Thermally Excited Low Frequency Magnetic Noise in CPP Structure MR Heads

1/f-type of low frequency noise is widely observed in today´s state-of-the-art tunneling-magneto-resistive (TMR) and current-perpendicular-to-plane (CPP), and giant-magneto-resistive (GMR) heads. In this paper, the 1/f-type magnetic noise arising from the thermally excited low frequency edge magnetization switch of the free layer is studied by micromagnetic simulation. It is found that although the free layer dimension is in the single domain regime, the edge magnetization random switching related to the thermal excitation and insufficient hard bias field on sensor edge can be a significant low frequency noise contributor in TMR/CPP GMR heads. A strong local hard bias (HB) field at sensor edge that pins the free layer edge magnetization is found to be more critical than a strong averaged HB field across the sensor to avoid magnetic 1/f-type noise. The simulation study shows that a high HB field gradient from sensor edge to center is preferred to maintain a low noise sensor while still achieving high sensitivity. This paper will also discuss the roles of free layer thickness, sensor geometry, current field, temperature, and air-bearing surface physical defect in the behavior of this edge flipping noise