Magnetocrystalline anisotropic effect in GdCo1-xFexAsO (x=0,0.05)

From a systematic study of the electrical resistivity ρ(T,H), magnetic susceptibility χ(T,H), isothermal magnetization M(H), and the specific heat C(T,H), a temperature-magnetic field (T-H) phase diagram has been established for GdCo1-xFexAsO (x=0 and 0.05) polycrystalline compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic (FM) transition of Co 3d electrons (TCCo) and antiferromagnetic (AFM) transition of Gd 4f electrons (TNGd). For the Fe-doped sample (x=0.05), an extra magnetic reorientation transition takes place below TNGd, which is likely associated with Co moments. The two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM) behavior in the magnetic susceptibility. Upon decreasing the temperature (T<TCCo), the magnetocrystalline anisotropy breaks up the FM order of Co by aligning the moments with the local easy axes of the various grains, leading to a spin reorientation transition at TRCo. By applying a magnetic field, TRCo monotonically decreases to lower temperatures, while the TNGd is relatively robust against the external field. On the other hand, the applied magnetic field pulls the magnetization of grains from the local easy direction to the field direction via a first-order reorientation transition, with the transition field (HM) increasing upon cooling the temperature.