Magnetic characterization of undoped and 15%-F doped LaFeAsO and SmFeAsO compounds

In this paper the magnetic behaviour of undoped and 15% F doped SmFeAsO (Sm-1111) and LaFeAsO (La-1111) samples is presented and discussed. Magnetization measurements are not a simple tool to use for the characterisation of the new family of Fe-based superconductors. In fact, during the preparation magnetic impurities can be formed, which exhibit large magnetic signals and may obscure the magnetic signal of the phase 1111, even if present in very small amount not easily detectable with standard techniques. If ferromagnetic impurities with Curie temperature higher than room temperature are present, their magnetic contribution may be estimated and subtracted, whereas this in not possible for other type of impurities (paramagnetic or antiferromagnetic). As a consequence, in some cases, the magnetic signal is misleading and useless. The undoped samples turned out to be cleaner than the doped ones: for these samples the magnetic characterization gather the main aspects of the physical behavior of the 1111 phase, i.e. the onset of the Spin Density Wave (SDW), the antiferromagnetic ordering at the Sm sublattice and the susceptibility increase with increasing temperature above the SDW temperature. In the doped samples, on the contrary, the presence of magnetic signals due to impurities is dominating in the normal state; however the superconducting behavior may be clearly observed and studied. In particular, in the Sm-1111 superconducting sample the coexistence-competition between superconductivity and antiferromagnetic ordering of the Sm ions was clearly observed.