  Precise estimation of spin Hall angle as well as successful maximization of
spin-orbit torque (SOT) form a basis of electronic control of magnetic
properties with spintronic functionality. Until now, current-nonlinear Hall
effect, or second harmonic Hall voltage has been utilized as one of the methods
for estimating spin Hall angle, which is attributed to the magnetization
oscillation by SOT. Here, we argue the second harmonic Hall voltage in
magnetic/nonmagnetic topological insulator (TI) heterostructures,
Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$. From the
angular, temperature and magnetic field dependence, it is unambiguously shown
that the large second harmonic Hall voltage in TI heterostructures is governed
not by SOT but mainly by asymmetric magnon scattering mechanism without
magnetization oscillation. Thus, this method does not allow an accurate
estimation of spin Hall angle when magnons largely contribute to electron
scattering. Instead, the SOT contribution in a TI heterostructure is
exemplified by current pulse induced non-volatile magnetization switching,
which is realized with a current density of $\sim 2.5 \times 10^{10}
\mathrm{A/m}^2$, showing its potential as spintronic materials.
