Revealing the origin of magnetoresistance and resistive switching in organic spin valves

自从2004年首次展示了有机自旋阀(OSV)以来，这些器件中的磁阻主要被解释为电注入和自旋极化电子的探测，从而导致一种巨磁电阻。然而，在许多实验中，我们已经证明，至少在一些典型的OSV中，磁阻不是源于有机材料中电子态的输运[2]，而是源于导致隧道磁阻(TMR)[3]和隧道各向异性磁阻(TAMR)[4]的隧道效应。此外，最近报道了有机自旋阀中的阻性开关[5]，因此除了磁阻特性之外，还增加了一种新的功能。然而，详细优化这些效应所需的基本物理一直没有得到充分的理解。本项目的目标是通过研究TAMR和TMR的贡献以及它们与电阻开关的相互作用，更好地理解导致OSV中磁阻和电阻开关的物理机制。只有确定不同的贡献及其来源，才有可能最大化每种效应，并最终了解OSV是否或以何种方式可以用于应用。为了实现这一目标，有必要回答三个问题：1)自旋阀的哪一部分负责每种效应的发生和大小(单界面、双界面、整体材料性质)？2)效应主要取决于其中一种材料还是仅仅取决于材料组合？3)制造细节起什么作用？界面准备对效果的出现至关重要吗(S)？这些问题的答案可以通过仔细改变材料和材料组合以及涉及的制造过程来找到。随着我们小组在过去几年中进行的实验，已经开发出用于研究这三种效应中的每一种的原型装置和过程，可以作为这些变化的起点。

After the first demonstration of an organic spin-valve (OSV) in 2004 [1] the magnetoresistance in these devices has mostly been interpreted as electrical injection and detection of spin polarized electrons leading to a kind of giant magnetoresistance. We have, however, shown in a number of experiments that at least in some prototypical OSV the magnetoresistance does not originate from transport through electronic states in the organic material [2] but rather from tunneling leading to tunneling magnetoresistance (TMR) [3] and tunneling anisotropic magnetoresistance (TAMR) [4]. In addition, resistive switching in organic spin valves has recently been reported [5] and thus added a new functionality besides the magnetoresistive properties. Nevertheless, the underlying physics which is necessary to optimize these effects in detail has never been sufficiently understood.The objectives of this project target a better understanding of the physics which lead to magnetoresistance and resistive switching in OSV by investigating the contributions of TAMR, TMR and their interplay with resistive switching. Only by identifying the different contributions and their origin will it be possible to maximize each of the effects and to finally understand whether or in what way OSV can be used for applications.To achieve this goal it is necessary to answer three questions:1) Which part of the spin valve is responsible for the occurrence and magnitude of each of the effects (single interface, both interfaces, bulk material properties)?2) Does the effect depend predominantly on one of the materials or merely on the material combination?3) What is the role of the fabrication details? Is the interface preparation crucial for the appearance of the effect(s)?The answers to these questions can be found by careful variation of the materials and material combinations as well as of the fabrication processes involved. With the experiments performed by our group over the last years, prototypical devices and processes for the investigation of each of the three effects have been developed which can be used as a starting point for these variations.