Ultrafast spin dynamics in molecular magnets

在上个世纪，磁性材料彻底改变了我们获取和利用信息的方式。数字信息存储在磁畴中的硬盘驱动器中，其中北极和南极代表二进制“1”和“0”。记录数据的速度受限于这些域的极点可以反转的速率。最近的进展使用激光脉冲短至百万分之十亿分之一秒（或飞秒），使其有可能克服这一限制，通过切换磁畴1000倍的速度比目前的技术可以实现。因此，超快磁性有可能大幅提高信息写入存储器的速度，并且是当前磁性研究的前沿之一。新的磁性材料和新的控制方法的持续发展将确保我们能够充分利用大数据集，这反过来将改善我们生活的许多方面，如医疗保健，政府，物流，并将减少全球能源消耗。另一个发展，但迄今为止尚未探索的超快磁性的背景下，是分子磁体的研究。这将克服将硬盘驱动器中的数据位大小减少到几个原子的问题，目前使用的材料已经达到了它们的大小限制。除了减小尺寸之外，分子磁体还显示出超快磁性的另一个优势。最近已经表明，具有局部磁矩的磁性材料有希望实现快速磁化反转。这些系统可以在产生更少热量的过程中更快地切换。由于分子磁体的磁性排序来自局部磁矩，这些系统非常有前途，因为它们的化学灵活性使得可以调节局部磁矩之间的相互作用，更重要的是，它们对光扰动的响应。这将使我们能够开发出可以使用超短激光脉冲进行开关的纳米材料。在这项提议中，我们将研究一系列模型化合物，在这些化合物中，可以系统地改变材料的元素组成和化学计量，以调整其磁性和光学性质。特别是，该项目将分为两个工作包（WP）：普鲁士蓝类似物的自旋翻转（WP 1）和光磁体动力学（WP 2）。在WP 1中，将研究普鲁士蓝类似物（PBAs）。众所周知，在光激发后，这些材料中可以发生非常快的自旋翻转。我们最近应用专门的方法直接观察飞秒时间尺度上的自旋翻转。我们将这些方法扩展到一系列的PBAs，以增加我们对磁矩之间的相互作用如何在局部化站点上自旋翻转后控制动力学的理解。在WP 2中，我们将建立在这一知识的基础上，研究基于Fe和Nb的类似系统。在光激发后，最初的反磁性（或“非磁性”）Fe（II）中心被切换，在类似的过程中，前面所描述的，但在这种情况下，自旋激发态被捕获后，光激发。这导致顺磁性Nb中心之间的磁相互作用，并最终发生宏观磁有序。目前还不知道磁有序过程发生的速度有多快，但是，我们的方法可以以前所未有的时间分辨率测量这一过程。这将使我们能够理解磁开关过程的机制，这对于优化过程以将材料和技术结合到未来的超快和超密度磁光数据存储设备中是必要的。EPSRC参考：EP/S 018824/1

Magnetic materials have completely changed how we can access and make use of information during the last century. Digital information is stored in hard-drives in magnetic domains, where the north and south poles represent binary "one" and "zero". How fast data can be recorded is limited to the rate at which the poles of these domains can be reversed. Recent advances using laser pulses as short as a millionth billionth of a second (or femtosecond) have made it possible to overcome this limitation by switching magnetic domains 1000 times faster than what current technology can achieve. Ultrafast magnetism therefore has the potential to drastically increase the rate of writing information to memories by orders of magnitude and is one of the frontiers in current magnetic research. A continued development of new magnetic materials and new ways of controlling them will ensure that we can make the most of large data sets, which in turn will improve many aspects of our lives such as health care, government, logistics and will reduce global energy consumption.Another development, but hitherto unexplored in the context of ultrafast magnetism, is the study of molecular magnets. These will overcome the problems with reducing the size of data bits in hard drives to that of a few atoms, where the materials currently used have reached their size limit. Besides from reducing the size, molecular magnets also show another advantage for ultrafast magnetism. It has recently been shown that magnetic materials with localised magnetic moments are promising for achieving fast magnetisation reversal. These systems can be switched much faster in a process that generates less heat. Since the magnetic ordering of molecular magnets are from localised magnetic moments, these systems are very promising because their chemical flexibility makes it is possible to tune the interaction between the localised moments, and more importantly, their response to light perturbation. This will allow us to develop nanomaterials that can be switched using ultrashort laser pulses.In this proposal, we will look at a series of model compounds, where it is possible to systematically change the elemental composition and stoichiometry of the materials to tune their magnetic and optical properties. In particular, the project will be split into two work packages (WPs): spin-flips in Prussian Blue Analogues (WP1) and dynamics of photomagnets (WP2). In WP1, Prussian blue analogues (PBAs) will be studied. It is known that very fast spin-flips can happen in these materials after light excitation. We have recently applied specialised methods to directly observe the spin-flip on a femtosecond timescale. We will extend these methods to a range of PBAs to increase our understanding of how the interaction between the magnetic moments govern the dynamics after the spin-flip on the localised sites. In WP2, we will build on this knowledge and study a similar system based on Fe and Nb. After light excitation, the initially diamagnetic (or "non-magnetic") Fe(II) centres are switched, in a similar process to what was described earlier, but in this case, the spin-excited state is trapped after photoexcitation. This leads to a magnetic interaction between paramagnetic Nb centres and eventually a macroscopic magnetic ordering takes place. It is not known how fast the magnetic ordering process takes place, however, our methods can measure this with unprecedented time resolution. This will allow us to understand the mechanisms for the magnetic switching process, which is necessary for optimising the process to incorporate both the materials and techniques in a future ultrafast and ultradense magneto-optical data storage devices. EPSRC Reference: EP/S018824/1