Recently a new class of materials called "topological insulators" has been discovered. These materials are insulating in their bulk but conducting at the surface. The conducting surface states arise due to significant spin-orbit coupling in these materials and possess the special property that their momentum is locked with the spin of the electrons [1]. This makes these materials interesting for so-called spintronics devices.
We study hybrid systems of topological insulators and ferromagnets. When one combines a topological insulator (TI) with a ferromagnet (FM) as shown in the picture, one can manipulate the properties of the surface states by changing the magnetization of the ferromagnet. We have studied thin strips of the topological insulator Bi₂Se₃ and could show that at a certain value of the magnetization the surface states undergo a phase transition into new states. For magnetization in z-direction a quantum Hall state appears, while for magnetization in x-direction a topological flat band appears [2,3]. A flat band is particularly interesting, because the electrons occupying such a band become slow and massive and the density of electronic states becomes high.
We have also shown that with the topological insulator material Bi₂Se₃ one can construct tunneling magnetoresistance (TMR) devices with extremely large TMR ratios [4]. Such devices could be used in future magnetic storage devices.