Gallina David, Pastor G M
Institut für Theoretische Physik, Universität Kassel, Heinrich-Plett-Straße 40, 34132 Kassel, Germany.
Nanomaterials (Basel). 2021 May 25;11(6):1392. doi: 10.3390/nano11061392.
Structural disorder has been shown to be responsible for profound changes of the interaction-energy landscapes and collective dynamics of two-dimensional (2D) magnetic nanostructures. Weakly-disordered 2D ensembles have a few particularly stable magnetic configurations with large basins of attraction from which the higher-energy metastable configurations are separated by only small downward barriers. In contrast, strongly-disordered ensembles have rough energy landscapes with a large number of low-energy local minima separated by relatively large energy barriers. Consequently, the former show good-structure-seeker behavior with an unhindered relaxation dynamics that is funnelled towards the global minimum, whereas the latter show a time evolution involving multiple time scales and trapping which is reminiscent of glasses. Although these general trends have been clearly established, a detailed assessment of the extent of these effects in specific nanostructure realizations remains elusive. The present study quantifies the disorder-induced changes in the interaction-energy landscape of two-dimensional dipole-coupled magnetic nanoparticles as a function of the magnetic configuration of the ensembles. Representative examples of weakly-disordered square-lattice arrangements, showing good structure-seeker behavior, and of strongly-disordered arrangements, showing spin-glass-like behavior, are considered. The topology of the kinetic networks of metastable magnetic configurations is analyzed. The consequences of disorder on the morphology of the interaction-energy landscapes are revealed by contrasting the corresponding disconnectivity graphs. The correlations between the characteristics of the energy landscapes and the Markovian dynamics of the various magnetic nanostructures are quantified by calculating the field-free relaxation time evolution after either magnetic saturation or thermal quenching and by comparing them with the corresponding averages over a large number of structural arrangements. Common trends and system-specific features are identified and discussed.
结构无序已被证明是二维（2D）磁性纳米结构的相互作用能景观和集体动力学发生深刻变化的原因。弱无序二维集合体具有一些特别稳定的磁构型，具有大的吸引盆，较高能量的亚稳构型与它们仅由小的向下势垒分隔。相比之下，强无序集合体具有粗糙的能量景观，有大量低能量局部极小值，由相对较大的能量势垒分隔。因此，前者表现出良好的结构寻求者行为，具有无阻碍的弛豫动力学，该动力学趋向于全局最小值，而后者表现出涉及多个时间尺度和捕获的时间演化，这让人联想到玻璃态。尽管这些一般趋势已明确确立，但在特定纳米结构实现中对这些效应程度的详细评估仍然难以捉摸。本研究量化了二维偶极耦合磁性纳米颗粒相互作用能景观中无序诱导的变化，作为集合体磁构型的函数。考虑了显示良好结构寻求者行为的弱无序方形晶格排列以及显示自旋玻璃状行为的强无序排列的代表性示例。分析了亚稳磁构型动力学网络的拓扑结构。通过对比相应的不连通图，揭示了无序对相互作用能景观形态的影响。通过计算磁饱和或热猝灭后的无场弛豫时间演化，并将它们与大量结构排列的相应平均值进行比较，量化了能量景观特征与各种磁性纳米结构的马尔可夫动力学之间的相关性。确定并讨论了共同趋势和特定于系统的特征。
