EXCHANGE-INDUCED ENHANCEMENT OF T-C IN CO1-X(EUS)(X) MACROSCOPIC FERRIMAGNETS

The temperature dependence of the magnetic-exchange behavior between t he EuS and the Co phase in the macroscopic ferrimagnet Co1-x(EuS)(x) h as 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 fi nd a strong exchange-induced enhancement of the Curie temperature of t he EuS phase for EuS concentrations x greater than or equal to 0.2 rea ching a maximum of 160 K in Co-0.6(EUS)(0.4). This is an order of magn itude higher than in bulk EuS. The maximum enhancement takes place at the concentration where a percolation from metallic to a semiconductin g 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 c rystallinity of the EuS ensures a high-spin Eu2+ state.