ROUGHNESS-INDUCED COUPLING BETWEEN FERROMAGNETIC-FILMS ACROSS AN AMORPHOUS SPACER LAYER

The exchange coupling between an amorphous ferromagnet and a polycryst alline Fe layer across an amorphous AuSn spacer is studied by spin-pol arized secondary-electron emission. No antiferromagnetic or oscillator y exchange coupling is observed. Instead, we find a coupling of unexpe ctedly long range which favors perpendicular alignment of the ferromag nets at larger spacer thicknesses. At a spacer thickness of 30 to 50 A ngstrom the magnetization direction of the top Fe layer continuously c hanges from ferromagnetic to 90 degrees alignment and remains in its p erpendicular orientation for AuSn films up to at least 100 Angstrom. T he rather weak thickness dependence of the observed coupling points at a recently proposed magnetic dipole interaction as possible mechanism . Following models by Demokritov er al. and by Neel and using the roug hness parameters extracted from scanning tunneling microscopy we calcu late the coupling strengths of the dipolar interactions. The experimen tal results can be described with the models. We confirm that in struc turally disordered multilayers roughness-induced magnetic dipolar inte ractions can cause biquadratic as well as bilinear coupling, while the quantum-mechanical exchange is strongly suppressed. The calculated co upling strengths set an upper bound of 0.002 erg/cm(2) for the quantum -mechanical exchange across this disordered metallic spacer.