  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 paper, we show that, at absolute zero
temperature, 100% spin injection efficiency from a non-ideal 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.
