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

自2004年首次展示有机自旋阀（OSV）以来，这些器件中的磁电阻大多被解释为电注入和自旋极化电子的检测导致一种巨磁电阻。然而，我们已经在一些实验中表明，至少在一些典型的OSV中，磁电阻不是来自有机材料[2]中的电子态输运，而是来自隧道导致隧道磁电阻（TMR）[3]和隧道各向异性磁电阻（TAMR）[4]。此外，有机自旋阀中的电阻开关最近被报道，从而在磁阻特性之外增加了一个新的功能。然而，对于优化这些效应所必需的基本物理原理，人们还没有得到充分的了解。该项目的目标是通过研究TAMR， TMR及其与电阻开关的相互作用，更好地理解OSV中导致磁阻和电阻开关的物理。只有通过识别不同的贡献及其来源，才能最大限度地发挥每种效果，并最终理解OSV是否可以或以何种方式用于应用程序。为了实现这一目标，有必要回答三个问题：1)自旋阀的哪个部分对每种效应（单界面，双界面，大块材料特性）的发生和大小负责？2)效果主要取决于其中一种材料还是仅仅取决于材料组合？3)制作细节的作用是什么？界面准备对效果的出现至关重要吗？这些问题的答案可以通过材料和材料组合的仔细变化以及所涉及的制造工艺来找到。随着我们小组在过去几年进行的实验，已经开发出用于调查这三种效应的原型设备和过程，可以用作这些变化的起点。

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.