Study of the Superconducting Proximity Effect Spin-Valve Phenomenon in Superconductor / Ferromagnet Nanolayered Structures

• Antragsteller: Professor Dr. Siegfried Rüdiger Horn
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2010 bis 2013
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 161046475

The main goal of the project is the experimental realization of a superconducting spin valve by apply-ing the unconventional physics of the superconductor/ferromagnet (S/F) proximity effect. The basis of this unconventional physics is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like superconducting state establishing in the F-material. It leads to an oscillating behaviour of the superconducting pairing wave function, while decaying into the ferromagnetic layer. This results in interference effects when the layer has a finite thickness, leading to an oscillation of the critical temperature or even an extinction and subsequent recovery of the superconducting state, i.e. to reentrant superconductivity, when the thickness of the thickness of the F-material is increased. For F/S/F trilayers a superconducting spin valve can be realized, that means the superconducting state can be switched off and on by changing the relative alignment of the magnetizations of the F-layers from antiparallel (AP) to parallel (P) and back. A necessary condition to achieve a large spin valve effect, i.e. a large difference between the critical temperatures of the AP and P case, is the realization of high amplitude critical temperature oscillations or (ideally) a reentrant behavior of the superconducting state. We could realize such critical temperature behavior in S/F bilayers (of Nb/Cu41Ni59) only. We are now able to observe the phenomena also in F/S bilayers, with the S-material now grown on top of the F-layer. This represents the second building block of the F/S/F spin valve core, which can be regarded as a mirror symmetric arrangement of two S/F layers. Moreover, the F/S/F spin valve core itself could be realized, showing the required non monotonous (oscillating) behaviour of the critical temperature including the reentrant case. The complete spin valve has not been realized so far. For the prepared samples, the pinning of the magnetization of one of the F-layers, while changing the magnetization direction of the other one in an applied magnetic field, was not possible yet, probably due to the fact that the exchange bias coupling of an antiferromagnetic (AF) cobalt monoxide sublayer to the lower F-layer was not strong enough. Our conclusion from this experience is that we first have to increase the exchange bias before we are able to fabricate a functioning AF-F/S/F spin valve. The solution of this problem appears to be much more time consuming as expected in our original proposal. Therefore, this task will be continued in the renewal proposal. In addition, we are developing an experimental realization of an S/F1/N/F2-AF spin valve structure, which is also sensitive to the relative orientation of the magnetizations of the two F-layers. Here, N represents a layer of nonmagnetic normal conducting metal, the thickness of which varies between several nanometers and zero. In first experiments we already observed a considerable exchange bias between the F2 and AF layers, consisting of cobalt and cobalt oxide. Detailed studies of this new spin valve structure we want to perform in the renewal proposal. Moreover, we plan to optimize both types of structures to get functioning spin valves with large critical temperature shifts. In addition, we plan to evaluate the transparency of the S/F boundary, entering the theory of the superconducting spin valve, by upper critical field measurements. Finally, we want to study the spin valve under operating conditions, i.e. under current transport. Particularly, we plan to measure the critical current, apply spin polarized current injection into the FFLO superconducting state, and measure the F/S boundary resistance. Thus, also problems of non-equilibrium superconductivity will be investigated, to learn about the behaviour of the spin valve under realistic, i.e. current carrying conditions, which will be present if applying the spin valve as storage or logic element. All steps are accompanied by a continuous detailed theoretical analysis.

• DFG-Verfahren: Sachbeihilfen