Evolution of Magneto-Orbital order Upon B-Site Electron Doping in Na_{1-x}Ca_{x}Mn_{7}O_{12} Quadruple Perovskite Manganites

Static and Resonant Properties and Magnetic Phase Diagram of LiMn2TeO6

Physical properties of the mixed-valent tellurate of lithium and manganese, LiMn2TeO6, were investigated in measurements of ac and dc magnetic susceptibility χ, magnetization M, specific heat Cp, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) in the temperature range 2−300 K under magnetic field up to 9 T. The title compound orders magnetically in two steps at T1 = 20 K and T2 = 13 K. The intermediate phase at T2 < T < T1 is fully suppressed by magnetic field µ0H of about 4 T. Besides magnetic phases transitions firmly established in static measurements, relaxation-type phenomena were observed well above magnetic ordering temperature in resonant measurements.

The rich physics of A-site-ordered quadruple perovskite manganites AMn7O12

Perovskite-structure AMnO₃ manganites played an important role in the development of numerous physical concepts such as double exchange, small polarons, electron-phonon coupling, and Jahn-Teller effects, and they host a variety of important properties such as colossal magnetoresistance and spin-induced ferroelectric polarization (multiferroicity). A-site-ordered quadruple perovskite manganites AMn₇O₁₂ were discovered shortly after, but at that time their exploration was quite limited. Significant progress in their understanding has been reached in recent years after the wider use of high-pressure synthesis techniques needed to prepare such materials. Here we review this progress, and show that the AMn₇O₁₂ compounds host rich physics beyond the canonical AMnO₃ materials.

Striping of orbital-order with charge-disorder in optimally doped manganites

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the [Formula: see text] doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn[Formula: see text]Mn[Formula: see text]O12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.

Phase Transitions in the Metastable Perovskite Multiferroics BiCrO3 and BiCr0.9Sc0.1O3: A Comparative Study

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr³⁺-to-Sc³⁺ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO₃ and BiCr_0.9Sc_0.1O₃ were obtained as metastable perovskite phases which are monoclinic C2/c with the √6a_p × √2a_p × √6a_p superstructure (where a_p is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr_0.9Sc_0.1O₃ is ∼1.3% larger than that of BiCrO₃. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO₃-type phase (orthorhombic Pnma, √2a_p × 2a_p × √2a_p) in the temperature ranges of 410-420 K (BiCrO₃) and 470-520 K (BiCr_0.9Sc_0.1O₃) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO₃ is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr_0.9Sc_0.1O₃ the conductivity increases. The onset of an antiferromagnetic order in BiCr_0.9Sc_0.1O₃ is observed at the Néel temperature (T_N) of about 85 K as compared with T_N = 110 K in BiCrO₃. In contrast to BiCrO₃, which exhibits a spin reorientation at T_sr ∼ 72 K, no such a transition occurs in BiCr_0.9Sc_0.1O₃.

Spin-Glass Magnetic Properties of A-Site Columnar-Ordered Quadruple Perovskites Y2MnGa(Mn4-xGax)O12 with 0 ≤ x ≤ 3

Y₂MnGa(Mn_4-xGa_x)O₁₂ solid solutions were synthesized at high pressure of ∼6 GPa and high temperature of ∼1570 K for the 0 ≤ x ≤ 3 compositional range. Synchrotron X-ray and neutron powder diffraction were used to study the crystal structures and cation distributions. These solutions adopt the parent structure of the A-site columnar-ordered quadruple perovskite family with space group P4₂/nmc (No. 137). They have lattice parameters of a = 7.36095 Å and c = 7.753 84 Å (x = 0), a = 7.361 68 Å and c = 7.716 16 Å (x = 1), a = 7.360 34 Å and c = 7.67142 Å (x = 2), and a = 7.363 93 Å and c = 7.616 85 Å (x = 3) at room temperature. The x = 0 sample has a cation distribution of [Y³⁺₂]_A[Mn³⁺]_A'[Ga³⁺_0.68Mn²⁺_0.32]_A″[Mn_3.68Ga_0.32]_BO₁₂ with a preferred localization of Ga³⁺ in the tetrahedral A″ site and with a small amount of Ga³⁺ in the octahedral B site. A complete triple A-site order, [Y³⁺₂]_A[Mn³⁺]_A'[Ga³⁺]_A″[Mn³⁺_4-xGa³⁺_x]_BO₁₂, is realized for x ≥ 1. All samples demonstrate spin-glass-like magnetic properties, and the absence of a long-range magnetic order at the ground state at 1.5 K was confirmed by neutron diffraction for the x = 1 sample. First-principles calculations indicated the spin-glass-like magnetic ordering is derived from the Ga substitution to the B sites and gave evidence that the ideal cation distribution could produce robust ferromagnetism in this family of perovskites.