Browsing by Author "Kayser, P"

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    The impact of chemical doping on the magnetic state of the Sr2YRuO6 double perovskite
    (Elsevier, 2017-05-01) Kayser, P; Ranjbar, B; Kennedy, BJ; Avdeev, M
    The impact of chemical doping of the type Sr2−xAxYRuO6 (A=Ca, Ba) on the low temperature magnetic properties of Sr2YRuO6, probed using variable temperature magnetic susceptibility, neutron diffraction and heat capacity measurements, are described. Specific-heat measurements of un-doped Sr2YRuO6 reveal two features at ∼26 and ∼30 K. Neutron scattering measurements at these temperatures are consistent with a change from a 2D ordered state to the 3D type 1 AFM state. Magnetic and structural studies of a number of doped oxides are described that highlight the unique low temperature behavior of Sr2YRuO6 and demonstrate that doping destabilizes the intermediate 2D ordered state. © 2017 Elsevier Inc.
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    Magnetic and structural studies of Sc containing ruthenate double perovskites A2ScRuO6 (A = Ba, Sr)
    (American Chemical Society, 2017-07-10) Kayser, P; Injac, S; Ranjbar, B; Kennedy, BJ; Avdeev, M; Yamaura, K
    Ruthenium-containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba, a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure is obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show that the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal–insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction and are described by k = (0,0,1) and k = (0,0,0) for the Ba and Sr oxides, respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient-pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner-sharing BO6 sites. The magnetic structure is described by k = (1/2,0,0) with the basis vector belonging to the irreducible representation Γ3. © 2017 American Chemical Society
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    Magnetodielectric effects in A-site cation-ordered chromate spinels LiMCr4O8 (M=Ga and In)
    (American Physical Society, 2016-08-15) Saha, R; Fauth, F; Avdeev, M; Kayser, P; Kennedy, BJ; Sundaresan, A
    We report the occurrence of a magnetodielectric effect and its correlation with structure and magnetism in the A-site ordered chromate spinel oxides LiMCr4O8 (M=Ga, In). In addition to magnetic and dielectric measurements, temperature dependent synchrotron and neutron diffraction experiments have been carried out for the Ga compound. The results are compared and contrasted with that of a corresponding conventional B-site magnetic chromate spinel oxide, ZnCr2O4. Like ZnCr2O4, the A-site ordered chromate spinels exhibit a magnetodielectric effect at the magnetic ordering temperature (TN∼13–15K), resulting from magnetoelastic coupling through a spin Jahn-Teller effect. While the presence of a broad magnetic anomaly, associated with a short-range magnetic ordering (TSO∼45K) in ZnCr2O4, does not cause any dielectric anomaly, a sharp change in dielectric constant has been observed in LiInCr4O8 at the magnetic anomaly, which is associated with the opening of a spin gap (TSG∼60K). Contrary to the In compound, a broad dielectric anomaly exists at the onset of short-range antiferromagnetic ordering (TSO∼55K) in LiGaCr4O8. The differences in dielectric behavior of these compounds have been discussed in terms of breathing distortion of the Cr4 tetrahedra. ©2016 American Physical Society
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    Observation of Nd ordering in a novel double perovskite Nd2MgRuO6 with weak exchange interaction at B-site
    (Elsevier, 2018-03-01) Sharannia, MP; Kayser, P; Savitha Pillai, S; Kennedy, BJ; Avdeev, M; Nirmala, R; Santhosh, PN
    Polycrystalline Nd2MgRuO6 has monoclinic P21/n structure with an ordered arrangement of Mg2+ and Ru4+ ions. Magnetic ordering is observed at 1.5 K where the Ru sublattice exhibits type-I antiferromagnetic order with the magnetic moments parallel to the c-axis. The Nd3+ cations show a ferromagnetic arrangement at this temperature. Nd2MgRuO6 exhibits semiconductor like behaviour. Impedance analysis shows the contribution of grain alone to the conduction process till 231 K, above which both the grain and grain boundary contribute. © 2017 Elsevier Inc.
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    Spin–orbit coupling controlled ground state in the Ir(V) perovskites A2ScIrO6 (A = Ba or Sr)
    (American Chemical Society, 2017-01-30) Kayser, P; Kennedy, BJ; Ranjbar, B; Kimpton, JA; Avdeev, M
    The structural and magnetic properties of the two Ir(V) perovskites Ba2ScIrO6 and Sr2ScIrO6 were established. The structures were refined using a combined neutron and synchrotron data set. At room temperature the former has a cubic structure in space group Fm3̅m with a = 8.1450(3) Å, and the latter is monoclinic in P21/n with a = 5.6606(3) Å, b = 5.6366(3) Å, c = 7.9720(4) Å, and β = 89.977(5)°. Magnetization measurements show both oxides have magnetic moments close to zero as a consequence of strong spin–orbit coupling that results in a Jeff ≈ 0 ground state. The distortion of the IrO6 octahedra in Sr2ScIrO6 is insufficient to generate crystal field splitting strong enough to quench the spin–orbit coupling. © 2017 American Chemical Society
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    Structural and magnetic properties of the osmium double perovskites Ba2–xSrxYOsO6
    (American Chemical Society, 2017-05-17) Kayser, P; Injac, S; Kennedy, BJ; Vogt, T; Avdeev, M; Maynard-Casely, HE; Zhang, Z
    The crystal and magnetic structures of double perovskites of the type Ba2–xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin–orbit coupling is constant and insignificant across the series. © 2017 American Chemical Society
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    Structural and magnetic studies of the ruthenium perovskites Ba2-xSrxHoRuO6
    (Elsevier, 2018-07-01) Injac, S; Kayser, P; Avdeev, M; Kennedy, BJ
    The series of ruthenate double perovskites Ba2-xSrxHoRuO6 (0 ≤ x ≤ 2) have been synthesized using solid state methods. The crystal structures of the series have been determined by high resolution synchrotron X-ray diffraction and show the sequence of structures to be Fm3̅m (a0a0a0) (0 ≤ x ≤ 0.6) → I4/m (a0a0c-) (x = 0.8) → I2/m (a0b-b-) (1.0 ≤ x ≤ 1.2) → P21/n (a-a-c+) (1.4 ≤ x ≤ 2.0). A similar progression of structures is observed in the sample BaSrHoRuO6 with increasing temperature. Magnetic characterisation of these materials was undertaken utilising variable temperature bulk magnetic susceptibility, isothermal magnetisation and low temperature neutron powder diffraction measurements. All members of the series order antiferromagnetically with the Ru and Ho sublattices ordering at different temperatures between 50 and 16 K. The presence of the magnetic Ho3+ at the perovskite B site stabilises the antiferromagnetic (AFM) ordering of the Ru sublattice. The addition of Sr for Ba beyond x = 1.2 weakens the AFM superexchange interactions through induced structural distortions, resulting in a canted AFM ground state. © 2018 Elsevier Inc.
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    Structural chemistry and magnetic properties of the hexagonal double perovskite Ba2CoOsO6
    (American Chemical Society, 2020-04-14) Kayser, P; Orlandi, F; Avdeev, M; Zhang, Z; Kimpton, JA; Kennedy, BJ
    The double perovskite Ba2CoOsO6, synthesized using solid-state methods at ambient pressure, is shown as a rare example of an oxide adopting the 6L-trigonal (S.G.: P3̅m1) perovskite structure. The structure, refined using a combination of X-ray and neutron diffraction data, showed the Co and Os were ordered over the two dimer sites with additional ordering over the corner-sharing sites. Bond valence calculations show the presence of the Co(II) and Os(VI) valence states, and the latter was confirmed using X-ray absorption spectroscopy. Bulk magnetic susceptibility measurements show Ba2CoOsO6 to undergo antiferromagnetic ordering near 100 K, and neutron diffraction showed an ordered moment on the Co3, Co4, and Os2 sites; whereas the Os1/Co1 remained disordered. © 2020 American Chemical Society
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    Structure and magnetism in Sr A_xTcO 3 perovskites: importance of the A-site cation
    (American Physical Society, 2017-02-27) Reynolds, EM; Avdeev, M; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kimpton, JA; Yu, M; Kayser, P; Kennedy, BJ
    The Sr1−xBaxTcO3 (x=0, 0.1, 0.2) oxides were prepared and their solid-state and magnetic structure studied as a function of temperature by x-ray and neutron powder diffraction. The refined Tc moments at room temperature and Néel temperatures for Ba0.1Sr0.9TcO3 and Ba0.2Sr0.8TcO3 were 2.32(14)μβ and 2.11(13)μβ and 714∘C and 702∘C, respectively. In contrast to expectations, the Néel temperature in the series Sr1−xAxTcO3 decreases with increasing Ba content. This observation is consistent with previous experimental measurements for the two series AMO3 (M=Ru, Mn; A=Ca, Sr, Ba) where the maximum magnetic ordering temperature was observed for A=Sr. Taken with these previous results the current work demonstrates the critical role of the A-site cation in the broadening of the π∗ bandwidth and ultimately the magnetic ordering temperature.©2017 American Physical Society