Effect of annealing and applied bias on barrier shape in CoFe/MgO/CoFe tunnel junctions

Abstract

Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions as a function of annealing and in-situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the asgrown sample. In-situ off-axis electron holography was employed to image the barrier potential profile of an MTJ directly, with the specimen under electrical bias. Varying the bias voltage from −1.5 V to +1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface. Introduction Metal-oxide interfaces are the subject of extensive experimental and theoretical research for next generation nano-scale spintronic devices that exploit spin as a degree of freedom for charged electrons. 1 They play a key role in metal-oxide based science and engineering, with applications including magnetic tunnel junctions (MTJs) 2 and other heterogeneous structures such as resistance switching oxides 3 with uses or potentials uses in low-power non-volatile memories. In its simplest form, the MTJ is a trilayer structure consisting of two ferromagnetic (FM) electrode layers separated by an ultra-thin dielectric layer. The electrical resistance across the insulating tunnel barrier is dependent upon the relative orientation of the magnetizations of the two ferromagnetic electrodes. In most cases, the electrical resistance is lower when the magnetization of the two ferromagnetic layers is

Resistance switching in transparent magnetic MgO films

“…An 80 nm MgO layer was first deposited at 450°C in an Ar-O 2 mixture. This temperature is close to the optimal annealing temperatures in usual MgO-based tunnel junctions [22].…”

Interfacial geometry dependence of the iron magnetic moment: The case of MgO/Fe/MgO

“…Annealing is found to decrease the resistance of the junction, although the effective barrier layer width expands. 47 This paradox is usually explained as being due to a reduction of the density of oxygen vacancies, though the resulting similarities with the Mg insertion layer case might point to their common origin. Please note that Mg can diffuse easily at the temperatures involved here, producing a Mg enrichment at the MgO surface.…”

The role of Mg interface layer in MgO magnetic tunnel junctions with CoFe and CoFeB electrodes

“…Figure 3 shows typical bias voltage dependences of differential resistance (dV/dI) and TMR for a MTJ at 5 K. The asymmetric TMR and dV/dI curves suggest that barrier profile is asymmetric. 30,31 At negative biases electrons are injected from the bottom electrode into the top one. Asymmetry can be ascribed to the poor quality of the upper MgO/CoFe interface presumably due to the propagation of the stacking faults, dislocations, and the lattice distortion.…”