Using window underlayer structures to protect near field transducers on heat assisted magnetic recording heads

A method, comprising:

• applying light to a near field transducer positioned toward a media facing surface of a magnetic head, the magnetic head having:
• a return pole;
• a main pole;
• a waveguide adjacent the near field transducer, wherein the waveguide extends away from the near field transducer along a direction perpendicular to the media facing surface;
• at least one cladding layer adjacent to the waveguide;
• an underlayer positioned behind the near field transducer with respect to the media facing surface, the underlayer extending away from the near field transducer along the direction perpendicular to the media facing surface;
• and
• a fill material at least partially surrounding the underlayer, the waveguide and the at least one cladding layer,
• wherein the underlayer has a lower coefficient of thermal expansion than the fill material,
• wherein upon illumination of the near field transducer, a portion of the media. facing surface of the magnetic head at the near field transducer exhibits less thermal protrusion toward a magnetic medium than another portion of the media facing surface.