An apparatus, system, and method for probing magnetic permeability of a material
located a distance from the apparatus comprises a first hard ferromagnetic layer,
a second hard ferromagnetic layer, a first soft ferromagnetic layer, a second soft
ferromagnetic layer, an intermediate layer disposed between the first hard ferromagnetic
layer and the first soft ferromagnetic layer, an insulating layer between the first
soft ferromagnetic layer and second soft ferromagnetic layer, and a spacer layer
disposed between the second soft ferromagnetic layer and the second hard ferromagnetic
layer, wherein the second soft ferromagnetic layer increases an on/off ratio of
a magnetic field in the first soft ferromagnetic layer, wherein the on/off ratio
is approximately 1.6, and wherein the second soft ferromagnetic layer pulls a magnetic
field of the apparatus into the first soft ferromagnetic layer.