Question about high stress of Mn-Co coating

Now I am doing the electrodeposition of Mn/Co alloys. For 1*1cm2 sample, I can get dense and uniform coating. However, for 1*1 inch2 sample, it is hard to get dense coating, or the stress in the coating is quite high, so when I try to get thicker coatings, the spallation will occur.
Then I add 4.5g/L into the bath, it still does not improve a lot. I wonder
(1) any other stress reducing agent will be useful?
(2) how to improve the current distribution for relatively big samples?
Thanks a lot.

I will guess that you are trying to use a chloride bath. This has such a high internal stress that any plating thicker than a flash will spall.
Can you go to a sulphate or a fluoboric solution? I did not find any reference to a sulfamate process, but you might be able to come up with a suitable one by making a cobalt bath and then adding Mn as a sulphate or similar compound if it is not soluble in sulfamic acid. This will probably have less stress than any other solution.
Look in "Modern Electroplating" [on AbeBooks, eBay, or Amazon affil links] by Lowenheim for 3 sets of references which contain many publication citations for help. Your Library will have or can get this book if your advisor does not have it.