Strain-controlled Domain Wall injection into nanowires for sensor applications

Abstract

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.