custom notation - Using subscript in a string

I am trying to create a string of the form

$0 < d_1 < d_2 < \ldots < d_n$

For a given n.

My closest attempt so far is

eqn[n_] :=
  For[i = 1; str = "0", i <= n, i++, 

   str = str <> "<" <> ToString[Subscript[d, i]]];

But for, say n=6, this gives me

"0 1 2 3 4 5 6"

Where it seems to have interpreted the string as "0.

Why is it doing this? How can I achieve the result I am looking for?