Ab initio calculations of the spin wave stiffness constant $D$ for elemental Fe and Ni performed by different groups in the past have led to values with a considerable spread of 50-100 %. We present results for the stiffness constant $D$ of Fe, Ni, and permalloy Fe$_{0.19}$Ni$_{0.81}$ obtained by three different approaches: (i) by finding the quadratic term coefficient of the power expansion of the spin wave energy dispersion, (ii) by a damped real-space summation of weighted exchange coupling constants, and (iii) by integrating the appropriate expression in reciprocal space. All approaches are implemented by means of the same Korringa-Kohn-Rostoker (KKR) Green function formalism. We demonstrate that if properly converged, all procedures yield comparable values, with uncertainties of 5-10 % remaining. By a careful analysis of the influence of various technical parameters we estimate the margin of errors for the stiffness constants evaluated by different approaches and suggest procedures to minimize the risk of getting incorrect results.
The way doping affects the spin wave stiffness and the exchange stiffness of permalloy (Py) is investigated via ab-initio calculations, using the Korringa-Kohn-Rostoker (KKR) Green function formalism. By considering various types of dopants of different nature (V, Gd, and Pt), we are able to draw general conclusions. To describe the trends of the stiffness with doping is it sufficient to account for the exchange coupling between nearest neighbors. The polarizability of the impurities is not an important factor for the spin wave stiffness. Rather, the decisive factor is the hybridization betwee…
In a recent paper, the authors propose to separately calculate the volumetric and chemical contributions to the elastic stiffness coefficients of systems containing solutes, as it is "computationally more efficient". We show that this is not the case and further that their methodology and hence their results are incorrect. There is no short cut for performing the desired calculations, if done rigorously, as we show in our 2012 work.
We present an ab initio theory of the spin-wave stiffness tensor for ordered and disordered itinerant ferromagnets with pair exchange interactions derived from a method of infinitesimal spin rotations. The resulting formula bears an explicit form of a linear-response coefficient which involves one-particle Green's functions and effective velocity operators encountered in a recent theory of electron transport. Application of this approach to ideal metal crystals yields more reliable values of the spin stiffness than traditional ill-convergent real-space lattice summations. The formalism can als…
This work proposes a new efficient approach for calculating the bending stiffness of two-dimensional materials using simple atomistic tests on small periodic unit cells. The tests are designed such that bending deformations are dominating and membrane deformations are minimized. Atomistic ab-initio simulations then allow for the efficient computation of bending energies. Density functional theory is used for this. Atomistic bending energies are then compared to classical models from structural mechanics. Two different models are considered for this -- one based on beam theory and one based on …
We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of Nd$_2$Fe$_{14}$B constructed from ab-initio calculations, and the LLG micromagnetics simulations are performed with the parameters obtained by the MC simulations. We clarify that the amplitude and the…
The (Cd, Be)Te semiconductor alloy that exhibits a dramatic mismatch in bond covalency and stiffness clarifying its vibrational and mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted (Zn, Be) chalcogenides. The test is done by way of experiment (x smaller than 0.11) combining Raman scattering with X ray diffraction at high pressure and ab initio calculations (x around 0, 0.5 and 1). The (macroscopic) bulk modulus B drops below the CdTe value on minor Be incorporation, a…
Forming a hetero-interface is a materials-design strategy that can access an astronomically large phase space. However, the immense phase space necessitates a high-throughput approach for optimal interface design. Here we introduce a high-throughput computational framework, InterMatch, for efficiently predicting charge transfer, strain, and superlattice structure of an interface by leveraging the databases of individual bulk materials. Specifically, the algorithm reads in the lattice vectors, density of states, and the stiffness tensors for each material in their isolated form from the Materia…
Ab initio calculations of the magnon dispersion in ferromagnetic materials typically rely on the adiabatic local density approximation (ALDA) in which the effective exchange-correlation field is everywhere parallel to the magnetization. These calculations, however, tend to overestimate the "magnon stiffness", defined as the curvature of the magnon frequency vs. wave vector relation evaluated at zero wave vector. Here we suggest a simple procedure to improve the magnon dispersion by taking into account gradient corrections to the ALDA at the exchange-only level. We find that this gradient corre…
Ab initio molecular and solid state studies of Fe$^{II}$ spin cross-over system [Fe(btz)$_2$(NCS)$_2$
本文报道了自旋交叉复合物[Fe（btz）$_{2}$（NCS）$_{2}$ ]的密度泛函理论从头计算，其中3d $^6$ Fe $^{II}$的特征是高自旋态（HS t $_{2g}^4$，e $_g^2$）和低自旋态（LS t $_{2g}^6$，e $_g^0$）.红外光谱和拉曼光谱的孤立的分子的结果是补充的结晶固体的电子能带结构特性的充分考虑：评估晶体场效应的状态密度和化学键指定一个特定的作用，Fe $^{II}$的协调领域内的Fe-N相互作用。
{\it Ab initio} studies of spin-spiral waves and exchange interactions in 3{\it d} transition metal atomic chains
采用广义梯度近似下的{ \it ab initio}密度泛函理论计算了3个$d$过渡金属原子链的横向自旋螺旋波的总能量随波矢的变化.据预测，在平衡键长，V，Mn和Fe链具有稳定的自旋螺旋结构，而磁性基态的Cr，Co和Ni链保持共线。此外，利用计算得到的自旋螺旋波的能量色散关系，计算了3个$d$过渡金属链的交换相互作用参数。有趣的是，发现V、Mn和Cr链中的磁耦合被抑制（即，第二近邻交换相互作用是反铁磁的），这导致在这些链中形成稳定的自旋螺旋结构。这3个$d$金属链的自旋波刚度常数也进行了评估，并被发现是小于其对应的散装和单层系统。在平均场近似下，还估计了这些原子链中可能的磁相变温度的上限（在100开尔文的数量级）。计算了自旋螺旋结构的电子能带结构。我们希望，在3个$d$过渡金属链中稳定的自旋螺旋结构和阻挫磁相互作用的有趣发现将刺激在这一领域的进一步理论和实验研究。
Single crystal elastic constants of bcc iron and bct FeC and FeN alloys, martensites, have been evaluated by ab initio calculations based on the density functional theory. The energy of a strained crystal has been computed using the supercell method at several values of the strain intensity, and the stiffness coefficient has been determined from the slope of the energy versus square of strain relation. Some of the third order elastic constants have also been evaluated. The absolute magnitudes of the calculated values for bcc iron are in fair agreement with experiment, including the third order…
The discovery of MAX phase borides has added a new dimension for research in the materials science community. In this paper, a first-principles study of the newly known MAX phase borides Zr2AB2 (A = In, Tl) has been carried out. The estimated lattice constants and volumes of the unit cell are found to be consistent with previous study. The dynamical and mechanical stability of the titled compounds have been checked. Fundamental insights into the stiffness constants, elastic moduli, hardness parameters, brittleness and anisotropy indices are presented. The Variation of these mechanical properti…
Temperature-dependent magnetic properties of Nd$_2$Fe$_{14}$B permanent magnets, i.e., saturation magnetization $M_\text{s}(T)$, effective magnetic anisotropy constants $K_i^\text{eff}(T)$ ($i=1,2,3$), domain wall width $δ_w(T)$, and exchange stiffness constant $A_\text{e}(T)$, are calculated by using \textit{ab-initio} informed atomistic spin model simulations. We construct the atomistic spin model Hamiltonian for Nd$_2$Fe$_{14}$B by using the Heisenberg exchange of Fe$-$Fe and Fe$-$Nd atomic pairs, the uniaxial single-ion anisotropy of Fe atoms, and the crystal-field energy of Nd ions which …
We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybr…
Within density-functional theory, perturbation theory~(PT) is the state-of-the-art formalism for assessing the response to homogeneous electric fields and the associated material properties, e.g., polarizabilities, dielectric constants, and Raman intensities. Here we derive a real-space formulation of PT and present an implementation within the all-electron, numeric atom-centered orbitals electronic structure code FHI-aims that allows for massively-parallel calculations. As demonstrated by extensive validation, this allows the rapid computation of accurate response properties of molecules and …
Novel 2D Altermagnetic Vanadium Oxide with a Buckled Lieb Structure
UNAM - National Nanotechnology Research Center and Institute of Materials Science and Nanotechnology, Bilkent University（比尔肯特大学纳米技术与材料科学研究所）；Linköping University（林雪平大学）；Uppsala University（乌普萨拉大学）；University of Iowa（爱荷华大学）
基于密度泛函理论第一性原理计算，发现单层V₂O晶体在翘曲Lieb晶格中是一种二维交变磁性材料，具有负泊松比、面外易磁化轴、1.2 eV动量依赖自旋劈裂以及大贝里曲率和约40 (ℏ/e) S cm⁻¹的自旋霍尔电导率。
