Our recent study on Th Co2 Ge2 and Th Co2 Si2 confirmed that ferromagnetic ordering of Co moments exists in both compounds below Curie temperature TC. This is clearly different from corresponding uranium compounds U Co2 Si2 and U Co2 Ge2 that show antiferromagnetic transition and no magnetic ordering of Co moments can be detected. In this work, we investigate the magnetic properties of (Th1-x Ux) Co2 Si2 and (Th1-x Ux) Co2 Ge2 solid solutions. The experimental results reveal complex magnetism, i.e., mixing of ferromagnetism and antiferromagnetism, in the compounds with middle compositions. For (Th1-x Ux) Co2 Si2, it is observed that TC is larger than TN, while increasing the uranium content enlarges the paramagnetic Curie temperature from θ<-3000 K (x=0.17) to θ>-280 K (x>0.69). In contrast, for (Th1-x Ux) Co2 Ge2, TC is smaller than TN and the paramagnetic Curie temperature reduces with increasing x from θ>0 K (x=0) to θ<-400 K (x>0.86).
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