  Spin filter tunnel junctions are based on selective tunneling of up and down
spin electrons controlled through exchange splitting of the band structure of a
ferromagnetic insulator. Therefore, spin filter efficiency can be tuned by
adjusting exchange strength of the tunnel barrier. We have observed that
magnetic field and bias voltage (current) can be used to regulate exchange
strength and consequently spin-filter efficiency in tunnel junctions with
ferromagnetic DyN and GdN tunnel barrier. In tunnel junctions with DyN barrier
we obtained $\sim$37$\%$ spin polarization of tunneling electrons at 11 K due
to a small exchange splitting ($ E_{ex}$) $\approx$5.6 meV of the barrier
height ($\Phi _0$) $\approx$60 meV. Huge spin-filter efficiency $\sim$97$\%$
was found for tunnel junctions with GdN barrier due to larger $E_{ex}$
$\approx$47 meV. In the presence of an applied magnetic field, barrier height
can further split due to magnetic field dependent exchange splitting $
E_{ex}(H)$. The spin filter efficiency in DyN tunnel junctions can be increased
up to $\sim$87$\%$ with magnetic field. Electric and magnetic field tuned
spin-filter efficiency of these tunnel junctions gives opportunity for
practical application of these devices with additional functionality.
