Abstract :
The functional properties of magnetic tunnel
junctions are critically dependant on the nanoscale
morphology of the insulating barrier (usually only a
few atomic layers thick) that separates the two ferromagnetic
layers. Three-dimensional atom probe analysis
has been used to study the chemistry of a magnetic
tunnel junction structure comprising an aluminium
oxide barrier formed by in situ oxidation, both in the
under-oxidised and fully oxidised states and before and
after annealing. Low oxidation times result in discrete
oxide islands. Further oxidation leads to a more continuous,
but still non-stoichiometric, barrier with evidence
that oxidation proceeds along the top of grain
boundaries in the underlying CoFe layer. Post-deposition
annealing leads to an increase in the barrier area,
but only in the case of the fully oxidised and annealed
structure is a continuous planar layer formed, which is
close to the stoichiometric Al:O ratio of 2:3. These
results are surprising, in that the planar layers are
usually considered unstable with respect to breaking up
into separate islands. Analysis of the various driving
forces suggests that the formation of a continuous layer
requires a combination of factors, including the strain
energy resulting from the expansion of the oxide during
internal oxidation on annealing
