Magnetic viscosity effects in the giant magnetoresistance of NiO/permalloy/Cu/permalloy exchange-biased films

We have examined the effects of magnetic viscosity (magnetic aftereffect) on the giant magnetoresistance (GMR) of NiO/Permalloy(45 Angstrom)/Cu(45 Angstrom)/Permalloy(45 Angstrom) exchange-biased spin-valve structures grown by direct current (de) magnetron sputtering on Si substrates. One of the samples had a 6 Angstrom Co layer on both sides of the Cu layer. GMR is a sensitive indicator of magnetic viscosity effects since any time dependence in the magnetization reversal of one of the layers is reflected directly in the GMR. Two different types of GMR measurements were made over the temperature range of -30 degrees C to 50 degrees C. The maximum time for the resistance to stabilize to 1 part in 10(4) ranged from about 70 to 200 s for both samples at the temperatures studied. The time dependency of the GMR was well described by C + S ln t, where S and C are constants; S is called the coefficient of magnetic viscosity. This equation characterizes the classic time dependent behavior of the magnetization. The maximum magnitude of S was found to decrease as the temperature was increased.