Spin torque oscillator with an antiferromagnetically coupled assist layer and methods of operating the same

A spin torque oscillator comprising:

• a first electrode;
• a second electrode;
• a device layer stack located between the first electrode and the second electrode, wherein the device layer stack comprises:
• a spin polarization layer including a first ferromagnetic material;
• an assist layer including a third ferromagnetic material;
• a ferromagnetic oscillation layer including a second ferromagnetic material located between the spin polarization layer and the assist layer;
• a nonmagnetic spacer layer located between the spin polarization layer and the ferromagnetic oscillation layer;
• and
• a nonmagnetic coupling layer located between the ferromagnetic oscillation layer and the assist layer, wherein the assist layer is antiferromagnetically coupled to the ferromagnetic oscillation layer through the non-magnetic coupling layer, and the assist layer has a magnetization that is coupled to a magnetization of the ferromagnetic oscillation layer,
• wherein:
• at least one of the first or second electrode comprises a ferromagnetic material having a fixed magnetization and comprising a ferromagnetic electrode layer and a ferromagnetic pillar that protrudes from the ferromagnetic electrode layer toward the device layer stack;
• the ferromagnetic pillar has substantially a same lateral width as a lateral width of the device layer stack;
• and
• a lateral width of the ferromagnetic electrode layer is larger than the lateral width of the ferromagnetic pillar, such that a ring shaped horizontal surface of the ferromagnetic electrode layer surrounds a base of the ferromagnetic pillar to provide a notch at a junction of the ferromagnetic electrode layer and the ferromagnetic pillar.