  We investigate experimentally the effects of externally applied strain on the
injection of 180$^\circ$ domain walls (DW) from a nucleation pad into magnetic
nanowires, as typically used for DW-based sensors. In our study the strain,
generated by substrate bending, induces in the material a uniaxial anisotropy
due to magnetoelastic coupling. To compare the strain effects,
$Co_{40}Fe_{40}B_{20}$, $Ni$ and $Ni_{82}Fe_{18}$ samples with in-plane
magnetization and different magnetoelastic coupling are deposited. In these
samples, we measure the magnetic field required for the injection of a DW, by
imaging differential contrast in a magneto-optical Kerr microscope. We find
that strain increases the DW injection field, however, the switching mechanism
depends strongly on the direction of the strain with respect to the wire axis.
We observe that low magnetic anisotropy facilitates the creation of a domain
wall at the junction between the pad and the wire, whereas a strain-induced
magnetic easy axis significantly increases the coercive field of the nucleation
pad. Additionally, we find that the effects of mechanical strain can be
counteracted by a magnetic uniaxial anisotropy perpendicular to the
strain-induced easy axis. In $Co_{40}Fe_{40}B_{20}$, we show that this
anisotropy can be induced by annealing in a magnetic field. We perform
micromagnetic simulations to support the interpretation of our experimental
findings. Our simulations show that the above described observations can be
explained by the effective anisotropy in the device. The anisotropy influences
the switching mechanism in the nucleation pad as well as the pinning of the DW
at the wire entrance. As the DW injection is a key operation for sensor
performances, the observations show that strain is imposing a lower limit for
the sensor field operating window.
