Zlata V. Pchelkina

The A- and B-site ordered quadruple perovskite oxide DyCu₃Fe₂Re₂O₁₂ with cubic Pn-3 symmetry was synthesized under high-pressure and high-temperature conditions. The material experiences a sharp long-range ferrimagnetic transition arising from the strong superexchange antiferromagnetic interactions of Cu-Re and Fe-Re at a high Curie temperature (T_C ≈ 660 K). Owing to the influence at lower temperatures of the antiferromagnetic ordering of the Dy³⁺ occupying the A-site, the susceptibility of DyCu₃Fe₂Re₂O₁₂ decreases continuously below 50 K, which is essentially different from that of the isostructural compound LaCu₃Fe₂Re₂O₁₂. Moreover, in the presence of a magnetic field, the A-site Dy³⁺ spins are readily transformed into the ferromagnetic state from the antiferromagnetic ground state. Thus, the saturated magnetic moment of DyCu₃Fe₂Re₂O₁₂ is sharply enhanced from 7.0 µ_B/f.u. at 300 K to 14 µ_B/f.u. at 2 K by applying a magnetic field of 7 T. Theoretical calculations suggest that DyCu₃Fe₂Re₂O₁₂ is a half-metallic ferrimagnet with a spin-up band gap of approximately 2.0 eV. The combination of the high Curie temperature, wide half-metallic energy gap, and large magnetic moment makes DyCu₃Fe₂Re₂O₁₂ promising for potential applications in advanced spintronic devices.

New nitrosonium manganese(II) nitrate, (NO)Mn-6(NO3)(13), has been synthesized and structurally characterized. In the temperature range of 45-298 K, the crystal is hexagonal (centrosymmetric sp. gr. P6(3)/m). Mn2+ ions are assembled into tubes along axis c with both NO3 - filling and coating. The nitrosonium cation is located in the framework cavity and is disordered by a 3-fold axis. At the temperature T-S1 = 190 K, a structural phase transition related to the libration of the intertube NO3 group and a small variation of Mn polyhedron is observed. Moreover, the anomalies in physical properties of (NO)Mn-6(NO3)(13) allow suggesting that ordering of NO+ units occurs at low temperatures. The antiferromagnetic ordering in this compound is preceded by the formation of a short-range correlation regime at about 25 K and takes place in two steps at T-N1 = 12.0 K and T-N2 = 8.4 K.

Cu₃TeO₃(SO₄)₂ features the coexistence of spin singlets and a long-range Néel order. An anomaly in its dielectric permittivity, which accompanies the magnetic order, allows considering it as a magnetoelectric multiferroic material of the second type.

PbCu(SeO₄)(OH)₂, the selenate sibling of the mineral linarite, was synthesized hydrothermally, investigated in measurements of magnetization M, specific heat C_p and dielectric permittivity ε, and analyzed within density functional theory formalism. This quasi-one-dimensional compound evidences formation of a short-range correlation regime at T* ~ 8 K and experiences a long-range magnetic order at T_N = 4.3 K. A magnetization saturation of approximately 1 µ_B is reached at µ₀H_flip ~ 16 T preceded by a jump at µ₀H_flop = 2.4 T. Additionally, there are multiple indicators of the formation of an additional electrically active phase above the Neel temperature, which suggests that PbCu(SeO₄)(OH)₂ is a multiferroic system.

A mixed-valence compound Pb₂Cu₁₀O₄(SeO₃)₄Cl₇ has a complex structure consisting of one nonmagnetic Cu⁺ (S = 0) ion and four nonequivalent magnetic Cu²⁺ (S = 1/2) ions.

The electronic and magnetic properties of clinopyroxene CaMnGe$_2$O$_6$ were studied using density function calculations within the GGA+U approximation. It is shown that anomalous ferromagnetic ordering of neighboring chains is due to a "common-enemy" mechanism. Two antiferromagnetic exchange couplings between nearest neighbours within the Mn-Mn chain and interchain coupling via two GeO$_4$ tetrahedra suppress antiferromagnetic exchange via single GeO$_4$ tetrahedron and stabilize ferromagnetic ordering of Mn chains.