Microscopic theory of ultrafast spin linear reversal

Abstract

A recent experiment (Vahaplar et al 2009 Phys. Rev. Lett. 103 117201) showed that a singlefemtosecond laser can reverse the spin direction without spin precession, or spin linearreversal (SLR), but its microscopic theory has been missing. Here we show that SLR doesnot occur naturally. Two generic spin models, the Heisenberg and Hubbard models,are employed to describe magnetic insulators and metals, respectively. We findanalytically that the spin change is always accompanied by a simultaneous excitation ofat least two spin components. The only model that has prospects for SLR isthe Stoner single-electron band model. However, under the influence of the laserfield, the orbital angular momenta are excited and are coupled to each other. If acircularly polarized light is used, then all three components of the orbital angularmomenta are excited, and so are their spins. The generic spin commutation relationfurther reveals that if SLR exists, it must involve a complicated multiple stateexcitation.