  GdCo$_5$ may be considered as two sublattices - one of Gd and one of Co -
whose magnetizations are in antiparallel alignment, forming a ferrimagnet.
Substitution of nickel in the cobalt sublattice of GdCo$_5$ has been
investigated to gain insight into how the magnetic properties of this prototype
rare-earth/transition-metal magnet are affected by changes in the transition
metal sublattice. Polycrystalline samples of GdCo$_{5-x}$Ni$_x$ for 0 $ \leq x
\leq $ 5 were synthesized by arc melting. Structural characterization was
carried out by powder x-ray diffraction and optical and scanning electron
microscope imaging of metallographic slides, the latter revealing a low
concentration of Gd$_2$(Co, Ni)$_7$ lamellae for $x \leq 2.5$. Compensation -
i.e. the cancellation of the opposing Gd and transition metal moments is
observed for $1 \leq x \leq 3$ at a temperature which increases with Ni
content; for larger $x$, no compensation is observed below 360 K. A peak in the
coercivity is seen at $x \approx 1$ at 10K coinciding with a minimum in the
saturation magnetization. Density-functional theory calculations within the
disordered local moment picture reproduce the dependence of the magnetization
on Ni content and temperature. The calculations also show a peak in the
magnetocrystalline anisotropy at similar Ni concentrations to the
experimentally observed coercivity maximum.
