Spin torque oscillator, method of manufacturing the same, magnetic recording head, magnetic head assembly, and magnetic recording apparatus

A method of manufacturing a spin torque oscillator, the method comprising:

• forming a metal layer to be converted into a current confinement layer on a stack including an oscillation layer comprising a magnetic material and a spin injection layer comprising a magnetic material;
• forming, on the metal layer, a mask layer with a tapered surface at an end thereof;
• and
• oxidizing or nitriding an end of the metal layer located under the mask layer formed to include the tapered surface at the end thereof, to form a current confinement layer comprising an insulating portion comprising an oxide or a nitride and a conductive portion comprising a nonmagnetic metal and penetrating the insulating portion in a direction of stacking.