Spin-transfer and fieldlike torques in antiferromagnets

The phenomenon of spin-transfer torque (STT) in an antiferromagnet (AFM) is reexamined in terms of the incoming and outgoing spin currents. In contrast to the conventional approach, our model treats the STT as a result of the electron spin angular momentum transfer to the entire monodomain magnetic structure rather than to the individual sublattice magnetizations. This treatment enables the analysis to account for not only the destructive role of electron spin relaxation but also the potentially incomplete loss of electron spin phases that can either enhance the STT or suppress it depending on the structure parameters. Application of the developed model to the dynamical equation of the AFM order parameter illustrates the qualitative and quantitative differences with the conventional approach. Unlike the latter, our results predict a strong dependence of the STT on the orientation of the electron spin polarization relative to the magnetic anisotropy axes of the AFM. A similar characteristic also reveals a complex interplay in the Neel vector dynamics that can lead to a sublinear response of the oscillation frequency to the strength of the spin current. The impact of the fieldlike torque that can arise likewise from the spin injection is examined as well for a comprehensive account. The numerical calculations elucidate further the conditions that can elicit efficient manipulation of the magnetic states in an AFM under a spin-polarized current.