arXiv subjects

Sergiu Arapan

Publications and source records attributed to Sergiu Arapan.

6 recordsLinked to original sources

arXiv2025

Opposite pressure effects on magnetic phase transitions in NiBr2

NiI2 and NiBr2 are archetypal van der Waals (vdW) triangular-lattice multiferroics that host incommensurate helimagnetic order at the lowest temperatures and undergo a transition to collinear antiferromagnetic order upon heating. Focusing on NiBr2, we reveal that both antiferromagnetic phases exhibit a pronounced sensitivity to hydrostatic pressure. The Neel temperature of the collinear phase increases steeply at 20 K/GPa, reaching 100 K at 3 GPa without any indication of saturation, whereas the helimagnetic phase is completely suppressed only above 0.8 GPa. This behavior contrasts sharply with NiI2, in which both helical and collinear phases are strengthened until a moderate pressure of 6 GPa, above which the helical phase instantly disappears. Ab initio calculations identify the second-nearest interlayer exchange interaction (j2') as the primary driver stabilizing the collinear AFM phase in NiBr2. In addition, the in-plane exchange ratio renders the helical order in NiBr2 considerably more fragile, enabling its suppression under relatively small pressures. These results underscore the dominant role of interlayer interactions in governing the distinct pressure responses of the magnetic phases in NiBr2 and NiI2.

cond-mat.mtrl-sci↗

arXiv2025

Effect of interface on magnetic exchange coupling in Co/Ru/Co trilayer: from ab-initio simulations to micromagnetics

Interfaces play a substantial role for the functional properties of structured magnetic materials and magnetic multilayers. Modeling the functional behavior of magnetic materials requires the treatment of the relevant phenomena at the device level. Properties predicted from the electronic structure and spin dynamics at the atomistic level have to be properly transferred into a continuum level treatment. In this work we show how Co/Ru/Co three layers can be simulated with the continuum theory of micromagnetism, with interface coupling energies and bulk intrinsic properties properly derived from the results of \emph{ab initio} and spin dynamics simulations at different temperatures.

cond-mat.mtrl-sci↗

arXiv2020

Large scale and linear scaling DFT with the CONQUEST code

We survey the underlying theory behind the large-scale and linear scaling DFT code, Conquest, which shows excellent parallel scaling and can be applied to thousands of atoms with exact solutions, and millions of atoms with linear scaling. We give details of the representation of the density matrix and the approach to finding the electronic ground state, and discuss the implementation of molecular dynamics with linear scaling. We give an overview of the performance of the code, focussing in particular on the parallel scaling, and provide examples of recent developments and applications.

physics.comp-ph↗

arXiv2019

Computational screening of Fe-Ta hard magnetic phases

In this work we perform a systematic calculation of the Fe-Ta phase diagram to discover novel hard magnetic phases. By using structure prediction methods based on evolutionary algorithms, we identify two new energetically stable magnetic structures: a tetragonal Fe$_3$Ta (space group 122) and cubic Fe$_5$Ta (space group 216) binary phases. The tetragonal structure is estimated to have both high saturation magnetization ($μ_0$M$_s$=1.14 T) and magnetocrystalline anisotropy (K$_1$=2.17 MJ/m$^3$) suitable for permanent magnet applications. The high-throughput screening of magneto-crystalline anisotropy also reveals two low energy metastable hard magnetic phases: Fe$_5$Ta$_2$ (space group 156) and Fe$_{6}$Ta (space group 194), that may exhibit intrinsic magnetic properties comparable to SmCo$_5$ and Nd$_2$Fe$_{14}$B, respectively.

cond-mat.mtrl-sci↗

arXiv2015

A linear scaling DFT study of the growth of a new \{105\} facet layer on a Ge hut cluster

Ge hut clusters, which appear during heteroepitaxy of Ge on Si(001), are prototypical examples of islands formed through strain. Experimentally, complete facets are observed to form rapidly, though the mechanism is unknown. We model the growth of new faces on Ge hut clusters, using linear scaling DFT. We build realistic small huts on substrates, and show that the growth of \{105\} facets proceeds from top-to-bottom, even for these small huts. The growth of the facet is driven by the reconstruction on the \{105\} facet, and nucleates at the boundaries of the facet. We thus resolve any ambiguities in kinetic models of hut cluster growth.

cond-mat.mtrl-sci↗

arXiv2011

Temperature driven $α$ to $β$ phase-transformation in Ti, Zr and Hf from first principles theory combined with lattice dynamics

Lattice dynamical methods used to predict phase transformations in crystals typically deal with harmonic phonon spectra and are therefore not applicable in important situations where one of the competing crystal structures is unstable in the harmonic approximation, such as the bcc structure involved in the hcp to bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an expression for the free energy that does not suffer from such shortcomings, and we show by self consistent {\it ab initio} lattice dynamical calculations (SCAILD), that the critical temperature for the hcp to bcc phase transformation in Ti, Zr and Hf, can be effectively calculated from the free energy difference between the two phases. This opens up the possibility to study quantitatively, from first principles theory, temperature induced phase transitions.

cond-mat.stat-mech↗