Studying Sub-Picosecond Photo-Magnetic Switching with Nanometer Resolution

所提出的实验涉及两个项目，涉及纳米级分辨率的磁态成像。在这两种情况下，我们建议使用X-射线透射显微镜研究的磁对比度的系统中的样品具有不同的条件下的磁历史。在第一种情况下的研究是有关成像的磁化状态在GdFeCo化合物的应用磁光激发后。表征和发展新的理解超快（亚皮秒）磁开关的磁性纳米粒子是一个重大的挑战。最近，它被证明，飞秒光脉冲的圆偏振已被证明产生，通过逆法拉第效应脉冲的有效磁场导致完全磁化反转的照射域的亚铁磁薄膜。虽然迄今为止还没有关于该过程的时间分辨测量的报道，但人们普遍认为，反转发生在亚皮秒的时间尺度上，超过了任何其他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的亚铁磁薄膜与强烈的超快光脉冲的圆偏振的照射效果，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，我们建议进行图案化阵列的NiFe的纳米磁性的研究。最近已经用宏观技术（诸如MOKE）观察到，图案中的元件的特定几何配置（即，正方形或六边形）可以导致与图案的物理结构各向异性相关的元件之间的磁相互作用。因此，作为磁记录中的基本属性的磁化反转可以受到图案的对称性的影响。使用XM可以非侵入性地直接观察磁畴结构，可以直接证明磁化强度如何受到局部磁场的影响，以及相邻磁矩之间的相互作用及其对称性是否在再磁化过程中起着至关重要的作用。XM测量的优点是可以根据所施加的磁场进行研究，从而可以观察到再磁化过程中磁结构的演变。

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.