Vladimir Antropov
Iowa State University, Physics and Astronomy, Faculty Member
A computational search for stable structures among both α and β phases of ternary ATB4 borides (A = Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that α-ATB4 compounds with A = Mg, Ca, Al,... more
A computational search for stable structures among both α and β phases of ternary ATB4 borides (A = Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that α-ATB4 compounds with A = Mg, Ca, Al, and T = V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of 3d transition metal atom dimers, which leads to the appearance of numerous Van Hove singularities in the electronic spectrum. The closeness of such singularities to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these singularities led to magnetic instabilities and magnetic ground states with a weakly metallic or semiconducting nature. Such states appear as non-trivial coexistence of the different spin ladders formed by magnetic dimers of 3d elements. These ma...
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The influence of on-site Hubbard correlations on electronic structure and magneto-optical spectra of CeSb have been studied. As the new key mechanism to produce the large magneto-optical signal is linked to the interaction between the... more
The influence of on-site Hubbard correlations on electronic structure and magneto-optical spectra of CeSb have been studied. As the new key mechanism to produce the large magneto-optical signal is linked to the interaction between the anisotropy of the Coulomb matrix and m-dependent p-f hybridization, this effect suggests a systematic search for materials with large magneto-optical effects.
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ABSTRACT
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Electronic structure and x-ray magnetic circular dichroism in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>YbAgC
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We describe techniques to determine magnetic exchange parameters using modern band structure methods. The analytical and numerical implementations of the linear-response method are discussed. The applications of the presented technique... more
We describe techniques to determine magnetic exchange parameters using modern band structure methods. The analytical and numerical implementations of the linear-response method are discussed. The applications of the presented technique have been done for 3d ferromagnets. Our results point out difficulties in defining a pairwise exchange coupling for itinerant magnets, while for systems with localized moments the traditional adiabatic approach is mostly valid.
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Abstract Non-collinear magnetic alignment is somewhat unusual for equilibrium, and those systems exhibiting such ground states are frequently `victims’ of competing forces. For non-equilibrium, or dynamical systems (e.g. T >0)... more
Abstract Non-collinear magnetic alignment is somewhat unusual for equilibrium, and those systems exhibiting such ground states are frequently `victims’ of competing forces. For non-equilibrium, or dynamical systems (e.g. T >0) non-collinear alignment is universal. We discuss such systems and present some basic formalism for evaluating from first principles the equation of motion and related relevant properties. Considerable discussion is also devoted to obtaining from first principles, parameters for model magnetic Hamiltonians. These models represent a convenient way to gain insight and to approximately evaluate magnetic interactions for very complex systems.
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Origin of MA in pure Ce<sub>2</sub>Co<sub>17</sub>: It had been argued that the dumbbell Co sites in Ce<sub>2</sub>Co<sub>17</sub> have very negative contribution to the perpendicular... more
Origin of MA in pure Ce<sub>2</sub>Co<sub>17</sub>: It had been argued that the dumbbell Co sites in Ce<sub>2</sub>Co<sub>17</sub> have very negative contribution to the perpendicular magnetic anisotropy (PMA), however a quantitative analysis of MA using DFT in this system is lacking. Using DFT, we resolve the magnetic anisotropy energy (MAE) into sublattice and investigate how MAE evolves when structure changes from CeCo<sub>5</sub> to Ce<sub>2</sub>Co<sub>17</sub>. By evaluating the matrix element of spin-orbit coupling energy, we confirmed that the dumbbell Co sites have a very negative contribution to PMA. We found that the pure Co basal plane no longer supports PMA as it did in CeCo<sub>5</sub> and its contribution to PMA becomes negligible or even slightly negative. The Ce-Co basal plane almost entirely provides the PMA in Ce<sub>2</sub>Co<sub>17</sub>.
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Self-consistent local GW method: Application to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>3</mml:mn><mml:mi>d</mml:mi></mml:m
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Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The... more
Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence $K(x)$ with a maximum near $x=0.3$ and a minimum near $x=0.8$ is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, $K$ depends strongly on the tetragonality and doubles under a modest 3% increase of the $c/a$ ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.
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The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong chemisorption-like bonding between Fe and As, which... more
The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong chemisorption-like bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping.
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Spin dynamics based on a quasiclassical approximation1 is used to study a single Fe or Ni magnetic impurity in ferromagnetic bcc iron. The optimization of the magnetic structure of the impurity includes quantum effects such as spatial... more
Spin dynamics based on a quasiclassical approximation1 is used to study a single Fe or Ni magnetic impurity in ferromagnetic bcc iron. The optimization of the magnetic structure of the impurity includes quantum effects such as spatial spin inhomogeneity and different orbital contributions. The Pauli equation for noncollinear magnetic configuration is solved for each impurity. When rotated from ferromagnetic ordering, Fe and Ni impurities show different behavior. The Fe moment remains almost unchanged through a rotation of 0.6π radians and drops to zero when the rotation is increased to 0.72π radians. This behavior is primarily due to the interplay of more localized eg states and itinerant t2g states. On the other hand, the Ni moment drops continuously with rotation, going to zero at 0.5π radians. The different degrees of localization of Ni and Fe moments are analyzed in terms of different spatial anisotropy of magnetization density. The inhomogeneous spin space configurations are presented and analyzed using corresponding space dependent exchange integrals.
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Reinvestigation of the intrinsic magnetic properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si33.svg"><mml:mrow><mml:msub><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:msub><mml:mrow><mml:mtext>Fe</mml:mtex
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Single-Crystal Permanent Magnets: Extraordinary Magnetic Behavior in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Ta</mml:mi></mml:math>-,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>C...more
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We present an overview of the locally self-consistent multiple scattering (LSMS) method. The method is based on real space multiple scattering theory, is naturally highly parallel, and has been implemented on Intel Paragon parallel... more
We present an overview of the locally self-consistent multiple scattering (LSMS) method. The method is based on real space multiple scattering theory, is naturally highly parallel, and has been implemented on Intel Paragon parallel platforms within the Center for Computational Sciences at Oak Ridge National Laboratory. O(N)-scaling is demonstrated for unit cells as large as 1000-atoms. We discuss in detail how the real space convergence properties of the method can be controlled by taking advantage of the stationary properties of a finite temperature Harris-Foulkes free energy functional. We show how the LSMS method can be combined with spin-dynamics to treat non-collinear magnetic states of materials. We show some preliminary results for the ground state magnetic structure of FCC Fe0.6 5Ni 0.35 alloys that indicate the possible existence of non-collinear arranges of magnetic moments in this system.
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We have investigated the use of recently developed stochastic and deterministic methods for treating the thermal properties and spin dynamics of magnetic systems. Several types of spin-hamiltonians considered. The parameters of such... more
We have investigated the use of recently developed stochastic and deterministic methods for treating the thermal properties and spin dynamics of magnetic systems. Several types of spin-hamiltonians considered. The parameters of such hamiltonians have been obtained from electronic structure calculations of real materials (Fe, Ni and others). Together with these 'second' principle calculations we performed model calculations for some magnetic clusters and the results are compared with analytical results and other simulations.
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Low-temperature measurements of the magnetocrystalline anisotropy energy K in (Fe1–xCox)2B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated... more
Low-temperature measurements of the magnetocrystalline anisotropy energy K in (Fe1–xCox)2B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence K(x) with a maximum near x = 0.3 and a minimum near x = 0.8 is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, K depends strongly on the tetragonality and doubles under a modest 3% increase of the c/a ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.