Try replacing D within the j-loop by D(j). Also add something like alpha(j) = a after the end of the i-loop (but inside the j-loop). Then you can plot D vs alpha.
I think you will need to increase your value of maxIter to get a converged solution.
You might also think of using a while loop, instead of the i-loop, with an error calculation in order to compare the error to your tola value (which is currently unused).