  Intending to optimize the giant magnetocaloric properties of HoB$_{2}$, we
synthesized and magnetocalorically characterized Ho$_{1-x}$Gd$_{x}$B$_{2}$(0.1
$\leq$ x $\leq$ 0.4) alloys. We found out that Gd enters stoichiometrically and
randomly into the Ho site, leading to a Vegard-type structural change. The
addition of spherical S$^{7/2}$ Gd$^{3+}$ moments prompts an enhancement in
Curie temperature, a reduction in peak value of the magnetic entropy change
while still being relatively high, and a broadening of the magnetic entropy
change curves. The overall influence is a relatively high refrigerant capacity
and relative cooling power, and an extension of the thermal working range to
higher temperatures; thus, electing Ho$_{1-x}$Gd$_{x}$B$_{2}$(as potential
candidates for cryogenic refrigeration applications.
