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We investigate the effect of alloying at the 3 d transition metal site of a rare-earth-transition metal oxide, by considering NdFe 0.5 Cr 0.5 O 3 mixed perovskite with two equal and random distribution of 3d ions, Cr and Fe, interacting with an early 4f rare earth ion, Nd. Employing temperature- and field- dependent magnetization measurements, temperature-dependent x-ray diffraction, neutron powder diffraction, and Raman spectroscopy, we characterize its structural and magnetic properties. Our study reveals bipolar magnetic switching (arising from negative magnetization) and magnetocaloric effect which underline the potential of the studied mixed perovskite in device application. The neutron diffraction study shows the absence of spin reorientation transition over the entire temperature range of 1.5-320 K, although both parent compounds exhibit spin orientation transition. We discuss the microscopic origin of this curious behavior. The neutron diffraction results also reveal the ordering of Nd spins at an unusually high temperature of about 40 K, which is corroborated by Raman measurements., (© 2024 IOP Publishing Ltd.)

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca 3 Ru 2 O 7 , which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90 ∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca 3 Ru 2 O 7 , using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO 6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them., (© 2023. Springer Nature Limited.)

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi 4 O 12 . This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn 2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below T N ≈12 K, Mn 2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu 2+ in the [EuF] + fluorite layers depending on the nature of the [TAs] - antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at T N ∼ 2.4-3 K due to an ordering of the Eu 2+ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k 1 = (½ ½ 0) stripe order pattern with magnetic moments within the a - b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu 2+ magnetic moments confined within the bc -plane. Additionally, the Mn 2+ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c -direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J 3 between the adjacent [EuF] + layers is comparable with in-plane J 1 - J 2 interactions already in the case of semiconducting fluorite layers [TAs] - (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs] - ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.

According to previous theoretical work, the binary oxide CuO can become a room-temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present Letter provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.

The annealing behavior of (1-x)BiMg0.5Ti0.5O3−xBiZn0.5Ti0.5O3 [(1-x)BMT−xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT−BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT−BZT system promising for the development of environmentally friendly piezoelectric ceramics.

A new member of A-site columnar-ordered A 2 A'A″B 4 O 12 quadruple perovskites with the composition of Y 2 CuGaMn 4 O 12 was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y 2 ] A [Cu 0.8 Mn 0.2 ] A' [Ga 0.8 Mn 0.2 ] A″ [Mn 3.6 Cu 0.2 Ga 0.2 ] B O 12 . It has the space group P 4 2 / nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at T C = 115 K with the ordered moment of 2.19 μ B at each B site and 0.89 μ B at the A' or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A' or A″ site are ordered in the opposite direction. Cu 2+ doping drastically changes magnetic properties as "parent" Y 2 MnGaMn 4 O 12 just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y 2 CuGaMn 4 O 12 : the lattice parameter a almost linearly decreases from 1.5 K to T C and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

Perovskite-structure AMnO 3 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 AMn 7 O 12 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 AMn 7 O 12 compounds host rich physics beyond the canonical AMnO 3 materials.

We show that a polar, pseudo-Jahn-Teller instability exists for the underbonded rare-earth A-site cations in the quadruple perovskite Dy&#95;{1-δ}Mn&#95;{7+δ}O&#95;{12}, which leads to the spontaneous formation of a dipolar glass. This observation alone expands the applicability of pseudo-Jahn-Teller physics in perovskite-derived materials, for which it is typically confined to B-site cations. We demonstrate that the dipolar glass order parameter is coupled to a ferrimagnetic order parameter via strain, leading to a first order magnetostructural phase transition that can be tuned by magnetic field. This phenomenology may emerge in a broad range of perovskite-derived materials in which A-site cation ordering and octahedral tilting are mutually tied to meet the criteria of structural stability.

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 BiMn 7 O 12 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., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The structural and physical properties of the β polymorph of iron tungstate Fe 2 WO 6 have been investigated by synchrotron and neutron diffraction vs temperature, combined with magnetization and dielectric properties measurements. The monoclinic P 2 1 / a crystal structure of β-Fe 2 WO 6 has been determined and consists of an original network of zigzag chains of FeO 6 and WO 6 octahedra sharing trans and skew edges, connected through corners into a 3D structure. Magnetization measurements indicate an antiferromagnetic transition at T N = 264 K, which corresponds to a ↑↑↓↓ nearly collinear ordering of iron moments inside sequences of four edge-sharing FeO 6 octahedra, as determined by neutron diffraction. A canting of the moments out of the ac plane is observed below 150 K, leading to a noncollinear antiferromagnetic structure, the P 2 1 / a ' magnetic space group remaining unchanged. These results are discussed in comparison with the crystal and magnetic structures of γ-Fe 2 WO 6 and with the magnetic couplings in other iron tungstates and trirutile Fe 2 TeO 6 .

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr 3+ -to-Sc 3+ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO 3 and BiCr 0.9 Sc 0.1 O 3 were obtained as metastable perovskite phases which are monoclinic C 2/ c with the √6 a p × √2 a p × √6 a p superstructure (where a p is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr 0.9 Sc 0.1 O 3 is ∼1.3% larger than that of BiCrO 3 . Both perovskites undergo a reversible structural transition into a nonpolar GdFeO 3 -type phase (orthorhombic Pnma , √2 a p × 2 a p × √2 a p ) in the temperature ranges of 410-420 K (BiCrO 3 ) and 470-520 K (BiCr 0.9 Sc 0.1 O 3 ) 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- C 2/ c crossover in BiCrO 3 is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr 0.9 Sc 0.1 O 3 the conductivity increases. The onset of an antiferromagnetic order in BiCr 0.9 Sc 0.1 O 3 is observed at the Néel temperature ( T N ) of about 85 K as compared with T N = 110 K in BiCrO 3 . In contrast to BiCrO 3 , which exhibits a spin reorientation at T sr ∼ 72 K, no such a transition occurs in BiCr 0.9 Sc 0.1 O 3 .

The formation of NaRMn 2 Ti 4 O 12 compounds (R = rare earth) under high pressure (about 6 GPa) and high temperature (about 1750 K) conditions was studied. Such compounds with R = Sm, Eu, Gd, Dy, Ho, Y adopt an A-site columnar-ordered quadruple-perovskite structure with the generic chemical formula A 2 A'A″B 4 O 12 . Their crystal structures were studied by powder synchrotron X-ray and neutron diffraction between 1.5 and 300 K. They maintain a paraelectric structure with centrosymmetric space group P 4 2 / nmc (No. 137) at all temperatures, in comparison with the related CaMnTi 2 O 6 perovskite, in which a ferroelectric transition occurs at 630 K. The centrosymmetric structure was also confirmed by second-harmonic generation. It has a cation distribution of [Na + R 3+ ] A [Mn 2+ ] A' [Mn 2+ ] A″ [Ti 4+ 4 ] B O 12 (to match with the generic chemical formula) with statistical distributions of Na + and R 3+ at the large A site and a strongly split position of Mn 2+ at the square-planar A' site. We found a C-type long-range antiferromagnetic structure of Mn 2+ ions at the A' and A″ sites below T N = 12 K for R = Dy and found that the presence of Dy 3+ disturbs the long-range ordering of Mn 2+ below a second transition at lower temperatures. The first magnetic transition occurs below 8-13 K in all compounds, but the second magnetic transition occurs only for R = Dy, Sm, Eu. All compounds show large dielectric constants of a possible extrinsic origin similar to that of CaCu 3 Ti 4 O 12 . NaRMn 2 Ti 4 O 12 with R = Er-Lu crystallized in the GdFeO 3 -type Pnma perovskite structure, and NaRMn 2 Ti 4 O 12 with R = La, Nd contained two perovskite phases: an AA' 3 B 4 O 12 -type Im 3̅ phase and a GdFeO 3 -type Pnma phase.

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls 1 and in related objects such as skyrmions 2 . However, chiral textures should also exist in other types of ferroic materials, such as antiferromagnets, for which theory predicts that they should move faster for lower power 3 , and ferroelectrics, where they should be extremely small and possess unusual topologies 4,5 . Here, we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO 3 . Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavourable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics.

Spin-reorientation phase transitions that involve the rotation of a crystal's magnetization have been well characterized in distorted-perovskite oxides such as orthoferrites. In these systems spin reorientation occurs due to competing rare-earth and transition metal anisotropies coupled via f-d exchange. Here, we demonstrate an alternative paradigm for spin reorientation in distorted perovskites. We show that the R&#95;{2}CuMnMn&#95;{4}O&#95;{12} (R=Y or Dy) triple A-site columnar-ordered quadruple perovskites have three ordered magnetic phases and up to two spin-reorientation phase transitions. Unlike the spin-reorientation phenomena in other distorted perovskites, these transitions are independent of rare-earth magnetism, but are instead driven by an instability towards antiferromagnetic spin canting likely originating in frustrated Heisenberg exchange interactions, and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies.

Y 2 MnGa(Mn 4- x Ga x )O 12 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 P 4 2 / 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 [Y 3+ 2 ] A [Mn 3+ ] A' [Ga 3+ 0.68 Mn 2+ 0.32 ] A″ [Mn 3.68 Ga 0.32 ] B O 12 with a preferred localization of Ga 3+ in the tetrahedral A″ site and with a small amount of Ga 3+ in the octahedral B site. A complete triple A-site order, [Y 3+ 2 ] A [Mn 3+ ] A' [Ga 3+ ] A″ [Mn 3+ 4- x Ga 3+ x ] B O 12 , 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.

A post-synthesis thermal treatment of metastable phases in the high-pressure stabilised perovskite BiFe1-yScyO3 system results in the irreversible formation of polymorphs which represent novel polar and antipolar structures with interesting magnetic properties. Such annealing-stimulated polymorphism is believed to be a general phenomenon which can be found in other systems.

Sm 2 MnMn(Mn 4- x Ti x )O 12 with 1 ≤ x ≤ 3 were prepared by a high-pressure, high-temperature method at 6 GPa and about 1570-1670 K. They belong to a family of A-site columnar-ordered quadruple perovskites A 2 A'A″B 4 O 12 , where A' is a site with a square-planar coordination and A″ is a site with a tetrahedral coordination. Their crystal structures were investigated using synchrotron X-ray and neutron powder diffraction. They crystallize in space group P4 2 / nmc (No. 137) with a = 7.41172 Å and c = 7.97131 Å for x = 1, a = 7.54945 Å and c = 7.76756 Å for x = 2, and a = 7.63949 Å and c = 7.70339 Å for x = 3 at 295 K. The determined charge and cation distributions are [Sm 3+ 1.88 Mn 2+ 0.12 ] A [Mn 3+ ] A' [Mn 2+ 0.88 Sm 3+ 0.12 ] A″ [Mn 3+ 3 Ti 4+ ] B O 12 for x = 1, [Sm 3+ 1.91 Mn 2+ 0.09 ] A [Mn 2+ ] A' [Mn 2+ 0.91 Sm 3+ 0.09 ] A″ [Mn 3+ 2 Ti 4+ 2 ] B O 12 for x = 2, and [Sm 3+ 1.88 Mn 2+ 0.12 ] A [Mn 2+ 0.88 Sm 3+ 0.12 ] A' [Mn 2+ ] A″ [Mn 2+ Ti 4+ 3 ] B O 12 for x = 3. Mn and Ti are distributed randomly in one B site in all compounds with the average oxidation state changing from +3.25 to +3.5 per one B atom, and such flexibility is realized because Mn at the A' site can change its oxidation state between +2 and +3. Sm and Mn are slightly disordered between the A and A″ sites for x = 1 and 2, and between the A and A' sites for x = 3. The x = 1 sample shows spin-canted antiferromagnetic properties with T N = 27 K, and the x = 2 sample, with T N = 62 K. On the other hand, the x = 3 sample is a ferrimagnet, confirmed by neutron diffraction, with T C = 40 K. The x = 3 sample shows relaxor-like dielectric properties below 220 K.

The scope of magnetic neutron scattering has been expanded by the observation of electronic Dirac dipoles (anapoles) that are polar (parity odd) and magnetic (time odd). A zero-magnetization ferromagnet Sm&#95;{0.976}Gd&#95;{0.024}Al&#95;{2} with a diamond-type structure presents Dirac multipoles at basis-forbidden reflections that include the standard (2, 2, 2) reflection. Magnetic amplitudes measured at four such reflections are in full accord with a structure factor calculated from the appropriate magnetic space group.

There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A 2 A'A''B 4 O 12 quadruple perovskites, which have an intrinsic triple order at the A sites. However, in many examples reported so far, A' and A'' cations are the same, and the intrinsic triple order is hidden. Here, we investigate structural properties of Dy 2 CuMnMn 4 O 12 (1) and Ho 2 MnGaMn 4 O 12 (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites. The cation distributions determined are [Ho 2 ] A [Mn] A' [Ga 0.66 Mn 0.34 ] A'' [Mn 3.66 Ga 0.34 ] B O 12 and [Dy 2 ] A [Cu 0.73 Mn 0.27 ] A' [Mn 0.80 Dy 0.20 ] A'' [Mn 1.89 Cu 0.11 ] B1 [Mn 2 ] B2 O 12 . There are clear signatures of Jahn-Teller distortions in 1 and 2, and the orbital pattern is combined with an original type of charge ordering in 1. Columnar-ordered quadruple perovskites represent a new playground to study complex interactions between different electronic degrees of freedom. No long-range magnetic order was found in 2 by neutron diffraction, and its magnetic properties in low fields are dominated by an impurity with negative magnetization or magnetization reversal. On the other hand, 1 shows three magnetic transitions at 21, 125, and 160 K., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The spin-1/2 kagome antiferromagnet is an archetypal frustrated system predicted to host a variety of exotic magnetic states. We show using neutron scattering measurements that deuterated vesignieite BaCu&#95;{3}V&#95;{2}O&#95;{8}(OD)&#95;{2}, a fully stoichiometric S=1/2 kagome magnet with <1% lattice distortion, orders magnetically at T&#95;{N}=9  K into a multi-k coplanar variant of the predicted triple-k octahedral structure. We find that this structure is stabilized by a dominant antiferromagnetic third-neighbor exchange J&#95;{3} with minor first- or second-neighbor exchanges. The spin-wave spectrum is well described by a J&#95;{3}-only model including a tiny symmetric exchange anisotropy.

The ABO 3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn 2 O 3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn 2 O 3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na&#95;{1-x}Ca&#95;{x}Mn&#95;{7}O&#95;{12} quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic, and orbital ordering across a wide range of compounds.

A-site-ordered quadruple perovskites RMn 7 O 12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at T N1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below T N2 ≈ 12 K were observed in SmMn 7 O 12 and EuMn 7 O 12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at T N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn 7 O 12 shows signs of nonstoichiometry expressed as (Tb 1- x Mn x )Mn 7 O 12 , and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn 7 O 12 and EuMn 7 O 12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn 7 O 12 and (Tb 0.88 Mn 0.12 )Mn 7 O 12 were studied by synchrotron X-ray powder diffraction at 295 K.

Cubic perovskite-type Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) is one of the mixed ionic-electronic conductors with the highest oxygen permeability known to date. It serves as a parent material for the development of functional derivatives for electrochemical applications including oxygen separation membranes, solid electrolyte cell electrodes and electrocatalysts for the oxygen evolution reaction. The present study is focused on the determination of the precise stability boundaries of cubic perovskite BSCF employing a coulometric titration technique in combination with thermogravimetric analysis, X-ray and neutron diffraction, and molecular dynamics simulations. Both the low-p(O 2 ) and high-p(O 2 ) stability boundaries at 700-950 °C were found to correspond to a fixed value of oxygen content in the perovskite lattice of 3 - δ = ∼2.13 and ∼2.515, respectively. The stability limits in this temperature range are expressed by the following equations: high-p(O 2 ) boundary: log p(O 2 ) (atm) (±0.1) = -10 150/T (K) + 8.055; low-p(O 2 ) boundary: log p(O 2 ) (atm) (±0.03) = -20 750/T (K) + 4.681. The p(O 2 )-T phase diagram of the BSCF system under oxidizing conditions is addressed in a wider temperature range and is shown to include a region of precipitation of a "low-temperature" phase occurring at 400-500 °C. The fraction of the low-temperature precipitate, which co-exists with the cubic perovskite phase and is stable up to 790-820 °C, increases upon increasing p(O 2 ) in the range 0.21-1.0 atm.

Electronic and magnetic properties of ferric ions (3d 5 ) in multiferroic ScFeO 3 are puzzling, in part because they are different from the only other multiferroic known to possess the same polar chemical structure, BiFeO 3 . Open questions about ScFeO 3 can be addressed by confronting observations with results for G-type antiferromagnetism allowed by the lithium niobate (LiNbO 3 )-like parent R3c structure. Calculated structure factors for resonant x-ray diffraction include all charge-like quadrupoles allowed by symmetry, and if experimental results for ScFeO 3 subsequently imply they are different from zero then ferric ions cannot be in the high-spin 6 S state. The same type of experiment can reveal the moment direction in the G-type antiferromagnetism, according to our calculations, and thereby contribute to understanding magnetic anisotropy. Furthermore, structure factors for magnetic neutron diffraction by ScFeO 3 include Dirac multipoles that are time-odd and parity-odd, e.g. dipoles that are often called anapoles or toroidal moments. Apart from Dirac multipoles, the conventional approach to the interpretation of neutron Bragg diffraction data will be inadequate if ferric ions (Fe 3+ ) are not in the high-spin 6 S state, because the scattering amplitude includes more than simple dipole moments in the general case.

Inverse trirutile Mn 2 TeO 6 was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P4 2 /mnm) to a monoclinic phase (P2 1 /c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO 6 octahedra alternate with more regular TeO 6 octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn-Teller origin, is driven by a single primary mode.

Charge and orbital degrees of freedom determine properties of many materials, and are central to many important phenomena. At high temperatures, thermal fluctuations overcome them, and high-symmetry structures are realized. On decreasing temperature, different charge- and orbital-order transitions take place accompanied by symmetry lowering. Remarkable exceptions to this general tendency, realized in Cu-doped BiMn 7 O 12 quadruple perovskites, are presented. Introduction of Cu 2+ produces mixtures of Mn 3+ and Mn 4+ and charge degree of freedom. BiCuMn 6 O 12 (and compositions in the vicinity) exhibits well-defined 1:3 charge order of Mn 4+ and Mn 3+ and orbital order of Mn 3+ near room temperature, but both charge and orbital orders collapse below about 115 K with the reentrance of the high-temperature cubic Im3‾ phase. What is interesting the collapse can be controlled by a magnetic field even without long-range magnetic order, and the collapsed phase shows nearly zero thermal expansion., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Scattering by magnetic charge formed by Dirac multipoles that are magnetic and polar is examined in the context of materials with properties that challenge conventional concepts. An order parameter composed of Dirac quadrupoles has been revealed in the pseudo-gap phase of ceramic, high-T c superconductors on the basis of Kerr effect and magnetic neutron Bragg diffraction measurements. Construction of Dirac quadrupoles that emerge from centrosymmetric sites used by Cu ions in the ceramic superconductor Hg1201 is illustrated, together with selection rules for excitations that will feature in neutron inelastic scattering, and RIXS experiments. We report magnetic scattering amplitudes for diffraction by polar multipoles that have universal value, because they are not specific to ceramic superconductors. To illustrate this attribute, we consider neutron Bragg diffraction from a magnetically ordered iridate (Sr 2 IrO 4 ) and discuss shortcomings in published interpretations of diffraction data.

In vortex-like spin arrangements, multiple spins can combine into emergent multipole moments. Such multipole moments have broken space-inversion and time-reversal symmetries, and can therefore exhibit linear magnetoelectric (ME) activity. Three types of such multipole moments are known: toroidal; monopole; and quadrupole moments. So far, however, the ME activity of these multipole moments has only been established experimentally for the toroidal moment. Here we propose a magnetic square cupola cluster, in which four corner-sharing square-coordinated metal-ligand fragments form a noncoplanar buckled structure, as a promising structural unit that carries an ME-active multipole moment. We substantiate this idea by observing clear magnetodielectric signals associated with an antiferroic ME-active magnetic quadrupole order in the real material Ba(TiO)Cu 4 (PO 4 ) 4 . The present result serves as a useful guide for exploring and designing new ME-active materials based on vortex-like spin arrangements.

The physical characterization and the extended crystallographic study of the double perovskite system Pb2Mn0.6Co0.4WO6 indicate an improper ferroelectric contribution to the polarization induced by the magnetic ordering. In the paramagnetic phase, the compound displays a centrosymmetric orthorhombic double perovskite structure with the Pmcn1' symmetry. The structure is strongly distorted by the lead stereoactivity. Magnetization measurements show two magnetic transitions at 188 and 9 K, but the time-of-flight neutron diffraction data provide evidence for a long-range magnetic ordering only below the second transition. Quantitative structure refinements combined with a comprehensive symmetry analysis indicate the Pm'c21' magnetic space group to be the adequate symmetry to describe the structural distortions and spin ordering in the ground state of the system. The symmetry implies a coexistence of a spontaneous ferromagnetic moment and a ferroelectric polarization along the orthogonal b- and c-axes, respectively, in the long-range ordered structure. Macroscopic measurements confirm the presence of the spontaneous polarization also below the first transition at 188 K, where only short-range magnetic correlations are evidenced by diffuse scattering in neutron diffraction.

A symmetry-based interpretation of published experimental results demonstrates that the pseudo-gap phase of underdoped HgBa2CuO(4+δ) (Hg1201) possesses an ordered state of magnetic charge epitomized by Cu magnetic monopoles. Magnetic properties of one-layer Hg1201 and two-layer YBa2Cu3O(6+x) (YBCO) cuprates have much in common, because their pseudo-gap phases possess the same magnetic space-group, e.g. both underdoped cuprates allow the magneto-electric (Kerr) effect. Differences in their properties stem from different Cu site symmetries, leaving Cu magnetic monopoles forbidden in YBCO. Resonant x-ray Bragg diffraction experiments can complement the wealth of information available from neutron diffraction experiments on five Hg1201 samples on which our findings are based. In the case of Hg1201 emergence of the pseudo-gap phase, with time-reversal violation, is accompanied by a reduction of Cu site symmetry that includes loss of a centre of inversion symmetry. In consequence, parity-odd x-ray absorption events herald the onset of the enigmatic phase, and we predict dependence of corresponding Bragg spots on magneto-electric multipoles, including the monopole, and the azimuthal angle (crystal rotation about the Bragg wavevector).

There is general agreement within the community of researchers that investigate high-Tc materials that it is most important to understand the pseudo-gap phase. To this end, many experiments on various cuprates have been reported. Two prominent investigations-Kerr effect and neutron Bragg diffraction-imply that underdoped YBCO samples possess long-range magnetic order of an unusual kind. However, other measurements do not support the existence of magnetic order. Here we show that the Kerr effect and magnetic Bragg diffraction data are individual manifestations of ordered magneto-electric quadrupoles at Cu sites. While the use of magneto-electric multipoles is new in studies of the electronic properties of cuprates, they are not unknown in other materials, including an investigation with x-rays of the parent compound CuO. We exploit the recent prediction that neutrons are deflected by magneto-electric multipoles. The outcome of our study is a theory for the order-parameter of the pseudo-gap phase without the aforementioned conflict with other measurements, and the first experimental evidence that neutrons interact with multipoles belonging to a state of magnetic charge.

Frustrated interactions exist throughout nature, with examples ranging from protein folding through to frustrated magnetic interactions. Whilst magnetic frustration is observed in numerous electrically insulating systems, in metals it is a rare phenomenon. The interplay of itinerant conduction electrons mediating interactions between localised magnetic moments with strong spin-orbit coupling is likely fundamental to these systems. Therefore, knowledge of the precise shape and topology of the Fermi surface is important in any explanation of the magnetic behaviour. PdCrO2, a frustrated metallic magnet, offers the opportunity to examine the relationship between magnetic frustration, short-range magnetic order and Fermi surface topology. By mapping the short-range order in reciprocal space and experimentally determining the electronic structure, we have identified the dual role played by the Cr electrons in which the itinerant ones on the nested paramagnetic Fermi surface mediate the frustrated magnetic interactions between local moments.

The conduction electrons in a metal experience competing interactions with each other and the atomic nuclei. This competition can lead to many types of magnetic order in metals. For example, in chromium the electrons order to form a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used to perturb or tune materials with delicately balanced electronic interactions. Here, we show that the application of a magnetic field can induce SDW magnetic order in a quasi-2D metamagnetic metal, where none exists in the absence of the field. We use magnetic neutron scattering to show that the application of a large (B ≈ 8 T) magnetic field to the perovskite metal Sr3Ru2O7 (refs 3-7) can be used to tune the material through two magnetically ordered SDW states. The ordered states exist over relatively small ranges in field (≲0.4 T), suggesting that their origin is due to a new mechanism related to the electronic fine structure near the Fermi energy, possibly combined with the stabilizing effect of magnetic fluctuations. The magnetic field direction is shown to control the SDW domain populations, which naturally explains the strong resistivity anisotropy or 'electronic nematic' behaviour observed in this material.

A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tensors, are absent from the so-called j(eff) = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr(2)IrO(4). The construct encompasses the j(eff) = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr(2)IrO(4).

A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba1-xNaxFe2As2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

We investigate how the order parameter of a continuous phase transition can be protected from view by symmetry in a magnetic crystal. The symmetry in question forbids atomic displacements and formation of magnetic dipoles, rendering the order parameter invisible in standard x-ray and magnetic neutron Bragg diffraction. Analysis of the allowed magnetic space-groups reveals exact properties of the hidden order parameter. We demonstrate that Bragg spots forbidden by the chemical structure can unveil magnetic hidden order. The method is applied to URu2Si2, which has been thoroughly investigated in the past few decades using all manner of experimental techniques. Starting from the established chemical structure of URu2Si2, we have performed a critical analysis of available data for magnetic neutron Bragg diffraction.

The magnetic properties of Sr(2)IrO(4), Na(2)IrO(3), Sr(3)Ir(2)O(7) and CaIrO(3) are discussed, principally in the light of experimental data in recent literature for Bragg intensities measured in x-ray diffraction with enhancement at iridium L-absorption edges. The electronic structure factors we report, which incorporate parity-even and acentric entities, serve the immediate purpose of making full use of crystal and magnetic symmetry to refine our knowledge of the magnetic properties of the four iridates from resonant x-ray diffraction data. They also offer a platform on which to interpret future investigations, using dichroic signals, resonant x-ray diffraction and neutron diffraction, for example, as well as ab initio calculations of electronic structure. Unit-cell structure factors, suitable for x-ray Bragg diffraction enhanced by an electric dipole-electric dipole (E1-E1) event, reveal exactly which iridium multipoles are visible, e.g., a magnetic dipole parallel to the crystal c-axis (z-axis) and an electric quadrupole with yz-like symmetry in the specific case of CaIrO(3). Magnetic space-groups are assigned to Sr(2)IrO(4), Sr(3)Ir(2)O(7) and CaIrO(3), namely, P(I)cca, P(A)ban and Cm'cm', respectively, in the Belov-Neronova-Smirnova notation. The assignment for Sr(2)IrO(4) is possible because of our new high-resolution neutron diffraction data, gathered on a powder sample. In addition, the new data are used to show that the ordered magnetic moment of an Ir(4+) ion in Sr(2)IrO(4) does not exceed 0.29(4) μ(B). Na(2)IrO(3) has two candidate magnetic space-groups that are not resolved with currently available resonant x-ray data.

We have performed dielectric measurements and neutron diffraction experiments on the delafossite AgFeO2. A ferroelectric polarization P is approximately equal to 300 μC/m2 was observed in a powder sample, below 9 K. The neutron diffraction experiment demonstrated successive magnetostructural phase transitions at T(N1)=15 K and T(N2)=9 K. The magnetic structure for 9 K≤T≤15 K is a spin-density wave with a temperature dependent incommensurate modulation k=(-1, q, 1/2), q is approximately equal to 0.384. Below 9 K, the magnetic structure turns into elliptical cycloid with the incommensurate propagation vector k=(-1/2,q,1/2), q is approximately equal to 0.2026 Based on the deduced magnetic point-group symmetry m1' of the low-temperature polar phase, we conclude that the ferroelectric polarization in AgFeO2 is perpendicular to the monoclinic b axis and is driven by the inverse Dzyaloshinskii-Moriya effect with two orthogonal components p1 is proportional to r(ij)×(S(i)×S(j)) and p2 is proportional to S(i)×S(j).