Can a Non-Ideal Metal Ferromagnet Inject Spin Into a Semiconductor With 100% Efficiency Without a Tunnel Barrier?

Current understanding of spin injection tells us that a metal ferromagnet can inject spin into a semiconductor with 100% efficiency if either the ferromagnet is an ideal half metal with 100% spin polarization or there exists a suitable tunnel barrier at the interface. In this article, we show that, at absolute zero temperature, 100% spin injection efficiency from a nonideal metal ferromagnet into a semiconductor quantum wire can be reached at certain injection energies without a tunnel barrier, provided there is an axial magnetic field along the direction of current flow as well as a spin orbit interaction in the semiconductor. At these injection energies, spin is injected only from the majority spin band of the ferromagnetic contact, resulting in 100% spin injection efficiency. This happens because of the presence of antiresonances in the transmission coefficient of the minority spins when their incident energies coincide with Zeeman energy states in the quantum wire. At absolute zero and below a critical value of the axial magnetic field, there are two distinct Zeeman energy states and therefore two injection energies at which ideal spin filtering is possible; above the critical magnetic field, there is only one such injection energy. The spin injection efficiency rapidly decreases as the temperature increases. The rate of decrease is slower when the magnetic field is above the critical value. The appropriate choice of semiconductor materials and structures necessary to maintain a large spin injection efficiency at elevated temperatures is discussed.