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

Abstract

Single crystals of two new intermetallic phases Sm₂Mn_1−xGa_6−yGe_y (x = 0.1–0.3, y = 0.6–1.0) and Sm₄MnGa_12−yGe_y (y = 3.0–3.5) were grown using a self-flux technique. According to single crystal X-ray diffraction data, Sm₄MnGa_12−yGe_y is characterised by the Y₄PdGa₁₂ structure type (a ∼ 8.65 Å; Imm), while Sm₂Mn_1−xGa_6−yGe_y formally adopts the K₂PtCl₆ structure type (a ∼ 8.71 Å; Fmm). The general features of both compounds with rather similar crystal structures are represented by the alternation of empty and Mn-filled p-element octahedra, the order of which is determined by the Mn concentration. The diffraction data for Sm₂Mn_1−xGa_6−yGe_y reveal a large concentration of Mn vacancies (x ∼ 0.3), which affects adjacent Ga/Ge atoms leading to their shift towards the vacancy. Both compounds demonstrate two ferromagnetic-like transitions and the presence of two interacting Mn and Sm magnetic sublattices. The Mn sublattice orders at T_C1 of 143 K and 318 K, while the Sm one orders at lower temperatures at T_C2 of 50 K and 280 K for Sm₄MnGa_8.6Ge_3.4 and Sm₂Mn_0.74Ga_5.1Ge_0.9, respectively. The increase in Mn content not only increases the ordering temperatures, but also dramatically decreases the coercivity μ₀H_C from 230 mT to just 6.5 mT at 2 K. Despite the presence of two magnetically active sublattices in Sm₂Mn_0.74Ga_5.1Ge_0.9, the magnetic entropy change is quite low and only reaches 0.3 J kg⁻¹ K⁻¹ at T = 300 K and μ₀H = 5 T, while the estimated relative cooling power (RCP) is about 36 J kg⁻¹ at 5 T.