Exchange-induced enhancement of TC in Co1-x(EuS)x macroscopic ferrimagnets

The temperature dependence of the magnetic-exchange behavior between the EuS and the Co phase in the macroscopic ferrimagnet Co1-x(EuS)(x) has been studied over a wide composition range, 0.1 less than or equal to x less than or equal to 0.6, using magnetooptic spectroscopy. We find a strong exchange-induced enhancement of the Curie temperature of the EuS phase for EuS concentrations x greater than or equal to 0.2 reaching a maximum of 160 K in Co-0.6(EUS)(0.4). This is an order of magnitude higher than in bulk EuS. The maximum enhancement takes place at the concentration where a percolation from metallic to a semiconducting behavior occurs. The results are explained by the nanometer size of the EuS crystallites, which leads to a large surface-to-volume ratio. This yields a strong exchange interaction to the Co matrix while the crystallinity of the EuS ensures a high-spin Eu2+ state.