  We study the current-driven skyrmion expulsion from magnetic nanostrips using
micromagnetic simulations and analytic calculations. We explore the threshold
current density for the skyrmion expulsion, and show that this threshold is
determined by the critical boundary force as well as the spin-torque
parameters. We also find the dependence of the critical boundary force on the
magnetic parameters; the critical boundary force decreases with increasing the
exchange stiffness and perpendicular anisotropy constants, while it increases
with increasing Dzyaloshinskii-Moriya interaction and saturation magnetization
constants. Using a simple model describing the skyrmion and locally-tilted edge
magnetization, we reveal the underlying physics of the dependence of the
critical boundary force on the magnetic parameters based on the relation
between the scaled Dzyaloshinskii-Moriya-interaction parameter and the critical
boundary force. This work provides a fundamental understanding of the skyrmion
expulsion and the interaction between the skymion and boundaries of devices and
shows that the stability of the skyrmion in devices can be related to the
scaled Dzyaloshinskii-Moriya-interaction parameter of magnetic materials.
