Effective Damping Constant and Current Induced Magnetization Switching of GdFeCo/TbFe Exchange-Coupled Bilayers

The Gilbert damping constants
$\alpha$
and effective perpendicular magnetic anisotropies
$K_{{\rm eff}}$
of amorphous GdFeCo (
$10 - {x}$
 nm)/TbFe (
$x$
 nm) exchange-coupled bilayers with various TbFe layer thicknesses
$x$
were measured. The products
$\alpha \times K_{{\rm eff}}$
of the bilayers were compared with the critical current densities
$J_{{\rm c}}$
for current-induced magnetization switching (CIMS) in giant magnetoresistance nanopillars with a GdFeCo/TbFe memory layer. The damping constant
$\alpha$
of the bilayers, estimated from time-resolved magneto-optical Kerr effect measurements, was 0.051 for
${x} = 0$
, and was significantly enhanced to 0.23 for
${x} = 1$
. The anisotropy constant
$K_{{\rm eff}}$
and the product of saturation magnetization times anisotropy field
M
_s

H
_k
also increased with increasing TbFe thickness
$x$
. The CIMS of the GdFeCo (9 nm)/TbFe (1 nm) bilayer exhibited 1.6 times larger
J
_c
than that of GdFeCo (10 nm), while the product
$\alpha \times K_{{\rm eff}}$
increased by a factor of 10.