Thermal and magnetic properties of La-doped Ce _1-yLa _yGe _1.80 with 0<y<1.0

CeGex with 1.71<x≤2.0 crystallizes in an orthorhombic structure and exhibits two magnetic transitions: an antiferromagnetic transition at TN =7 K and a ferromagnetic transition at TC =4.3 K. When x decreases below 1.71, CeGex transforms to a tetragonal structure and exhibits only an antiferromagnetic transition at TN =7 K. We have further studied the physical properties of La-doped Ce1-y Lay Ge1.80 with 0<y<1.0. The magnetization and specific-heat measurements show that with initial La substitution for Ce, TN decreases monotonically while TC increases slightly, and TN and TC merge to become a single ferromagnetic state at TC =4.8 K in the vicinity of y=0.22. With further La doping, TC decreases slowly and linearly from 4.8 to 1.75 K at y=0.65. The x-ray measurements show that the lattice parameters remain nearly unchanged in the two magnetic transition regimes with 0<y<0.22 and increase monotonically with further La substitution for Ce (0.22<y≤1.0). The entropy removal and magnetic moment calculations are also discussed.