  Magnetic garnets such as yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG) are
widely used in spintronic and magnonic devices. Their magnetic and
magneto-optical properties can be modified over a wide range by tailoring their
chemical composition. Here, we report the successful growth of Al-substituted
yttrium iron garnet (YAlIG) thin films via radio frequency sputtering in
combination with an ex situ annealing step. Upon selecting appropriate process
parameters, we obtain highly crystalline YAlIG films with different Al$^{3+}$
substitution levels on both, single crystalline Y$_3$Al$_5$O$_{12}$ (YAG) and
Gd$_3$Ga$_5$O$_{12}$ (GGG) substrates. With increasing Al$^{3+}$ substitution
levels, we observe a reduction of the saturation magnetisation as well as a
systematic decrease of the magnetic ordering temperature to values well below
room temperature. YAlIG thin films thus provide an interesting material
platform for spintronic and magnonic experiments in different magnetic phases.
