Relative importance of surface anisotropy and defects on loop shifts in antiferromagnetic nanoparticles

The shifts in field-cooling hysteresis loops of antiferromagnetic nanoparticle systems have been widely observed in experiments. Here, numerical calculation is used to study the influence of the surface anisotropy and defects. It is found that surface defects alone are sufficient to yield the loop shifts (LS) at proper field angles. Neither the disordered nor the radial surface anisotropy is the indispensable ingredient to generate the LS. However, both of them play a role in enhancing the LS. The possible mechanisms are discussed.