Evaluation of the Exchange Stiffness Constants of Itinerant Magnets from the FirstPrinciples Calculations
© The Physical Society of Japan
This article is on
Evaluation of the Exchange Stiffness Constants of Itinerant Magnets at Finite Temperatures from the FirstPrinciples Calculations
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 93, 054705 (2024).
Using firstprinciples calculations, we evaluated the exchange stiffness constants of ferromagnetic metals at finite temperatures. The constants can be used as parameters in the Landau–Lifshitz–Gilbert equation.
Firstprinciples calculations based on electronic theory are powerful method for quantitatively representing the individuality of materials without the need for adjustable parameters. However, most industrial materials are far from perfect crystals and exhibit many defects such as vacancies, grain boundaries, and strains. Currently, it is difficult to describe the properties of these practical materials solely via firstprinciples calculations, and coarse graining based on the stiffness or characteristic length of order parameters is required. In such scale regions, the equations, such as the LandauLifshitzGilbert (LLG) equation for magnetism, are effective. This equation enables visualization of the macroscopic behavior of the order parameters (magnetization) in complex environments. Based on this perspective, the role of the firstprinciples calculations is to provide the parameters (magnetic constants) constituting the LLG equation.
The magnetic constants that constitute the LLG equation include the exchange stiffness constant (𝐴), magnetic anisotropy constant (K), and Gilbert damping constant (α). The issue arises because the interest in material properties predominantly focuses on room temperature or higher temperature regimes. Hence, the challenge involves determining the magnetic constants at finite temperatures from a microscopic standpoint. With respect to general metallic ferromagnets, functional integral methods have been powerful prescriptions for the finite temperature magnetism.
In this study, we focus on the firstprinciples calculations of the exchange stiffness constant 𝐴 at finite temperatures based on the functional integral method. The exchange stiffness constant is a measure of the rigidity (or hardness) of regular spin arrangements (such as ferromagnetic alignments) arising from the exchange interaction between spins. To evaluate 𝐴(𝑇), a spiral magnetic structure characterized by wavevector 𝑞 is generated in the presence of spin fluctuations. Subsequently, we can obtain 𝐴(𝑇) from the increase in free energy.
Specifically, we calculate 𝐴(𝑇) of Permalloy (Fe_{0.2}Ni_{0.8}) and L10type FePt. For Fe_{0.2}Ni_{0.8}, the calculated magnetic moment is approximately 1 Bohr magneton (µ_{B}) at 𝑇 = 300 K, closely reproducing the measured values. The Curie temperature is approximately 550 K, slightly lower than the measured value (670 K). The obtained 𝐴(𝑇) falls within the range of values empirically used in the LLG equation (several meV/Å to 10 meV/Å), and 𝐴(𝑇) at room temperature appears to match well with the measured values. Additionally, 𝐴(𝑇) of FePt shows similar temperature dependence, aligning well with room temperature measurements. The results highlight the structure’s anisotropy, with significant directional dependence in 𝐴(𝑇).
Written by A. Sakuma
Evaluation of the Exchange Stiffness Constants of Itinerant Magnets at Finite Temperatures from the FirstPrinciples Calculations
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 93, 054705 (2024).
Share this topic
Fields
Related Articles

Which is Moving?—Pinning Down the Origin of Fluctuations in Muon Spin Relaxation—
Structure and mechanical and thermal properties in condensed matter
Crossdisciplinary physics and related areas of science and technology
2024328
The study demonstrated that we can distinguish between the diffusion motion of the muon itself and the motion of the surrounding ions in muon spin relaxation.

Variety of Mechanically Induced Spin Currents in Rashba Systems
Electronic transport in condensed matter
Magnetic properties in condensed matter
Structure and mechanical and thermal properties in condensed matter
2024322
Various types of spin currents, including unconventional types, are generated in Rashba spinorbit coupled systems by dynamic lattice distortions associated with, for example, surface acoustic waves.

Relation between MeanField Theory and Atomic Structures in Chalcogenide Glasses
Structure and mechanical and thermal properties in condensed matter
202421
The authors conducted various of Xray and neutron scattering experiments on typical chalcogenide glasses and clarified the relationship between the atomic structure and simple rigidity percolation theory.

Possible Origin of High Thermoelectric Power Factor in Ultrathin FeSe: A Twoband Model
Electronic structure and electrical properties of surfaces and nanostructures
Structure and mechanical and thermal properties in condensed matter
Crossdisciplinary physics and related areas of science and technology
20231221
The high thermoelectric power factor observed in ultrathin FeSe can be theoretically explained by a twoband model with chemical potential between upper and lower band bottoms.

Exploring Recent Advances in the Physics of Biofluid Locomotion
Measurement, instrumentation, and techniques
Crossdisciplinary physics and related areas of science and technology
Electromagnetism, optics, acoustics, heat transfer, and classical and fluid mechanics
Statistical physics and thermodynamics
Mathematical methods, classical and quantum physics, relativity, gravitation, numerical simulation, computational modeling
Structure and mechanical and thermal properties in condensed matter
2023128
This Special Topics Edition of the JPSJ describes the latest advances in the field of biofluid locomotion, shedding light on the underlying physics behind the movement of organisms that swim and fly.