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Planar Hall effect (PHE) sensors with the junction size of 50 mum times 50 mum were fabricated successfully by using spin-valve thin films Ta(5)/NiFe(x) /Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) with x = 4, 8, 10, 12, 16. The magnetic field sensitivity of the PHE sensors increases with increasing thickness of ferromagnetic (FM) free layer. The sensitivity of about 95.5 m Omega/(kA/m) can be obtained when the thickness of the FM-free layer increases up to 16 nm. The enhancement of sensitivity is explained by the shunt current from other layers. The PHE profiles are well described in terms of the Stoner-Wohlfarth energy model. The detection of magnetic micro-beads label Dynabeadsreg M-280 is demonstrated and the results revealed that the sensor is feasible for high-resolution biosensor applications.