A design for a microwave assisted magnetic recording device is disclosed wherein a spin torque oscillator (STO) between a main pole and write shield has a spin polarization (SP) layer less than 30 Angstroms thick and perpendicular magnetic anisotropy (PMA) induced by an interface with one or two metal oxide layers. Back scattered spin polarized current from an oscillation layer is used to stabilize SP layer magnetization. One or both of the metal oxide layers may be replaced by a confining current pathway (CCP) structure. In one embodiment, the SP layer is omitted and spin polarized current is generated by a main pole/metal oxide interface. A direct current or pulsed current bias is applied across the STO. Rf current may also be injected into the STO to reduce critical current density. A write gap of 25 nm or less is achieved while maintaining good STO performance.