Impact of Ge Doping on Structural and Magnetic Ordering in RMn_xGa₃ and R₄Mn_1–xGa_12–yGe_y (R = Tb, Dy; x ≤ 0.25, y ≈ 1.0–3.3)

Abstract

Single crystals of RMn x Ga 3 and their new quaternary derivatives R 4 Mn 1−x Ga 12−y Ge y (R = Tb, Dy, x ≤ 0.25, y ≈ 1.0−3.3) were grown from a Ga flux. The compounds are derivatives of cubic RGa 3 phases, with Mn atoms filling the Ga 6 voids. RMn x Ga 3 formally adopts a cubic ABO 3 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 R 4 Mn 1−x Ga 12−y Ge y , which can be formally described in the Y 4 PdGa 12 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 .

Interplay of two magnetic sublattices in related compounds Sm2Mn1−xGa6−yGey (x = 0.1–0.3, y = 0.6–1.0) and Sm4MnGa12−yGey (y = 3.0–3.5) with different ordering of empty and filled (Ga,Ge)6 octahedra

“…14 For example, the addition of Mn atoms promotes the appearance of a magnetoactive sublattice of Mn, which exhibits ferromagnetic ordering as was observed in Y 4 Mn 0.74 Ga 12 13 and in our previous study of RMn x Ga 3 (R = Tb, Dy, x = 0–0.2). 14 The insertion of a transition metal can also stabilise a derivative structure, where the cubic RGa 3 phase itself is either unknown or not stable at room temperature (for a specific R element). 13–16,18…”

“…The flux growth technique, developed for the quaternary phases R 4 Mn 1− x Ga 12− y Ge y (R = Tb, Dy, x = 0–0.25, y = 1.0–3.3), 14 was used for growing high quality single crystals suitable for X-ray diffraction experiments and measurements of magnetic properties. The starting elements Sm (99.9%), Mn powder (99.9%), Ge (99.999%) and Ga (99.999%) mixed in the ratios 2R : 2Mn : 25Ga : n Ge ( n = 0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3) were placed inside silica tubes.…”

“…The first leads to the formation of a structural family of compounds with the general formula R n TGa 3 n +2 ( n = 1 and 2). 2–6 In the second case, the voids can be filled either stochastically leading to the RT x Ga 3 phases 7,8 or in an ordered fashion creating the R 4 TGa 12 9–14 and R 2 TGa 6 15,16 superstructures.…”

“…14 The insertion of a transition metal can also stabilise a derivative structure, where the cubic RGa 3 phase itself is either unknown or not stable at room temperature (for a specific R element). 13–16,18…”

“…14 The p -element substitution can also enhance both the value of the Mn magnetic moment and the T C of the Mn sublattice, 13,14 which causes the partial ferromagnetic ordering of R atoms above T N . 14…”