Sudden critical current drops induced in S/F structures

Abstract

Abstract. In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness ds ≈ 2ξS. Nb/Co and Nb/Py (Permalloy Ni80Fe20) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness dF > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to TC show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness dF ∼ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.

Sudden magnetization drops in superconductor/ferromagnet bilayered structures

“…The thickness transition between these two types is determined by demagnetizing fields which means that it becomes increasingly energetically favourable for domain walls to mutate from out of plane to in-plane direction as films become thinner. This has been shown in experiments in Co, Permalloy (Py) and Ni samples [1,7,[30][31][32] where the transition between these two type of domain walls occurs at around 30 nm for Co and 20 nm for Ni.…”

“…Our results show sudden drops in the magnetization around coercive fields of the ferromagnet where M ∼ 0 and B ∼ H. This is the response of a normal state material with a weak or no diamagnetic response where the external field can easily penetrate. This results in a reentrant behavior to superconductivity as seen in magnetoresistance measurements in [1]. Similar properties of so called field-induced superconductivity (FIS) have only been realized by magnetic dots array deposited on top of a superconducting Pb film [4].…”

“…The Nb/Co bilayers were grown using a UHV DCmagnetron sputtering system in a chamber cooled to −100 • C using liquid nitrogen as reported in [1]. The base pressure prior to Ar insertion was less than 2 × 10 −9 Torr.…”

“…In experiments where the usual proximity effects (due to singlet pairs) were studied [14,[16][17][18][19][20]33] the chosen ferromagnet thickness was of the order of the ferromagnetic coherence length ξ F ; in this range one usually expects domain walls to lie in the plane. For thick ferromagnets Bloch domain walls (BDW) dominate and may generate vortices in the superconductor in the arXiv:1901.06330v1 [cond-mat.supr-con] 18 Jan 2019 out of plane direction [34] affecting its critical current and magnetoresistance [1,3,7]. Here, given that the thickness of the ferromagnet was much greater than ξ F , effects due to Bloch domain walls (BDW) prevail over the so-called Fulde-Ferrel-Larkin-Ovchinnilov (FFLO) state, effects which have been damped at the ferromagnet coherence length scale.…”

Vortex flipping in superconductor/ferromagnet spin-valve structures

“…As the ferromagnet thickness decreases, demagnetizing fields increase, which means that it becomes increasingly energetically favorable for domain walls to mutate from the out-of-plane to in-plane direction. This has been demonstrated in experiments in Co, Permalloy (Py), and Ni samples, [19][20][21][22] where it has been observed that the transition between these two types of domain walls occurs at around 30 nm for Co and 20 nm for Ni.…”