Print the AdamN tile
CJam, 20 bytes

Laq~{'a+_@f*\f+z}/N*


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La                      Push [[]]
  q~                    Push input n
    {           }/      For i in 0..n-1 ...
     'a+                  Add to char 'a to give current char
        _@f*              Join each row by char
            \f+           Add char to the end of each row as well
               z          Zip to transpose
                  N*    Join result by newlines


20/21-byte alternatives:

Laaq~{'a+aff+:sz}/N*
Laaq~{'a+\Laf+f*z}/N*
Laq~{'a+f{_@*\+}z}/N*