Spin-lattice model simulations of tetragonal FePt system

TL;DR

This work addresses magnetoelastic coupling modeling in tetragonal FePt with a scalable spin-lattice approach. It develops a tetragonal spin-lattice Hamiltonian $H_{SL}$ that couples interatomic potentials $\mathcal{V}_{ij}(r_{ij})$ with magnetic terms $\mathcal{H}_Z$, $\mathcal{H}_{ex}$, $\mathcal{H}_{di}$, and $\mathcal{H}_{MCA}$, with exchange $J_{ij}(r_{ij})$ and pseudo-dipolar $l_{ij}(r_{ij})$ parameterized by Bethe-Slater curves $\Phi(r_{ij})$. A key contribution is linking the pseudo-dipolar contributions to the magnetoelastic constants $b_i$ and adjusting $\tilde{K}_1$ to offset interference, validated by comparing with ab initio MAE and $b_i$ from VASP. The model is implemented in LAMMPS with a custom RF-MEAM potential to reproduce the energy-volume curve and elastic constants, enabling accurate, large-scale simulations of magnetoelastic phenomena in tetragonal systems such as L1$_0$ FePt.

Abstract

Magnetic materials play a key role in the contemporary industry, providing unique features with a wide application potential. To study physical phenomena and design new materials, it is important to possess an appropriate tool, a model allowing simulation of desired behavior. Spin-lattice model simulations can be used to investigate spacious systems containing thousands of atoms, while complex phenomena arising from the interplay of lattice and spin dynamics can be modeled. One of the important phenomena that can be modeled in the spin lattice simulation is magnetoelastic behavior, offering direct conversion between the mechanical and magnetic energy. However, so far, only models for systems with cubic symmetry have been introduced. Therefore, here, a spin-lattice model for a system with tetragonal symmetry is proposed, where its strength is manifested by simulation of magnetoelastic properties of a characteristic representative L1$_{0}$ FePt system.

Figures (6)

• Figure 1: Tetragonal supercell 2x2x2 with depicted pseudo-dipolar interactions $l_{i}(r_{i})$
• Figure 2: Volume dependence of the total energy difference in FePt calculations. (filled point) DFT result calculated in VASP. (empty triangle) LAMMPS simulation with an alloy potential. (empty square) LAMMPS simulation with DFT-based potential, (empty circle) LAMMPS simulation with DFT-based potential and SL model included.
• Figure 3: Magnetocrystalline anisotropy simulation for FePt system obtained by ab initio VASP calculations (filled points) and by the SL-model simulations (empty points).
• Figure 4: FePt 2$\times$2$\times$2-supercell. Exchange interaction directions for the five nearest Fe-Fe neighbors depicted. (plotted in VESTA 3 VESTA)
• Figure 5: FePt magnetic exchange coupling parameters. (a) radial dependance of the exchange coupling $J(d)$. (b-f) Exchange coupling $J_{i}(d)$ under volume change of the FePt cell.