Studying Sub-Picosecond Photo-Magnetic Switching with Nanometer Resolution

拟议的实验与两个项目有关，这些项目涉及纳米分辨率的磁态成像。在这两种情况下，我们建议使用X射线透射显微镜来研究具有不同磁史条件的样品中系统的磁对比度。在第一种情况下，研究涉及施加磁光激发后GdFeCo化合物的磁化态的成像。磁性纳米颗粒中的超快(亚皮秒)磁开关的表征和新认识的发展是一个主要的挑战。最近的研究表明，圆偏振的飞秒光脉冲可以通过有效磁场的逆法拉第效应脉冲产生，从而使亚铁磁性薄膜的辐照磁化区域完全反转。尽管到目前为止还没有关于这一过程的时间分辨测量的报道，但人们普遍认为，这种逆转发生在亚皮秒的时间尺度上，超过了任何其他已报道的磁开关机制的速度限制。在这个项目中，我们试图以纳米分辨率成像强的超快圆偏振光脉冲辐照亚铁磁性薄膜的效应，从而揭示这种超快磁化反转的物理机制--光磁开关。在第二个实验中，我们计划对NiFe的图案化阵列的纳米磁性进行研究。最近用诸如MOKE的宏观技术观察到，图案中元素的特定几何构型(即正方形或六边形)可能导致与图案的物理结构各向异性相关的元素之间的磁相互作用。结果，作为磁记录的基本特性的磁化反转可能受到图案的对称性的影响。利用XM可以非侵入性地直接观察磁化结构，可以直接说明磁化如何受到局部磁场的影响，以及相邻磁矩和它们的对称性之间的相互作用是否在重新磁化过程中起到关键作用。XM测量的优点是可以作为外加磁场的函数来进行研究，从而允许观察在再磁化过程中磁组态的演变。

The proposed experiments are related to two projects involving imaging of magnetic states with nanoscopic resolution. In both cases we propose to use X-ray transmission microscopy to study the magnetic contrast of the systems in samples with different conditions of magnetic history. In the first case the study is related to imaging of the magnetisation states in GdFeCo compounds after application of magneto-optical excitations. The characterization and thereby the development of new understanding of ultrafast (sub-picosecond) magnetic switching in magnetic nanoparticles is a major challenge. Recently it was demonstrated that femtosecond optical pulses of circular polarization have been shown to generate, via the Inverse Faraday Effect pulses of effective magnetic field leading to complete magnetization reversal of the irradiated domains of ferrimagnetic films. Although no time resolved measurement of this process has been reported so far, it is widely believed that the reversal occurs on sub-picosecond time scales, surpassing the speed limitations of any other reported mechanism of magnetic switching. In this project, we seek to image with nanometer resolution the effect of irradiation of ferrimagnetic films with intense ultrafast optical pulses of circular polarization and thereby to shed light upon physical mechanisms underlying this kind of ultrafast magnetization reversal - the photo-magnetic switching. In the second experiment we propose to carry out studies of nanomagnetic properties of patterned arrays of NiFe. It has been recently observed with macroscopic techniques, such as MOKE, that a specific geometric configuration of elements in a pattern (i.e. square or hexagonal) may lead to magnetic interactions between the elements anisotropically correlated with the physical structure of the pattern. As a result, magnetisation reversal, which is the fundamental property in magnetic recording, can be affected by the symmetry of the pattern. A direct observation of the magnetic domain structure, which can be done non-invasively with XM can directly demonstrate how the magnetisation is affected by the local magnetic field and whether the interactions between the neighboring moments and their symmetry play a crucial role in the remagnetisation process. The advantage of XM measuremnts that the investigation can be performed as function of the applied field allowing to observe the evolution of the magnetic configurations during the remagnetising process.