Spin current propagation through epitaxial antiferromagnetic thin films

Lead Research Organisation: UNIVERSITY OF EXETER
Department Name: Physics and Astronomy


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.


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