The Influence of Edge Inhomogeneities on Vortex Hysteresis Curves in Magnetic Tunnel Junctions

Magnetoresistive sensors using a tunnel magnetoresistance (TMR) spin valve structure with CoFe and CoFeB free layer (FL) are investigated. Lateral dimension, thickness, and magnetic properties of the circular-shaped FL energetically favor the nucleation of a magnetic vortex. In the vortex configuration, the sensors show the expected hysteresis-free transfer curve. Distinct differences between CoFe and CoFeB in the vortex typical parameters are observed. The experimental results are compared to micromagnetic simulations. Influence of saturation magnetization ${M}_{s}$ on the vortex transfer curve is studied in order to reproduce the experimental data. However, only by adjusting ${M}_{s}$ iteratively the experimental data cannot be fully reproduced: The vortex annihilation ( ${H}_{\mathrm {an}}$ ) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at ${H}_{\mathrm {an}}$ is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at ${H}_{\mathrm {an}}$ . This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in ${H}_{\mathrm {an}}$ can be explained for 1.1 $\mu \text{m}$ FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at ${H}_{\mathrm {an}}$ can be understood by introducing an electrically inactive edge area. The analysis shows that for CoFe and CoFeB different edge inhomogeneity effects are present or edge inhomogeneities have different impacts.