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

超导/铁磁体纳米层结构中的超导邻近效应自旋阀现象的研究

• 批准号: 161046475
• 负责人: Professor Dr. Siegfried Rüdiger Horn
• 金额: --
• 依托单位: Institut für Materials Resource Management
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2010
• 资助国家: 德国
• 起止时间: 2009-12-31 至 2012-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/161046475?language=en
• 关键词: Study; Superconducting; Proximity; Effect; Spin

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.

该项目的主要目标是通过应用超导体/铁磁体（S/F）邻近效应的非常规物理学来实验实现超导自旋阀。这种非常规物理的基础是在F材料中建立的类Fulde-Ferrell-Larkin-Ovchinnikov（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。当层具有有限厚度时，这导致干涉效应，从而导致临界温度的振荡，或者当F材料的厚度增加时，甚至导致超导状态的消失和随后的恢复，即，导致折返超导性。对于F/S/F三层，可以实现超导自旋阀，这意味着可以通过将F层的磁化的相对取向从反平行（AP）改变到平行（P）并返回来切换超导状态。实现大自旋阀效应的必要条件，即AP和P情况的临界温度之间的大差异，是实现高振幅临界温度振荡或（理想地）超导状态的重入行为。我们只能在Nb/Cu_（41）Ni_（59）的S/F双层膜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。这表示F/S/F自旋阀核心的第二构建块，其可以被视为两个S/F层的镜像对称布置。此外，F/S/F自旋阀核心本身可以实现，显示出所需的非单调（振荡）行为的临界温度，包括重入的情况。到目前为止，完整的自旋阀尚未实现。对于所制备的样品，钉扎的F-层的磁化，而改变另一个在施加的磁场中的磁化方向，是不可能的，可能是由于这样的事实，即反铁磁（AF）一氧化钴子层的交换偏置耦合到较低的F-层是不够强。我们的结论是，我们首先要增加交换偏置之前，我们能够制造一个功能性的AF-F/S/F自旋阀。这个问题的解决方案似乎比我们原来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，我们正在开发一个实验实现的S/F1/N/F2-AF自旋阀结构，这也是敏感的两个F层的磁化的相对取向。这里，N表示一层垂直导电金属，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和AF层之间相当大的交换偏置，由钴和钴氧化物组成。我们希望在更新提案中对这种新的自旋阀结构进行详细研究。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自旋阀。此外，我们计划评估的S/F边界的透明度，进入超导自旋阀的理论，通过上临界场测量。最后，我们想研究自旋阀的工作条件下，即在电流传输。特别地，我们计划测量临界电流，将自旋极化电流注入到FFLO超导态，并测量F/S边界电阻。因此，还将研究非平衡超导性的问题，以了解自旋阀在现实的，即载流条件下的行为，如果将自旋阀用作存储或逻辑元件，则将存在这种行为。所有步骤都伴随着连续详细的理论分析。

项目成果

Experimental Observation Of The Triplet Spin-Valve Eect In A Superconductor-Ferromagnet Heterostructure

• DOI: 10.1103/physrevb.87.144507
• 发表时间: 2012-06
• 作者: V. Zdravkov;J. Kehrle;G. Obermeier;D. Lenk;H. V. Nidda;C. Muller;A. Sidorenko;S. Horn;R. Tidecks;L. Tagirov

Reentrant superconductivity and superconducting critical temperature oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 grown on cobalt oxide

氧化钴上生长的 Cu41Ni59/Nb/Cu41Ni59 F/S/F 三层材料的重入超导性和超导临界温度振荡

• DOI: 10.1063/1.4813131
• 发表时间: 2013
• 期刊: Journal of Applied Physics
• 影响因子: 3.2
• 作者: V. I. Zdravkov;J. Kehrle;D. Lenk;G. Obermeier;A. Ullrich;C. Müller;H.-A. Krug von Nidda;R. Morari;A. S. Sidorenko;L. R. Tagirov;S. Horn;R. Tidecks
• 通讯作者: R. Tidecks

Critical temperature oscillations and reentrant superconductivity due to the FFLO like state in F/S/F trilayers

F/S/F 三层中的 FFLO 状态导致临界温度振荡和重入超导性

• DOI: 10.1002/andp.201100133
• 发表时间: 2012
• 期刊: Annalen der Physik
• 影响因子: 2.4
• 作者: J. Kehrle;V. I. Zdravkov;G. Obermeier;J. Garcia-Garcia;A. Ullrich;C. Müller;R. Morari;A. S. Sidorenko;S. Horn;L. R. Tagirov;R. Tidecks
• 通讯作者: R. Tidecks