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What Is A Topological Insulator?
That’s one of my favorite topics to talk about. One of the most striking properties of topological insulators is that t are insulating in their volume, but their surface is conducting, what is called topological surface states (TSS). Due to a huge spin-orbit coupling, in the TSS the spin and momentum of the electron are coupled perpendicular to each other, the famous spin-momentum locking. When one drives a current on the TSS, the Fermi surface will be displaced leading to a momentum imbalance. Since spin and momentum are locked, it is predicted that a momentum imbalance also leads to a spin imbalance, generating a spin-polarized current. So, this is great, right? We were able to generate a spin current only by means of an electric field!! Now, since spins carry angular momentum, it could be nice if we could transfer it for a ferromagnetic material adjacent to the TI and manipulate its magnetization. And this is basically the idea. This kind of experiments are called current-induced spin-transfer torques. The advantage of the TIs compared to the STT-MRAMs we have nowadays is that, if STT with TIs work, it dissipates less power than the conventional devices since the electrons don’t have to go physically through the ferromagnetic layers of the material.
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In this way, scientists at the University of Copenhagen have now produced a first TIS torques on ferromagnetic materials, leading to the observation that spin currents generate magnetization. In addition, since in the TIS both the spin and momentum can be manipulated to generate an electric field, one is capable of creating a magnetic field inside the material. The experiments have shown that the TIS can even create a strong magnetic field in a ferromagnetic material. The researchers concluded in their theoretical study that “the observed effects can be obtained by using a ferromagnetic conductor as an intermediary between the electric field and the local magnetic field in the TiS2. “The use of ferromagnetic materials as part of an STT framework may be of enormous relevance to the future application of magnetic resonance techniques in material sciences. [emphasis added] Source: University of Copenhagen So, the scientists found that when an electric.