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

该项目的主要目标是利用超导体/铁磁体（S/F）接近效应的非常规物理原理，实验实现超导自旋阀。这种非常规物理学的基础是在f -材料中建立的富尔德-费雷尔-拉金-奥夫钦尼科夫（FFLO）超导态。它导致超导配对波函数的振荡行为，同时衰减到铁磁层。这导致了当层厚度有限时产生干涉效应，导致临界温度的振荡，甚至当f -材料厚度增加时超导状态的消失和随后的恢复，即重入超导。对于F/S/F三层，可以实现超导自旋阀，这意味着可以通过改变F层的相对磁化方向，从反平行（AP）到平行(P)再到反平行(P)来开关超导状态。实现大自旋阀效应（即AP和P的临界温度之间的大差异）的必要条件是实现高振幅的临界温度振荡或（理想情况下）超导态的可重入行为。我们只能在S/F双层（Nb/Cu41Ni59）中实现这种临界温度行为。我们现在也能够在F/S双层中观察到这种现象，S材料现在生长在F层的顶部。这是F/S/F自旋阀芯的第二个构件，可以看作是两个S/F层的镜像对称排列。此外，F/S/F自旋阀芯本身可以实现，显示所需的临界温度的非单调（振荡）行为，包括重入情况。完整的自旋阀至今尚未实现。对于所制备的样品，由于反铁磁性（AF）一氧化碳亚层与下层f层的交换偏置耦合不够强，在外加磁场中，不可能在改变另一层磁化方向的同时固定其中一层的磁化。我们从这个经验得出的结论是，在我们能够制造一个功能的AF-F/S/F自旋阀之前，我们首先必须增加交换偏置。这个问题的解决似乎比我们最初的建议所预期的要耗时得多。因此，这项任务将在更新提案中继续进行。此外，我们正在开发S/F1/N/F2-AF自旋阀结构的实验实现，该结构对两个f层的相对磁化方向也很敏感。这里，N代表一层无磁性的正常导电金属，其厚度在几纳米和零之间变化。在第一个实验中，我们已经观察到F2层和AF层（由钴和钴氧化物组成）之间存在相当大的交换偏置。详细研究了这种新型自旋阀的结构，并提出了更新建议。此外，我们计划优化这两种类型的结构，以获得具有大临界温度变化的功能旋转阀。此外，我们计划通过上临界场测量来评估进入超导自旋阀理论的S/F边界的透明度。最后，我们要研究在运行条件下的自旋阀，即在电流输运下的自旋阀。特别是，我们计划测量临界电流，在FFLO超导态注入自旋极化电流，并测量F/S边界电阻。因此，也将研究非平衡超导性的问题，以了解自旋阀在实际情况下的行为，即电流承载条件下，如果将自旋阀用作存储或逻辑元件，将会出现这种情况。所有步骤都伴随着连续的详细理论分析。

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.