Pinholes and temperature-dependent transport properties of MgO magnetic tunnel junctions

Abstract

Magnetic tunnel junctions (MTJs) with thin crystalline MgO(001) barriers displaying large tunnel magnetoresistance (TMR) and low resistance-area product $(RA)$ will likely be used as the next generation sensors in read heads of ultrahigh-density hard drives. However, the thin insulating barrier may result in the presence of metallic pinholes joining the two electrodes. Here we study the transport properties of thin MgO-based low resistance MTJs (barrier thickness $t=7.5\text{ }\text{\AA{}}$), deposited by magnetron sputtering, with $RA$ values of $\ensuremath{\sim}40\text{ }\ensuremath{\Omega}\ensuremath{\mu}{\text{m}}^{2}$, reaching TMR values of $\ensuremath{\sim}60--75%$ at room temperature. We performed temperature-dependent (300--20 K) resistance $(R)$ measurements and observed different behaviors for different magnetic states: positive $dR/dT$ for the parallel (P) state, attributed to the presence of pinholes in the barrier, but a mixed character in the antiparallel (AP) state, with $dR/dT$ changing from negative to positive with decreasing temperature. This indicates an interesting competition between tunneling and metallic transport in the studied samples. To explain this transport behavior, we developed a simple model with the two conducting channels, tunnel and metallic, in parallel. The model assumes a linear variation of the electrical resistance with temperature for both conducting channels and its dependence on the MTJ magnetic state (P and AP). The modeled results show that the sign of $dR/dT$ does not give an indication of the dominant conductance mechanism and that the crossover temperature at which $dR/dT$ changes sign depends strongly on the linear electrical resistance--temperature coefficients. We performed fits to our experimental $R(T)$ data, using the proposed model, and observed that such fits reproduced the data quite well, illustrating the validity of the model.