Abstract

LnFeAsO (Ln= Rare Earth) type rare earth pnictide compounds have generated a great excitement in the condensed matter physics community with the discovery of superconductivity in the temperature range of 23 to 54 K. Fe sublattice exhibits interesting spin density wave (SDW) type antiferromagnetism or ferromagnetism in the LnFeAsO compounds and this series has been investigated by a variety of experimental methods to explore the origin and mechanisms of magnetism of the transition metal Fe. An interesting series of magnetic ordering and multiple magnetic transitions of Co and rare earth ions have also been observed in a similar group of compounds, LnCoAsO (Ln=rare earth). The magnetic ordering of Co in these compounds is distinctly different from the magnetic ordering of Fe in LnFeAsO (Ln= Rare Earth). Co is found to order ferromagnetically in LaCoAsO. In NdCoAsO, ferromagnetic ordering of Co moments is observed below 69 K, followed by a low temperarute transition to antiferromagnetic ordering within the Co sublattice below 14 K and a second antiferromagnetic transition with the antiferromagnetic ordering in both Co and Nd sub-lattices below 3.5 K. In this talk, I will present the experimental determination of the magnetic moments of Co and Nd atoms from x-ray magneic circular dichroism spectroscopy measurements. The temperature and magnetic field dependence of magnetism at the Co and Nd sites will also be discussed.