Bias Voltage Dependence of Magnetic Tunnel Junctions Comprising Double Barriers and CoFe/NiFeSiB/CoFe Free Layer

The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO
$_ x$
/free layer/AlO
$_x$
/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an Ni
$_16$
Fe
$_62$
Si
$_8$
B
$_14$
7 nm, Co
$_90$
Fe
$_10$
(fcc) 7 nm, or CoFe t
$_1$
/NiFeSiB t
$_2$
/CoFe t
$_1$
layer in which the thicknesses t
$_1$
and t
$_2$
are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of 86 k
$Omega mu$
m
$^2$
, a coercivity
$(H_c)$
of 11 Oe, and an interlayer coupling field
$(H_i)$
of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of 68 k
$Omega mu$
m
$^2$
, and a
$H_c$
of 11 Oe, but an increased
$H_i$
of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that
$H_i$
increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and
$H_i$
became large because of the magnetostatic NÉel coupling.