  We present an investigation into the intrinsic magnetic properties of the
compounds YCo5 and GdCo5, members of the RETM5 class of permanent magnets (RE =
rare earth, TM = transition metal). Focusing on Y and Gd provides direct
insight into both the TM magnetization and RE-TM interactions without the
complication of strong crystal field effects. We synthesize single crystals of
YCo5 and GdCo5 using the optical floating zone technique and measure the
magnetization from liquid helium temperatures up to 800 K. These measurements
are interpreted through calculations based on a Green's function formulation of
density-functional theory, treating the thermal disorder of the local magnetic
moments within the coherent potential approximation. The rise in the
magnetization of GdCo5 with temperature is shown to arise from a faster
disordering of the Gd magnetic moments compared to the antiferromagnetically
aligned Co sublattice. We use the calculations to analyze the different Curie
temperatures of the compounds and also compare the molecular (Weiss) fields at
the RE site with previously published neutron scattering experiments. To gain
further insight into the RE-TM interactions, we perform substitutional doping
on the TM site, studying the compounds RECo4.5Ni0.5, RECo4Ni, and RECo4.5Fe0.5.
Both our calculations and experiments on powdered samples find an
increased/decreased magnetization with Fe/Ni doping, respectively. The
calculations further reveal a pronounced dependence on the location of the
dopant atoms of both the Curie temperatures and the Weiss field at the RE site.
