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Träfflista för sökning "WAKA:ref ;pers:(Eriksson Olle);pers:(Iusan Diana)"

Sökning: WAKA:ref > Eriksson Olle > Iusan Diana

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1.
  • Eriksson, Olle, et al. (författare)
  • Ordering in diluted magnetic semiconductors : A magnetic percolation phenomenon (invited)
  • 2007
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 101:9, s. 09H114
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped III-V and II-VI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electron-electron interaction.
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2.
  • Iusan, Diana, et al. (författare)
  • Effect of diffusion and alloying on the magnetic and transport properties of Fe/V/Fe trilayers
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 75:2, s. 024412
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic and transport properties of the Fe/V/Fe(001) trilayers were studied using the self-consistent Green's function technique based on the tight-binding linear muffin-tin orbital method in the atomic-sphere approximation. The coherent potential approximation was used to describe the effects of interdiffusion and alloying at the interfaces on the properties of the semi-infinite bcc Fe(001)/mFe/nV/mFe/Fe(001) trilayers. The electric conductance was calculated using the Kubo-Landauer formalism, in the current-perpendicular-to-plane geometry. It is shown that a dipole moment is created at the Fe/V interface due to the charge transfer from vanadium to iron, and a small induced magnetic moment is present in the first vanadium layer and is antiparallel to that of iron. The interlayer exchange coupling shows rapid oscillations for small spacer thicknesses, and the interdiffusion and alloying at the interface stabilize the ferromagnetic coupling. Moreover, the interdiffusion reduces the vanadium-induced magnetic moment and increases the iron magnetic moment at the interface. The giant magnetoresistance (GMR) ratio presents damped oscillations as a function of the vanadium spacer thickness. The interdiffusion and the presence of Mn impurities at the interface reduce considerably the GMR ratio and produce results that are in agreement with experimental data.
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3.
  • Iusan, Diana, et al. (författare)
  • Effects of strain on ferroelectric polarization and magnetism in orthorhombic HoMnO3
  • 2013
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:1, s. 014403
  • Tidskriftsartikel (refereegranskat)abstract
    • We explore how the ferroelectric polarization of antiferromagnetic E-type orthorhombic HoMnO3 can be increased, by investigating the effects of in-plane strain on both the magnetic properties and the ferroelectric polarization, using combined density functional theory calculations and a model Hamiltonian technique. Our results show that the net polarization is strongly enhanced under compressive strain, due to an increase of the electronic contribution to the polarization. In contrast, the ionic contribution is found to decrease. We identify the electron-lattice coupling, due to Jahn-Teller (JT) distortions, and its response to strain, to be responsible for the observed behavior. The JT-induced orbital ordering of occupied Mn-e(g)(1) electrons in alternating 3x(2)-r(2)/3y(2)-r(2) orbital states in the unstrained structure, changes under in-plane compressive strain to a mixture with x(2)-z(2)/y(2)-Z(2) states. The asymmetric hopping of e(g) electrons between Mn ions along zigzag spin chains (typical of the AFM-E spin configuration) is therefore enhanced under strain, explaining the large value of the polarization. Using a degenerate double-exchange model including electron-phonon interaction, we reproduce the change in the orbital ordering pattern. In this picture, the orbital ordering change is related to a change of the Berry phase of the e(g) electrons. This causes an increase of the electronic contribution to the polarization.
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4.
  • Iusan, Diana, et al. (författare)
  • Electronic structure and chemical and magnetic interactions in ZnO doped with Co and Al : Experiments and ab initio density-functional calculations
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 78:8, s. 085319
  • 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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5.
  • Iusan, Diana, et al. (författare)
  • Influence of defects on the magnetism of Mn-doped ZnO
  • 2007
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 101:9, s. 09H101
  • Tidskriftsartikel (refereegranskat)abstract
    • The properties of dilute magnetic semiconductors are usually strongly influenced by the defects present in the system. Ab initio calculations may provide valuable insight for the microscopic understanding of the interactions with defects. Here, we present studies of Mn-doped ZnO in the presence of several defects by a combined approach of ab initio electronic structure calculations using Korringa-Kohn-Rostoker-coherent potential approximation and Monte Carlo simulations (MCSs). Electronic structure and magnetic interactions have similar trends for wurtzite and zinc-blende crystal structures. A weak antiferromagnetic interaction has been found for 5% Mn doping in defect-free ZnO. Defects such as O vacancies and Zn interstitials lead to antiferromagnetic interactions between the Mn atoms, while Zn vacancies and oxygen substitution by nitrogen yield ferromagnetic interactions. As the concentration of Mn is low and the exchange interactions are short ranged, MCSs show small values of Curie temperatures (not more than 50 K). However, for a few cases with codoping of Mn and defects, we obtained higher Curie temperatures (around 130 K). Estimates of the Curie temperatures, assuming an average separation of the Mn atoms in the mean-field solution of Heisenberg model, are in very good agreement with the results obtained from MCS.
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6.
  • Iusan, Diana, et al. (författare)
  • Microscopic picture of Co clustering in ZnO
  • 2009
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - The American Physical Society. - 1098-0121. ; 79:12, s. 125202
  • Tidskriftsartikel (refereegranskat)abstract
    • Density functional theory was applied to study the chemical and magnetic interactions between Co atoms doped in ZnO. It was found that the Co impurities tend to form nanoclusters and the interactions between these atoms are antiferromagnetic within the local spin-density approximation (LSDA)+Hubbard U approach. The extracted interatomic exchange parameters agree reasonably well with recent experimental results. We have analyzed and compared the electronic structure obtained using the LSDA and LSDA+U approaches and found that the LSDA+U gives the most reasonable result, highlighting the importance of short-ranged antiferromagnetic interactions due to superexchange.
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7.
  • Iusan, Diana, et al. (författare)
  • Role of defects on the magnetic interactions in Mn-doped ZnO
  • 2007
  • Ingår i: Physica status solidi. A, Applied research. - 0031-8965. ; 204:1, s. 53-60
  • Tidskriftsartikel (refereegranskat)abstract
    • Oxide based diluted magnetic semiconductors are highly controversial from the point of view of intrinsic ferromagnetism brought in by doping with transition metal ions. By ab-initio Korringa-Kohn-Rostoker-Coherent-Potential-Approximation (KKR-CPA) calculations in the framework of density functional theory, we have obtained the electronic structure and magnetic exchange interaction parameters of Mn-doped ZnO in the presence and absence of several types of defects. A weak antiferromagnetic exchange interaction is observed in pure Mn-doped ZnO in the dilute limit with an increase in the strength of interactions with increasing concentration of Mn. In the presence of donor defects, such as oxygen vacancies and interstitial Zn, the interactions remain antiferromagnetic whereas in case of acceptor defects like Zn vacancies and N substitution of O, ferromagnetic interactions are observed. We have used the ab-initio exchange parameters in Monte-Carlo simulations for calculating finite temperature properties. Due to the short ranged behavior of interactions and disorder effects, the Curie temperatures calculated from Monte Carlo simulations show moderate values, not exceeding 50 K when the Mn content is 5%.
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8.
  • Iusan, Diana, et al. (författare)
  • Theoretical study of the magnetism of Mn-doped ZnO with and without defects
  • 2006
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 74:23, s. 235208
  • Tidskriftsartikel (refereegranskat)abstract
    • We calculate the exchange interaction parameters of a classical Heisenberg Hamiltonian for Mn-doped ZnO (Mn concentration between 5% and 20%) by an ab initio Korringa-Kohn-Rostoker coherent-potential-approximation method in the framework of density functional theory. A weak antiferromagnetic exchange interaction is observed in pure Mn-doped ZnO in the dilute limit with an increase in the strength of interactions with increasing concentration of Mn. In the presence of donor defects, such as oxygen vacancies and interstitial Zn, the interactions remain antiferromagnetic, whereas in case of acceptor defects like Zn vacancies and N substitution of O, ferromagnetic interactions are observed. Due to the short-ranged character of interactions and disorder effects, the Curie temperatures calculated from Monte Carlo simulations yield low values (∼45  K). However, in a few combinations of Mn and defect concentrations, the calculated Curie temperature can be as high as 135  K. If clustering of Mn atoms on a zinc-blende lattice is taken into account, the Mn-Mn spin correlations within a cluster are found to persist up to 600  K. Finally, we have shown that a modified mean-field theory, which we refer to as the “average mean field” estimate, yields values of the ordering temperature that are in good agreement with Monte Carlo simulations.
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9.
  • Sanyal, Biplab, et al. (författare)
  • Electronic structure of Co doped ZnO : Theory and experiment
  • 2008
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 103:7, s. 07D130
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the electronic structure of Co doped ZnO by theory and experiment. Photoelectron and x-ray absorption spectroscopy experiments were performed on 5% Co doped ZnO thin films. The results show that Co-3d states lie in the valence band and hybridize considerably with O-2p states. These results compare qualitatively with our density functional calculations combined with a Hubbard approach to include strong electron correlations within the Co-3d state.
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10.
  • Sanyal, Biplab, et al. (författare)
  • Inhomogeneity in Co Doped ZnO Diluted Magnetic Semiconductor
  • 2008
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 103:7, s. 07D131
  • Tidskriftsartikel (refereegranskat)abstract
    • Here, we have studied the chemical and magnetic interactions in Co doped ZnO diluted magnetic semiconductor by ab initio density functional calculations. The calculated chemical pair interaction parameters suggest a strong tendency of clustering between Co atoms. Both chemical and magnetic pair interaction parameters are short ranged with a large first neighbor interaction. Monte Carlo simulations show that the Curie temperature for a homogeneous sample is low, whereas allowing for an inhomogeneous growth results in very high Curie temperatures. It is argued that these inhomogeneities may be the reason for the observation of high Curie temperatures in Co doped ZnO. It is also demonstrated that the electronic structure is modified significantly in presence of Co clusters.
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