Thermally Activated Bilinear and Biquadratic Exchange Coupling Across Amorphous 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 Å and around 45 Å with a range of ferromagnetic coupling at thicknesses between them. We also observe biquadratic coupling at the first crossover between ferro- and antiferromagnetic orientations. The small bilinear coupling strength of about 5 · 10 -6 J/m 2 can reversibly be enhanced upon heating. Irreversible thermal annealing, on the other hand, generally reduces the coupling and in some cases even changes its sign. From the reversible and irreversible temperature dependences we infer that resonant tunnelling through defect states in the spacer material mediates the magnetic exchange.