Competing exchange interactions and magnetic anisotropy of La_1−xTb_xMn₂Si₂

Crystal structure, magnetization and magnetic susceptibility have been studied for the La_1−xTb_xMn₂Si₂ (0≤x≤1) polycrystalline and quasi-single crystalline samples. It has been shown that, at low temperature T =4.2 K, substitution of the terbium for lanthanum leads to recurred change of the type of interlayer Mn-Mn magnetic ordering. For the compounds with x<0.2 ≈ x_c1 the manganese magnetic moments of adjacent layers are ordered ferromagnetically, in the concentration range 0.2<x<x_c2 (the critical concentration x_c2 is between 0.4 and 0.6) the antiferromagnetic order is realized, while for x>x_c2 the Mn sublattice is again ferromagnetically ordered and, due to the negative Tb–Mn interaction, ferrimagnetic structure is formed. Using the magnetization data, the concentration magnetic phase diagram has been suggested. The observed variation of the type of magnetic ordering has been explained in terms of the change of interatomic Mn-Mn distances and a competition of the Tb–Mn, Mn–Mn and Tb–Tb interlayer exchange interactions.