Showing papers by "J. Paul Attfield published in 2015"
TL;DR: A ferrimagnetic double perovskite, Mn2 FeReO6, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μB which greatly exceed those for other double perOVskite ferrimagnets.
Abstract: Ferrimagnetic A2BB′O6 double perovskites, such as Sr2FeMoO6, are important spin-polarized conductors. Introducing transition metals at the A-sites offers new possibilities to increase magnetization and tune magnetoresistance. Herein we report a ferrimagnetic double perovskite, Mn2FeReO6, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μB which greatly exceed those for other double perovskite ferrimagnets. A novel switching transition is discovered at 75 K where magnetoresistance changes from conventional negative tunneling behavior to large positive values, up to 265 % at 7 T and 20 K. Neutron diffraction shows that the switch is driven by magnetic frustration from antiferromagnetic Mn2+ spin ordering which cants Fe3+ and Re5+ spins and reduces spin-polarization. Ferrimagnetic double perovskites based on A-site Mn2+ thus offer new opportunities to enhance magnetization and control magnetoresistance in spintronic materials.
66 citations
TL;DR: The thermal evolution of electronic order in the complex Verwey ground state of magnetite has been determined through 22 high-accuracy synchrotron x-ray structure refinements using three $10--40\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ grains of stoichiometric magnetite as discussed by the authors.
Abstract: The thermal evolution of electronic order in the complex Verwey ground state of magnetite $(\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4})$ has been determined through 22 high-accuracy synchrotron x-ray structure refinements using three $10--40\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ grains of stoichiometric magnetite. A robust fitting function is introduced to extract values of order parameterlike quantities at zero temperature and at the upper limit of the Verwey phase ${T}_{\mathrm{u}}=123.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The low-temperature structural distortion is found to be almost frozen below the Verwey transition but small changes in lattice and local mode amplitudes and Fe-Fe distances reveal an increase in electron localization on cooling. These distortions confirm that electron localization within trimerons is the driving force behind the Verwey transition. Electron localization is also revealed by anomalous decreases in the largest principal thermal displacement factors of Fe cations as electron-phonon decoupling occurs on cooling.
39 citations
TL;DR: The crystal and magnetic structures of charge-disproportionated Ca2FeMnO6 were analyzed by neutron powder diffraction and the unique magnetic structure is described as a result of two-dimensional localization of the ligand holes with effective spins.
Abstract: The crystal and magnetic structures of charge-disproportionated Ca2FeMnO6 were analyzed by neutron powder diffraction. Ca2FeMnO6 is a layered double perovskite oxide with a two-dimensional arrangement of Mn4+ and unusual high valence Fe4+ at room temperature. When cooled, the compound shows charge disproportionation followed by magnetic transition. Around 200 K, the Fe4+ shows the charge disproportionation to Fe3+ and Fe5+, which are ordered in a checkerboard pattern in the two-dimensional FeO6 octahedral layers. The magnetic transition occurs at 95 K, which is much lower than the charge disproportionation temperature. The magnetic structure is commensurate but noncollinear, and the antiferromagnetic coupling of Fe3+ and Fe5+ spins in the FeO6 octahedral layers gives the ferrimagnetic moments. The unique magnetic structure is described as a result of two-dimensional localization of the ligand holes with effective spins.
35 citations
TL;DR: In this article, a random-bond spinglass configuration was obtained at high pressure using high pressure Neutron and synchrotron diffraction data with a = 5.5947(1) A, b = 7.7613(1), and c = 4.3882(1).
Abstract: The BiCr(0.5)Ni(0.5)O(3) perovskite has been obtained at high pressure. Neutron and synchrotron diffraction data show a Pnma orthorhombic structure with a = 5.5947(1) A, b = 7.7613(1) A, and c = 5.3882(1) A at 300 K and random B-site Cr/Ni distribution. Electron diffraction reveals an incommensurate modulation parallel to the b axis. The combination of either Cr-O-Ni (J > 0) or Cr-O-Cr/Ni-O-Ni (J < 0) nearest-neighbor spin interactions results in a random-bond spin-glass configuration. Magnetization, neutron diffraction, and muon-spin-relaxation measurements demonstrate that variations in the local bonding and charge states contribute to the magnetic frustration.
10 citations
TL;DR: In this paper, powder neutron diffraction and magnetic susceptibility measurements at 10-300 K were used to determine the crystal and magnetic structures of brownmillerite type Ca2Cr2O5, which was obtained by reduction of the high pressure phase CaCrO3 through hard soft chemistry.
Abstract: Powder neutron diffraction and magnetic susceptibility measurements at 10–300 K have been used to determine the crystal and magnetic structures of brownmillerite type Ca2Cr2O5, which was obtained by reduction of the high pressure phase CaCrO3 through hard–soft chemistry The ambient temperature crystal structure of Ca2Cr2O5 is refined in space group I2mb and the unusual tetrahedral coordination of Cr3+ results in local structural distortions Cr3+ spins order antiferromagnetically below 220 K and a substantial observed canting of moments shows that Heisenberg exchange is weak or frustrated and competes with antisymmetric Dzialoshinskii–Moriya interactions
10 citations
TL;DR: The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.
Abstract: The 2D-Ising ferromagnetic phase BaFe(2+)2(PO4)2 shows exsolution of up to one-third of its iron content (giving BaFe(3+)1.33(PO4)2) under mild oxidation conditions, leading to nanosized Fe2O3 exsolved clusters. Here we have prepared BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+); y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 °C thermal treatment in air, a complete oxidation of Fe(2+) to Fe(3+) leaves stable M(2+) ions, as verified by (57)Fe Mossbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric α-Fe2O3 clusters coating the main phase strongly depends on the yM metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic α-Fe2O3 particles (100% for BaFe(0.5-x)Co(1.5)(PO4)2) was detected even at 78 K. Although Ni(2+) and Co(2+) ions tend to block Fe diffusion, the crystal structure of BaFe(0.67)Co1(PO4)2 demonstrates a fully ordered rearrangement of Fe(3+) and Co(2+) ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.
9 citations
TL;DR: In this paper, polycrystalline samples of Ca2FeMnO6 were obtained by oxidation of brownmillerite with O3 (200 °C, 6 h).
Abstract: Polycrystalline samples of Ca2FeMnO6 are prepared by oxidation of the brownmillerite Ca2FeMnO5 with O3 (200 °C, 6 h).
5 citations
TL;DR: In this paper, an oxygen-deficient material with limiting composition 5H-BaCrO 2.8 was synthesised at 1200°C under ambient pressure, which contains double tetrahedral Cr 4+ layers and orders antiferromagnetically below 260k with a (0.0.1/2) magnetic structure.
5 citations
TL;DR: Mn2FeReO6 is prepared by hot pressing of a stoichiometric mixture of Mn3O4, Fe3O 4, and ReO2 (Pt capsule in multianvil press, 11 GPa, 1400 °C, 20 min) as mentioned in this paper.
Abstract: Mn2FeReO6 is prepared by hot pressing of a stoichiometric mixture of Mn3O4, Fe3O4, and ReO2 (Pt capsule in multianvil press, 11 GPa, 1400 °C, 20 min).
2 citations
TL;DR: The 6-layered hexagonal polymorph of 6H-BaCrO3 was prepared by hot isostatic pressing of Ba3Cr2O8 and Cr2O3 in the molar ratio of 2:1.
Abstract: The 6-layered hexagonal polymorph of 6H-BaCrO3 is prepared by hot isostatic pressing of Ba3Cr2O8 (from BaCO3 and Cr2O3, 1000 °C, 12 h) and Cr2O3 in the molar ratio of 2:1 (multianvil press, 900 °C, 9 GPa, 30 min).