  Er3+ plus V5+, and Er3+ plus Nb5+ co-doped CaWO4, formulas Ca1-xErxW1-xMxO4,
were synthesized in air by a conventional solid-state method. A color change
from white to pink was observed in the final products. An equal fraction of
dopants was employed to obtain charge neutrality, and the limits of the
solubility for our conditions are lower than x=0.15. The magnetic
susceptibility data shows that that the magnetic coupling becomes increasingly
antiferromagnetic with increasing Er3+content. The Curie-Weiss fit and
isothermal magnetization imply that different degrees of spin-orbit coupling
appear to be present in the two doping systems. No transitions were observed in
the heat capacity data above 0.4 K.
