Direct measurements of heat capacity from 80 to 540 K of antiferromagnetic superlattices of NiO (high Néel temperature $T_N$), CoO (low $T_N$), and MgO (nonmagnetic) are used to study the effect of exchange coupling and layer thickness on magnetic ordering. NiO/CoO superlattices with thin layers show a single heat capacity peak similar to a N${\mathrm{i}}_{0.5}$C${\mathrm{o}}_{0.5}$O alloy; with increasing layer thickness, the peak splits into two maxima. Finite size effects are seen in uncoupled NiO and CoO. Observed shifts in $T_N$ show the importance of correlation lengths and spin fluctuations in the ordering.

• Received 13 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3451