  Materials with spin-momentum locked surface or interface states provide an
interesting playground for studying physics and application of charge-spin
current conversion. To characterize their non-equilibrium magnetic and
transport properties in the presence of a time-dependent external magnetic
field and a spin injection from a contact, we introduce three macroscopic
variables: a vectorial helical magnetization, a scaler helical magnetization,
and the conventional magnetization. We derive a set of closed dynamic equations
for these variables by using the spinor Boltzmann approach with the collision
terms consistent with the symmetry of spin-momentum locked states. By solving
the dynamic equations, we predict several intriguing magnetic and transport
phenomena which are experimentally accessible, including magnetic resonant
response to an AC applied magnetic field, charge-spin conversion, and spin
current induced by the dynamics of helical magnetization.
