A technique for magneto-optic sensing of film switching in 5 nsec over an area 20-μ in diameter has been reported by Thompson and Change (Ref. 1). The technique, when used in conjunction with inductive sensing, enables the analysis of the switching behavior of each layer in a coupled film structure. An experimental structure consists of an insulated ground plane on top of which two Permalloy layers (125-μ wide, 900 Å/layer) sandwiching a word conductor (125-μ wide, 2-μ thick, 6 μ above ground) are deposited with each axis along the strips. Two strands of lines (100-μ wide on 200-μ centers) on glass are connected in parallel to form a bit/sense line. The sense line is 40 μ above the coupled film lines. The openings between strands provide optic access for Kerr magneto-optic sensing of the top film. The word current and its ground image create fields of the same polarity at the bottom film, but of different polarities at the top film. It has been surmised that only the bottom film is switched by the drive current and its image, while the top film is switched by the stray field from the switched bottom film. Experimental results, indeed, confirm the above. Furthermore, the switching speed of the bottom film is found to be limited by flux trapping in the ground plane. During read, the switching speed of the top film is limited by its own demagnetizing field cancelling the stray field. During write, the switching speeds of both films are limited by the subtraction of shape-anisotropy field from induced-anisotropy field due to hard direction-flux closure. The device signal is found to be only slightly altered by a hard-direction bias field in the range of ±10 Oe. © 1967 The American Institute of Physics.