TY - JOUR

T1 - Phonons in magnetically disordered materials

T2 - Magnetic versus phononic time scales

AU - Dutta, Biswanath

AU - Körmann, Fritz

AU - Ghosh, Subhradip

AU - Sanyal, Biplab

AU - Neugebauer, Jörg

AU - Hickel, Tilmann

PY - 2020

Y1 - 2020

N2 - The lattice dynamics in magnetic materials, such as Fe depends on the degree of disorder of the atomic magnetic moments and the time scale of spin fluctuations. Using first-principles methods, we have studied this effect by determining the force constant matrix in two limits: (i) When spin fluctuations are much faster than the atom vibrations, their combined impact is captured by a spin-space averaged force constant matrix, (ii) when individual spin fluctuations are sufficiently slow to scatter the phonon modes, the itinerant coherent potential approximation with spin-pair resolved force constants (i.e., φ↑↑,φ↓↓, and φ↑↓) is employed in this paper. The physical consequences for the vibrational spectral functions are analyzed by systematically modifying the input parameters (magnetization and ratio of force constants betweens atoms with equal and opposite spin directions) and by deriving them for the prototype material system bcc Fe from first-principles calculations. In the paramagnetic regime, the two limits yield identical phonon spectra. Below the Curie temperature, however, there are regions in the parametric study that show qualitative differences, including a broadening of the phonon peaks. For bcc Fe, however, the quantitative modifications of phonon frequencies turn out to be small.

AB - The lattice dynamics in magnetic materials, such as Fe depends on the degree of disorder of the atomic magnetic moments and the time scale of spin fluctuations. Using first-principles methods, we have studied this effect by determining the force constant matrix in two limits: (i) When spin fluctuations are much faster than the atom vibrations, their combined impact is captured by a spin-space averaged force constant matrix, (ii) when individual spin fluctuations are sufficiently slow to scatter the phonon modes, the itinerant coherent potential approximation with spin-pair resolved force constants (i.e., φ↑↑,φ↓↓, and φ↑↓) is employed in this paper. The physical consequences for the vibrational spectral functions are analyzed by systematically modifying the input parameters (magnetization and ratio of force constants betweens atoms with equal and opposite spin directions) and by deriving them for the prototype material system bcc Fe from first-principles calculations. In the paramagnetic regime, the two limits yield identical phonon spectra. Below the Curie temperature, however, there are regions in the parametric study that show qualitative differences, including a broadening of the phonon peaks. For bcc Fe, however, the quantitative modifications of phonon frequencies turn out to be small.

UR - http://www.scopus.com/inward/record.url?scp=85083178238&partnerID=8YFLogxK

U2 - 10.1103/PhysRevB.101.094201

DO - 10.1103/PhysRevB.101.094201

M3 - Article

AN - SCOPUS:85083178238

VL - 101

JO - Physical Review B

JF - Physical Review B

SN - 2160-3308

IS - 9

M1 - 094201

ER -