C under transverse magnetic field. Both the magneto-impedance maximum and the anisotropy field depend on the aspect ratio of the structure what highlights the influence of the shape anisotropy. The narrower structures give the highest maximum of the magneto-impedance (170% of variation of impedance) and to the higher anisotropy field (4200 A/m).