Geometry effects on low frequency noise in giant magnetoresistance (GMR) sensors

Microfabricated patterned NiFeCo/Cu GMR multilayers have been studied by varying device sensing area. A bridge sensor configuration was used to measure signal and noise as a function of the device size at room temperature, in a dc magnetic field and with a dc sense current. Wide-band noise from 0.1 Hz to 100 Hz was found to be field dependent for both easy and hard axis directions of applied bias field. The noise showed a broad peak which coincides with the highest signal sensitivity for both easy and hard axis cases in agreement with work on other multilayer systems. The demagnetizing fields cause the peak signal to decrease with stripe width. The signal to noise ratio was found to be proportional to the square root of the device area. Narrow-band fluctuation noise in the presence of an ac drive signal in these GMR devices was found to have a sharp peak near to zero de applied field. This was explained in terms of a spin-flop mechanism.