finalSF{e} should work. If you're getting an error, it's probably because finalSF already exists before the loop as something else than a cell array. To fix that:
finalSF = cell(1, 25); %before the loop.
Another option is to declare finalSF as a 3d array, since all your phi are the same size:
finalSF = zeros(500, 500, 25);
for idx = 1:25
%calculate phi
finalSF(:, :, e) = phi;
end
