Impact of Ge Doping on Structural and Magnetic Ordering in RMn x Ga3 and R4Mn1- xGa12- yGe y (R = Tb, Dy; x ≤ 0.25, y ≈ 1.0-3.3)

Abstract

Single crystals of RMn_xGa₃ and their new quaternary derivatives R₄Mn_1-xGa_12-yGe_y (R = Tb, Dy, x ≤ 0.25, y ≈ 1.0-3.3) were grown from a Ga flux. The compounds are derivatives of cubic RGa₃ phases, with Mn atoms filling the Ga₆ voids. RMn_xGa₃ formally adopts a cubic ABO₃ perovskite structure, in which the presence of Mn atoms results in a shift of the neighboring Ga atoms from their ideal position. A partial substitution of Ga by Ge leads to a higher Mn content, resulting in structural ordering of the latter and the formation of the superstructure phases R₄Mn_1-xGa_12-yGe_y, which can be formally described in the Y₄PdGa₁₂ structure type. The presence of Mn vacancies, which was observed for R = Tb, and Ga/Ge mixing lead to a noticeable deviation from the idealized structure. The compounds contain two magnetic sublattices: the R sublattice, which orders antiferromagnetically near 20 K, and the Mn sublattice, which orders ferromagnetically at T_C = 125-225 K with the Ge doping resulting in higher T_C. The two sublattices are not independent, as the Mn sublattice induces partial ferromagnetic ordering of the rare earth atoms below T_C, at least for the Ge-doped phases. Near T_N, both magnetic susceptibility and heat capacity reveal complex behavior, indicating changes in magnetic structures below T_N.