THERMALLY ACTIVATED BILINEAR AND BIQUADRATIC EXCHANGE COUPLING ACROSSAMORPHOUS-SILICON

Ferromagnetic Fe and Ni films separated by a spacer layer of amorphous Si evaporated at T = 40 K exhibit oscillatory exchange coupling. Two antiferromagnetic-coupling regions are found for Si thicknesses around 16 angstrom and around 45 angstrom with a range of ferromagnetic coup ling at thicknesses between them. We also observed biquadratic couplin g at the first crossover between ferro- and antiferromagnetic orientat ions. The small bilinear coupling strength of about 5.10(-6) J/m2 can reversibly be enhanced upon heating. Irreversible thermal annealing, o n the other hand, generally reduces the coupling and in some cases eve n changes its sign. From the reversible and irreversible temperature d ependences we infer that resonant tunnelling through defect states in the spacer material mediates the magnetic exchange.