Spin current propagation through epitaxial antiferromagnetic thin films

现代电子设备的运行依赖于传输电子电荷的电流。然而，电子也拥有固有的角动量，也就是所谓的“自旋”，这是它的磁矩的原因。自旋是具有两个允许值的量子力学量。因此，我们可以认为电子是北极指向上或下的最小的条形磁铁。通常情况下，电流在上态和下态中传输相同数量的电子。然而，在铁磁材料内部，处于上态的电子比处于下态的电子更多；这就是其磁性行为的根源。这意味着从铁磁体汲取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向下自旋的电子朝着相反的方向移动，这样就有了自旋角动量流，而没有任何电荷流。这就是纯自旋电流的含义。在铁磁体中，有一个额外的机制可以用来传输自旋电流。我们可以想象，一个电子的自旋从上到下翻转，而不是电子在运动，而自旋翻转的位置从一个原子移动到下一个原子。这种机制即使在材料是电绝缘体的情况下也存在，被称为“自旋波”。铁磁体只是具有磁性的多种材料中的一种。这一建议主要涉及反铁磁性材料，其中自旋的方向在连续的原子层之间上下交替。反铁磁体没有净磁矩，因为相邻原子上的磁矩相互抵消，所以通常更难研究，长期以来被认为在实际应用方面是无用的。然而，在反铁磁体中也会出现自旋波，因此反铁磁体可以用来传输纯的自旋电流。最近观察到，通过在铁磁和非磁层中插入薄的反铁磁层可以增强自旋电流的幅度。我们已经证明，反铁磁层能够传输直流和交流自旋电流，这证实了一个模型，该模型还预测，如果仔细选择反铁磁层的厚度，自旋电流可以放大至少10倍。这个额外的角动量是从晶格中提取的。考虑到通常需要较小的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使用自旋电流的器件的能效可以显著提高。一个直接的例子是一种磁随机存取存储器(MRAM)，其中自旋电流被注入到铁磁层中以反转其磁化，从而以二进制码表示0或1。在这个项目中，我们将使用一种超快激光测量技术来观察反铁磁性薄膜中存在的自旋波模式，这种模式的厚度可能是10个原子直径的数量级。这将是一项重大成就，因为超薄膜可以表现出与块状晶体非常不同的行为，而且观察其自旋波的方法尚未得到证明。一旦我们有了这些信息，我们就能够设计多层堆栈，在其中观察自旋流的传播和放大。具体地说，我们将在我们已经开发和演示的同步加速器源上使用时间分辨X射线测量技术。最后，我们将探索如何对堆栈进行优化，以便将其用于实际应用，如MRAM。

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.