The magnetic dipolar coupling of head-to-head domain walls is studied in 350nm wide NiFe and Co nanostructures by high resolution magnetic imaging. We map the stray field of a domain wall directly with sub-10-nm resolution using off-axis electron holography and find that the field intensity decreases as 1r with distance. By using x-ray magnetic circular dichroism photoemission electron microscopy, we observe that the spin structures of interacting domain walls change from vortex to transverse walls, when the distance between the walls is reduced to below (77±5)nm for 27nm thick NiFe and (224±65)nm for 30nm thick Co elements. Using measured stray field values, the energy barrier height distribution for the nucleation of a vortex core is obtained.

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Assuming a similar dependence of the stray field for NiFe, a Gaussian distribution is found for the energy barrier distribution for NiFe as well.

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