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Interlayer exchange coupling is an interaction that favors particular relative orientations of magnetization in different magnetic layers. In a metallic multilayer, spacer electrons can mediate coupling even when a nonmagnetic layer separates the magnets.

A simple model writes energy per area as $E/A=-J{\hat{\mathbf{m}}}_1\cdot {\hat{\mathbf{m}}}_2$. Positive $J$ favors parallel alignment and negative $J$ favors antiparallel alignment. Spacer material and thickness affect the coupling; magnetic-field sweeps can reveal the field needed to overcome it. Correlated interface roughness can instead produce orange-peel coupling, so a preferred alignment alone does not uniquely establish exchange mediation. ^[ref]

Roughness can produce a distribution of local switching fields and nonuniform reversal. The abrupt edges in the ideal pseudo spin valve loops then become rounded or sloping, the antiparallel plateau becomes less flat, and overlapping layer-reversal distributions can narrow or partly eliminate the clear high-resistance window. The middle panel of the original diagram illustrates only this broadening mechanism.

Correlated roughness on the two interfaces can also introduce orange-peel coupling, a magnetostatic coupling that favors parallel alignment. Such coupling shifts switching fields and can suppress independent reversal. Sufficiently severe roughness or pinholes can have a similar qualitative effect by directly coupling the ferromagnets.

Changes in amplitude require care. Some spin-dependent interface scattering contributes to giant magnetoresistance, so a small increase in roughness is not guaranteed to lower the signal monotonically. Intermixing, spin-flip scattering and an incompletely antiparallel state generally reduce useful contrast. The detailed width, displacement and height of $R(H)$ therefore depend on how the interfaces become rough, rather than on a single roughness number.