Inverse tunnel magnetoresistance of magnetic tunnel junctions with a NiCo2O4 electrode

Inverse spinel oxide NiCo2O4 (NCO) is known to exhibit ferrimagnetic characteristics and electrical conductivity. First-principles calculations predict NCO to be a half-metal with a negative polarization of -100%. In this study, we fabricated epitaxial NCO/MgO/Fe magnetic tunnel junctions by reactive molecular beam epitaxy and observed an inverse tunnel magnetoresistance (TMR) effect of -19.1% at 14 K, indicating that NCO has negative spin polarization. The TMR ratio monotonically decreased with increasing temperature, which was attributed to the temperature dependence of the NCO surface magnetization due to the thermal excitation of spin waves. In addition, the TMR ratio displayed strong bias voltage dependence, decreasing to less than half of the maximum value at +20 and -30 mV. These findings support the use of NCO in spintronic devices and should lead to further developments in oxide spintronics.