Effect of exchange-correlation spin–torque on spin dynamics

Abstract

A recently proposed exchange-correlation functional with in density functional theory, which ensures that the exchange-correlation magnetic field is source-free, is shown to give non-zero internal spin-torque. This spin-torque is identically zero for all conventional local and semi-local functionals. Extension of this source-free functional to the time domain is used to study the effect of the internal spin-torque on the laser induced spin-dynamics in bulk Co, Ni and interfaces of these metals with Pt. It is shown that the internal spin-torque contribute significantly to spin-dynamics only when the magneto crystalline anisotropy energy is small, as in the case of cubic bulk materials. For surfaces or interfaces, where the anisotropy energy is large, these torques are too small to cause any significant precession of spins in early times (< 100fs). Further more it is shown that the spin-dynamics caused by the internal spin-torque is slow compared to the inter-site spin transfer and spin-orbit mediated spin-flips.

Element Specificity of Transient Extreme Ultraviolet Magnetic Dichroism

“…A Schrödingerlike equation is then used to propagate 2-component Pauli spinors in time, from which the expectation value of observables, such as the magnetic moment, may be calculated. Real time (RT) TDDFT has proved successful in predicting and understanding ultrafast spin dynamics in bulk ferromagnetics [8,12], Heusler compounds [13], Co/Cu interfaces [14], and Ni/Pt multilayers [15][16][17]. Furthermore, linear response (LR) TDDFT was recently shown to successfully calculate the static MCD spectra for Fe, Co and Ni which requires a good description of the quantum mechanical electronic structure [18].…”

Spin vacuum switching

“…At no time during this ultrafast switching does moment perpendicular to the original magnetization axis develop: femtosecond scale magnetization switching without out-of-axis rotation (this is explicitly shown in section S7, where we plot the magnetization vector field in all three directions). To ensure that the conclusions that we draw (i.e., switching of spin orientation without rotation) are not an artifact of approximations used in the theory, which does not allow for local exchange-correlation spin torques ( 12 , 13 ), we have performed calculations that explicitly include such torques (see Methods for details) ( 14 ) and find exactly the same physics of switching without rotation (section S7).…”

“…This implies that there is no local xc spin-torque at any time. To ensure that the physics is robust, we have also performed calculations with the source-free functional ( 14 ), which explicitly allows for and leads to spin-torques ( 12 , 13 ).…”