Spin current propagation through epitaxial antiferromagnetic thin films

自旋电流通过外延反铁磁薄膜的传播

• 批准号: EP/W006006/1
• 负责人: Robert Hicken
• 金额: $ 71.3万
• 依托单位: UNIVERSITY OF EXETER
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FW006006%2F1
• 关键词: Spin; current; propagation; through; epitaxial

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.

现代电子设备的运行取决于传输电子电荷的电流。但是，电子还具有固有的角动量，称为“自旋”，这是其磁矩的原因。自旋是具有两个允许值的量子力学数量。因此，我们可以将电子视为最小的钢筋磁铁，其北极向上或向下指向。通常，电流在上下状态中传输相等数量的电子。但是，在铁磁物质内部，在上升状态下的电子多于下降状态。这是其磁性行为的起源。这意味着从铁磁铁中抽取的电流将具有大量的UP旋转。实际上，在某些情况下，在非磁金属中，我们可以安排相等数量的带有上下旋转的电子向相反的方向移动，以便没有任何电荷流动的旋转角动量流动。这就是纯旋转电流的含义。在Ferromagnet中，可以使用额外的机制来运输自旋电流。我们可以想到一个电子从向下到向下旋转，而不是电子移动，而是将其转移的位置从一个原子移动到另一个原子。即使材料是电绝缘子，也称为“自旋波”，也存在该机制。铁磁体只是具有磁性顺序的多种类型的材料之一。该提案主要与抗铁磁材料有关，其中自旋的方向在连续的原子层之间在上下交替。抗铁磁铁没有净磁矩，因为相邻原子上的那些会取消，因此通常很难研究，并且长期以来被认为在实际应用方面是无用的。然而，旋转波也出现在抗铁磁铁中，因此可以使用抗铁磁体来运输纯自旋电流。最近观察到，通过将薄抗铁磁性层插入一堆铁磁性和非磁性层，可以通过将自旋电流的振幅提高。我们已经表明，抗铁磁层能够同时运输直流和交流旋转电流，从而确认了一个模型，该模型还可以预测，如果仔细选择了层的厚度，则可以至少将旋转电流放大至少10倍。这种额外的角动量是从晶格中得出的。鉴于通常需要一个小的电流才能产生纯自旋电流，因此在抗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源效率可以显着提高。一个直接的示例是一种磁随机访问存储器（MRAM），其中旋转电流被注入铁磁层以逆转其磁化，以代表二进制代码中的0或1。将功耗减少2倍，已经使MRAM成为数据中心应用程序中动态随机访问记忆（DRAM）的有吸引力的替代品。在本项目中，我们将使用超快激光测量技术来首先观察抗铁磁薄膜中存在的旋转波模式，这可能是10位厚度厚度的十个ATOMIC直径的顺序。这将是一项重大成就，因为超薄薄膜的行为与散装晶体的行为有很大不同，并且观察其自旋波的方法尚未得到证明。一旦获得了这些信息，我们将能够设计多层堆栈，以观察自旋电流的传播和扩增。具体而言，我们将在已经开发和演示的同步子源上使用时间解决的X射线测量技术。最后，我们将探索如何优化堆栈，以便可以在MRAM等实用应用中使用它们。