Spin-Torque and Spin Polarisation in Epitaxial Magnetic Silicides

铁和硅是地壳中最丰富的两种元素。然而，这两种元素中最简单的化合物——单硅化铁（FeSi），却拥有奇怪的电子和磁性，这让研究人员困惑了几十年。在低温下，它是具有窄间隙的非磁性半导体。在变暖的情况下，大多数材料变得更难磁化：FeSi变得更容易磁化，并转变为一种重电子金属。虽然在实验上已经知道了40多年，但正确的理论描述仍然没有解决。当钴取代铁时，事情就变得更有趣了。理论预测（在块状晶体上进行的间接实验似乎也证实了这一点），每个Co原子贡献一个携带电流的电子，以及一个电子自旋的磁性，这表明这是一种完美的极化磁性半导体——更重要的是，这是一种基于Si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，这种薄膜看起来像是epilayers：单晶，其中每个原子都与衬底定义的晶格一致。自旋极化是所有自旋电子材料的关键性能指标，所有自旋电子效应都随着极化的增加而增加。因此，以硅为基础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及其结构、磁性和电子性能。我们还将研究一种更简单、更快速的形成薄膜的方法，即溅射。然后，我们将继续对这种非凡材料的自旋极化进行首次直接测量，而且，在技术上至关重要的硅晶片薄膜形式上进行测量。然而，在另一方面，这种吸引力确实是非凡的。这种材料的晶体结构非常不寻常，因为它缺乏镜像对称性，因此一种在几乎所有其他磁性材料中都被抑制的模糊效应开始发挥作用：所谓的Dzyaloshinskii-Moriya相互作用。与铁磁体中通常的均匀状态不同，这一项使自旋彼此绕成螺旋状。它可以在足够大的磁场中达到均匀饱和状态，但在此过程中，另一个几乎被遗忘的理论物理学部分开始发挥作用。有一种中间状态是由称为skyrmions的磁漩涡晶格形成的，skyrmions是一种拓扑结构，最初是在20世纪60年代发明的，用于描述介子的场。去年有研究表明（使用一种相关化合物单硅化锰的块状晶体），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过晶体的电流来启动，其电流密度比传统磁性材料中移动漩涡所需的电流密度小100万倍左右。我们将在硅化晶片样品中寻找这些磁性粒子，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，用元素周期表中的相邻元素锗取代硅，也可以将硅化铁转化为一种helmagnetic金属，完全取代后，这种结构在摄氏零度以上几度的温度下仍能保持不变。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推到室温以上，达到技术上有用的值。

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.