A magnetic tunnel junction (MTJ) device usable as a magnetic memory cell or
magnetoresistive sensor, such as a MTJ read head for magnetic recording,
has the free ferromagnetic layer located on the bottom of the device, the
bottom free layer being formed on a special underlayer. The MTJ read head
may be a flux-guided head that uses the free layer as a flux guide for
directing magnetic flux from the magnetic media to the sensing region of
the MTJ. The special underlayer for the growth of the free layer is an
alloy comprising Mn, one of Pt, Ni, Ir and Os, and an additive X selected
from Ta, Al, Ti, Cu, Cr and V. Without the additive, the underlayer alloy
is antiferromagnetic. The additive is present in an amount sufficient to
render the alloy to have no magnetic ordering, i.e., it is neither
antiferromagnetic nor ferromagnetic, but without substantially affecting
the preferred crystalline texture and unit cell size so that the
underlayer is well-suited as a growth-enhancing underlayer for the free
layer.
