The use of magnetic particles or ferromagnetic (FM) domains as pinning centers for vortices in the ferromagnetic/superconducting (FM/S) hybrid layered structures has been demonstrated in recent years. The goal is the creation of new devices in which this type of magnetic pinning (MP) enhances the control of the critical current in the S layer. However, the influence of various materials parameters on the MP is not yet well understood.

In this work we investigate the effect of the shape of the FM domains on the MP in a series of FM/S bilayers which consist of niobium film as the S layer, and Co/Pt multilayer with perpendicular magnetic anisotropy as the FM layer. First, using the MFM technique we show that the variation of the thickness of platinum in the Co/Pt multilayer in the range 0.3 < t < 2 nm leads to a change of the domain pattern, from less dense collection of domains of diverse sizes when t=0.3 nm, to a more dense collection of uniform smaller domains when t increases. We then use the array of small Hall sensors to study the flux penetration into the FM/S bilayers built of these Co/Pt multilayers with various t. We find that the enhancement of the critical current induced by the MP is present in all cases. However, the pattern which contains diverse sizes, but fewer domains induces smaller critical current enhancement than the pattern with dense small domains. In addition, large flux instabilities are observed during the flux penetration in the first case, while in the second case the flux penetration is smoother. The origins of these effects will be discussed.

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