Vortex flipping in superconductor/ferromagnet spin-valve structures

Abstract

We report in-plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni below the superconducting transition T c , our results show strong evidence of vortex flipping driven by ferromagnet magnetization. This is a direct consequence of the proximity effect that leads to vortex supercurrent leakage into the ferromagnets. Here, the polarized electron spins are subject to the vortices' magnetic field, occasioning vortex flipping. Such a novel mechanism has been made possible by fabrication of the ferromagnet/superconductor/ferromagnet/antiferromagnet multilayered spin valves with an S layer thin enough to barely confine vortices inside as well as F layers thin enough to align and control magnetization within the plane. When Co is used, the vortex flipping effect is not observed. This is attributed to the shorter coherence length of Co. Interestingly, a reduction in the pinning field of about 400 Oe is observed instead when the Nb layer is in the superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts, any explanation must be directly related to the proximity effect and thus a remarkable phenomenon that deserves further investigation.

Effects of Nb buffer layer on superconducting and magnetic behavior of IrMn/NiFe/Nb/NiFe spin-valves

“…4(a), we have a characteristic loop of a type II superconductor, i.e., at this temperature the response to magnetic field is dominated by the contribution from the Nb superconductor and "pinning" in the IrMn/NiFe is minimal. The shape of this curve is related to vortex processes of a type II superconductor: vortex penetration, vortex pinning, and vortex creation-annihilation [23]. This fact discards the possibility to estimate the FM parameters with accuracy (H C and H EB ).…”

Sudden magnetization drops in superconductor/ferromagnet bilayered structures

“…As we shall see, the effect of vortex injection from the Co layer leads to a sudden drop in magnetization of the superconducting layer. A reference Nb/Co bilayer with a thin Co layer of 3 nm was also studied [24] to compare our results; this eliminated effects arising from Bloch domain walls while keeping the usual proximity effect. Furthermore this Nb/Co bilayer is a good reference sample for obtaining characteristic parameters of the ultra-thin Nb film.…”

“…Most of the previous works dealt with transport measurements of S/F structures and only a few [2,24,26,27] investigated magnetic properties of the superconductor under the influence of the ferromagnet. Indeed, as recent experiments suggest the magnetic properties of F/S/F spin valve structures may strongly be affected by proximity effect leading to strong variations of the superconductor's magnetization [24].…”

“…Most of the previous works dealt with transport measurements of S/F structures and only a few [2,24,26,27] investigated magnetic properties of the superconductor under the influence of the ferromagnet. Indeed, as recent experiments suggest the magnetic properties of F/S/F spin valve structures may strongly be affected by proximity effect leading to strong variations of the superconductor's magnetization [24]. Here using a thin superconductor it was possible to squeeze vortices between two ferromagnets allowing a direct control the vortex orientation by ferromagnets' relative magnetic orientation leading to vortex flipping.…”

Nonlocal Spin Transport Mediated by a Vortex Liquid in Superconductors

“…There can be other mechanisms that can give rise to the interconversion. For example, a vortex in a superconductor can harbor spin-polarized normal quasiparticles in its core, which tend to align along the magnetic flux associated with the vortex via the Zeeman coupling as demonstrated in the vortex-flipping experiments [25]. The spin angular momentum of quasiparticles in the vortex core can be directly transferred to a magnetic insulator when the vortex is annihilating at its interface.…”