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- 2019
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Tidskriftsartikel (refereegranskat)
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- 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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| 3. |
- 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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| 4. |
- 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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| 10. |
- Herrera-Marschitz, M, et al.
(författare)
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On the origin of extracellular glutamate levels monitored in the basal ganglia of the rat by in vivo microdialysis
- 1996
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Ingår i: Journal of Neurochemistry. - : Wiley. - 0022-3042 .- 1471-4159. ; 66:4, s. 1726-1735
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Tidskriftsartikel (refereegranskat)abstract
- Several putative neurotransmitters and metabolites were monitored simultaneously in the extracellular space of neostriatum, substantia nigra, and cortex and in subcutaneous tissue of the rat by in vivo microdialysis. Glutamate (Glu) and aspartate (Asp) were at submicromolar and gamma-aminobutyric acid (GABA) was at nanomolar concentrations in all brain regions. The highest concentration of dopamine (DA) was in the neostriatum. Dynorphin B (Dyn B) was in the picomolar range in all brain regions. Although no GABA, DA, or Dyn B could be detected in subcutaneous tissue, Glu and Asp levels were 5 and approximately 5 and approximately 0.4 microM, respectively. Lactate and pyruvate concentrations were approximately 200 and approximately 10 microM in all regions. The following criteria were applied to ascertain the neuronal origin of substances quantified by microdialysis: sensitivity to (a) K+ depolarization, (b) Na+ channel blockade, (c) removal of extracellular Ca2+, and (d) depletion of presynaptic vesicles by local administration of alpha-latrotoxin. DA, Dyn B, and GABA largely satisfied all these criteria. In contrast, Glu and Asp levels were not greatly affected by K+ depolarization and were increased by perfusing with tetrodotoxin or with Ca2+-free medium, arguing against a neuronal origin. However, Glu and Asp, as well as DA and GABA, levels were decreased under both basal and K+-depolarizing conditions by alpha-latrotoxin. Because the effect of K+ depolarization on Glu and Asp could be masked by reuptake into nerve terminals and glial cells, the reuptake blocker dihydrokainic acid (DHKA) or L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was included in the microdialysis perfusion medium. The effect of K+ depolarization on Glu and Asp levels was increased by DHKA, but GABA levels were also affected. In contrast, PDC increased only Glu levels. It is concluded that there is pool of releasable Glu and Asp in the rat brain. However, extracellular levels of amino acids monitored by in vivo microdialysis reflect the balance between neuronal release and reuptake into surrounding nerve terminals and glial elements.
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| 12. |
- 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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| 13. |
- 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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