Spin current propagation through epitaxial antiferromagnetic thin films

现代电子学的操作依赖于传递电子电荷的电流。然而，电子也拥有固有的角动量，被称为“自旋”，这是它的磁矩的原因。自旋是一个有两个允许值的量子力学量。因此，我们可以把电子想象成尽可能小的条形磁铁，它的北极要么向上，要么向下。通常，电流在上下状态下传输的电子数量相等。然而，在铁磁性材料内部，处于上态的电子多于处于下态的电子；这就是它的磁性的起源。这意味着来自铁磁体的电流将具有向上自旋的优势。事实上，在某些情况下，在非磁性金属中，我们可以安排相同数量的自旋向上和自旋向下的电子朝相反的方向运动，这样就有了自旋角动量的流动，而没有任何电荷的流动。这就是所谓的纯自旋电流。在铁磁体内部，有一个附加的机制可以传输自旋电流。而不是电子移动，我们可以想象一个电子从上到下翻转它的自旋，这个翻转自旋的位置从一个原子移动到另一个原子。即使材料是电绝缘体，这种机制也存在，被称为“自旋波”。铁磁体只是许多具有磁性有序的材料中的一种。这一建议主要涉及反铁磁材料，其中自旋方向在连续的原子层中上下交替。反铁磁体没有净磁矩，因为相邻原子上的磁矩相互抵消了，所以通常更难研究，并且在很长一段时间内被认为在实际应用方面毫无用处。然而，自旋波也出现在反铁磁体中，因此反铁磁体可以用来传输纯自旋电流。最近观察到自旋电流的振幅可以通过在铁磁层和非磁层堆叠中插入薄的反铁磁层来增强。我们已经证明，反铁磁层能够传输直流和交流自旋电流，证实了一个模型，该模型还预测，如果仔细选择层的厚度，自旋电流可以被放大至少10倍。这个额外的角动量是从晶格中得到的。考虑到产生纯自旋电流通常需要很小的电流，在反铁磁层中放大自旋电流的能力意味着使用自旋电流的设备的能量效率可以显着提高。一个直接的例子是一种磁性随机存取存储器（MRAM），其中自旋电流注入铁磁层以反转其磁化，从而在二进制代码中表示0或1。将功耗降低1 / 2就已经使MRAM成为数据中心应用中动态随机存取存储器（DRAM）的一个有吸引力的替代方案。在本项目中，我们将使用超快激光测量技术，首先观察存在于厚度为10原子直径数量级的反铁磁薄膜中的自旋波模式。这将是一项重大成就，因为超薄薄膜的行为与大块晶体非常不同，而且观察其自旋波的方法尚未得到证实。一旦我们有了这些信息，我们就可以设计多层堆叠来观察自旋电流的传播和放大。具体来说，我们将在同步加速器源上使用我们已经开发和演示的时间分辨x射线测量技术。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.