Spin-chain magnetism in Eu-doped Ca<SUB>3</SUB>Co<SUB>2</SUB>O<SUB>6</SUB> and Ca<SUB>3</SUB>CoRhO<SUB>6</SUB> investigated by Mossbauer spectroscopy

We report the results of 151Eu Mossbauer effect and magnetization measurements in the Eu-doped Ca3Co2O6 and Ca3CoRhO6, which are of great current interest in the fields of spin-chain magnetism and geometrical frustration. We find that there is a pronounced increase in the linewidth of the Mossbauer spectra below a certain temperature (T∗) that is well above the one at which three-dimensional magnetic ordering features set in. This unusual broadening of the spectra indicates the existence of a characteristic temperature in these "exotic" magnetic systems, which is attributable to the onset of incipient one-dimensional magnetic order. This is inferred from an intriguing correlation of T∗ with the paramagnetic Curie temperature (a measure of intrachain coupling strength in these cases)