| 1. |
- Azad, A.K., et al.
(författare)
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Spin-glass transition in a La-doped Sr2MnWO6 double perovskite
- 2008
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Ingår i: PHYSICAL REVIEW B. ; 77:6, s. 064418-
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Tidskriftsartikel (refereegranskat)abstract
- Polycrystalline Sr2-xLaxMnWO6 (x=0.25 and 0.5) materials have been prepared by a traditional solid state sintering method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in monoclinic symmetry (space group P2(1)/n). The unit cell volume decreases with increasing La3+ concentration at the A site. The crystal structure contains alternating MnO6 and WO6 octahedra, considerably tilted due to the relative small size of the cations that occupy the A sublattice of the perovskite. ac and dc magnetization measurements show a destruction of the low temperature antiferromagnetic phase by electron doping through substitution of Sr2+ for La3+. The electron doping creates a mixed tungsten valence (W6+/W5+) resulting in a low temperature spin-glass state.
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| 2. |
- Dahbi, Mohammed, et al.
(författare)
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Electrochemical behavior of LiNi1-y-zCoyMnzO2 probed through structural and magnetic properties
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:2, s. 023904-
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated LixNi1-y-zCoyMnzO2 compounds with y = 1/3, 0.25, 0.2, 0.1 and z = 1/3, 0.2, 0.1, 0.05 in order to study the influence of Ni and Mn concentration, cationic disorder, and crystallite size on the magnetic and charge/discharge behavior. The samples have been studied by means of x-ray diffraction, scanning electron microscopy, voltammetry, cycling capacity, and magnetometry. The discharge capacity increases with increasing Ni concentration as does the number of ferromagnetic interactions. With higher Mn concentration a higher capacity is observed together with formation of strong antiferromagnetic interactions driving the magnetic frustration to lower temperatures. Our results show that for sufficiently low Co concentrations a stable and magnetically more ordered structure can be obtained with excellent electrochemical properties, although a relatively large amount of Ni is present.
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| 3. |
- Iusan, Diana, et al.
(författare)
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Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al : Experiments and ab initio density-functional calculations
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:8, s. 085319-
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Tidskriftsartikel (refereegranskat)abstract
- We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thin-film samples fabricated by solution-based methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valence-band edge with pinning of the Fermi level primarily due to native defects yielding a heavily n-doped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the local-density approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.
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| 4. |
- Karimipour, Masoud, et al.
(författare)
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Effect of Annealing Temperature on the Structural and Magnetic Properties of Co-Doped TiO2 Nanoparticles via Complex-Polymer Sol-Gel Method
- 2012
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 12:2, s. 950-954
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Tidskriftsartikel (refereegranskat)abstract
- Co-doped TiO2 nanoparticles were synthesized via non hydrous complex-polymer sol gel method. A series of Co-x:Ti1-xO2 samples with x = 0.01, 0.03, 0.05, 0.08 and 0.10, were prepared and subsequently annealed at 400 degrees, 600 degrees and 800 degrees C. Structural and magnetic properties of Co-x:Ti1-xO2 have been studied by means of X-ray diffraction and DC magnetometry. All samples annealed at 400 degrees C show a paramagnetic behavior with an average grain size of 11 nm. With increasing annealing temperatures a complete crystallization is seen with growth of the cluster size up to 31 nm with clear evidence of a presence of CoTiO3. For all concentrations and annealing conditions no sign of a metallic phase, even at x = 0.10, is seen.
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| 5. |
- Karimipour, Masoud, et al.
(författare)
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Nanoparticles of Ni/NiO embedded in TiO(2) synthesized by the complex-polymer sol-gel method
- 2011
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 84:3, s. 035702-
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Tidskriftsartikel (refereegranskat)abstract
- Ni-doped TiO(2) nanoparticles were synthesized by the non-hydrous complex-polymer sol-gel method. Titanium isopropoxide was used as precursor and acetyl acetone (AcAc) and citric acid were used as polymer and complex agent, respectively. A series of Ni(x) : Ti(1-x)O(2) samples with x = 0.01, 0.03, 0.05 and 0.10 were prepared and subsequently annealed at 400, 600 and 800 degrees C. The structural and magnetic properties of Ni(x) : Ti(1-x)O(2) were studied by means of x-ray diffraction (XRD) and dc magnetometery. Samples annealed at 400 degrees C attained a particle size of 12 nm with structural phases of anatase, rutile and cubic NiO. From the blocking temperatures of the zero-field-cooled and field-cooled measurements, similar particle sizes (as from XRD) were obtained. The presence of ferro- and antiferromagnetic (AFM) interactions further confirms that the particles have different structural compositions. At 800 degrees C the particles have grown to about 30 nm with either a rutile or NiTiO(3)-type structure. The presence of NiTiO(3) is also seen in the magnetic measurements, which show the characteristic AFM peak at 23 K.
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| 6. |
- Knut, Ronny, et al.
(författare)
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Magnetic and electronic characterization of highly Co-doped ZnO : An annealing study at the solubility limit
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:9, s. 094438-
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Tidskriftsartikel (refereegranskat)abstract
- We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co3+, indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.
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| 7. |
- Wikberg, J Magnus, et al.
(författare)
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Annealing effects on structural and magnetic properties of Co implanted ZnO single crystals
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:8, s. 083918-
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Tidskriftsartikel (refereegranskat)abstract
- Single crystals of ZnO were implanted with 100 keV-Co ions at room temperature with a fluence of 4.8 x 10(16) cm(-2) and subsequently annealed at different temperatures up to 800 degrees C. The samples were analyzed by Rutherford backscattering spectrometry, secondary ion mass spectrometry, X-ray diffraction, X-ray photoemission spectroscopy and magnetometry. The as-implanted Co:ZnO crystal shows a homogeneous distribution of Co in the near surface region of the crystal. Upon annealing, clear evidence of secondary phases is found. At the highest annealing temperature (800 degrees C) a ferromagnetic behavior is observed at room temperature with a coercive field of 120 Oe assigned mainly to metallic fcc Co nano-crystallites. We find that for the annealed samples, the temperature dependent magnetization cannot be explained within a model containing only a paramagnetic contribution due to well dispersed Co ions and a ferromagnetic contribution due to Co nano-crystallites, at least one more ferromagnetic contribution is needed for a consistent explanation of the experimental results.
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| 8. |
- Wikberg, J. Magnus, et al.
(författare)
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Dimensionality crossover and frustrated spin dynamics on a triangular lattice
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:22, s. 224411-
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of the magnetic behavior of the layered oxide, LiNi0.65Co0.25Mn0.10O2, through ac and time-dependent susceptibility, dc linear and nonlinear susceptibility as well as neutron-diffraction measurements are presented. A ferrimagneticlike spin ordering appears at 119 K with a spontaneous magnetization coexisting with spin frustration in two dimensions (2D). At lower temperature, a cluster-glass transition is found at 17.4 K indicating a transformation to a completely frustrated state in three dimensions (3D). A dimensionality crossover with temperature, from 2D to 3D, in a magnetically frustrated system has been demonstrated. The observed magnetic behavior is believed to originate from a percolating system of spin clusters defined by disordered and frustrated exchange interactions and the findings conform well with predictions of the percolation cluster model.
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| 9. |
- Wikberg, J. Magnus, et al.
(författare)
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Magnetic order, aging, and spin frustration in a percolating spin system, LiNi0.8Co0.1Mn0.1O2
- 2010
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108:8, s. 083909-
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Tidskriftsartikel (refereegranskat)abstract
- Structural and magnetic properties of the Li layered oxide, LiNi0.8Co0.1Mn0.1O2, have been studied by means of x-ray diffraction, dc magnetization, ac linear, and nonlinear susceptibility as well as by magnetic aging and temperature cycling experiments. A percolating spin system interacting via antiferromagnetic and ferromagnetic superexchange interactions of different strength induce a ferrimagnetic quasilong-range order with a transition temperature Tc ≈ 70 K, a Weiss temperature of −25 K and an effective magnetic moment of 2.07 μB. On cooling below Tc the two-dimensional (2D) triangular lattice built up of edge-sharing metal oxide octahedras first exhibits a 2D spin glass like behavior followed by complete spin frustration in three dimensions below 30 K. The findings correlate well with expectations for a ferrimagnetic reentrant cluster glass system as well as with predictions of the percolating cluster model.
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| 10. |
- Wikberg, J. Magnus, et al.
(författare)
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Magnetic order and frustrated dynamics in Li(Ni0.8Co0.1Mn0.1)O-2 : a study by mu+SR and SQUID magnetometry
- 2012
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Ingår i: 12TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE (MUSR2011). - : Elsevier BV. ; , s. 202-205
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Konferensbidrag (refereegranskat)abstract
- Recently, the mixed transition metal oxides of the form Li(Ni1-y-zCoyMnz)O-2, have become the center of attention as promising candidates for novel battery material. These materials have also revealed very interesting magnetic properties due to the alternate stacking of planes of metal oxides on a 2D triangular lattice and the Li-layers. The title compound, Li(Ni0.8Co0.1Mn0.1)O-2, has been investigated by both magnetometry and measurements and mu+SR. We find the evolution of localized magnetic moments with decreasing temperature below 70 K. The magnetic ground state (T = 2 K) is, however, shown to be a frustrated system in 3D, followed by a transition into a possible 2D spin-glass above 22 K. With further increasing temperature the compound show the presence of remaining correlations with increasing effective dimensionality all the way up to the ferrimagnetic transition at T-C = 70 K.
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