Ultrafast helicity-dependent all-optical switching in hybrid magnetic nanomaterials

信息技术改变了我们的生活，将我们联系在一起，并提供了获取以前属于专家和学者的知识的途径。然而，今天的技术只是一个开始。嵌入信息收集和远程控制我们使用或佩戴的对象将导致物联网，极大地扩展我们需要处理和存储的信息量。要实现这一愿景，底层硬件的能力必须继续以惊人的速度发展。这个项目的主要关注点是我们未来如何存储信息。今天，我们越来越依赖云计算来存储和检索包含大量硬盘驱动器(HDD)阵列的数据中心的信息。我们已经习惯了这样的想法，即每个硬盘的容量每年都会增加，因此可以通过定期更换每个硬盘来满足增加的存储容量需求。然而，这一战略的成功现在还不确定，因为增加产能所需的技术的持续小型化已经达到了不易克服的物理限制。具体地说，用于存储一位信息(1或0)的磁盘表面区域的大小已经变得如此之小，以至于在10年的时间尺度上是不稳定的，这是数据保留的工业标准。已经开发出具有增强稳定性的材料，但是不可能利用从传统磁记录头获得的磁场来切换其磁化，即写入数据。这引起了人们对新的磁开关机制的浓厚兴趣，这种机制可以绕过这个看似不可避免的瓶颈。这个项目将探索如何利用光来切换新磁性材料的磁化强度，在这种材料中，电子和磁性可以被定制为光学控制。2004年，研究表明，通过一种称为机械剥离的技术，石墨烯的原子单分子层可以从碳的一种形式的石墨晶体中剥离出来。这项技术是有效的，因为石墨烯层只有在范德华力的作用下才能与相邻的层弱结合。事实上，还有许多其他具有类似成键的晶体，其中很少有单层膜可能被剥离。通过剥离不同晶体的层并将它们堆叠成多层，有可能创造出迄今未知的杂化材料，这种材料可以结合其母晶体的良好性能。此外，连续层之间的界面可以非常干净和有序。这里的目的是将具有永久磁序的二维铁磁性(2dFM)层与半导体过渡金属二卤化物(TMDC)层相结合，在TMDC层中，电子可以非常高的效率被光学激发。此外，有可能激发具有磁矩的电子，该磁矩与它们的量子力学“自旋”有关，其方向由入射光的偏振决定。将进行这样的实验：使用持续时间小于1万亿秒的超快激光脉冲来激发TMDC层中的电子，以便它们的磁矩可以与2dFM层中的磁矩相互作用。通过光的偏振来控制激发磁矩的方向，目的是随意地前后切换2dFM的磁化强度。此外，磁化强度变化的方式和时间尺度将通过使用第二个激光脉冲在我们选择的时间询问瞬时磁状态来确定。虽然最初的目标是观察和理解全光交换的机制，但将许多不同材料组合在一起的能力将有助于寻找最适合数据存储应用的组合。

Information technology has transformed our lives, connecting us and providing access to knowledge that was previously the preserve of experts and scholars. And yet today's technology is only a beginning. Embedding information gathering and remote control of objects that we either use or wear will lead to an Internet of Things that greatly extends the amount of information that we need to process and store. For this vision to be realised, the capabilities of the underlying hardware must continue to advance at breakneck speed. The principal focus of this project is on how we might store information in future.Today we are increasingly dependent upon cloud computing that stores and retrieves information from data centres that contain enormous arrays of magnetic hard disk drives (HDDs). We have become accustomed to the idea that the capacity of each HDD will increase every year so that increased demand for storage capacity can be met by regularly replacing each HDD. However, the success of this strategy is now uncertain because the continued miniaturisation of technology needed to increase capacity has reached physical limits that are not easily overcome. Specifically, the size of the region on the surface of a disk that is used to store one "bit" of information (1 or 0), has become so small as to be unstable on the timescale of 10 years that is the industrial standard for date retention. Materials with enhanced stability have been developed, but it is not possible to switch their magnetization, i.e. write data, with the magnetic field available from a conventional magnetic recording head. This has generated intense interest in new mechanisms for magnetic switching that can bypass this seemingly unavoidable bottleneck.This project will explore how light may be used to switch the magnetization of new magnetic materials in which the electronic and magnetic properties can be tailored to optical control. In 2004 it was shown that atomic monolayers of graphene can be peeled from a crystal of graphite, a form of carbon, by a technique known as mechanical exfoliation. The technique is effective because graphene layers are only weakly bonded to their neighbours by what are known as van der Waals forces. In fact, there are many other crystals with similar bonding from which few and monolayer films may be exfoliated. By exfoliating layers from different crystals and stacking them to form a multilayer, it is possible to create hitherto unknown hybrid materials that can combine the favourable properties of their parent crystals. Furthermore, the interface between successive layers can be extremely clean and well ordered. Here the aim is to combine 2 dimensional ferromagnetic (2dFM) layers that have permanent magnetic order with semiconducting transition metal dichalcogenide (TMDC) layers in which electrons can be optically excited with very high efficiency. Furthermore, it is possible to excite electrons that have a magnetic moment, which is associated with their quantum mechanical "spin", with direction determined by the polarization of the incident light.Experiments will be performed in which an ultrafast laser pulse with duration less than 1 trillionth of a second is used to excite electrons in the TMDC layer so that their magnetic moments can interact with the magnetic moments in the 2dFM layer. By controlling the direction of the excited magnetic moments through the polarization of the light, the aim is to switch the magnetization of the 2dFM backwards and forwards at will. Furthermore, the manner and timescales on which the magnetization changes will be determined by using a second laser pulse to interrogate the instantaneous magnetic state at a time of our choosing. While the initial goal is to observe and understand the mechanism of all-optical switching, the ability to combine many different materials will facilitate the search for the combinations that are best suited to data storage applications.