Spin current propagation through epitaxial antiferromagnetic thin films

现代电子学的运行依赖于传输电子电荷的电流。然而，电子还具有固有的角动量，称为“自旋”，这是其磁矩的原因。自旋是一个有两个允许值的量子力学量。因此，我们可以把电子看作是最小的条形磁铁，它的北极不是朝上就是朝下。通常，电流在上升态和下降态传输相同数量的电子。然而，在铁磁材料内部，处于向上状态的电子比处于向下状态的电子多;这是其磁性行为的起源。这意味着从铁磁体引出的电流将具有向上自旋的优势。事实上，在非磁性金属的某些情况下，我们可以安排相同数量的上下自旋电子沿相反方向运动，这样就有了自旋角动量流，而没有任何电荷流。这就是所谓的纯自旋流，在铁磁体中，还有一种额外的机制可以用来传输自旋流。我们可以想象一个电子从上到下翻转它的自旋，而不是电子移动，翻转的自旋的位置从一个原子移动到另一个原子。即使当材料是电绝缘体时，这种机制也存在，并且被称为“自旋波”。铁磁体只是具有磁序的许多类型材料中的一种。这个提议主要涉及反铁磁材料，在反铁磁材料中，连续的原子层的自旋方向在上下之间交替。反铁磁体没有净磁矩，因为相邻原子上的磁矩相互抵消，所以通常更难以研究，并且很长一段时间被认为在实际应用方面毫无用处。然而，自旋波也发生在反铁磁体中，因此反铁磁体可以用来传输纯自旋电流。最近观察到，通过在铁磁层和非磁性层的堆叠中插入薄的反铁磁层，可以增强自旋电流的幅度。我们已经表明，反铁磁层能够传输直流和交流自旋电流，确认一个模型，也预测自旋电流可以放大至少10倍，如果层的厚度是仔细选择。这个额外的角动量来自晶格。考虑到通常需要小电流来产生纯自旋电流，在反铁磁层中放大自旋电流的能力意味着可以显著提高使用自旋电流的器件的能量效率。一个直接的示例是一种类型的磁性随机存取存储器（MRAM），其中自旋电流被注入到铁磁层中以反转其磁化，从而以二进制代码表示0或1。在这个项目中，我们将使用超快激光测量技术首先观察反铁磁薄膜中存在的自旋波模式，该薄膜的厚度可能是10个原子直径的数量级。这将是一个重大的成就，因为超导薄膜的行为与大块晶体非常不同，观察它们的自旋波的方法还有待证明。一旦我们有了这些信息，我们就能够设计多层堆叠，以观察自旋电流的传播和放大。具体来说，我们将使用时间分辨的X射线测量技术在同步辐射源，我们已经开发和演示。最后，我们将探讨如何优化堆栈，以便它们可以用于MRAM等实际应用。

The operation of modern day electronics depends upon electric currents that transport electron charge. However, the electron also possesses intrinsic angular momentum, known as "spin", that is responsible for its magnetic moment. Spin is a quantum-mechanical quantity with two allowed values. We can therefore think of the electron as the smallest possible bar magnet with its north pole pointing either up or down. Ordinarily an electric current transports equal numbers of electrons in the up and down states. However, inside a ferromagnetic material there are more electrons in the up state than the down state; this is the origin of its magnetic behaviour. This means an electric current drawn from a ferromagnet will have a preponderance of up spins. In fact, under certain circumstances in non-magnetic metals, we can arrange for equal numbers of electrons with up and down spins to move in opposite directions so that there is a flow of spin angular momentum without any flow of charge. This is what is meant by a pure spin current.Within a ferromagnet an additional mechanism is available to transport spin current. Rather than the electrons moving, we can think of one electron flipping its spin from up to down and the location of this flipped spin moving from one atom to the next. This mechanism is present even when the material is an electrical insulator and is known as a "spin wave". Ferromagnets are only one of many types of material that have magnetic order. This proposal is concerned primarily with antiferromagnetic materials, where the direction of the spin alternates between up and down for successive layers of atoms. Antiferromagnets have no net magnetic moment, because those on adjacent atoms cancel out, so are generally more difficult to study, and for a long time were thought to be useless in terms of practical applications. However, spin waves also occur in antiferromagnets and so antiferromagnets can be used to transport pure spin current.It was recently observed that the amplitude of a spin current can be enhanced by the insertion of thin antiferromagnetic layers into a stack of ferromagnetic and non-magnetic layers. We have shown that the antiferromagnetic layer is able to transport both dc and ac spin currents, confirming a model that also predicts that spin currents could be amplified by at least a factor of 10 if the thickness of the layer is chosen carefully. This additional angular momentum is drawn from the crystal lattice. Given that a small electric current is usually required to generate a pure spin current, the ability to amplify spin current in the antiferromagnetic layer means that the energy efficiency of devices using spin currents could be significantly improved. One immediate example is a type of magnetic random access memory (MRAM), where spin current is injected into a ferromagnetic layer to reverse its magnetization so as to represent a 0 or 1 in binary code. Reducing power consumption by just a factor of 2 would already make MRAM an attractive alternative to dynamic random access memory (DRAM) within data centre applications.In this project, we will use an ultrafast laser measurement technique to first observe the spin wave modes that exist within antiferromagnetic thin films that may be the order of 10 atomic diameters in thickness. This will be a major achievement since ultrathin films can behave very differently to bulk crystals, and methods for observing their spin waves have yet to be demonstrated. Once we have this information, we will then be able to design multi-layered stacks in which to observe the propagation and amplification of spin currents. Specifically, we will use a time resolved x-ray measurement technique at a synchrotron source that we have already developed and demonstrated. Finally, we will explore how the stacks can be optimised so that they can be used in practical applications such as MRAM.