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 Ms 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
_an
) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at H
_an
is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at H
_an
. This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in H
_an
can be explained for 1.1 μm FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at H
_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.