  We demonstrate interface energy level engineering, exploiting the
modification in energy band structure across Verwey phase transition of
$Fe_{3}O_{4}$ electrode, in a $Fe_{3}O_{4}$(111)/$Alq_{3}$/Co vertical
spin-valve (SV) device. Experimental results on device characteristics I-V)
study exhibit a transition in conduction mode from carrier injection to
tunneling across Verwey transition temperature ($T_{V}$) of $Fe_{3}O_{4}$
electrode. Both giant magneto-resistance (GMR) and tunneling MR (TMR) have been
observed in a single SV device as a function of temperature, below and above
$T_{V}$, respectively. Appearance of GMR, accompanied by injection limited
natural Schottky-like I-V characteristics, provide evidences of spin injection
at electrode/$Alq_{3}$ interface and transport through molecular orbitals in
this SV device. Features of TMR exhibit significant differences from that of
GMR. This is due to the dominant hyperfine-field interaction in the multi-step
tunneling regime. We have achieved room-temperature SV operation in our device.
A phenomenological model for device operation has been proposed to explain the
transition in the conduction mode and associated MR features across $T_{V}$. We
propose that the tuning of charge gap at Fermi level across Verwey transition
due to charge ordering on the octahedral iron sites of $Fe_{3}O_{4}$ results in
a corresponding tuning of conduction mode causing this unique cross over from
GMR to TMR in this ferrite-based organic SV.
