| 1. |
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- 2019
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Gao, S., et al.
(författare)
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Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2 O4
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:13
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Tidskriftsartikel (refereegranskat)abstract
- In spinels ACr2O4(A=Mg, Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002)PODIE20885-715610.1154/1.1479738], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(12120),k2=(1012) appear. The ordered moment reaches 1.94(3) μB/Cr3+ for k1 and 2.08(3) μB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c component. We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.
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| 3. |
- Kalvius, G. M., et al.
(författare)
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Magnetism of Pd1-xNix alloys near the critical concentration for ferromagnetism
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:6, s. 064418-
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Tidskriftsartikel (refereegranskat)abstract
- We report results of a muon spin rotation and relaxation (mu SR) study of dilute Pd1-xNix alloys, with emphasis on Ni concentrations x = 0.0243 and 0.025. These are close to the critical value x(cr) for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. Additional control data were taken for pure nonmagnetic Pd, and for an alloy where ferromagnetism is well established (x = 0.05). The 2.43 and 2.5 at.% Ni alloys exhibit similar mu SR properties. Both samples are fully magnetic, with average zero-temperature muon local fields < B-loc(T = 0)> = 2.0 and 3.8 mT and Curie temperatures T-C = 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively. The temperature dependence of < B-loc > suggests ordering of Ni spin clusters rather than isolated spins. Just above T-C, the temperature where LRO vanishes, a two-phase region is found with coexisting separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above T-C. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of T-C extrapolate to x(cr) = 0.0236 +/- 0.0027. The dynamic muon spin relaxation in the vicinity of T-C differs for the two samples: a relaxation-rate maximum at T-C is observed for x = 0.0243, reminiscent of critical slowing down, whereas for x = 0.025 no dynamic relaxation is observed within the mu SR time window. The data suggest a mean-field-like transition in this alloy.
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| 6. |
- Eremina, R. M., et al.
(författare)
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Magnetization and specific heat of the dimer system CuTe(2)O(5)
- 2011
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 84:3, s. 391-395
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Tidskriftsartikel (refereegranskat)abstract
- We report on magnetization and specific heat measurements on single-crystalline CuTe(2)O(5). The experimental data are directly compared to theoretical results for two different spin structures, namely an alternating spin-chain and a two-dimensional (2D) coupled dimer model, obtained by Das et al. [Phys. Rev. B 77, 224437 (2008)]. While the analysis of the specific heat does not allow to distinguish between the two models, the magnetization data is in good agreement with the 2D coupled dimer model.
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| 7. |
- Kalvius, G. M., et al.
(författare)
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Spin-Lattice Instability in the Chromium Sulfur Spinel Fe0.5Cu0.5Cr2S4
- 2008
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:25, s. 252204-
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Tidskriftsartikel (refereegranskat)abstract
- Zero field mu SR spectroscopy was applied to a polycrystalline sample of the ferrimagnetic sulfur spinel Fe0.5Cu0.5Cr2S4 between 5 and 315 K. The temperature dependence of the interstitial magnetic field B-mu as well as the transverse and longitudinal relaxation rates were deduced. At around 100 K, the temperature dependence of the interstitial field exhibits a strong deviation from the expected Brillouin-like behavior together with a maximum of the transverse relaxation rate. These features are characteristic for a spin reorientation transition. This instability of the ferrimagnetic spin-lattice has not been reported previously. Fe-57 Mossbauer data from the same sample show no irregularity in the temperature dependence of the Fe hyperfine field which could indicate that the spin reorientation involves primarily the Cr sublattice. Above and below the spin reorientation regime, disorder in the spin-lattice is sizable, but not excessive. At low temperatures, the spins are essentially static, spin dynamics sets in above the reorientation range. The mu SR data are also complemented by new susceptibility and magnetization data taken on similar material.
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| 8. |
- Sherman, A, et al.
(författare)
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Dielectric properties of C-70-solvate crystals grown from a benzene solution
- 1998
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. ; 102:39
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- C-70 crystals with size up to 3 x 3 x 0.2 mm were grown from benzene solution. The real and imaginary parts of the complex conductivity of this crystals were studied in the frequency range 20 Hz-1 MHz and the temperature range from 10 K to the room temper
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| 9. |
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| 10. |
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| 11. |
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| 12. |
- Kalvius, G. M., et al.
(författare)
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Frustration driven magnetic states of A-site spinels probed by mu SR
- 2010
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 77:1, s. 87-100
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Tidskriftsartikel (refereegranskat)abstract
- The normal A-site spinels MnAl2O4, FeAl2O4, CoAl2O4, as well as related mixed (Mn0.5Fe0.5Al2O4) and partially inverted (Fe1.4Al1.6O4) spinels have been studied by mu SR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the mu SR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T (M) is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl2O4, Fe1.4Al1.6O4) or the formation of a spin liquid phase (FeAl2O4, CoAl2O4, Mn0.5Fe0.5Al2O4). The magnetic ground state of MnAl(2)O(4)aEuroEuroshows coexistence of antiferromagnetic and spin liquid phases. In FeAl(2)O(4)aEuroEuroand CoAl(2)O(4)aEuroEurolong-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn(0.5)Fe(0.5)Al(2)O(4)aEuroEuroprevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe(1.4)Al(1.6)O(4)aEuroEurostrengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (similar to 100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.
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| 13. |
- Kalvius, G. M., et al.
(författare)
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Low temperature incommensurately modulated and noncollinear spin structure in FeCr2S4
- 2010
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:5, s. 052205-
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Tidskriftsartikel (refereegranskat)abstract
- FeCr2S4 orders magnetically at T-N approximate to 170 K. According to neutron diffraction, the ordered state down to 4.2 K is a simple collinear ferrimagnet maintaining the cubic spinel structure. Later studies, however, claimed trigonal distortions below similar to 60 K coupled to the formation of a spin glass type ground state. To obtain further insight, muon spin rotation/relaxation (mu SR) spectroscopy was carried out between 5 and 200 K together with new Fe-57 Mossbauer measurements. Below similar to 50 K, our data point to the formation of an incommensurately modulated noncollinear spin arrangement like a helical spin structure. Above 50 K, the spectra are compatible with collinear ferrimagnetism, albeit with a substantial spin disorder on the scale of a few lattice constants. These spin lattice distortions become very large at 150 K and the magnetic state is now better characterized as consisting of rapidly fluctuating short-range ordered spins. The Neel transition is of second order, but ill defined, extending over a range of similar to 10 K. The Mossbauer data around 10 K confirm the onset of orbital freezing and are also compatible with the noncollinear order of iron. The absence of a major change in the quadrupole interaction around 50 K renders the distortion of crystal symmetry to be small.
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| 14. |
- Kalvius, G. M., et al.
(författare)
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Magnetism of frustrated A-site spinels (Mn, Fe, Co)Al2O4
- 2009
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 404:5-7, s. 660-662
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Tidskriftsartikel (refereegranskat)abstract
- In the A-site spinels MAl2O4 (M = Mn, Fe, Co) the A-site sublattice is subjected to strong frustration, putting all three compounds on the verge between long-range and short-range magnetic order. The mu SR spectra of the three compounds show two signals at all temperatures examined and a dramatic change in spectral shape at characteristic temperatures. Above this temperature, one signal (slowly relaxing) is typical for free paramagnetic spins, the other (fast relaxing) for highly correlated paramagnetic spins. For M = Mn the ratio of their intensities is 1:3 independent of temperature and the spectral change occurs around 40 K (T-N from neutron data) where the strongly correlated fraction enters into long-range order and the free paramagnetic fraction into a glassy spin state. For M = Co or Fe, the free paramagnetic fraction increases from similar to 20% to 85% with rising temperature. At the cusp temperatures observed in magnetization (5 or 12 K) the dominating, highly correlated paramagnetic fraction turns into a dynamic short-range ordered magnetic ground state. In all three compounds substantial local spin disorder and persistent spin fluctuations characterize their magnetic ground states, typical features of highly frustrated magnets.
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| 15. |
- Kalvius, G. M., et al.
(författare)
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Magnetism of the chromium thio-spinels Fe1-xCuxCr2S4 studied using muon spin rotation and relaxation
- 2013
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:18, s. 186001-
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Tidskriftsartikel (refereegranskat)abstract
- Powder samples of Fe1-xCuxCr2S4 with x = 0, 0.2, 0.5, 0.8 were studied, between 5 and 300 K. The results reveal that for x < 1, the magnetic order in the series is more varied than the simple collinear ferrimagnetic structure traditionally assumed to exist everywhere from the Curie point to T -> 0. In FeCr2S4 several ordered magnetic phases are present, with the ground state likely to have an incommensurate cone-like helical structure. Fe0.8Cu0.2Cr2S4 is the compound for which simple collinear ferrimagnetism is best developed. In Fe0.5Cu0.5Cr2S4 the ferrimagnetic spin structure is not stable, causing spin reorientation around 90 K. In Fe0.2Cu0.8Cr2S4 the ferrimagnetic structure is at low temperatures considerably distorted locally, but with rising temperature this disorder shows a rapid reduction, coupled to increased spin fluctuation rates. In summary, the present data show that the changes induced by the replacement of Fe by Cu have more profound influences on the magnetic properties of the Fe1-xCuxCr2S4 compounds than merely a shift of Curie temperature, saturation magnetization and internal field magnitude.
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| 16. |
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| 18. |
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| 19. |
- Höfner, S., et al.
(författare)
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Dynamical Atmospheres and Winds of AGB Stars
- 2003
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Ingår i: Proc. of IAU Symposium 210, Modelling of Stellar Atmospheres. - : Astronomical Society of the Pacific. ; , s. 353-365
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Konferensbidrag (refereegranskat)
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| 20. |
- Laurita, N. J., et al.
(författare)
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Singlet-Triplet Excitations and Long-Range Entanglement in the Spin-Orbital Liquid Candidate FeSc2S4
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 114:20
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical models of the spin-orbital liquid (SOL) FeSc2S4 have predicted it to be in close proximity to a quantum critical point separating a spin-orbital liquid phase from a long-range ordered magnetic phase. Here, we examine the magnetic excitations of FeSc2S4 through time-domain terahertz spectroscopy under an applied magnetic field. At low temperatures an excitation emerges that we attribute to a singlet-triplet excitation from the SOL ground state. A threefold splitting of this excitation is observed as a function of applied magnetic field. As singlet-triplet excitations are typically not allowed in pure spin systems, our results demonstrate the entangled spin and orbital character of singlet ground and triplet excited states. Using experimentally obtained parameters we compare to existing theoretical models to determine FeSc2S4's proximity to the quantum critical point. In the context of these models, we estimate the characteristic length of the singlet correlations to be xi / (a/2) approximate to 8.2 (where a/2 is the nearest neighbor lattice constant), which establishes FeSc2S4 as a SOL with long-range entanglement.
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| 21. |
- Sillrén, Per, 1982, et al.
(författare)
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Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy
- 2014
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 140:12
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Tidskriftsartikel (refereegranskat)abstract
- Liquid monohydroxy alcohols exhibit unusual dynamics related to their hydrogen bonding induced structures. The connection between structure and dynamics is studied for liquid 1-propanol using quasi-elastic neutron scattering, combining time-of-flight and neutron spin-echo techniques, with a focus on the dynamics at length scales corresponding to the main peak and the pre-peak of the structure factor. At the main peak, the structural relaxation times are probed. These correspond well to mechanical relaxation times calculated from literature data. At the pre-peak, corresponding to length scales related to H-bonded structures, the relaxation times are almost an order of magnitude longer. According to previous work [C. Gainaru, R. Meier, S. Schildmann, C. Lederle, W. Hiller, E. Rössler, and R. Böhmer, Phys. Rev. Lett.105, 258303 (2010)] this time scale difference is connected to the average size of H-bonded clusters. The relation between the relaxation times from neutron scattering and those determined from dielectric spectroscopy is discussed on the basis of broad-band permittivity data of 1-propanol. Moreover, in 1-propanol the dielectric relaxation strength as well as the near-infrared absorbance reveal anomalous behavior below ambient temperature. A corresponding feature could not be found in the polyalcohols propylene glycol and glycerol.
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| 22. |
- Warzanowski, P., et al.
(författare)
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Multiple spin-orbit excitons and the electronic structure of alpha-RuCl3
- 2020
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- The honeycomb compound alpha-RuCl3 is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled j = 1/2 moments arising for a t(2g)(5) electron configuration with strong spin-orbit coupling lambda and a large cubic crystal field. The actual low-energy electronic structure of alpha-RuCl3, however, is still puzzling. In particular, infrared absorption features at 0.30, 0.53, and 0.75 eV seem to be at odds with a j = 1/2 scenario. Also the energy of the spin-orbit exciton, the excitation from j = 1/2 to 3/2, and thus the value of lambda, are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find lambda = 0.16 eV and Delta = 42(4) meV for the observed noncubic crystal-field splitting, supporting the validity of the j = 1/2 picture for alpha-RuCl3. The unusual strength of the double excitation is related to the underlying hopping interactions, which form the basis for dominant Kitaev exchange.
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