Heat capacity measurements of NiO/CoO superlattices (invited) (abstract)

Abstract

We present heat capacity measurements of NiO/CoO superlattices grown by reactive sputtering. Neutron diffraction studies of similar superlattices have shown antiferromagnetic ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. Specific heat measurements were made using a unique thin film microcalorimeter capable of measuring the heat capacity of thin films from 1.5 K to well above room temperature. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers. For thin bilayers (26 Å), we observe a single broad heat capacity peak similar to a Ni0.5Co0.5O alloy. This peak is at a temperature which corresponds to the superlattice magnetic blocking temperature. For thicker bilayers (≳50 Å), two broad maxima occur which approach the individual Néel temperatures of CoO and NiO with increasing bilayer thickness. The ordering temperature of the NiO layers is more suppressed than expected, indicating a more pronounced size effect compared to the CoO layers. The magnetic entropy S=R(ln 3+ln 2)/2 for the superlattices, within the uncertainties of the measurement, is conserved. We compare the temperature dependences of the specific heats to various models.