Spin-sensitive scanning tunneling microscope using GaAs optically pumped tips

Ultrathin Co layers grown on Au(1 1 1) surface were observed by means of a scanning tunneling microscope (STM) equipped with a p-type GaAs tip. The Ga As tips were made either by in-air cleavage or micro-fabrication process an d were illuminated through the sample by a lie-Ne laser or a GaAlAs laser t o excite spin-polarized electrons inside. The spin polarization was modulat ed by modulating laser beam polarization. During a scanning of the GaAs tip over the as-grown Co ultrathin films of 3 ML thickness, modulation frequen cy component of the tunneling current was recorded to construct an image. T he image showed domain like structure with typical lateral size of about 15 0-300 nm. This size coincides with the magnetic domain size which was obser ved by a magnetic force microscope for the same sample (or a sample that wa s made by the same procedure). The imaging mechanism is discussed taking sp in dependent tunneling and several parasitic effects such as (magnetic) cir cular dichroism into account. (C) 1999 Elsevier Science B.V. All rights res erved.