  We study the ground-state phase diagram of a Heisenberg model with spin
$S=\frac{1}{2}$ on a diamond-like decorated square lattice. A diamond unit has
two types of antiferromagnetic exchange interactions, and the ratio $\lambda$
between the length of the diagonal bond and that of the other four edges
determines the strength of frustration. It has been pointed out [J. Phys. Soc.
Jpn {\bf85}, 033705 (2016)] that the so-called tetramer-dimer states, which are
expected to be stabilized in an intermediate region of $\lambda_{\rm
c}<\lambda<2$, are identical to the square-lattice dimer covering states, which
ignited renewed interest in high-dimensional diamond-like decorated lattices.
In order to determine the phase boundary $\lambda_{\rm c}$, we employ the
modified spin wave method to estimate the energy of the ferrimagnetic state and
obtain $\lambda_{\rm c}=0.974$. Our obtained magnetizations for
spin-$\frac{1}{2}$ sites and for spin-1 sites are $m=0.398$ and
$\tilde{m}=0.949$, and spin reductions are 20 \% and 5\%, respectively. This
indicates that spin fluctuation is much smaller than that of the
$S=\frac{1}{2}$ square-lattice antiferromagnet: thus, we can consider that our
obtained ground-state energy is highly accurate. Further, our numerical
diagonalization study suggests that other cluster states do not appear in the
ground-state phase diagram.
