Dipolar coupling

There is no dipolar coupling between perfectly smooth, uniformly magnetized ferromagnetic layers because they create no stray field. However, rough surfaces do couple via dipolar fields. This is known as the orange-peel effect. When the roughness of the two layers is correlated, the coupling is ferromagnetic. The magnitude of the dipolar coupling associated with surface roughness was calculated by Kools, who found

where ts is the thickness of the spacer layer, δ_s is the surface roughness of the spacer and l is the period of the roughness. Some typical values for a rough film are t_s = 5 nm, δ_s = 1 nm l =20 nm, which gives σ_d = 0.03 mJ m⁻² for a film with μ₀M_s = 1 T. This is not insignificant compared with the exchange coupling.

Dipolar coupling is also significant in magnetic nanopillars, where each nanostructured layer creates a stray field.