| 1. |
- Islam, Fhokrul, et al.
(author)
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Ab initio calculations of the magnetic properties of Mn impurities on GaAs (110) surfaces
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; B 85, s. Article ID: 155306-
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Journal article (peer-reviewed)abstract
- We present a computational study of individual and pairs of substitutional Mn impurities on the (110) surface of GaAs samples based on density functional theory. We focus on the anisotropy properties of these magnetic centers and their dependence on on-site correlations, spin-orbit interaction, and surface-induced symmetry-breaking effects. For a Mn impurity on the surface, the associated acceptor-hole wave function tends to be more localized around the Mn than for an impurity in bulk GaAs. The magnetic anisotropy energy for isolated Mn impurities is of the order of 1 meV, and can be related to the anisotropy of the orbital magnetic moment of the Mn acceptor hole. Typically Mn pairs have their spin magnetic moments parallel aligned, with an exchange energy that strongly depends on the pair orientation on the surface. The spin magnetic moment and exchange energies for these magnetic entities are not significantly modified by the spin-orbit interaction, but are more sensitive to on-site correlations. Correlations in general reduce the magnetic anisotropy for most of the ferromagnetic Mn pairs.
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| 2. |
- Islam, Fhokrul, et al.
(author)
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First-principles study of spin-electric coupling in a {Cu3} single molecular magnet
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:15
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Journal article (peer-reviewed)abstract
- We report on a study of the electronic and magnetic properties of the triangular antiferromagnetic {Cu3} single-molecule magnet, based on spin-density-functional theory. Our calculations show that the low-energy magnetic properties are correctly described by an effective three-site spin s = 1/2 Heisenberg model, with an antiferromagnetic exchange coupling J approximate to 5 meV. The ground-state manifold of the model is composed of two degenerate spin S = 1/2 doublets of opposite chirality. Due to lack of inversion symmetry in the molecule these two states are coupled by an external electric field, even when spin-orbit interaction is absent. The spin-electric coupling can be viewed as originating from a modified exchange constant delta J induced by the electric field. We find that the calculated transition rate between the chiral states yields an effective electric dipole moment d = 3.38 x 10(-33) C m approximate to e10(-4)a, where a is the Cu separation. For external electric fields epsilon approximate to 10(8) V/m this value corresponds to a Rabi time tau approximate to 1 ns and to a delta J on the order of a few mu eV.
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| 3. |
- Islam, Fhokrul, et al.
(author)
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On the enhancement of magnetic anisotropy in cobalt clusters via non-magnetic doping
- 2014
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 26:125303
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Journal article (peer-reviewed)abstract
- We show that the magnetic anisotropy energy (MAE) in cobalt clusters can be significantlyenhanced by doping them with group IV elements. Our firstprincipleselectronic structurecalculations show that Co4C2 and Co12C4 clusters have MAEs of 25 K and 61 K, respectively. The large MAE is due to controlled mixing between Co dandC pstatesand can be furthertuned by replacing C by Si. Larger assemblies of such primitive units are shown to be stablewith MAEs exceeding 100 K in units as small as 1.2 nm, in agreement with the recentobservation of large coercivity. These results may pave the way for the use of nanoclustersinhigh density magnetic memory devices for spintronics applications
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| 4. |
- Islam, Fhokrul
(author)
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Stable magnetic order and charge induced rotation of magnetizationin nano-clusters
- 2014
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 105:152409
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Journal article (peer-reviewed)abstract
- Efficient control of magnetic anisotropy and the orientation of magnetization are of centralimportance for the application of nanoparticles in spintronics. Conventionally, magnetization iscontrolled directly by an external magnetic field or by an electric field via spin-orbit coupling.Here, we demonstrate a different approach to control magnetization in small clusters. We firstshow that the low magnetic anisotropy of a Co5 cluster can be substantially enhanced by attachingbenzene molecules due to the mixing between p states of C and the d states of Co sites. We thenshow that the direction of magnetization vector of Co5 sandwiched between two benzene moleculesrotates by 90 when an electron is added or removed from the system. An experimental set up torealize such effect is also suggested.
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| 5. |
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| 6. |
- Mahani, Mohammad Reza, et al.
(author)
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Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface
- 2014
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:16, s. Article ID: 165408-
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Journal article (peer-reviewed)abstract
- Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically theelectronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+](0) plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity d electrons are in good agreement with approaches that treat the spin ofthe impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+](0) and the ionized [Fe2+](-)states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+, h](0), the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+, h](0). In contrast, for [Fe3+](0) electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e. g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.
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| 7. |
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| 8. |
- Mahani, Mohammad Reza, et al.
(author)
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Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator
- 2014
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90, s. Article ID: 195441-
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Journal article (peer-reviewed)abstract
- We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi$_2$Se$_3$ topological insulator. Combining \textit{ab initio} calculations with microscopic tight-binding modeling, we identify the effects of inversion-symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in ${+2}$ valence state and introduces an acceptor in the bulk band gap. The Mn-acceptor has predominantly $p$--character, and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$--levels and the $p$--levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasi-resonant coupling and the strong real-space overlap between the spin-chiral surface states and the mid-gap spin-polarized Mn-acceptor states.
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| 9. |
- Mahani, Mohammad Reza, et al.
(author)
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The role of d levels of substitutional magnetic impurities at the (110) GaAs surface
- 2013
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Conference paper (other academic/artistic)abstract
- The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field known as magnetic solotronics, bearing exciting prospects for novel spintronics devices at the atomic scale. Advances in different STM based techniques allowed experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of details [1]. Theoretical studies based both on microscopic tight-binding (TB) models and DFT techniques have contributed in elucidating the experimental findings. In particular, for the case of Mn dopants on the (110) GaAs surface, TB models [2] have provided a quantitative description of the properties of the associated acceptor states. Most of these TB calculations ignore dealing explicitly with the Mn d-levels and treat the associated magnetic moment as a classical vector. However recent STM experiments [3] involving other TM impurities, such as Fe, reveal topographic features that might be related to electronic transitions within the d-level shell of the dopant. In this work we have included explicitly the d levels in the Hamiltonian. The parameters of the model have been extracted from DFT calculations. We have investigated the role that d levels play on the properties of the acceptor states of the doped GaAs(110) surface, and analyzed their implications for STM spectroscopy.
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| 10. |
- Mahani, Mohammad Reza, et al.
(author)
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Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators
- 2013
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In: MRS Online Proceedings Library/Volume 1564/2013. - : Materials Research Society.
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Conference paper (peer-reviewed)abstract
- We present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant. In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap. We present explicit results for BiSe antisite defects and compare with STM experiments.
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