In most of the Co/Mn-based double perovskite, a cusp like behavior in ZFC magnetization occurs just below the magnetic transition temperature. What is the reason behind this?
If your material enters an ordered state the magnetic susceptibility becomes anisotropic. If say you have a collinear antiferromagnet with moments aligned along a and you apply an external magetic field along that direction it will take a large field to realign the spins and the susceptibility is suppressed. If however you apply the field perpendicular to the easy axis the spins will be happy to realign smoothly. Depending on the relative magnitude of collinear and perpendicular susceptibility you can get a cusp in your magnetization data. A small point of clarification: There is almost always a small AC or DC field applied when doing magnetization measurements even if formally they are called ZFC.