Magnetic properties of undoped and 15%F doped SmFeAsO compounds

In this paper the magnetic behaviour of SmFeAsO and SmFeAs(O0.85F0.15) samples is presented and discussed. Molar susceptibility of SmFeAsO exhibits a local peak at T around 140K due to the establishment of a long range antiferromagnetic ordering of the Fe moments in Fe-As layers. This feature has already been observed with different techniques, and frequently ascribed to the onset of a Spin Density Wave (SDW). At TN around 6K another peak, which we attribute to the establishment of antiferromagnetic ordering of Sm ion sublattice, is observed. Furthermore, a temperature independent signal (Pauli paramagnetism, Landau and core diamagnetism...) is also present in the magnetic behaviour of this sample. In SmFeAs(O0.85F0.15) the antiferromagnetic ordering in Fe-As plane is suppressed and superconductivity occurs at T = 52 K, whereas the antiferromagnetic ordering of Sm ions at low temperature persists, leading to the coexistence and competition between superconducting and magnetic orderings. Above the transition temperature, after the subtraction of the Sm ion sublattice paramagnetic contribution and of the temperature independent contribution to the experimental susceptibility data, a Curie-Weiss behaviour for Fe is observed, with a magnetic moment of 1.4 Bohr magneton.