  In order to enhance spin injection efficiency from ferromagnetic (FM) metal
into a two-dimensional electron gas (2DEG), we introduce another FM metal and
two tunnel barriers (I) between them to investigate the current polarization in
such ballistic FM/I/FM/I/2DEG junction. Our treatment is based on the
free-electron scattering theory. It is found that due to quantum interference
effect, the magnitude and sign of the current polarization exhibits periodical
oscillating behavior with variation of the thickness of the middle FM metal
layer or its exchange energy strength. For some suitable parameters, the spin
injection efficiency may arrive over 80% in this junction and can also be
controlled by the electron density of 2DEG. Our results may shed light on the
development of new spin-polarized device.
