  The issue of whether a thermal gradient acts like a magnetic field or an
electric current in the domain wall (DW) dynamics is investigated. Broadly
speaking, magnetization control knobs can be classified as energy-driving or
angular-momentum driving forces. DW propagation driven by a static magnetic
field is the best-known example of the former in which the DW speed is
proportional to the energy dissipation rate, and the current-driven DW motion
is an example of the latter. Here we show that DW propagation speed driven by a
thermal gradient can be fully explained as the angular momentum transfer
between thermally generated spin current and DW. We found DW-plane rotation
speed increases as DW width decreases. Both DW propagation speed along the wire
and DW-plane rotation speed around the wire decrease with the Gilbert damping.
These facts are consistent with the angular momentum transfer mechanism, but
are distinct from the energy dissipation mechanism. We further show that
magnonic spin-transfer torque (STT) generated by a thermal gradient has both
damping-like and field-like components. By analyzing DW propagation speed and
DW-plane rotation speed, the coefficient ( \b{eta}) of the field-like STT
arising from the non-adiabatic process, is obtained. It is found that \b{eta}
does not depend on the thermal gradient; increases with uniaxial anisotropy
K_(||) (thinner DW); and decreases with the damping, in agreement with the
physical picture that a larger damping or a thicker DW leads to a better
alignment between the spin-current polarization and the local magnetization, or
a better adiabaticity.
