Effect of pressure on the magnetic and superconducting transitions of GdFe_{1-x Co x}AsO (x = 0, 0.1, 1) compounds

We have investigated the effect of applied pressure (P) on the magnetic and superconducting transitions of GdFeCo_xAsO (x = 0, 0.1, 1) compounds by measuring the temperature dependence of resistivity. At ambient pressure, GdFeAsO undergoes a spin-density wave transition at associated with the 3d electrons of Fe. Under external pressure, TN decreases from 128.2 K at 0 GPa to 114.5 K at 2.5 GPa at the rate = 6 K/GPa. Unlike GdFeAsO, the isostructural GdCoAsO exhibits a ferrimagnetic ordering of the Co moments at 76 K which decreases from 76 K to 71 K as pressure increases from 0 to 2.5 GPa. For the GdFeCoAsO sample, the superconducting onset temperature T_c ^on decreases monotonically from 19 to 17.1 K, and the temperature T_c ^zero at which the resistivity disappears decreases from 16.7 to 10.5 K as pressure increases from 0 to 2.9 GPa. For all the three samples, both the value of resistivity and the strength of electron-electron correlation decrease monotonically with increasing pressure.