  The ground state of the frustrated A-site magnetic spinel CoAl2O4 has been a
controversial issue whether it is a collinear antiferromagnetic ordering or a
spiral spin - liquid state, as the ratio of the two competing interactions,
J2/J1 lies close to the boundary between these two ground states. Here, we
address the magnetic ground state in CoAl2O4 with different amount of Co2+/Al3+
site disorder from the study of magnetoelectric effect and Monte Carlo
simulations. CoAl2O4 with low site disorder exhibits linear magnetoelectric
effect below the magnetic ordering temperature. With increasing disorder, the
magnetoelectric effect is suppressed and the sample with 14% disorder exhibits
a spin glass behavior without the magnetoelectric effect. Monte Carlo
simulations support the experimental findings and suggest that the site
disorder suppresses long - range antiferromagnetic order and induces a spin
glass state. Since the linear magnetoelectric effect requires a long - range
magnetic ordering, we suggest that the ground state of CoAl2O4 with low site
disorder is a collinear antiferromagnet.
