Improvement of Magnetoresistive Character of NiO/NiFeCo/Cu/NiFeCo Exchange-Biased Spin-Valves Through Annealing in Magnetic Field

NiO(70 nm)/NiFeCo(5 nm)/Cu(2 nm)/NiFeCo(5 nm) spin-valve multilayers were prepared by electron beam evaporation. It is found that the magnetoresistive (MR) ratio of the as-deposited films is relatively small (about 1.6% ) and there is no apparent biasing in MR curves of as-deposited multilayers due to weak exchange coupling. Through heat treatment at a proper temperature in a magnetic field, the exchange field of the materials increases and the multilayers show good exchange-biased spin-valve MR character with MR value reaching about 2.5%. The proper temperature range for annealing is 200-250^oC. With further increasing temperature, MR ratio drops rapidly.