Kirill Belashchenko PublicationsCopyright (c) 2015 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsbelashchenko
Recent documents in Kirill Belashchenko Publicationsen-usThu, 29 Oct 2015 01:46:45 PDT3600<i>Ab Initio</i> Construction of Magnetic Phase Diagrams in Alloys: The Case of Fe<sub>1−x</sub>Mn<sub>x</sub>Pt
http://digitalcommons.unl.edu/physicsbelashchenko/29
http://digitalcommons.unl.edu/physicsbelashchenko/29Tue, 27 Oct 2015 13:51:25 PDT
A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. Application to the Fe_{1−x}Mn_{x}Pt “magnetic chameleon” system yields the sequence of magnetic phases at T=0 and the c − T magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of the magnetic phase diagram is demonstrated.
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B. S. Pujari et al.Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb
http://digitalcommons.unl.edu/physicsbelashchenko/28
http://digitalcommons.unl.edu/physicsbelashchenko/28Tue, 27 Oct 2015 13:46:12 PDT
Effects of thermal spin disorder and excessMn on the electronic spectrum of half-metallic NiMnSb are studied using first-principles calculations. Temperature-dependent spin disorder, introduced within the vector disordered local moment model, causes the valence band at the Γ point to broaden and shift upwards, crossing the Fermi level and thereby closing the half-metallic gap above room temperature. The spectroscopic signatures of excess Mn on the Ni, Sb, and empty sites (Mn_{Ni}, Mn_{Sb}, andMn_{E}) are analyzed. Mn_{Ni} is spectroscopically invisible. The relatively weak coupling of Mn_{Sb} and Mn_{E} spins to the host strongly deviates from the Heisenberg model, and the spin of MnE is canted in the ground state. While the half-metallic gap is preserved in the collinear ground state of Mn_{Sb}, thermal spin disorder of the weakly coupled Mn_{Sb} spins destroys it at low temperatures. This property of MnSb may be the source of the observed low-temperature transport anomalies.
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K. D. Belashchenko et al.Microscopic first-principles model of strain-induced interaction in concentrated size-mismatched alloys
http://digitalcommons.unl.edu/physicsbelashchenko/27
http://digitalcommons.unl.edu/physicsbelashchenko/27Tue, 27 Oct 2015 13:40:31 PDT
The harmonic Kanzaki-Krivoglaz-Khachaturyan model of strain-induced interaction is generalized to concentrated size-mismatched alloys and adapted to first-principles calculations. The configuration dependence of both Kanzaki forces and force constants is represented by real-space cluster expansions that can be constructed based on the calculated forces. The model is implemented for the fcc lattice and applied to Cu_{1−x}Au_{x} and Fe_{1−x}Pt_{x} alloys for concentrations x = 0.25, 0.5, and 0.75. The asymmetry between the 3d and 5d elements leads to large quadratic terms in the occupation-number expansion of the Kanzaki forces and thereby to strongly non-pairwise long-range interaction. The main advantage of the full configuration-dependent lattice deformation model is its ability to capture this singular many-body interaction. The roles of ordering striction and anharmonicity in Cu-Au and Fe-Pt alloys are assessed. Although the harmonic force constants defined with respect to the unrelaxed lattice are unsuitable for the calculation of the vibrational entropies, the phonon spectra for ordered and disordered alloys are found to be in good agreement with experimental data. The model is further adapted to concentration wave analysis and Monte Carlo simulations by means of an auxiliary multiparametric real-space cluster expansion, which is used to find the ordering temperatures. Good agreement with experiment is found for all systems except CuAu_{3} (due to the known failure of the generalized gradient approximation) and FePt_{3}, where the discrepancy is likely due to the neglect of magnetic disorder.
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I. A. Zhuravlev et al.First-principles microscopic model of exchange-driven magnetoelectric response with application to Cr<sub>2</sub>O<sub>3</sub>
http://digitalcommons.unl.edu/physicsbelashchenko/26
http://digitalcommons.unl.edu/physicsbelashchenko/26Tue, 27 Oct 2015 13:35:00 PDT
The exchange-driven contribution to the magnetoelectric susceptibility α is formulated using a microscopic model Hamiltonian coupling the spin degrees of freedom to lattice displacements and electric field, which may be constructed from first-principles data. Electronic and ionic contributions are sorted out, and the latter is resolved into a sum of contributions from different normal modes. If intrasublattice spin correlations can be neglected, the longitudinal component α becomes proportional to the product of magnetic susceptibility and sublattice magnetization in accordance with Rado’s phenomenological model. As an illustration, the method is applied to analyze the temperature dependence of the longitudinal magnetoelectric susceptibility of Cr_{2}O_{3} using first-principles calculations and the quantum pair cluster approximation for magnetic thermodynamics. A substantial electronic contribution is found, which is opposite to the ionic part. The sensitivity of the results to the Hubbard U parameter and the sources of error are studied. It is also found that non-Heisenberg interactions are too weak to account for the sign change of α in Cr_{2}O_{3}.
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Sai Mu et al.Deviations from Matthiessen’s rule and resistivity saturation effects in Gd and Fe from first principles
http://digitalcommons.unl.edu/physicsbelashchenko/25
http://digitalcommons.unl.edu/physicsbelashchenko/25Tue, 27 Oct 2015 13:31:13 PDT
According to earlier first-principles calculations, the spin-disorder contribution to the resistivity of rare-earth metals in the paramagnetic state is strongly underestimated if Matthiessen’s rule is assumed to hold. To understand this discrepancy, the resistivity of paramagnetic Fe and Gd is evaluated by taking into account both spin and phonon disorder. Calculations are performed using the supercell approach within the linear muffin-tin orbital method. Phonon disorder is modeled by introducing random displacements of the atomic nuclei, and the results are compared with the case of fictitious Anderson disorder. In both cases, the resistivity shows a nonlinear dependence on the square of the disorder potential, which is interpreted as a resistivity saturation effect. This effect is much stronger in Gd than in Fe. The nonlinearity makes the phonon and spin-disorder contributions to the resistivity nonadditive, and the standard procedure of extracting the spin-disorder resistivity by extrapolation from high temperatures becomes ambiguous. An “apparent” spin-disorder resistivity obtained through such extrapolation is in much better agreement with experiment compared to the results obtained by considering only spin disorder. By analyzing the spectral function of the paramagnetic Gd in the presence of Anderson disorder, the resistivity saturation is explained by the collapse of a large area of the Fermi surface due to the disorder-induced mixing between the electron and hole sheets.
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James K. Glasbrenner et al.Electronic structure and magnetic properties of Gd-doped and Eu-rich EuO
http://digitalcommons.unl.edu/physicsbelashchenko/24
http://digitalcommons.unl.edu/physicsbelashchenko/24Tue, 27 Oct 2015 13:25:00 PDT
The effects of Gd doping and O vacancies on the magnetic interaction and Curie temperature T_{C} of EuO are studied using first-principles calculations. Linear response calculations in the virtual crystal approximation show a broad maximum in the Curie temperature as a function of doping, which results from the combination of the saturating contribution from indirect exchange and a decreasing contribution from the f -d hopping mechanism. Non-Heisenberg interaction at low doping levels and its effect on T_{C} are examined. The electronic structure of a substitutional Gd and of an O vacancy in EuO are evaluated. When the 4f spins are disordered, the impurity state goes from single to double occupation, but correlated bound magnetic polarons are not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films, the impurity states broaden into bands and remain partially filled. To go beyond the homogeneous doping picture, magnetostructural cluster expansions are constructed, which describe the modified exchange parameters near Gd dopants or O vacancies. Thermodynamic properties are studied using Monte Carlo simulations. The Curie temperature for Gd-doped EuO agrees with the results of the virtual crystal approximation and shows a maximum of about 150 K. At 3.125% vacancy concentration the T_{C} increases to 120 K, consistent with experimental data for Eu-rich film samples.
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J. M. An et al.Effects of alloying and strain on the magnetic properties of Fe<sub>16</sub>N<sub>2</sub>
http://digitalcommons.unl.edu/physicsbelashchenko/23
http://digitalcommons.unl.edu/physicsbelashchenko/23Tue, 27 Oct 2015 13:15:36 PDT
The electronic structure and magnetic properties of pure and doped Fe_{16}N_{2} systems have been studied in the local-density (LDA) and quasiparticle self-consistent GW approximations. The GW magnetic moment of pure Fe_{16}N_{2} is somewhat larger compared to LDA but not anomalously large. The effects of doping on magnetic moment and exchange coupling were analyzed using the coherent potential approximation. Our lowest estimate of the Curie temperature in pure Fe_{16}N_{2} is significantly higher than the measured value, which wemainly attribute to the quality of available samples and the interpretation of experimental results.We found that different Fe sites contribute very differently to the magnetocrystalline anisotropy energy (MAE), which offers a way to increase the MAE by small site-specific doping of Co or Ti for Fe. The MAE also increases under tetragonal strain.
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Liqin Ke et al.Effect of substitutional doping on the Néel temperature of Cr<sub>2</sub>O<sub>3</sub>
http://digitalcommons.unl.edu/physicsbelashchenko/22
http://digitalcommons.unl.edu/physicsbelashchenko/22Tue, 27 Oct 2015 13:09:15 PDT
First-principles calculations are used to explore the possibility of enhancing the Néel temperature T_{N} of the magnetoelectric antiferromagnet Cr_{2}O_{3} by substitutional doping.We describe the electronic structure of transition metal (V, Ti, Mn, Fe, Co, and Ni) and anion (N and B) impurities and evaluate their effect on the exchange interaction.We find that, although transition-metal impurities and N are likely to reduce T_{N}, substitution of O by B is likely to increase it. Both N and B impurities introduce impurity states mediating strong hybridization and magnetic interaction between the neighboring Cr ions. For N impurities, this leads to magnetic frustration, but in the case of B substitution, the stability of the ground antiferromagnetic state is enhanced.
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Sai Mu et al.Spin-disorder resistivity of ferromagnetic metals from first principles: The disordered-local-moment approach
http://digitalcommons.unl.edu/physicsbelashchenko/21
http://digitalcommons.unl.edu/physicsbelashchenko/21Tue, 27 Oct 2015 13:04:00 PDT
The paramagnetic spin-disorder resistivity (SDR) of transition-metal ferromagnets Fe, Co, Ni, ordered transition metal alloys Ni_{3}Mn and Fe_{3}Si as well as Ni_{2}MnX (X = In,Sn,Sb) Heusler alloys is determined from first principles. SDR is evaluated similar to the residual resistivity by using the disordered local moment (DLM) model combined with the Kubo-Greenwood linear response calculation. The electronic structure is determined within the tight-binding linear muffin-tin orbital method and the coherent potential approximation (CPA) applied to the DLM state. We also estimate the temperature dependence of the resistivity below the Curie temperature using a simple model. The results agree well with the supercell Landauer-Buttiker calculations and, generally, with experimental data. For the Ni_{2}MnSb Heusler alloy it is necessary to include substitutional disorder of B2-type to explain the experimental data.
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J. Kudrnovsky et al.Spin injection from a half-metal at finite temperatures
http://digitalcommons.unl.edu/physicsbelashchenko/20
http://digitalcommons.unl.edu/physicsbelashchenko/20Tue, 27 Oct 2015 12:59:19 PDT
Spin injection from a half-metallic electrode in the presence of thermal spin disorder is analyzed using a combination of random matrix theory, spin-diffusion theory, and explicit simulations for the tight-binding s-d model. It is shown that efficient spin injection from a half-metal is possible as long as the effective resistance of the normal metal does not exceed a characteristic value, which does not depend on the resistance of the half-metallic electrode but, rather, is controlled by spin-flip scattering at the interface. This condition can be formulated as α < l/ lN sf T_{c}^{-1}, where α is the relative deviation of the magnetization from saturation, l and lN sf are the mean-free path and the spin-diffusion length in the nonmagnetic channel, and T_{c} is the transparency of the tunnel barrier at the interface (if present). The general conclusions are confirmed by tight-binding s-d model calculations. A rough estimate suggests that efficient spin injection from true half-metallic ferromagnets into silicon or copper may be possible at room temperature across a transparent interface.
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K. D. Belashchenko et al.Microscopic origin of the structural phase transitions at the Cr<sub>2</sub>O<sub>3</sub> (0001) surface
http://digitalcommons.unl.edu/physicsbelashchenko/19
http://digitalcommons.unl.edu/physicsbelashchenko/19Tue, 27 Oct 2015 12:51:27 PDT
The surface of a Cr_{2}O_{3} (0001) film epitaxially grown on Cr undergoes an unusual reentrant sequence of structural phase transitions (1 × 1 → √ 3 × √ 3 → 1 × 1). In order to understand the underlying microscopic mechanisms, the structural and magnetic properties of the Cr_{2}O_{3} (0001) surface are here studied using firstprinciples electronic structure calculations. Two competing surface Cr sites are identified. The energetics of the surface is described by a configurational Hamiltonian with parameters determined using total-energy calculations for several surface supercells. Effects of epitaxial strain and magnetic ordering on configurational interaction are also included. The thermodynamics of the system is studied using Monte Carlo simulations. At zero strain the surface undergoes a 1 × 1 → √ 3 × √ 3 ordering phase transition at T_{c} ~ 165 K. Tensile epitaxial strain together with antiferromagnetic ordering drive the system toward strong configurational frustration, suggesting the mechanism for the disordering phase transition at lower temperatures.
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A. L. Wysocki et al.First-principles study of phase stability of Gd-doped EuO and EuS
http://digitalcommons.unl.edu/physicsbelashchenko/18
http://digitalcommons.unl.edu/physicsbelashchenko/18Tue, 27 Oct 2015 12:47:33 PDT
Phase diagrams of isoelectronic Eu_{1−x}Gd_{x}O and Eu_{1−x}Gd_{x}S quasibinary alloy systems are constructed using first-principles calculations combined with a standard cluster-expansion approach and Monte Carlo simulations. The oxide system has a wide miscibility gap on the Gd-rich side but forms ordered compounds on the Eu-rich side, exhibiting a deep asymmetric convex hull in the formation enthalpy diagram. The sulfide system has no stable compounds. The large difference in the formation enthalpies of the oxide and sulfide compounds is attributed to the contribution of local lattice relaxation, which is sensitive to the anion size. The solubility of Gd in both EuO and EuS is in the range of 10%–20% at room temperature and quickly increases at higher temperatures, indicating that highly doped disordered solid solutions can be produced without the precipitation of secondary phases. We also predict that rocksalt GdO can be stabilized under appropriate experimental conditions.
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J. M. An et al.Consistent model of magnetism in ferropnictides
http://digitalcommons.unl.edu/physicsbelashchenko/17
http://digitalcommons.unl.edu/physicsbelashchenko/17Tue, 27 Oct 2015 12:42:24 PDT
The discovery of superconductivity in LaFeAsO introduced the ferropnictides as a major new class of superconducting compounds with critical temperatures second only to cuprates. The presence of magnetic iron makes ferropnictides radically different from cuprates. Antiferromagnetism of the parent compounds strongly suggests that superconductivity and magnetism are closely related. However, the character of magnetic interactions and spin fluctuations in ferropnictides, in spite of vigorous efforts, has until now resisted understanding within any conventional model of magnetism. Here we show that the most puzzling features can be naturally reconciled within a rather simple effective spin model with biquadratic interactions, which is consistent with electronic structure calculations. By going beyond the Heisenberg model, this description explains numerous experimentally observed properties, including the peculiarities of the spin wave spectrum, thin domain walls, crossover from first to second order phase transition under doping in some compounds, and offers new insight in the occurrence of the nematic phase above the antiferromagnetic phase transition.
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A. L. Wysocki et al.Robust isothermal electric control of exchange
bias at room temperature
http://digitalcommons.unl.edu/physicsbelashchenko/16
http://digitalcommons.unl.edu/physicsbelashchenko/16Tue, 27 Oct 2015 12:39:08 PDT
Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with highspeed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr_{2}O_{3} has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr_{2}O_{3} single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.
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Xi He et al.Biquadratic magnetic interaction in parent
ferropnictides
http://digitalcommons.unl.edu/physicsbelashchenko/15
http://digitalcommons.unl.edu/physicsbelashchenko/15Tue, 27 Oct 2015 12:31:18 PDT
We discuss an effective spin Hamiltonian with biquadratic interaction for ferropnictide superconductors from the point of view of band structure theory and available experimental data. This model is consistent with electronic structure calculations and captures many observed magnetic properties, including the anisotropy of the exchange coupling, thin domain walls, and the crossover from first to second-order phase transition under doping. The parameters of the model are analyzed as a function of the local spin moment using first-principles calculations. Calculations show the biquadratic coupling is negative in stoichiometric KFe_{2}Se_{2}, and the phase diagram is extended into this region. We also consider magnetic short-range order and discuss the limitations of this model in comparison with experiment.
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A. L. Wysocki et al.Spin-dependent tunneling from clean and oxidized Co surfaces
http://digitalcommons.unl.edu/physicsbelashchenko/14
http://digitalcommons.unl.edu/physicsbelashchenko/14Tue, 27 Oct 2015 12:23:47 PDT
Transmission through a sufficiently thick vacuum barrier is factorized in the product of two ‘‘surface transmission functions’’ and a vacuum decay factor. Based on this factorization, we study the spin polarization of the tunneling current from clean and oxidized (1 1 1) FCC Co surfaces through vacuum into Al. The conductance is calculated using the principal-layer Green’s function approach within the tight-binding LMTO scheme. We find that for typical vacuum barrier thicknesses the tunneling current from the clean surface is dominated by minority-spin electrons. A monolayer of oxygen on top of the surface completely changes the shape of k_{ll}-resolved transmission and makes the tunneling current almost 100% majority-spin polarized.
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K. D. Belashchenko et al.