A method of forming a spin transfer torque reversal assisted magnetic recording (STRAMR) writer is disclosed wherein a spin torque oscillator (STO) has a flux guiding layer (FGL) wherein magnetization flips to a direction substantially opposing the write gap (WG) field when sufficient current (IB) density is applied across the STO between a trailing shield and main pole (MP) thereby enhancing the MP write field. The FGL has a center portion with a larger magnetization saturation×thickness (MsT) than in FGL outer portions proximate to STO sidewalls. Accordingly, lower IB density is necessary to provide a given amount of FGL magnetization flipping and there is reduced write bubble fringing compared with writers having a FGL with uniform MsT. Lower MsT is achieved by partially oxidizing FGL outer portions. In some embodiments, there is a gradient in outer FGL portions where MsT increases with increasing distance from FGL sidewalls.