Pablo Nieves

Interatomic potentials are essential for molecular dynamics simulations of magnetic materials, yet incorporating magnetic features into potentials for complex antiferromagnets remains challenging. Nickel oxide (NiO), a prototypical cubic antiferromagnet, exemplifies this difficulty. Here, we develop a methodology to integrate magnetic properties into interatomic potentials for cubic antiferromagnets by adding a magnetic Hamiltonian, which includes both the Heisenberg exchange and the Néel model. We apply this approach to NiO by constructing two potentials: one based on the Born model of ionic solids and another using a reference-free modified embedded atom method. The models are validated against density functional theory calculations and experimental data, showing excellent agreement in mechanical and magnetic properties across both zero and finite temperatures, correctly capturing thermal expansion and the temperature dependence of elastic constants. Furthermore, we demonstrate the sensitivity of the potential to symmetry-breaking lattice distortions (tetragonal, shear, and trigonal), providing a predictive framework for controlling the Néel vector via strain engineering in antiferromagnetic spintronics. These models enable large-scale simulations of magnetoelastic phenomena in antiferromagnets and open avenues for molecular dynamics studies involving coupled electric and magnetic fields in metal oxides.

Recently, there has been renewed interest in the magnetism of FeRh that extends well beyond the extensively studied, yet not fully understood, antiferromagnetic–ferromagnetic transition. Despite its popularity, fundamental magnetic properties—including magnetocrystalline anisotropy and magnetoelasticity—remain insufficiently characterized from both theoretical and experimental studies. In this work, we first used Density Functional Theory to analyze these properties in single crystal FeRh for both antiferromagnetic and ferromagnetic phases at zero temperature, revealing significant qualitative and quantitative differences between them. We then attempted to develop a magnetic interatomic potential for a classical spin-lattice model, coupling spin dynamics and molecular dynamics, that reproduces the magnetoelastic phenomenology obtained from first principles calculations. Our results constitute a step towards a better understanding of the complex magnetoelasticity of FeRh and open the door to more accurate large-scale atomistic modeling of similar materials undergoing antiferromagnetic-ferromagnetic first-order phase transitions.

The origin of magnetocrystalline anisotropic energy (MAE) guided by spin–orbit coupling in the L1₀-FePt alloy was analyzed. Correlation of MAE with the calculated magnetoelastic constants in the single and polycrystal model was determined by means of first-principles calculations using density functional theory (DFT). More specifically, a systematic analysis of the large MAE and magnetostriction coefficients (λ’s) were done by means of post-processing of eigenvalues (orbital energies) and eigenfunctions (orbital occupancies). Our numerical analysis includes the convolution of the projected wave-function (density of states) of each orbital of the Fe and Pt sub-lattices into their orbital energies, which in principle should be valid for any solid regardless of the strength of the MAE. The determined anisotropic magnetostriction coefficients are of the λ ∼ 10 − 4 –10^–3 order for several crystallographic directions; which is in accord to the known experimental data. However, the polycrystalline model based on the uniform stress approximation leads to a decrease of the λ by two orders in the overall magnetostrictive behavior ( λ ∼ 10 − 5 ). Such a large difference is not unexpected as the magnetostriction for polycrystalline FePt thin films were recorded as very low, thus validating the robustness of the current theoretical proposition of analyzing MAE.

Based on the idea of creating sensors sensitive to the magnetic component of terahertz electromagnetic radiation, we consider the potential possibilities of obtaining information about studied terahertz pulse by using spin-based sensors. We propose a way to analytically estimate the magnitude of the magnetic response to the action of a terahertz pulse depending on its parameters, analyze some ferromagnetic materials as possible candidates for creating the mentioned sensors, and analyze the difficulties and limitations that appear when using different types of ferromagnets.

Picosecond ultrasonics is a fast growing and advanced research field with broad application to the imaging and characterization of nanostructured materials as well as at a fundamental level. The aim of this paper is to provide an advanced 3D model based on atomistic spin-lattice simulations of the laser-induced elastic response in ferromagnetically ordered fcc Ni. The advantage of such an approach is the possibility to take into account the laser radiation interaction with the spins and thus characterize the magnetic contribution to the total stress. We analyze the atomic displacements caused both by the ultrafast thermal expansion of the crystal lattice and by the demagnetization process due to the heating of a certain area of the sample by an ultrashort laser pulse. Subsequently, an attempt is made to propose mathematical expressions for describing the corresponding total stress. The lattice and magnetic contributions have been evaluated, whereupon the former is found to be much greater than the latter. Published by the American Physical Society 2024