The magnetoresistive sensors made from Permalloy (Ni₈₀Fe₂₀) thin films and Permalloy based multilayered structures are very attractive for detection of low magnetic fields in lab-on-a-chip applications. We exploit the anisotropic magnetoresistance effect (AMR), which appears in these structures, to build a microcompass for detection of the position and rotation rate of a micro rotor used to study the dynamics of the biological fluids. For this reason, we used the planar Hall effect (PHE) to get direct access to the anisotropic part of the resistance with the advantage of a reduced thermal drift of the output signal. The multilayer structure presents, in addition to the AMR effect, the giant magnetoresistance effect. The samples have circular shape geometry with approximately 5 mm in diameter. The four-lead setup consists of 4 Cu strips forming a square of 4 mm each side. We used a computer controlled measurement system. Because of the contacts misalignment the angular behaviour of the PHE voltage is distorted. To compensate these errors and to increase the sensor sensitivity we used a special setup in which we made, sequentially, two measurements of the PHE for each angle over two orthogonal directions. Also, we performed micromagnetic simulations to discuss the effect of the magnetic field strength on the shape of the angular dependence of PHE. To make these simulations we used complex structures of Permalloy single domains which interact between them and with the applied magnetic field. From experimental measurements and micromagnetic simulations, made on the multilayered structure, results a distortion of the angular dependence of the PHE for low magnetic fields (less than 200 Oe) because the magnetization vector can not follow the direction of the applied magnetic field.

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1. MAGNETORESISTANCE AND HALL EFFECT CHARACTERISATION OF SOME MAGNETIC MULTILAYER STRUCTURES
2. Characterisation of Permalloy based thin films and magnetic multilayers using the magnetoresistance and Hall effect measurements