  We consider a new type of magnetic tunnel junction, which consists of two
ferromagnetic tunnel barriers acting as spin filters (SFs), separated by a
nonmagnetic metal (NM) layer. Using the transfer matrix method and the
free-electron approximation, the dependence of the tunnel magnetoresistance
(TMR) on the thickness of the central NM layer, bias voltage and temperature in
the double SF junction are studied theoretically. It is shown that the TMR and
electron-spin polarization in this structure can reach very large values under
suitable conditions. The highest value of the TMR can reach 99%. By an
appropriate choice of the thickness of the central NM layer, the degree of spin
polarization in this structure will be higher than that of the single SF
junctions. These results may be useful in designing future spin-polarized
tunnelling devices.
