Temperature dependence of tunnel magnetoresistance

Abstract

Electric transport through magnetic tunnel junctions (MTJs) has been studied at various temperatures to gain understanding of the transport mechanisms in such devices. Between 15 and 400 K, MTJs with Al2O3 barriers have been tested at low voltage (barrier height: 2.0–2.1 eV, barrier width: 1.5 nm). For the soft-magnetic electrode a sputtered 1 nm Co/6 nm Fe double layer was used. The hard-magnetic electrode is realized with a 1.5 nm Co/1.0 nm Cu/1.0 nm Co system. Antiferromagnetic coupling between the two Co layers leads to a high saturation field. The 1.5 nm Co layer is used as the second electrode of the MTJ. The conductance increases with growing temperature while the tunnel magnetoresistance (TMR) shows a slight decrease. For interpretation of the results, the temperature dependence of direct tunneling, of the hopping conductance via trapped states, and of the interface magnetization have to be taken into consideration. The dominant factor for the TMR is proportional to 1−BT3/2 and follows the temperature dependence of the interface magnetization. The experimental data allow us to separate transport mechanisms and characterize the junction quality. At room temperature the spin-independent hopping conductance of our junctions is calculated to be less than 10% of the total conductance. Concerning the magnetic properties, a ferromagnetic orange-peel coupling corresponding to a field of about 4 Oe (0.3 kA/m) was found at 15 K, which decays exponentially with increasing temperature to less than 0.6 Oe (0.05 kA/m) at 300 K. The coercive field of the soft layer also shows an exponential decay.