Detection of Fe<inf>3</inf>O<inf>4</inf> Magnetic Nanoparticles using Giant Magnetoresistance (GMR) Sensor Based on Multilayer and Spin Valve Thin Films by Wheatstone Bridge Circuit

We have been demonstrated the detection of Fe 3 O 4 nanoparticles by using giant magnetoresistance (GMR) sensor based on multilayer and spin valve thin films by using in Wheatstone bridge. The thin films were fabricated by DC magnetron sputtering method. The structure of multilayer thin film is [Co (1,5 nm)/ Cu (1,0 nm)]20 and spin valve thin film is IrMn (10 nm) / CoFe (3 nm) / Cu (2.2 nm) / CoFeB (10 nm). The Fe 3 O 4 nanoparticles were synthesized by chemical coprecipitation method. The nanoparticles show superparamagnetic properties which saturation magnetization of 77 emu/gram and average particle size of 10 nm. The crystal structure of Fe 3 O 4 magnetic nanoparticles was inverse spinel. The output voltage of Wheatstone bridge is linear to the Fe 3 O 4 concentrations (0.1 mg/ml; 1 mg/ml; 10 mg/ml; 100 mg/ml). That gradient was 0.20 for spin valve thin film and 0.07 for multilayer thin film. The gradient shows that spin valve thin film more sensitive than multilayer. The magnetic moment of spin valve thin film was saturated easier than multilayer thin film. The magnetization of spin valve thin film could be saturated by external field of 20 Oe, while multilayer was 5000 Oe. Magnetoresistance of spin valve was 6%, and magnetoresistance of multilayer was 4%.