Majorana fermions, Andreev reflection and magnetoresistance effect in three-dimensional topological insulators

We study the transport properties of ferromagnet/ferromagnet/s-wave superconductor junctions formed on the surface of a three-dimensional topological insulator in an experimentally relevant regime. It is found that the total Andreev reflection happens at zero bias and the zero-bias conductance in the junctions is not dependent on the magnetizations. We show that the features are strongly related to the formation of Majorana fermions in the middle ferromagnet. However, for finite bias, the conductance is very sensitively controlled by the relative orientation of the magnetizations and this leads to a controllable magnetoresistance effect in the system. These results provide guidance for searching the Majorana fermion and possess potential applications in superconducting spintronics.