Crossover from injection to tunneling conduction mode and associated magneto-resistance in a single $Fe_{3}O_{4}$(111)/$Alq_{3}$/Co spin-valve device

TL;DR

This study demonstrates a temperature-driven transition from injection to tunneling conduction in a $Fe_{3}O_{4}$/Alq$_{3}$/Co spin-valve, revealing distinct magnetoresistance behaviors and room-temperature operation due to interface energy level engineering across the Verwey transition.

Contribution

It introduces a novel organic spin-valve device exhibiting a crossover from GMR to TMR at the Verwey transition, with a phenomenological model explaining the conduction mode change.

Findings

Observation of GMR below $T_{V}$ indicating spin injection.

Detection of TMR above $T_{V}$ due to tunneling conduction.

Room-temperature operation of the spin-valve device.

Abstract

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.