The shape of the Hanle curve in spin-transport structures in the   presence of the ac drive
  Resistance between two ferromagnetic electrodes coupled to a normal channel depends on their relative magnetizations. The spin-dependent component, R, of the resistance changes with magnetic field, B, normal to the directions of magnetizations. In the field of spin transport, this change, R(B), originating from the Larmour spin precession, is called the Hanle curve. We demonstrate that the shape of the Hanle curve evolves upon application of an ac drive and study this evolution theoretically as a function of the amplitude, B_1, and frequency, w, of the drive. If the distance between the electrodes, L, is smaller than the spin-diffusion length, l_s, the prime effect of a weak circular-polarized drive is the shift of the center of the curve to the value of B for which the Larmour frequency, w_L, is ~B_1^2 w. Magnetic resonance at w_L~w manifests itself in the derivative, dR/dB. For large L >> l_s the ac drive affects the Hanle curve if the drive amplitude exceeds the spin relaxation rate, 1/t_s, i.e. at B_1 t_s > 1. The prime effect of the drive is the elimination of a minimum in R(B). Linearly polarized drive has a fundamentally different effect on the Hanle curve, affecting not its shape, but rather its width. 
R. C. Roundy, M. C. Prestgard, A. Tiwari, and M. E. Raikh
cond-mat.mes-hall