Spin-momentum helical locking induced spin-valve effects in topological insulator/ferromagnet heterostructures

Topological insulator is composed of an insulating bulk state and an odd number of massless spin-helical Dirac cone formed two dimensional surface state. Here we report a novel spin-valve effect in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$/CoFe heterostructures. This effect indicates the spin-momentum helical locking on the Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ topological surface state. It is also indicated that the characteristics of helical surface can be preserved at topological insulator/ferromagnet interface although the ferromagnetism can break the time reversal symmetry and therefore generate an energy gap at the topological Dirac cone.