The magnetic phase diagram has been mapped out via the measurements of electronic resistivity, magnetization and specific heat in the cobalt-based layered LnCo1-xFexAsO (Ln=La, Sm) compounds. The ferromagnetic (FM) transition at 63 K for LaCoAsO is r
apidly suppressed upon Fe doping, and ultimately disappears around x=0.3 in the LaCo1-xFexAsO system. When La is replaced by magnetic rare earth element Sm, the 3d electrons first undergo a FM transition at Tc = 75 K, followed by an antiferromagnetic (AFM) transition at a lower temperature TN1 = 45 K. With partial Fe doping on the Co site, both FM (Tc) and AFM (TN1) transition temperatures are significantly suppressed, and finally approach zero kelvin at x = 0.3 and 0.2, respectively. Meanwhile, a third magnetic transition at TN2 = 5.6 K for SmCoAsO, associated with the AFM order of the Sm3+ 4f-oments, is uncovered and TN2 is found to be almost robust against the small Fe-doping. These results suggest that the 4f electrons of Sm3+ have an important effect on the magnetic behavior of 3d electrons in the 1111 type Co-based LnCo1-xFexAsO systems. In contrast, the magnetism of the f-electrons is relatively unaffected by the variation of the 3d electrons. The rich magnetic phase diagram in the Co-rich side of the LnCo1-xFexAsO system, therefore, is established.
