Linear magnetoelectric effect as a signature of long-range collinear antiferromagnetic ordering in the frustrated spinel CoAl2O4

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.