Electromagnetic properties of undoped LaFePnO (Pn P, As)

Undoped LaFePnO (Pn = P, As) shows metallic resistivity and a tetragonal phase at room temperature. The electrical resistivity (?) of undoped LaFeAsO exhibits a sudden decrease around Tanom ~160 K with decreasing the temperature and increases with decreasing the temperatures (T) below another threshold temperature (Tmin). It makes a contrast to undoped LaFePO, which shows neither of Tanom nor Tmin. The Tanom corresponds to a crystallographic phase transition: undoped LaFeAsO shows an orthorhombic phase at temperatures < 160 K. A positive magnetoresistance (MR = (?-?0) / ?0) also appears below Tanom. DFT calculations indicate that spin moment of LaFeAsO is 1.56 ?B / Fe in an antiferromagnetic spin configuration in a tetragonal phase, while spin moment of LaFePO is almost quenched.