Flux linked with smaller loop
\(\phi = B({l^2}) = \frac{{{\mu _0}}}{{4\pi }}\frac{{8\pi i{l^2}}}{L}\)
\(\therefore \phi = Mi \Rightarrow M = \frac{\phi }{i} = \frac{{{\mu _0}}}{{4\pi }}.\frac{{8\sqrt 2 {l^2}}}{L}\)
\(\Rightarrow M \propto \frac{{{l^2}}}{L}\)