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。skyrmionics的新兴领域，旨在创造和操纵skyrmions的外部刺激，标志着从纯粹的基础研究过渡到真实的技术应用。到目前为止，相应的原理验证研究主要集中在手性立方化合物纳米结构和薄膜中的布洛赫型skyrmion以及界面处出现的skyrmion上。在这两种情况下，由于主体材料的导电性质，对skyrmion的操纵是通过电流实现的。本研究小组最近发现的Néel型skyrmions和StuartParkin及其同事今年对反skyrmions的观察为skyrmions的研究开辟了新的视角。在本项目中，我们计划研究多铁性绝缘体纳米结构和外延膜中skyrmions和反skyrmions的电场控制。这种方法具有几个技术优点：首先，电场控制与电流控制相比需要低得多的能量消耗。此外，存在于具有轴对称的晶体中的尼尔型斯基尔米恩和反斯基尔米恩在热稳定性方面比布洛赫型斯基尔米恩更稳健。在该项目的一部分，我们的目标是实现电场控制的Néel型skyrmions在纳米结构的腔隙尖晶石，这是我们最近建立的一类新的多铁性化合物。我们将利用铁电畴的电压控制以及磁畴壁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，并实现它们的电操纵。作为最终目标，我们将演示通过电场产生单个Néel型skyrmions。 在该项目的另一部分，利用腔隙尖晶石的结构多态性，我们的目标是通过单轴应变和电场在这些化合物的单晶和外延膜中实现Néel型skyrmion和反skyrmion晶格状态之间的切换。这种skyrmion-antiskyrmion相变不仅是基本的兴趣，但代表了一个技术上的重要问题，因为两种类型的对象的内部结构的差异促进其外部控制的不同方式。这些目标将通过结合几种实验方法来实现，包括介电谱，电子自旋共振谱，磁电，磁输运和磁光测量，以及真实的和倒易空间成像技术（MFM、LTEM、SANS）。所有这些都已成功地应用于我们的小组或我们的合作者在SPP的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.