Nonlinear conductance in nanoscale CoFeB/MgO magnetic tunnel junctions with perpendicular easy axis

Magnetic tunnel junctions (MTJ) exhibit spin-dependent conductance that governs their performance in various applications. While the transport characteristics are known to show nonlinearity, their behavior and underlying mechanism have not yet understood well. Here we investigate nonlinear conductance at a low bias regime in nanoscale MTJs with a perpendicular magnetic easy axis and various junction sizes, by measuring current-voltage (IV ) characteristics and ferromagnetic resonance. We evaluate IV properties as I = G1V +G2V2 + G3V3 under various external magnetic fields and examine the correlations among G1, G2, and G3. We find that G2 increases with decrease in the junction size, G3 has a negative correlation with G1, and 8G3/8G1 (= k) has a positive correlation with G2. These results can be explained by considering the spin flip during the tunneling and a modulation of material properties at the device edge caused by the nanofabrication process. Ferromagnetic resonance measurements support the physical picture suggested by the transport measurements. Our findings shed light on the mechanism of electron transport in nanoscale MTJs and facilitate the establishment of a rigorous model describing their nonlinear conductance.