Publications by authors named "Roger D Johnson"

The physics of spin-orbit entangled magnetic moments of 4d and 5d transition metal ions on a honeycomb lattice has been much explored in the search for unconventional magnetic orders or quantum spin liquids expected for compass spin models, where different bonds in the lattice favour different orientations for the magnetic moments. Realising such physics with rare-earth ions is a promising route to achieve exotic ground states in the extreme spin-orbit limit; however, this regime has remained experimentally largely unexplored due to major challenges in materials synthesis. Here we report the successful synthesis of powders and single crystals of β-NaPrO, with 4f Pr j = 1/2 magnetic moments arranged on a hyperhoneycomb lattice with the same threefold coordination as the planar honeycomb.

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Article Synopsis
  • The study investigates the magnetic properties of ErCuMnMnO using techniques like magnetic susceptibility, specific heat, and neutron powder diffraction, revealing four distinct magnetic phase transitions.
  • The complex structure of quadruple perovskites leads to interactions between various magnetic sublattices, which cause spin reorientation transitions due to competing magnetic forces within the manganese sublattice.
  • At low temperatures, one Er sublattice becomes magnetically ordered, while the other remains non-magnetic until a final phase transition, highlighting intricate interactions between transition metal and rare-earth sublattices, which poses challenges for understanding the entire RCuMnMnO family.
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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 O . 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 ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass.

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  • Perovskite-structure AMnO manganites have contributed to important physical concepts like double exchange and Jahn-Teller effects, leading to properties like colossal magnetoresistance and multiferroicity.
  • A-site-ordered quadruple perovskite manganites AMnO were identified but initially received little attention until recent advances in high-pressure synthesis techniques helped improve understanding.
  • Recent studies reveal that AMnO compounds exhibit complex physics that goes beyond traditional AMnO materials, indicating a promising area for further research.
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Through , high-pressure X-ray diffraction experiments we have shown that the homoleptic perovskite-like coordination polymer [(CH)NH]Cu(HCOO) undergoes a pressure-induced orbital reordering phase transition above 5.20 GPa. This transition is distinct from previously reported Jahn-Teller switching in coordination polymers, which required at least two different ligands that crystallize in a reverse spectrochemical series.

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The [ZnNi(HF)(pyz)]SbF ( = 0.2; pyz = pyrazine) solid solution exhibits a zero-field splitting () that is 22% larger [ = 16.2(2) K (11.

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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 BiMnO In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities.

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The formation of NaRMnTiO 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 AA'A″BO. Their crystal structures were studied by powder synchrotron X-ray and neutron diffraction between 1.

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Article Synopsis
  • - ABO perovskite oxides are being studied for their unique physical properties and potential uses in various industries.
  • - A unique binary perovskite manganite, MnO, can be created under extreme conditions and shows both magnetically driven ferroelectricity and a significant magnetoelectric effect at low temperatures.
  • - Neutron powder diffraction studies have identified complex antiferromagnetic structures in MnO, revealing the interplay of spin, charge, and orbital factors, highlighting the material's potential for advanced electric and magnetic applications.
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There is an emerging topic in the science of perovskite materials: A-site columnar-ordered A A'A''B O 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 CuMnMn O (1) and Ho MnGaMn O (2) by neutron and X-ray powder diffraction and prove the triple order at the A sites.

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A-site-ordered quadruple perovskites RMnO 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).

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The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity.

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Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.

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Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectly--via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) introduce strong spin-orbit coupling directly, such that these numbers become entwined together and the Mott physics attains a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spin-anisotropic magnetic interactions, coupling the spin to a given spatial direction of exchange and leading to strongly frustrated magnetism.

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Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are mechanistically distinct DNA repair pathways that contribute substantially to double-strand break (DSB) repair in mammalian cells. We have combined mutations in factors from both repair pathways, the HR protein Rad54 and the DNA-end-binding factor Ku80, which has a role in NHEJ. Rad54(-/-)Ku80(-/-) mice were severely compromised in their survival, such that fewer double mutants were born than expected, and only a small proportion of those born reached adulthood.

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