Low-frequency resistance noise of several NiFe giant magnetoresistance systems

Resistance noise has been observed in several NiFe giant magnetoresistance systems as a function of magnetic fields applied transverse to device stripes at a variety of temperatures and bias currents. A constant DC current bias is applied to thin film multilayer devices and the voltage fluctuations across these devices are measured as a noise power spectrum, SV, and interpreted as a resistance noise power spectrum, SR. The devices measured are stripes of annealed and unannealed NiFe/Ag multilayers and NiFe/Cu spin-valves with a FeMn pinning layer. Annealing causes the formation of weakly magnetostatically coupled discontinuous layers in the NiFe/Ag. Random Telegraph Fluctuators are observed in NiFe/Ag discontinuous multilayer devices of 2 mum and 1 mum stripe widths. Analysis of these fluctuators gives estimates of their magnetic moments of 106--107 muB. 1/ f noise is observed in NiFe/Ag discontinuous multilayers of 8 mum and 16 mum stripe widths, with occasional observation of Random Telegraph Fluctuators. High bias currents, 107 A/cm2, broaden the magnetoresistive response of 8 mum and 16 mum NiFe/Ag stripes and cause the appearance of peaks in the magnetoresistance associated with each NiFe layer. Increased noise and magnetic susceptibility, as observed by the magnetooptical Kerr effect, are observed in association with these peaks. A partition function model is reviewed to explain the resistance peaks, increased noise, and susceptibility. Fitting this model to the observed resistance peaks gives an estimate for the fluctuators' magnetic moment of 10 6 muB.;Noise measured in NiFe/Cu spin-valves is found to be 1/f-like and present only in the response region. This noise is broadened and its amplitude decreased by application of longitudinal fields parallel to the device stripes. A few Lorentzian noise traces are found on a 4 mum spin-valve. The field and temperature at which the Lorentzians are observed gives an estimate for the fluctuating magnetic moment in the spin-valve of 104 mu B. Several simple spin-valve models are considered to compare with the measured noise: a random telegraph fluctuators model, a dynamic layer magnetization rotation model, and a spin-valve free magnetic layer partition function model.