Structural and magnetic changes in MgO-based magnetic tunneling junctions during the early stages of annealing

We have studied the effects of the initial stages of the annealing on magnetic tunnel junctions with MgO barriers and CoFeB electrodes. We report changes in the resistance-voltage characteristics and tunneling magnetoresistance for patterned transport junctions, and correlate these with the observed changes in the structural and magnetic interface morphologies determined by soft X-ray resonant magnetic scattering from sheet films from the same wafer. An important feature of our experiment was that all measurements were carried out within the soft X-ray diffractometer on samples from the same wafer subjected to simultaneous annealing cycles, so that our magnetotransport and scattering data are directly comparable. The as-grown junction showed a tunneling magnetoresistance ratio of 5.5%, and a specific barrier resistance of 85.6kΩμm2. A 200 deg. C anneal for 1 h resulted in a small rise in barrier resistance and magnetoresistance coupled with a smoothing of the magnetic interfaces, consistent with the healing of barrier defects and removal of tunneling hot-spots. A subsequent 300 deg. C anneal for a further hour resulted in further smoothing, and a rise in the magnetoresistance ratio to 72%, and a much weaker dependence of the parallel state resistance upon voltage bias, indicating the development of (001) crystallographic texture in the electrodes. Annealing to 325 deg. C yielded a further decrease in magnetic interface width (the quadrature sum of roughness and intermixing length scales). The reduction in interface width for Co species occurred at higher temperatures than for Fe throughout the experiments.