  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.

Mesoscale and Nanoscale Physics