Domain and wall structures in films with helical magnetization profile

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.

Research highlights
► This paper is devoted to the peculiar domain structure resulting from an anisotropy distribution in the thickness of the sample, realized through specific elaboration conditions.
► The helical magnetization profile obtained leads to a complex dynamic behaviour described and modelled in Phys.Rev. B 80, 134412 (published in October 2009) which has been already cited three times.
► This paper sheds light on of the demagnetized state of such samples: a variety of domains structure has been observed by Kerr microscopy, under various saturation fields.
► The most striking conclusion is driven by the analysis of the magnetization process which implies the co-existence of two types of domain walls in the sample, with four possible directions for the mean magnetization. The magnetization profile of the two walls has been confirmed by numerical simulation.