Spin-Torque and Spin Polarisation in Epitaxial Magnetic Silicides

铁和硅是地壳中含量最丰富的两种元素。然而，这两种元素最简单的化合物，单硅化铁(FeSi)，具有奇怪的电子和磁性，几十年来一直困扰着研究人员。在低温下，它是一种窄带隙的非磁性半导体。在变暖时，大多数材料变得更难磁化：FeSi变得更容易，它会转变为一种重金属电子金属。虽然在实验上已经知道了40多年，但对这一现象的正确理论描述仍然没有确定下来。当钴取代铁时，事情就变得更加有趣了。理论预测(对大块晶体的间接实验似乎也证实了)，每个Co原子贡献一个载流电子，以及一个电子自旋的磁性，这意味着一个完美的极化磁性半导体-更重要的是，一个基于硅的半导体。事实上，我们最近已经能够在商业硅片上制备这种材料的薄膜，看起来像是外延层：单晶体，其中每个原子都与衬底定义的晶格登记在一起。自旋极化是所有自旋电子材料的关键指标，所有自旋电子效应都随着极化的增加而增加。因此，拥有一种硅基的高极化材料是一个非常诱人的前景。在我们项目的第一部分，我们将确认我们的薄膜的性质以及它们的结构、磁性和电子性质。我们还将研究一种更简单、更快速的薄膜形成方法，即溅射法。然后，我们将继续对这种非凡材料的自旋极化进行第一次直接测量，而且是以技术上至关重要的薄膜形式在硅片上进行测量。然而，磁性在另一方面确实是非凡的。这种材料的晶体结构非常不寻常，因为它缺乏镜像对称性，因此一种几乎在所有其他磁性材料中都被抑制的晦涩效应开始发挥作用：所谓的Dzyaloshinskii-Moriya相互作用。这一术语不同于铁磁体中通常的均匀状态，而是使自旋以螺旋形式相互旋转。这可以在足够大的磁场中达到均匀的饱和状态，但在此过程中，另一项基本上被遗忘的理论物理学开始发挥作用。有一种中间状态是由磁旋涡的晶格形成的，称为Skyrmions，这种拓扑结构最早是在20世纪60年代发明的，用于描述pi介子场。去年的研究表明(使用一种相关化合物锰单硅化物的大块晶体)，由于这种特殊的拓扑结构，这些旋转的磁性结构可以通过流经晶体的电流启动，电流密度大约是传统磁性材料中移动漩涡所需电流密度的一百万倍。我们将在硅化物晶片样品中寻找这些有磁性的天空离子物体，并测量移动它们所需的电流密度。不幸的是，这种材料只有在绝对零度以上几十度的温度下才具有磁性，而且在达到室温之前，所有的磁性都会消失。然而，用元素周期表中的邻居锗取代硅，也可以将硅化铁转变为一种日磁金属，完全取代也可以将这种结构保存到摄氏零上几度的温度。我们将通过在这种材料中掺杂钴来完成我们的项目，并看看是否可以将临界温度提高到高于室温的技术实用值。

Iron and silicon are two of the most abundant elements in the earth's crust. Nevertheless, the simplest chemical compound of these two elements, iron monosilicide (FeSi), possesses bizarre electronic and magnetic properties that have confounded researchers for decades. At low temperatures it is a non-magnetic semiconductor with a narrow gap. On warming, most materials become harder to magnetise: FeSi becomes easier, and it transforms into a heavy electron metal. Although known experimentally for over four decades, the proper theoretical description of this is still not settled. When cobalt is substituted for iron things get even more interesting. Theory predicts (and indirect experiments on bulk crystals seem to confirm) that each Co atom contributes one current carrying electron, and also one electron spin's worth of magnetism, suggesting a perfectly polarised magnetic semiconductor - and what is more, one based on Si. Indeed, we have recently been able to prepare thin films of this material on commercial silicon wafer that appear to be epilayers: single crystals where every atom is in register with the lattice defined by the substrate. Spin polarisation is the key figure of merit for all spintronic materials, with all spintronic effects growing as the polarisation increases. Having a high polarisation material that is silicon-based is therefore a very tantalising prospect. In the first part of our project we will confirm the nature of our thin films and their structural, magnetic, and electronic properties. We will also investigate a simpler and quicker way of forming films known as sputtering. We will then go on to make the first direct measurements of the spin polarisation of this remarkable material, and moreover, do so in the technologically vital thin film form on Si wafer.The magnetism is truly remarkable in another way, however. The crystal structure of this material is very unusual in that it lacks mirror symmetry, and so an obscure effect that is suppressed in almost every other magnetic material comes into play: the so-called Dzyaloshinskii-Moriya interaction. Instead of the usual uniform state in a ferromagnet, this term causes the spins to spiral around each other in a helix. This can be brought to a uniform saturated state in a large enough magnetic field, but on the way another largely forgotten piece of theoretical physics comes into play. There is an intermediate state formed from a lattice of magnetic vortices called skyrmions, a topological structure first invented to describe fields of pi-mesons in the 1960s. Last year it was shown (using bulk crystals of a related compound, manganese monosilicide) that because of this special topology, these swirling magnetic structures can be set into motion by a current flowing through the crystal at a current density around one million times smaller than that needed to move a vortex in a conventional magnetic material. We shall seek these magnetic skyrmion objects in our silicide wafer samples and measure the current density needed to move them. Unfortunately, this material is only magnetic at temperatures a few tens of degrees above absolute zero, and all magnetic properties are lost well before room temperature is reached. Nevertheless, replacing silicon with its neighbour in the periodic table, germanium, can also transform iron silicide into a helimagnetic metal, with complete replacement preserving this structure up to a temperature a few degrees above zero Celsius. We shall complete our project by doping this material with cobalt and see if the critical temperature can be pushed above room temperature to technologically useful values.