  We have performed an experimental and theoretical investigation on the spin
reorientation transitions Ho$_{1-y}$Gd$_y$Al$_2$ compounds. Crystallographic,
magnetic and calorimetric measurements have been performed for five selected
samples in the Ho$_{1-y}$Gd$_y$Al$_2$ series ($y$ = 0, 0.25, 0.5, 0.75 and
1.0). From the results, a linear increase in the lattice parameter as a
function of Gd-content ($x$) is observed. Magnetization measurements show that
all samples present collinear ferromagnetism. Specific heat measurements were
performed in order to detail the first-order spin reorientation transition
(FOSRT) that appears for HoAl$_2$ around 20 K. Furthermore, we investigated how
the increase in Gd content affects the character of such phase transition. Our
results show a slightly modification in the spin reorientation transition
temperature, which moves to lower temperatures as Gd is increased in HoAl$_2$,
whereas for the selected concentrations up to x = 0.75 the first-order
character of the transition is maintained. Such behavior may lead to a large
refrigerant capacity (RC) over a wide temperature range, which is an important
parameter to evaluate the magnetocaloric effect (MCE) of such compounds. The
theoretical analysis by means of a model Hamiltonian that includes crystal
field, exchange and Zeeman interactions reveals a modification in the
crystal-field acting on Ho ions as Gd-content is increased.
