| 1. |
- Jayakumar, O. D., et al.
(författare)
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1D Morphology Stabilization and Enhanced Magnetic Properties of Co : ZnO Nanostructures on Codoping with Li: A Template-Free Synthesis
- 2009
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Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 9:10, s. 4450-4455
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Tidskriftsartikel (refereegranskat)abstract
- ID natiostructures of Zn1-xCoxO (x = 0, 0.03 and 0.05) and Co and Li codoped ZnO (Zn0.85Li0.10Co0.05O) were prepared by a soft chemical method. We report a very interesting observation of morphological control and transformation of ZnO nanorods to spherical particles induced by Co substitution. It is also remarkable to note that the morphology completely reverts back to rod shape by Li incorporation. In addition to this unusual observation, the Li incorporation enhances the room-temperature ferromagnetic (RTFM) properties. These experimental observations are well-supported by theory work as well. These results are significant, as the I D RTFM will have implications in spintronic devices.
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| 2. |
- Jayakumar, O. D., et al.
(författare)
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Enhancement of ferromagnetic properties in Zn0.95Co0.05O nanoparticles by indium codoping : An experimental and theoretical study
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:23, s. 232510-
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Tidskriftsartikel (refereegranskat)abstract
- Nanoparticles of Zn0.95-xCo0.05InxO (x = 0.0 to 0.07) were synthesized by the pyrolitic reaction of sol-gels obtained from respective metal precursors. X-ray diffraction and high-resolution transmission electron microscopy studies confirm the formation of impurity-free wurtzite type ZnO structure for all the compositions. While pristine ZnO is diamagnetic, Zn0.95Co0.05O nanoparticles show weak paramagnetic behavior at room temperature. When "In." is codoped with Co with x = 0.0 to 0.07 in Zn0.95-xCo0.05InxO, a systematic increase in magnetic moment is observed up to x = 0.07. First-principles modeling supports that the ferromagnetic phase become more favorable at higher indium doping concentrations.
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| 3. |
- Jayakumar, O. D., et al.
(författare)
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Experimental and theoretical investigations of dopant, defect, and morphology control on the magnetic and optical properties of transition metal doped ZnO nanoparticles
- 2014
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Ingår i: Springer Series in Materials Science. - New Delhi : Springer Nature. - 0933-033X. ; 180, s. 341-370
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Tidskriftsartikel (refereegranskat)abstract
- The control of size, shape, and physical properties by surface modifications are of immense interest in materials which are of technological importance. The ZnO-based wide bandgap semiconductor nanoparticles have gained significant interest in the research community due to its large exciton binding energy (60 meV). Further substantial renewed interest in ZnO-based compounds is due to the possible realization of p-type conduction and ferromagnetic behavior when doped with transition metals. In this report we present interesting results on the ZnO nanoparticle system in which the control of dopants, morphology, and the surface modification can influence significantly the physical properties of the ZnO nanoparticles. First, we present the methods to control the morphology of the ZnO particle to obtain nanorods. As an example we show the effect of Li dopant on the morphology control of Co and Ni doped ZnO. The effect of morphology on the magnetic properties of these compounds is discussed further. We also demonstrate the effect of the n-type charge carriers on the magnetic and optical properties by doping aliovalent cations in Zn(Co)O. Following this we comment on the magnetic property manipulations by surfactant treatment of transition metal (TM) doped ZnO and defect stabilization in ZnO by Mg doping. The magnetic coupling is RKKY-like both with and without Li co-doping. Finally, we provide the significant implications of these results on the nanorods structures of room temperature ferromagnetic materials by first-principles modeling. These theoretical analyses demonstrate that Li co-doping has primarily two effects in bulk Zn1-xMxO (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. Second, Li-on- Zn prefer to be closer to the M atoms to compensate the M-O bonds and to locally depopulate the 3d states, and this will help in forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped Zn1-xMxO nanorods can therefore be explained by the better rod morphology in combination with locally ionizing the magnetic M atoms.
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| 4. |
- Jayakumar, O. D., et al.
(författare)
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Experimental and theoretical investigations on magnetic behavior of (Al,Co) co-doped ZnO nanoparticles
- 2010
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 2:8, s. 1505-1511
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Tidskriftsartikel (refereegranskat)abstract
- We present the structural and magnetic properties of Zn0.95-xCo0.05AlxO (x = 0.0 to 0.1) nanoparticles, synthesized by a novel sol-gel route followed by pyrolysis. Powder X-ray diffraction data confirms the formation of a single phase wurtzite type ZnO structure for all the compositions. The Zn0.95Co0.05O nanoparticles show diamagnetic behavior at room temperature. However, when Al is co-doped with Co with x = 0.0 to 0.10 in Zn0.95-xCo0.05AlxO, a systematic increase in ferromagnetic moment is observed up to x = 0.07 at 300 K. Above x = 0.07 (e.g. for x = 0.10) a drastic decrease in ferromagnetic nature is observed which is concomitant with the segregation of poorly crystalline Al rich ZnO phase as evidenced from TEM studies. Theoretical studies using density functional calculations on Zn0.95-xCo0.05AlxO suggest that the partial occupancy of S2 states leads to an increased double exchange interaction favoring the ferromagnetic ground states. Such ferromagnetic interactions are favorable beyond a threshold limit. At a high level doping of Al, the exchange splitting is reduced, which suppresses the ferromagnetic ordering.
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| 5. |
- Jayakumar, O. D., et al.
(författare)
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Tunable Ferromagnetism accompanied by Morphology Control in Li-doped Zn0.97Ni0.03O
- 2010
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:41, s. 17428-17433
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Tidskriftsartikel (refereegranskat)abstract
- We report morphological and ferromagnetic property control in ZnO nanorod structures by an optimum doping of Ni and Li. Nanostructures of Zn0.97-xNi0.03LixO (x = 0, 0.03, 0.05, 0.08, and 0.10) are prepared by a solvothermal method. High aspect ratio (5-15) ZnO nanorods transform to particles (with 1-3 aspect ratio) influenced by 3 at. % Ni substitution in ZnO (Zn0.97Ni0.03O). It is remarkable to note that the Zn1.97Ni0.03O particles completely retain the nanorod shape with significantly increased aspect ratio (15-30) when 3 at.c/a Li ions are codoped in (Zn0.99Li0.03Ni0.30O). Li substitution tits enhances ferromagnetism with largest magnetization (0.8 emu.g(-1)) observed for Zn0.94Li0.03Ni0.03O. For Li concentration >3 at.%, the aspect ratio as well as the magnetization decreased considerably. These experimental observations are explained by first-principles modeling. At low Li-on-Zn acceptor concentrations, the total magnetization is increased by lower Ni d-state populations, whereas at higher Li concentrations the population of ZnO host states decreases the ferromagnetism by induced magnetic moments on the oxygens. We discuss the significant implications of these results on the nanorods structures of room temperature ferromagnetic materials, which are expected to play pivotal role in developing spintronic devices.
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| 6. |
- Persson, Clas, et al.
(författare)
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Morphology and Magnetic Coupling in ZnO:Co and ZnO:Ni Co-Doped with Li
- 2011
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Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 119:2, s. 95-98
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Tidskriftsartikel (refereegranskat)abstract
- Zn0.95Co0.05O and Zn0.97Ni0.03O nanorods, prepared by a solvothermal method, show intriguing morphology and magnetic properties when co-doped with Li. At low and moderate Li incorporation (below 10 and 3 at.% Li in the Co- and Ni-doped samples, respectively) the rod aspect ratio is increased and room temperature ferromagnetic properties are enhanced, whereas the ferromagnetic coupling in Zn0.97Ni0.03O is decreased for Li concentrations > 3 at.%. First-principles theoretical analyses demonstrate that Li co-doping has primarily two effect 3 in bulk Zn1-xMxO (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. The magnetic coupling is Ruderman-Kittel-Kasuya-Yosida-like both without and with Li co-doping. Second, Li-on-Zn prefer to be close to the M atoms to compensate the M-O bonds and to locally depopulate the 3d states, and this will help forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped Zn1-xMxO nanorods can therefore be explained by the better rod morphology in combination with ionizing the magnetic M atoms.
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| 7. |
- Upadhyay, R. V., et al.
(författare)
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Effect of rare-earth Ho ion substitution on magnetic properties of Fe3O4 magnetic fluids
- 2006
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:8
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Tidskriftsartikel (refereegranskat)abstract
- Rare-earth atoms play an important role in determining the magnetocrystalline anisotropy in 4f-3d intermetallic compounds. Recently we reported on the synthesis and magnetic properties of Gd-substituted Mn-Zn ferrite nanoparticles as potentially suitable for magnetic fluid hyperthermia (MFH). In MFH the specific power absorption rate is related to the lossy magnetocrystalline anisotropic properties of the magnetic fluids. In this paper we report the role of Ho substitution in magnetite nanoparticles, which is found to enhance the KV product arising from the large anisotropy of Ho3+ moments. The zero-field-cooled magnetization data is then simulated by assuming noninteracting magnetic particles with uniaxial anisotropy and lognormal particle size distribution. The fit parameters give the values of particle diameter (D) 9 nm, standard deviation 0.3 in ln(D), and the anisotropy constant K to be 3.5x10(4) J/m(3). The value of K thus obtained is an order of magnitude larger than the value known for the undoped for magnetite (10(4) J/m(3)).
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