Finite size effects on the moment and ordering temperature in antiferromagnetic CoO layers

The relationship between magnetic properties and microstructure of thin antiferromagnetic CoO layers in CoO(X)/SiO₂(50)₂₅ multilayers has been investigated. The temperature decay of the thermoremanent moment, zero-field-cooled/field-cooled magnetization measurements, and specific heat were evaluated as indicators of the magnetic ordering temperature. The temperatures associated with each decreased slightly with decreasing CoO layer thickness from 100 to 30 {}, but then exhibited a sharp decrease for CoO layer thickness below 20 {}. This decrease has been previously observed, and was attributed to intrinsic finite size effects associated with broken magnetic bonds at the surfaces. In the present investigation, it was determined that the CoO layer was amorphous in these thinner layers, accounting for the dramatic drop in N\'eel temperature. For thicker CoO layers, all measures of magnetic ordering coincide, indicating a true Neel temperature, whereas they do not for the thinner films. The structural change of CoO from crystalline to amorphous also causes a significant change in the temperature dependence of the magnetization, due to an increased number of weakly coupled uncompensated spins.