Browsing by Author "Ren, W"

Now showing 1 - 9 of 9
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    Cooling field tuned magnetic phase transition and exchange bias-like effect in Y0.9Pr0.1CrO3
    (AIP Publishing, 2015-09-11) Deng, DM; Zheng, JS; Yu, DH; Wang, BM; Sun, DH; Avdeev, M; Feng, Z; Jing, C; Lu, B; Ren, W; Cao, SX; Zhang, JC
    Cooling magnetic field dependence of magnetic phase transition has been observed in Y0.9Pr0.1CrO3. GzFx order (spin structure of PrCrO3) is dominant after zero field cooling (ZFC), whereas GxFz order (spin structure of YCrO3) is dominant after cooling under a field higher than 100 Oe. Positive/negative exchange bias-like effect, with large vertical shift and small horizontal shift, has been observed after FC/ZFC process. The vertical shift can be attributed to the frozen ordered Pr3+ and Cr3+ spins in magnetic domains, because of the strong coupling between Pr3+ and Cr3+ sublattices; while the horizontal shift is a result of the pinning of spins at the interfaces. The frozen structure is generated by the field used for the measurement of the initial magnetization curve of M(H) for the ZFC cooled sample, while it is generated by the cooling field for the sample cooled under a cooling field higher than 100 Oe. © 2015 AIP Publishing LLC.
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    Giant shifts of crystal-field excitations in ErFeO3 driven by internal magnetic fields
    (Cornell University, 2021-09-16) O'Brien, J; Deng, GC; Ma, XX; Feng, ZJ; Ren, W; Cao, SX; Yu, DH; McIntyre, GJ; Ulrich, C
    Crystal-field excitations in transition-metal oxides where -rare-earth elements locate in the space between the transition-metal-oxide tetrahedra and octahedra, are assumed to be robust with respect to external perturbations such as temperature. Using inelastic neutron-scattering experiments, a giant shift of the energy of the lowest crystal-field excitation of Er3+ (4I15/2) in ErFeO3 from 0.35 meV to 0.75 meV was observed on cooling from 10K to 1.5K through the magnetic ordering temperature of Er3+ at 4.1 K. A crystal-field model was proposed to explain the observed crystal field excitations in this work. The model indicates the lowest-energy crystal-field excitation in ErFeO3 is the first Kramers doublet above the ground state. Its energy substantially shifts by the internal field induced by the ordered Er3+ magnetic moments. Further magnetic-field-dependent measurements provide strong supportive evidence for this scenario. By fitting the external magnetic-field dependency of the crystal-field excitation energy, the internal field generated by Er3+ magnetic moments was derived to be ~0.33meV. The result indicates that the internal field of Er3+ magnetic moments contribute to the energy shift of the crystal-field excitations. The giant energy shift under fields could be attributed to the anisotropy of the large effective g-factor. CC BY: Creative Commons Attribution
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    Large easy-plane anisotropy induced spin reorientation in magnetoelectric materials (Co4−xMnx)Nb2O9
    (IOP Science, 2019-03-29) Deng, GC; Yu, YS; Cao, YM; Feng, ZJ; Ren, W; Cao, SX; Studer, AJ; Hester, JR; Kareri, Y; Ulrich, C; McIntyre, GJ
    Neutron powder diffraction experiments were carried out on the magnetoelectric compound series (Co4−xMnx)Nb2O9 (x  =  0, 1, 2, 3, 3.5, 3.9, 3.95 and 4) from base temperature to above their Neel temperatures. Their magnetic structures were analysed by using the irreducible representation analysis and Rietveld refinement method. Similar to Co4Nb2O9, the compounds with x  ⩽  3.9 have noncollinear in-plane magnetic structures (Γ6) with magnetic moments lying purely in the ab plane with certain canting angles. Mn4Nb2O9 has a collinear antiferromagnetic structure (Γ2) with magnetic moments aligning along the c axis. The compound of x  =  3.95 shows two magnetic phases in the magnetization, which was confirmed to have the Γ2 magnetic structure above 60 K and develop a second Γ6 local phase in addition to the main Γ2 phase due to doping. This study indicates 2.5 at% Co2+ doping is sufficient to alter the collinear easy-axis magnetic structure of Mn4Nb2O9 into the noncollinear easy-plane magnetic structure, which is attributed to the large easy-plane anisotropy of Co2+ and relative small Ising-like anisotropy of Mn2+. The doping effects on the Néel temperature and occupancy are also discussed. © 2019 IOP Publishing Ltd
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    Magnetic structure and spin dynamics of multiferroic system Co4Nb2O9
    (Australian Institute of Physics, 2017-02-01) Deng, GC; Cao, YM; Ren, W; Cao, SX; Gauthier, N; Kenzelmann, M; Studer, AJ; Rule, KC; Gardner, JS; Davison, G; Imperia, P; McIntyre, GJ
    Co4Nb2O9, was recently reported to have large magneto-dielectric coupling effect under a certain magnetic field. This compound has a corundum-type crystal structure of space group P-3c1 of ref. and undergoes antiferromagnetic phase transition around 27 K. It was previously believed that the magnetic moments of Co2+ order into a collinear antiferromagnetic structure in which Co2+ spins order parallel to the c-direction and form ferromagnetic chains with antiparallel inter-chain coupling. However, the recent study has shown that this magnetic structure model is incorrect. In this study, we found that the Co2+ magnetic moments align in the ab plane with a non-collinear configuration. Using inelastic neutron scattering, we measured the spin wave excitation from its magnetic phase along (h00) and (00l). A magnetic model was proposed to explain the observed spin dynamical behavior. There are two inequivalent Co sites, which form spin chains in an alternative way along c axis. Each Co2+ moment couples with its two inequivalent neighbors on the same chain with ferromagnetic interactions. Co2+ moments from each site form a zig-zag hexagonal ring perpendicular to the c axis, where antiferromagnetic interactions dominate. On the basis of this model, the observed spin wave spectra can be well simulated by SpinW.
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    The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO3
    (AIP Publishing, 2015-04-29) Deng, GC; Guo, P; Ren, W; Cao, SX; Maynard-Casely, HE; Avdeev, M; McIntyre, GJ
    Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO3, have been reinvestigated in detail. The spin-reorientation transition of the Fe3+ magnetic phase and the low-temperature magnetic ordering of Er3+ were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe3+ align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Néel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe3+ moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er3+ moments align antiferromagnetically with a Cy mode below 4.5 K. For the Fe3+ moments, an additional Cx mode is induced by the ordering of the Er3+ moments. Namely, they change from GyFz mode into CxGyFz mode in the Pnma space-group setting. This study resolves the long-lasting dispute about the magnetic structure of ErFeO3 at low temperature. © 2015 AIP Publishing LLC.
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    Origin of magnetoelectric coupling effect and spin dynamics of multiferroic system Co4Nb2O9
    (International Conference on Neutron Scattering, 2017-07-12) Deng, GC; Cao, YM; Ren, W; Cao, SX; Studer, AJ; Gauthier, N; Kenzelmann, M; Davison, G; Rule, KC; Gardner, JS; Imperia, P; Ulrich, C; McIntyre, GJ
    Co4Nb2O9,was recently reported to have large magnetoelectric coupling effect under a certain magnetic field. This compound has a crystal structure (space group P-3c1) derived from corundum structure and undergoes antiferromagnetic phase transition around 27K. It was previously believed that the magnetic moments of Co2+ order into a collinear antiferromagnetic structure in which magnetic moments are parallel to the c axis and form ferromagnetic chains with antiparallel inter-chain coupling. However, the recent study has shown that this magnetic structure model is incorrect. In this study, we found that the Co2+magnetic moments on both Co1 and Co2 sites align in the ab plane with a non-collinear configuration. Using inelastic neutron scattering, we measured the spin wave excitation from its magnetic phase along (h00) and (00l). A spin dynamic model proposed in this study is able to explain the observed spin dynamical behavior quite well. The nearest and next nearest neighbor interactions (NN and NNN) along the c axis are ferromagnetic. The interaction on the zig-zag ring of Co1 perpendicular to the c axis is highly frustrated while that of the zig-zag ring of Co2 is antiferromagnetic. The single ion anisotropy and Dzyaloshinskii-Moriya (DM) interaction contribute to the spin dynamics of Co4Nb2O9 as well. The simulated spin wave excitation by using SpinW[5] matches the experimental data very well. The DM interaction, which is most probably due to the triangle Co2-O-Co2 bond, was found to be the origin of the magnetoelectric coupling in this compound.
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    Spin dynamics and magnetoelectric coupling mechanism of Co4Nb2O9
    (American Physical Society, 2018-02-28) Deng, GC; Cao, YM; Ren, W; Cao, SX; Studer, AJ; Gauthier, N; Kenzelmann, M; Davidson, G; Rule, KC; Gardner, JS; Imperia, P; Ulrich, C; McIntyre, GJ
    Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a noncollinear in-plane magnetic structure with Co2+ moments lying in the ab plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest- and next-nearest-neighbor exchange interactions, in-plane anisotropy, and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of Co4Nb2O9 is attributed to the large in-plane anisotropy, while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure. ©2018 American Physical Society
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    Spin dynamics of edge-sharing spin chains in SrCa13Cu24O41
    (American Physical Society, 2018-11-12) Deng, GC; Yu, DH; Mole, RA; Pomjakushina, E; Conder, K; Kenzelmann, M; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, HQ; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; McIntyre, GJ
    The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet SrCa13Cu24O41 was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbor interaction Jc as 5.4 meV and the interchain interaction Ja=4.4meV. Jc is antiferromagnetic and its value is close to the nearest-neighbor interactions of the similar edge-sharing spin-chain systems such as CuGeO3. Comparing with the hole-doped spin chains in Sr14Cu24O41, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in SrCa13Cu24 O41 show a gapless excitation in this paper. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping. ©2018 American Physical Society
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    Spin dynamics of quasi-one-dimensional spin-ladder system SrCa13Cu24O41 in the long-range magnetic ordering state
    (Australian Institute of Physics, 2018-01-31) Deng, GC; Yu, DH; Mole, RA; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, H; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ
    Sr14-xCaxCu24O41 is a quasi-one-dimensional magnet, which consists of two sublattices: spin ladder and spin chain, forming an incommensurate crystal structure along the c axis, namely, the ladder leg or chain direction. The highly Ca-doped compounds undergo a superconducting phase transition under hydrostatic pressure ~ 3GPa, which is really intriguing since the hole-doped even-leg spin-ladder system was theoretically predicted as superconductors by charge-pairing mechanism through antiferromagnetic interaction on the rungs of spin ladders. In the previous study, we discovered that all compounds with different Ca content have a singlet ground state with a spin-gap ~ 32meV. In the highly Ca-doped sample SrCa13Cu24O41, a long-range magnetic ordering takes place at ~ 4.2K. Interestingly, the singlet spin-liquid state and the long-range magnetic ordering coexist in this compound. In this study, we further investigated its spin dynamics in the ordered phase by using inelastic neutron scattering on PELICAN and SIKA at OPAL. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis, indicating the nature of a 2D magnet. A dynamic model has been proposed to fit the experimental data, indicating three major exchange interactions along rungs (JR), legs (JL) and between neighbor ladders (JInter). This study helps us to understand the origin of the spin liquid ground state in this low-dimensional magnet, in which hole-doping should be attributed to induce the long-range magnetic ordering due to the disorder-induced order effect.