query regarding MTJ (magnetic tunnel junction)

[nanomtj]

In the magneto tunnel junction builder, there is an option to specify the layer separation of the barrier (central region). How do we set the layer separation value..?? On what grounds do we set the value of layer separation for any material..?? Also, there is an option to select the electrode/oxygen distance. How do we know what values will be most apt..?? Kindly let me know..

[kstokbro]

You can try to find these values in theoretical or experimental reference papers.
However, I suggest that you just think about the generated geometry as a starting guess for a geometry optimization with ATK. With ATK 11.2 you can automatically relax the size of the central cell to relax the stress of the interfaces.

[nanomtj]

So you mean to say that those parameters may vary from one experiment to another..???That means they are not constants..??