  The Gd3Ga5-xAlxO12 (0 < x < 5) solid solution has been prepared using ceramic
synthesis routes and the structural and magnetic properties investigated using
X-ray diffraction, magnetic susceptibility, chi, and isothermal magnetisation,
M(H), measurements. Our results indicate a contraction of the unit cell and
more significant antiferromagnetic interactions as x increases. Despite the
decrease in the magnetic polarisation on application of a field and the
corresponding decrease in the change in the magnetic entropy we find that
Gd3Al5O12 has a significantly higher observed (17%) and theoretical (14%) DS
per unit mass than Gd3Ga5O12. Per unit volume the theoretical increase in DS
(7%) is offset by the increased antiferromagnetic interactions in Gd3Al5O12.
The differences in DS are driven by a decrease in both the mass and density as
Al ions replace Ga ions. These results highlight the importance of changes to
the crystal structure when considering materials for solid state magnetic
cooling.
