Electric Control of Skyrmions and Antiskyrmions in Multiferroic Nanostructures and Epitaxial Films

对纳米结构和薄膜中Skyrmions的行为和控制的详细了解是它们在未来存储器件中作为磁性比特应用的先决条件。旨在通过外部刺激创造和操纵天空微子的新兴领域标志着从纯粹的基础研究到真正的技术应用的转变。到目前为止，相应的原理证明研究主要集中在手性立方化合物的纳米结构和薄膜中的Bloch型Skyrmions以及出现在界面上的Skyrmions。在这两种情况下，由于主体材料的导电性质，对Skyrmions的操纵都是通过电流实现的。本课题组最近实现的奈尔型天米子，以及斯图尔特·帕金和他的同事今年对反天米子的观察，开启了天微子研究的新视角。在这个项目中，我们计划研究多铁性绝缘体纳米结构和外延薄膜中天米子和反天米子的电场控制。这种方法有几个技术优势：第一，电场控制比电流控制需要更低的能耗。此外，Néel型Skyrmions和反Skyrmions存在于具有轴对称的晶体中，在热稳定性方面比Bloch型Skyrmions更强健。在该项目的一个部分，我们致力于在空位尖晶石纳米结构中实现Néel型Skyrmions的电场控制，这是我们最近建立的一类新的多铁化合物。我们将利用铁电畴的电压控制以及磁性和铁电畴壁之间的强纠缠来研究几何约束对Néel型Skyrmion阵列的影响，并实现对它们的电控。作为最终目标，我们将演示如何通过电场创造单个内尔型天象。在本项目的另一部分中，我们利用空位尖晶石的结构多态，通过单晶和外延薄膜中的单轴应变和电场，实现了Néel型Skyrmion和反Skyrmion晶格态之间的转换。这种天米子-反天米子的相变不仅是一个基本的兴趣，而且是一个重要的技术问题，因为这两类天体内部结构的差异促进了它们不同的外部控制方式。这些目标将通过结合几种实验方法来实现，包括介电光谱、电子自旋共振光谱、磁电、磁输运和磁光测量，以及实空间成像技术和倒易空间成像技术(MFM、LTEM、SANS)。它们都被我们的团队或SPP的合作者成功地应用于Skyrmions的研究。

The detailed knowledge about the behavior and the control of skyrmions in nanostructures and thin films is a prerequisite for their applications as magnetic bits in future memory devices. The emerging field of skyrmionics, aiming at the creation and the manipulation of skyrmions by external stimuli, signals a transition from purely fundamental research towards real technical applications. The corresponding proof-of-principles studies have been focused so far on Bloch-type skyrmions in nanostructures and thin films of chiral cubic compounds as well as on skyrmions emerging at interfaces. In both cases, the manipulation of skyrmions was achieved by electric currents, due to the conducting nature of the host materials. The very recent realization of Néel-type skyrmions by our group and the observation of antiskyrmions by Stuart Parkin and coworkers this year opened new perspectives in skyrmionics.Within this project, we plan to investigate the electric-field control of skyrmions and antiskyrmions in nanostructures and epitaxial films of multiferroic insulators. This approach has several technological advantages: First, the electric-field control requires much lower energy consumption as compared to the electric-current control. Moreover, Néel-type skyrmions and antiskyrmions, present in crystals with axial symmetry, are more robust than Bloch-type skyrmions in terms of thermal stability. In one part of the project, we aim at the realization of the electric-field control of Néel-type skyrmions in nanostructures of lacunar spinels, which we recently established as a novel class of multiferroic compounds. We are going to exploit the voltage-control of the ferroelectric domains together with the strong entanglement between the magnetic and ferroelectric domain walls to study the effect of geometrical constrains on Néel-type skyrmion arrays and to achieve their electric manipulation. As the ultimate goal, we are going to demonstrate the creation of individual Néel-type skyrmions by electric fields. In the other part of the project, taking advantage of the structural polymorphism of lacunar spinels, we aim at the switching between the Néel-type skyrmion and the antiskyrmion lattice states via uniaxial strain and electric field in single crystals and epitaxial films of these compounds. Such skyrmion-antiskyrmion phase transformations are not only of fundamental interest, but represent a technologically important issue, since differences in the internal structures of the two types of objects promote different ways of their external controls.These goals will be reached by combining several experimental methods including dielectric spectroscopy, electron spin resonance spectroscopy, magnetoelectric, magneto-transport and magneto-optical measurements, as well as real- and reciprocal-space imaging techniques (MFM, LTEM, SANS). All of them have been applied successfully for the study of skyrmions by our group or by our collaborators within the SPP.