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Topological insulators open new avenues for spin-transfer torque and second-generation spintronics

November 2, 2012 - In the recent  Physical Review Letters article, graduate student Farzad Mahfouzi and Prof. Nikolic, in collaboration with Prof. Naoto Nagaosa  from the University of Tokyo and RIKEN Advanced Science Institute in Japan, have predicted a new type of spin-transfer torque in vertical heterostructures involving recently discovered three-dimensional topological insulators (TIs).  

The spin-transfer torque (STT) is a phenomenon in which spin current of large enough density injected into a  ferromagnetic (F) layer either switches its magnetization from one static configuration to another or generates a dynamical situation with steady-state precessing magnetization. The origin of STT is absorption of itinerant flow of angular momentum components normal to the magnetization direction. It represents one of the central phenomena of the second-generation spintronics, focused on manipulation of coherent spin states. The reduction of current densities (currently of the order 106-108 A/cm2)  required for STT-based magnetization switching is expected to bring commercially viable magnetic random access memory (MRAM). 

Most of transport experiments on three-dimensional TIs, which possess a usual band gap in the bulk while hosting metallic surfaces whose massless Dirac electrons have spins locked with their momenta due to the strong Rashba-type spin-orbit coupling, have been focused on injecting current through their surface. However, the pursued topological transport regime  is obfuscated by the presence of bulk charge carriers due to unintentional doping of the interior of realistic devices made of Bi2Se3

On the other hand, the TI-based devices discussed in the article have current flowing perpendicularly through the metallic surface states, and they  do not require perfectly insulating bulk. This geometry generates two peculiar contributions to the torque arising from the spin-polarizing effect of the bulk of the TI slab and the TI/F interface. Theoretical description of this phenomenon has required development of a new approach to the computatation of STT in the presence of spin-orbit coupling. Besides fundamental interest into the rich nonequilibrium physics arising in the interplay of spin currents carried by fast conduction electrons (described by quantum mechanics) and slow magnetization dynamics (described by classical mechanics),  the article has suggested (see Supplemental Material) that coating conventional magnetic tunnel junctions with a thin layer of TI could greatly improve switching times in STT-MRAM devices.

For more information about theoretical and experimental research on spintronic devices involving TIs, see Websites of Nikolic group at UD and FIRST program QS2C project in Japan.