arXiv subjects

Mojtaba Alaei

Publications and source records attributed to Mojtaba Alaei.

29 records · Page 2Linked to original sources

arXiv2020

Magnetic and structural properties of Ni-substituted magnetoelectric Co$_4$Nb$_2$O$_9$

The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds.

cond-mat.mtrl-sci↗

arXiv2020

Ab initio determination of magnetic ground state of pyrochlore Y$_2$Mn$_2$O$_7$

There are two discrepant experimental results on the magnetic ground state of Y$_{2}$Mn$_{2}$O$_{7}$, one study proposes a spin glass state, while another introduces the material as a ferromagnet. In this study, we attempt to resolve this issue by employing density functional theory and Monte Carlo simulations. We derive different spin models by varying the Hubbard $U$ parameter in ab initio GGA+$U$ calculations. For the most range of Hubbard $U$, We obtain that the leading terms in the spin Hamiltonian are bi-quadratic and the nearest neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations of these models with the experiments, we find a ferromagnetic ground state for Y$_{2}$Mn$_{2}$O$_{7}$ as the most compatible with experiments. We also consider Y$_{2}$Mo$_{2}$O$_{7}$ as a prototype of the defect-free pyrochlore system with spin-glass behavior and compare it with Y$_{2}$Mn$_{2}$O$_{7}$. The orbital degrees of freedom are considered as a leading factor in converting a defect-free pyrochlore such as Y$_{2}$Mn$_{2}$O$_{7}$ to a spin glass system. By changing the $d$ orbital occupations of Mo atoms, our GGA+$U$ calculations for Y$_{2}$Mo$_{2}$O$_{7}$ indicate many nearly degenerate states with different $d$ orbital orientations which reveals $d$ orbital degrees of freedom in this material. While for Y$_{2}$Mn$_{2}$O$_{7}$, we find a single ground state with a fixed orbital orientation. Consequently, all of our ab initio approaches confirm Y$_{2}$Mn$_{2}$O$_{7}$ as a ferromagnetic system.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio investigation of magnetic ordering in the double perovskite Sr$_{2}$NiWO$_{6}$

{\it Ab initio} calculations, GGA/GGA+$U$, are used to propose a spin Hamiltonian for the B-site ordered double perovskite, Sr$_{2}$NiWO$_{6}$. Our results show that the exchange interaction constants between the next nearest neighbors in both intra- and inter- $ab$ plane ($J_2$ and $J_{2c}$) are an order of magnitude larger than the ones between the nearest neighbors ($J_1$ and $J_{1c}$). Employing the Monte Carlo simulation, we show that the obtained Hamiltonian properly describes the finite temperature properties of Sr$_{2}$NiWO$_{6}$. Our {\it ab initio} calculations also reveal a small magnetic anisotropy and non-trivial bi-quadratic interaction between the nearest inter-$ab$ plane neighbors, which play essential roles in stabilizing the type-II anti-ferromagnetic ground state of Sr$_{2}$NiWO$_{6}$.

cond-mat.str-el↗

arXiv2018

Effects of strain on electronic and magnetic properties of Co/WS$_2$ junction: a density functional and Monte Carlo study

In this work, density functional computations and Monte Carlo simulations are performed to investigate structural, electronic, magnetic, and thermodynamic properties of Co/WS$_2$ junction, a semiconductor WS$_2$ monolayer covered by a ferromagnetic cobalt monolayer. In addition to a conventional semilocal exchange-correlation functional, three nonlocal functional, including the novel ACBN0 scheme, are applied to obtain reliable electronic and magnetic properties. It is argued that the ACBN0 scheme, is very efficient for first principles description of the Co/WS$_2$ junction. The obtained electronic structures evidence a trustworthy half-metallic gap in the majority spin channel of the lowest energy configuration of the junction, promising for spintronic applications. The obtained magnetic thermodynamics properties from Monte Carlo simulations predict a Curie temperature of about 110\,K, which is far small for device applications of this junction. The electronic and magnetic properties of the system are calculated under various compressive and tensile strains and it is shown that a tensile strain of about 4\% may effectively improve thermal stability of half-metallic ferromagnetism in the Co/WS$_2$ junction.

cond-mat.mtrl-sci↗

arXiv2018

Crossover between tricritical and Lifshitz points in pyrochlore FeF$_{3}$

Pyrochlore FeF$_{3}$ (pyr-FeF$_{3}$) is a Heisenberg anti-ferromagnetic (AF) with a magnetic susceptibility deviating from the Curie-Weiss law, even at the room temperature. This compound shows a transition to a long-range ordered state with all-in all-out (AIAO) spin configuration. The critical properties of this transition have remained a matter of dispute. In this work, to gain more insight into the critical properties of pyr-FeF$_{3}$, using ab initio density functional theory (DFT), we obtain spin Hamiltonian of this material under the relative volume change with respect to the experimental volume ($\frac{ΔV}{V_0}$) from $-0.2$ to $0.2$. We show that the relevant terms in the spin Hamiltonians are the AF exchange up to third neighbors, the nearest neighbor bi-quadratic and the direct Dyzaloshinski-Moriya (DM) interactions and find how these coupling constants vary under the volume change. Then we study the effect of volume change on the finite temperature critical behavior, using classical Monte Carlo (MC) simulation. We show that the spin system undergoes a weakly first order transition to AIAO at small volumes which turns to a second order transition close to the experimental structure. However, increasing $\frac{ΔV}{V_0}$ to $\sim0.2$, systems shows a transition to a modular spin structure. This finding suggests the existence of a Lifshitz point in pyr-FeF$_{3}$ and may explain the unusual critical exponents observed for this compound.

physics.comp-ph↗

arXiv2017

Origin of magnetic frustration in Bi$_3$Mn$_4$O$_{12}$(NO$_3$)

Bi$_3$Mn$_4$O$_{12}$(NO$_3$) (BMNO) is a honeycomb bilayers anti-ferromagnet, not showing any ordering down to very low temperatures despite having a relatively large Curie-Weiss temperature. Using ab initio density functional theory, we extract an effective spin Hamiltonian for this compound. The proposed spin Hamiltonian consists of anti-ferrimagnetic Heisenberg terms with coupling constants ranging up to third intra-layer and fourth inter-layer neighbors. Performing Monte Carlo simulation, we obtain the temperature dependence of magnetic susceptibility and so the Curie-Weiss temperature and find the coupling constants which best matches with the experimental value. We discover that depending on the strength of the interlayer exchange couplings, two collinear spin configurations compete with each other in this system. Both states have in plane N{é}el character, however, at small interlayer coupling spin directions in the two layers are antiparallel (N$_1$ state) and discontinuously transform to parallel (N$_2$ state) by enlarging the interlayer couplings at a first order transition point. Classical Monte Carlo simulation and density matrix renormalization group calculations confirm that exchange couplings obtained for BMNO are in such a way that put this material {at the phase boundary of a first order phase transition}, where the trading between these two collinear spin states prevents it from setting in a magnetically ordered state.

cond-mat.str-el↗

arXiv2016

First-principles structure search for the stable isomers of stoichiometric WS2 nano-clusters

In this paper, we employ evolutionary algorithm along with the full-potential density functional theory (DFT) computations to perform a comprehensive search for the stable structures of stoichiometric (WS2)n nano-clusters (n=1-9), within three different exchange-correlation functionals. Our results suggest that n=3, 5, 8 are possible candidates for the low temperature magic sizes of WS2 nano-clusters while at temperatures above 600 Kelvin, n=5 and 7 exhibit higher relative stability among the studied systems. The electronic properties and energy gap of the lowest energy isomers were computed within several schemes, including semilocal PBE and BLYP functionals, hybrid B3LYP functional, many body based DFT+GW approach, and time dependent DFT calculations. Vibrational spectra of the lowest lying isomers, computed by the force constant method, are used to address IR spectra and thermal free energy of the clusters. Time dependent density functional calculation in real time domain is applied to determine the full absorption spectra and optical gap of the lowest energy isomers of the WS2 nano-clusters.

cond-mat.mtrl-sci↗

arXiv2015

On the Classical Vibrational Coherence of Carbonyl Groups in the Selectivity Filter Backbone of KcsA Ion Channel

It has been suggested that quantum coherence in the selectivity filter of ion channel may play a key role in fast conduction and selectivity of ions. However, it has not been clearly elucidated yet why classical coherence is not sufficient for this purpose. In this paper, we investigate the classical vibrational coherence between carbonyl groups oscillations in the selectivity filter of KcsA ion channels based on the data obtained from molecular dynamics simulations. Our results show that classical coherence plays no effective role in fast ionic conduction.

q-bio.BM↗

arXiv2015

Spin Hamiltonian, {Order Out of a Coulomb Phase} and Pseudo-Criticality in the Highly Frustrated Pyrochlore Heisenberg Antiferromagnet FeF$_{\bf 3}$}

FeF$_3$, with its half-filled Fe$^{3+}$ $3d$ orbital, hence zero orbital angular momentum and $S=5/2$, is often put forward as a prototypical highly-frustrated classical Heisenberg pyrochlore antiferromagnet. By employing {\it ab initio} density functional theory (DFT), we obtain an effective spin Hamiltonian for this material. This Hamiltonian contains nearest-neighbor antiferromagnetic Heisenberg, bi-quadratic and Dzyaloshinskii-Moriya interactions as dominant terms and we use Monte Carlo simulations to investigate the nonzero temperature properties of this minimal model. We find that upon decreasing temperature, the system passes through a Coulomb phase, composed of short-range correlated coplanar states, before transforming into an "all-in/all-out" (AIAO) state via a very weakly first order transition at a critical temperature $T_c\approx 22$ K, in good agreement with the experimental value for a reasonable set of Coulomb interaction $U$ and Hund's coupling $J_{\rm H}$ describing the material. Despite the transition being first order, the AIAO order parameter evolves below $T_c$ with a power-law behavior characterized by a pseudo "critical exponent" $β\approx 0.18$ in accord with experiment. We comment on the origin of this unusual $β$ value.

cond-mat.str-el↗

arXiv2015

First-principles insights into f magnetism, a case study on some magnetic pyrochlores

First-principles calculations are performed to investigate f magnetism in A$_2$Ti$_2$O$_7$ (A=Eu, Gd, Tb, Dy, Ho, Er, Yb) magnetic pyrochlore oxides. The Hubbard U parameter and the relativistic spin orbit correction is applied for more accurate description of the electronic structure of the systems. It is argued that the main obstacle for first-principles study of these systems is the multi-minima solutions of their electronic configuration. Among the studied pyrochlores, Gd$_2$Ti$_2$O$_7$ shows the least multi-minima problem. The crystal electric field theory is applied for phenomenological comparison of the calculated spin and orbital moments with the experimental data.

cond-mat.str-el↗

arXiv2015

Density-functional study of the pure and palladium doped small copper and silver clusters

The size-dependent electronic, structural, magnetic and vibrational properties of small pure cop- per and silver clusters and their alloys with one and two palladium atoms are studied by using full-potential all-electron density functional computations. The stable isomers of these clusters are identified and their theoretical magic numbers are determined via the analysis of the second differ- ence of their minimized energy. We discuss that the doped Pd atoms generally prefer to sit in the high coordination sites of the pure clusters. It is argued that Pd doping influences the structural properties and the two dimensional to three dimensional structural cross over in the small Cu and Ag clusters. The many body based GW correction is applied for more accurate determination of the electron affinity and ionization potential of these systems. Magnetic and vibrational properties of the pure and doped clusters are presented and discussed.

cond-mat.mtrl-sci↗