Thermally excited Skyrmions: from individual to collective dynamics

在这个项目中，我们将使用理论和实验相结合的方法来了解热激发天子的静力学和动力学。我们将超越目前研究的铁磁系统，探索具有多个反平行耦合的磁子晶格的系统，包括合成的和本征的亚铁磁和反铁磁天子，这些系统具有由多个亚晶格产生的复杂性质。第一步是根据我们对铁磁天子的联合工作和首次观察到的亚铁磁和合成反铁磁天米子系统中的扩散，确定单个天米子的热力学。随着对低净磁矩的强增强扩散动力学的令人兴奋的预测，这对于基于扩散的非常规计算来说是个好兆头。因此，我们将探索从大的铁磁矩到零净磁矩的天米子扩散，在零净磁矩下，亚晶格完全相互补偿。我们将通过调节温度来改变亚铁磁体中的净磁矩，并通过选择合适的层厚来改变合成反铁磁体中的净磁矩。在第二步中，我们将探索可以形成不同相的Skyrmion系综的集体动力学，例如2D系统特有的六方相和拓扑相变。我们从统计分析样本中的天米子位置开始，这使我们能够量化天米子感受到的潜在景观，包括对反铁磁性天米子增强尝试频率的探测预测，以及支配由此产生的相变的天米子相互作用。然后通过分析它们的局域取向有序参数，并与数值模拟进行比较，来探讨相变和相变。最后，我们将利用Skyrmion独特的大小和形状的动态可调性来研究相形成和相变的动力学。通过突变天米子的大小和形状，我们可以研究时间分辨的平衡过程，以确定驱动相变的关键过程。我们将使用一系列精选的实验磁技术来实施这项工作，这些技术在可变温度下工作，并结合可变的外加磁场。我们将主要使用实时磁性显微镜进行直接成像，并辅之以磁散射技术。从理论上讲，我们使用一种独特的数值模拟技术组合，使我们能够从电子尺度跨越到介观尺度。从原子自旋模拟中，我们将确定反铁磁Skyrmions的本征热力学，然后允许我们提取真实的参数，使用微磁方法来模拟实验上可访问的Skyrmion大小，最终使分子动力学模拟尽可能大的Skyrmion系综来模拟相和相变。

In this project we will use a combined theoretical and experimental approach to understand the statics and dynamics of thermally excited skyrmions. We will go beyond currently studied ferromagnetic systems and explore systems with multiple magnetic sublattices that are coupled antiparallelly, including synthetic as well as intrinsic ferrimagnetic and antiferromagnetic skyrmions with complex properties resulting from their multiple sub-lattices.The first step is to ascertain the thermal dynamics of individual skyrmions based on our joint work on ferromagnetic skyrmions and first observations of diffusion in ferrimagnetic and synthetic antiferromagnetic skyrmion systems. With exciting predictions of strongly enhanced diffusive dynamics for low net magnetic moments, this bodes well for diffusion-based non-conventional computing. Thus, we will probe the skyrmion diffusion from the regime of large ferromagnetic moments down to zero net moment where the sub-lattices fully compensate each other. We will vary the net magnetic moment in ferrimagnets by tuning the temperature and in synthetic antiferromagnets by choosing appropriate thicknesses of the layers. In the second step, we will probe the collective dynamics of skyrmion ensembles that can form different phases, such as the hexatic phase and topological phase transitions unique to 2D systems. We start by statistically analysing the skyrmion positions in a sample, which allows us to quantify the potential landscape that the skyrmions feel, including probing predictions of enhanced attempt frequencies for antiferromagnetic skyrmions and the skyrmion interactions that govern the resulting phase transitions. The phases and phase transitions will then be probed by analysing their local orientational order parameter in comparison to numerical simulations. Finally, we will use the unique dynamic tunability of skyrmion sizes and shapes to study the dynamics of the phase formation and phase transition. By abruptly varying the skyrmion size and shape, we can study the time-resolved equilibration process to identify key processes driving phase transitions.We will implement the work using a range of selected experimental magnetic techniques that operate at variable temperature in combinations with variable applied magnetic fields. We will use primarily real-time magnetic microscopy for direct imaging complemented by magnetic scattering techniques. Theoretically we use a unique combination of numerical simulation techniques that allow us to span from the electronic to the mesoscopic scale. From atomistic spin simulations we will ascertain the intrinsic thermal dynamics of antiferromagnetic skyrmions, which then allows us to extract realistic parameters to model experimentally accessible skyrmion sizes using micromagnetic approaches, finally enabling molecular dynamics simulations of as large ensembles of skyrmions as necessary to model phases and phase transitions.