arXiv subjects

V. P. Antropov

Publications and source records attributed to V. P. Antropov.

At least 19 recordsLinked to original sources

arXiv2017

Spin-density fluctuations and the fluctuation-dissipation theorem in 3d ferromagnetic metals

Spatial and time scales of spin density fluctuations (SDF) were analyzed in 3d ferromagnets using ab initio linear response calculations of complete wavevector and energy dependence of the dynamic spin susceptibility tensor. We demonstrate that SDF are spread continuously over the entire Brillouin zone and while majority of them reside within the 3d bandwidth, a significant amount comes from much higher energies. A validity of the adiabatic approximation in spin dynamics is discussed. The SDF spectrum is shown to have two main constituents: a minor low-energy spin wave contribution and a much larger high-energy component from more localized excitations. Using the fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the related effective fluctuating moment were properly evaluated and their universal dependence on the 3d band population is further discussed.

cond-mat.mtrl-sci↗

arXiv2017

Tunable dimensional crossover and magnetocrystalline anisotropy in Fe$_2$P-based alloys

Electronic structure calculations are used to examine the magnetic properties of Fe$_2$P-based alloys and the mechanisms through which the Curie temperature and magnetocrystalline anisotropy can be optimized for specific applications. It is found that at elevated temperatures the magnetic interaction in pure Fe$_2$P develops a pronounced two-dimensional character due to the suppression of the magnetization in one of the sublattices, but the interlayer coupling is very sensitive to band filling and structural distortions. This feature suggests a natural explanation of the observed sharp enhancement of the Curie temperature by alloying with multiple elements, such as Co, Ni, Si, and B. The magnetocrystalline anisotropy is also tunable by electron doping, reaching a maximum near the electron count of pure Fe$_2$P. These findings enable the optimization of the alloy content, suggesting co-alloying of Fe$_2$P with Co (or Ni) and Si as a strategy for maximizing the magnetocrystalline anisotropy at and above room temperature.

cond-mat.mtrl-sci↗

arXiv2017

Prediction of a new efficient permanent magnet SmCoNiFe3

We propose a new efficient permanent magnet, SmCoNiFe3, that is a breakthrough development of the well-known SmCo5 prototype. More modern neodymium magnets of the Nd-Fe-B type have an advantage over SmCo5 because of their greater maximum energy products due to their iron-rich stoichiometry. Our new magnet, however, removes most of this disadvantage of SmCo5 while preserving its superior high-temperature efficiency over the neodymium magnets.

cond-mat.mtrl-sci↗

arXiv2015

Ab initio construction of magnetic phase diagrams in alloys: The case of Fe$_{1-x}$Mn$_x$Pt

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.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic anisotropic effects and electronic correlations in MnBi ferromagnet

The electronic structure and numerous magnetic properties of MnBi magnetic systems are investigated using local spin density approximation (LSDA) with on-cite Coulomb correlations (LSDA+$U$) included. We show that the inclusion of Coulomb correlations provides a much better description of equilibrium magnetic moments on Mn atom as well as the magnetic anisotropy energy (MAE) behavior with temperature and magneto-optical effects. We found that the inversion of the anisotropic pairwise exchange interaction between Bi atoms is responsible for the observed spin reorientation transition at 90 K. This interaction appears as a result of strong spin orbit coupling on Bi atoms, large magnetic moments on Mn atoms, significant $p-d$ hybridization between Mn and Bi atoms, and it depends strongly on lattice constants. A better agreement with the magneto-optical Kerr measurements at higher energies is obtained. We also present the detailed investigation of the Fermi surface, the de Haas-van Alphen (dHvA) effect and the X-ray magnetic circular dichroism in MnBi.

cond-mat.str-el↗

arXiv2013

Comparative study of the electronic structure, phonon spectra and electron-phonon interaction of ZrB2 and TiB2

The electronic structure, optical and x-ray absorption spectra, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface, phonon spectra, electron-phonon Eliashberg and transport spectral functions, temperature dependence of electrical resistivity of the MB2 (M=Ti and Zr) diborides were investigated from first principles using the full potential linear muffin-tin orbital method. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. A good agreement with experimental data of optical and x-ray absorption spectra, phonon spectra, electron-phonon spectral functions, electrical resistivity, cyclotron masses and extremal cross sections of the Fermi surface was achieved.

cond-mat.supr-con↗

arXiv2011

Magnetic Exchange Interactions in BaMn2As2: A Case Study of the J1-J2-Jc Heisenberg Model

BaMn2As2 is unique among BaT2As2 compounds crystallizing in the body-centered-tetragonal ThCr2Si2 structure, which contain stacked square lattices of 3d transition metal T atoms, since it has an insulating large-moment (3.9 muB/Mn) G-type (checkerboard) antiferromagnetic AF ground state. We report measurements of the anisotropic magnetic susceptibility chi versus temperature T from 300 to 1000 K of single crystals of BaMn2As2, and magnetic inelastic neutron scattering measurements at 8 K and 75As NMR measurements from 4 to 300 K of polycrystalline samples. The Neel temperature determined from the chi(T) measurements is TN = 618(3) K. The measurements are analyzed using the J1-J2-Jc Heisenberg model. Linear spin wave theory for G-type AF ordering and classical and quantum Monte Carlo simulations and molecular field theory calculations of chi(T) and of the magnetic heat capacity Cmag(T) are presented versus J1, J2 and Jc. We also obtain band theoretical estimates of the exchange couplings in BaMn2As2. From analyses of our chi(T), NMR, neutron scattering, and previously published heat capacity data for BaMn2As2 on the basis of the above theories for the J1-J2-Jc Heisenberg model and our band-theoretical results, our best estimates of the exchange constants in BaMn2As2 are J1 = 13 meV, J2/J1 = 0.3 and Jc/J1 = 0.1, which are all antiferromagnetic. From our classical Monte Carlo simulations of the G-type AF ordering transition, these exchange parameters predict TN = 640 K for spin S = 5/2, in close agreement with experiment. Using spin wave theory, we also utilize these exchange constants to estimate the suppression of the ordered moment due to quantum fluctuations for comparison with the observed value and again obtain S = 5/2 for the Mn spin.

cond-mat.str-el↗

arXiv2010

Consistent model of magnetism in ferropnictides

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.

cond-mat.supr-con↗

arXiv2010

Low Energy, Coherent, Stoner-like Excitations in CaFe$_{2}$As$_{2}$

Using linear-response density-functional theory, magnetic excitations in the striped phase of CaFe$_{2}$As$_{2}$ are studied as a function of local moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the N{é}el temperature, originating largely from a narrow band of Fe $d$ states near the Fermi level, and coexist with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model.

cond-mat.mtrl-sci↗

arXiv2010

Competition between antiferromagnetic instability and frustrations in Fe-Se

Using linear-response density-functional theory, we obtain the magnetic interactions in the several iron pnictides. The ground state has been found to be non-collinear in FeSe, with a large continuum of nearly degenerate states lying very close to the magnetic "striped" structure. The presence of non-collinearity also seems to be a generic feature of iron pnictides when the Fe moment is small. At small R_{Fe-Se} the system is itinerant: strong frustration give rise to excess of spin entropy, long ranged interactions create incommensurate orderings and strong biquadratic (or ring) couplings violate the applicability of Heisenberg model. There is a smooth transition to more localized behavior as R_{Fe-Se} increases: stable magnetic orbital order develops which favor long range AFM stripe ordering with strongly anisotropic in-plane exchange couplings. The stabilization of the stripe magnetic order is accompanied by the inversion of the exchange coupling.

cond-mat.mtrl-sci↗

arXiv2010

Effects of Spin Fluctuations and Anomalous Thermal Expansion of delta-Plutonium

We suggest a model for the magnetic dynamics of - plutonium and its alloys in order to show that the dynamical fluctuations of the magnetization density, or spin fluctuations, may be responsible for the anomalies of their observed thermal expansion. We show that due to strong magneto-elastic coupling, spin fluctuations may essentially contribute to the volume strain by giving a negative magneto-volume contribution that is proportional to the squared local magnetic moment and the magnetic Gruneisen constant which is negative in - plutonium. In the presented model, the local magnetic moment increases as the temperature rises, resulting in the interplay between the positive contributions to the volume strain from the lattice and the negative contribution from spin fluctuations, and finally leads to the Invar anomaly or to the negative coefficient of thermal expansion. Our results agree closely with the measured thermal expansion data for Pu-Ga alloys.

cond-mat.mtrl-sci↗

arXiv2010

Phenomenological Model of Longitudinal Spin Fluctuations in Itinerant Antiferromagnets

We present the phenomenological analysis of the spectrum of longitudinal spin fluctuations in isotropic itinerant electron antiferromagnets with account of spin anharmonicity giving rise to coupling of transverse and longitudinal normal modes. The spectrum consists of a quasielastic part forming a central peak or a dip, depending on temperature and the Landau relaxation rate. Effects of spin fluctuation coupling also give rise to an inelastic part of the spectrum which has a form of resonances or antiresonances near the magnon frequencies related to non-propagating longitudinal excitations.

cond-mat.mtrl-sci↗

arXiv2009

Itinerant magnetic excitations in antiferromagnetic CaFe2As2

Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab-initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

cond-mat.supr-con↗

arXiv2008

Role of covalent Fe-As bonding in the magnetic moment formation and exchange mechanisms in iron-pnictide superconductors

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 covalent 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.

cond-mat.mtrl-sci↗

arXiv2008

Character of magnetic instabilities in CaFe2As2

The density functional non-interacting susceptibility has been analyzed in different phases of CaFe2As2 and compared with similar data for pure d-metals. The conditions for the "no local moment" itinerant state with large frustrations are found for the "collapsed" phase (corresponding to superconducting phase). This itineracy determines the instability versus the incommensurate magnetic order for the narrow region of wave vectors. For the ambient pressure phase, the local moments on Fe atoms with much less frustrated antiferromagnetic interactions are stabilized and a magnetic short or long range order for all wave vectors is developed.

cond-mat.supr-con↗

arXiv2008

Anisotropic three-dimensional magnetism in CaFe2As2

Inelastic neutron scattering measurements on the low energy spin waves in CaFe2As2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data.

cond-mat.supr-con↗

arXiv2005

Quantum Spin Effect and Short-Range Order above the Curie Temperature

Using quantum Heisenberg model calculations with Green's function technique generalized for arbitrary spins, we found that for a system of small spins the quantum spin effects significantly contribute to the magnetic short-range order and strongly affect physical properties of magnets. The spin dynamics investigation confirms that these quantum spin effects favor the persistence of propagating spin-wave excitations above the Curie temperature. Our investigation suggests a reconsideration of prevailing point of view on finite temperature magnetism to include quantum effects and the magnetic short-range order.

cond-mat.other↗

arXiv2005

Electronic structure and anisotropic transport properties in hexagonal YPtIn and LuAgGe ternary compounds

We present anisotropic, zero applied magnetic field, temperature dependent resistivity measurements on hexagonal, non-magnetic, YPtIn and LuAgGe single crystals. For these materials the in-plane resistivity, $ρ_{ab}$, is significantly higher than the $c$ - axis one, $ρ_c$, with $ρ_{ab}/ρ_c \approx 1.4$ for YPtIn and $\approx 4.2 - 4.7$ for LuAgGe. The connection between the electronic structure and the anisotropic transport properties is discussed using density functional calculations that link the observed anisotropy with a specific shape of Fermi surface and anisotropy of the Fermi velocities.

cond-mat.mtrl-sci↗