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Suppression of the superconducting critical current of Nb in bilayers of Nb/SrRuO3

In bilayers consisting of ferromagnetic and superconducting films, the ferromagnetic film in its domain state induces inhomogeneous distribution of magnetic fields in the superconducting film. When the ferromagnetic film has bubble magnetic domains in a labyrinth structure, it has been found that the pinning of the vortices increases; hence, the critical current of the superconducting film becomes larger. Here we study the effect of parallel ferromagnetic domain structure in Nb/SrRuO3 on the critical current of Nb with current flowing perpendicularly to the domains and find that in this case the ferromagnetic domain structure decreases the critical current.


I. INTRODUCTION:
The critical current (I c ) in type II superconductors is limited by the pinning strength of the vortices, since the current applies force on the vortices leading to vortex motion which dissipates energy and destroys superconductivity. Different techniques for creating vortex pinning were proposed, including thickness modulation [1], columnar defects [2] and introduction of magnetic particles on the superconducting film [3,4]. Most of those techniques were based on local suppression of the superconducting state, since it is favorable for the vortex normal core to be located in areas which are normal anyway; hence saving the condensation energy.
Recently it was proposed to pin the magnetic flux of vortices rather than their normal core [5]. This magnetic pinning can be achieved in superconductor -ferromagnet (SC/FM) multilayers. The magnetostatic interaction between magnetic flux of the vortices and magnetization of the FM layer gives rise to pinning potential across the domain structures that increases the superconducting critical current [6,7,8]. This method was demonstrated in systems where the FM film had bubble magnetic domains in a labyrinth structures, such as CoPt or BaFe 12 O 19 [6,7,8]. On the other hand, no additional pinning force is expected in FM/SC multilayers if the vortex motion is parallel to the domain structure [6].
Here we study the influence of in-plane magnetic stripe domain structure of SrRuO 3 on the critical current of Nb in SC/FM bilayers with current flowing perpendicularly to the domain structure (Fig. 1). In this configuration vortices are forced to move parallel  To measure the I-V curves of the Nb film with no field penetrating from the SrRuO 3 film, the sample was cooled from above the Curie temperature of SrRuO 3 (∼ 150 K) in a field of 2 T down to 10 K. This prevented the formation of magnetic domains in the SrRuO 3 film.
At 10 K the field was set to zero and the SrRuO 3 remained fully magnetized in a single domain state. The sample was then cooled down under the T c of Nb (7 K) at zero field.
The I-V curves were measured at different temperatures from 2 K up to the T c of Nb. We call those measurements field cooled measurement (FC). Between every two sequential I-V measurements the sample was heated above the T c of Nb so that each measurement started with no penetration of vortices into the Nb film. In the second type of measurements the sample was cooled from above the Curie temperature of SrRuO 3 at zero field to below the T c of Nb and I-V curves were measured at the same range of temperatures. We call those measurements zero field cooled measurement (ZFC). In those measurements magnetic domains formed inside the SrRuO 3 film and magnetic flux penetrated into the Nb film. To estimate the value of the effective magnetic field induced by the magnetic domain structure of SrRuO 3 on the Nb film we measured I-V curve with the SrRuO 3 film fully magnetized and external magnetic field, H, applied in the plane of the sample. As it is seen in Figure 2, the effect of the domain structure of SrRuO 3 on the I c of Nb is similar to that of an applied magnetic field of 1600 Oe. Such a field is consistent with our estimations based on calculations of Sonin [11] (Fig 3), who calculated the fringing field created by the magnetic structure at the interface between FM and SC layers in the SC/FM bilayers.
Despite the difference in the field distribution in case of the field produced by the stripe domain structure and the uniform applied field, we note that the results are of the same order of magnitude. We have shown that in bilayers of Nb/SrRuO 3 with current flowing perpendicularly to the domain walls there is a temperature range in which clear suppression of the critical current is observed.