Effect of Magnetic Coupling Between Two CoFeB Layers on Thermal Stability in Perpendicular Magnetic Tunnel Junctions With MgO/CoFeB/Insertion Layer/CoFeB/MgO Free Layer

We investigated the effect of magnetic coupling (the energy constant of unit area
$J_{\text {cpl}}$
) between CoFeB free layers on thermal stability factor
$\Delta $
in magnetic tunnel junction (MTJ) with CoFeB/MgO double interfacial perpendicular magnetic anisotropy (IPMA). We newly introduced
$J_{\text {cpl}}$
in models for calculations, which were based on magnetic domain propagation model when MTJ diameter
$d$
is larger than critical diameter
$d_{\text {c}}$
and magnetization coherent rotation model when
$d \le d_{\text {c}}$
. With increasing
$J_{\text {cpl}}$
,
$\Delta $
increases and saturates when
$J_{\text {cpl}}$
is over a critical value
$J_{\mathrm {cpl\_{}c}}$
. Magnetostatic coupling constants (
$J_{\text {stat}}$
) between the free layers were also calculated.
$J_{\text {stat}}$
is much smaller than
$J_{\mathrm {cpl\_{}c}}$
and cannot maximize
$\Delta $
by itself, so an interlayer exchange coupling (the critical energy constant of unit area
$J_{\mathrm {ex\_{}c}}$
) is required to cover the difference between
$J_{\mathrm {cpl\_{}c}}$
and
$J_{\text {stat}}$
.
$J_{\mathrm {ex\_{}c}}$
also rapidly increases with the decrease in
$d$
and reaches a plateau of 0.15 mJ/m
²
in
$d \le 30$
nm. MTJ devices with the smallest
$J_{\text {ex}}\,\,(=0.01 \text {mJ/m}^{2}$
) and a moderate PMA (constant per unit area
$K_{\text {eff}} t^{\ast }$
) were made and
$\Delta s$
were evaluated by experiments. The experimental
$\Delta s$
were fitted by the calculations with adjusting stiffness constants
$A_{s}$
. The calculated
$\Delta s$
and the experimental
$\Delta s$
were in good agreement when
$A_{s}$
had a smaller value than that of the blanket film. Since the experimentally used
$J_{\text {ex}}$
value was almost 0,
$\Delta $
values were estimated when
$J_{\text {ex}}$
was a greater value of 0.15 mJ/m
²
. As a result, 30% enhancement of
$\Delta $
was obtained at the smallest
$d = 20$
nm.