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.
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