Pt Content Dependence of Magnetic Properties of CoPt/Ru Patterned Films

The magnetic properties of dot arrays made of CoPt/Ru perpendicular films (20 nm thickness) were examined as a function of Pt content. The CoPt dot arrays with a dot size D of 140 nm showed a single domain state, after removal of the applied field equal to H
_r
. H
_r
decreased from 5.2 kOe to 3.0 kOe as the Pt content decreased from 20 at% to 14 at%. The angular dependence of H
_r
for these dot arrays indicated coherent rotation of the magnetization during nucleation. The effective magnetic anisotropy, including the demagnetizing energy due to the dot shape, K
_u

^eff
, decreased as the Pt content decreased, resulting in the H
_r
reduction. The values of the switching volume for nucleation, V
_sw
, evaluated from the stabilizing energy barrier E
₀
, were a few percent of the dot volume. The switching diameter for nucleation, D
_sw
, increased slightly as the Pt content decreased, which was qualitatively in good agreement with the increase in the exchange length of magnetization. The value of E
₀
/k
_B
T (k
_B
is the Boltzmann constant and T is the absolute temperature) reduced as the Pt content decreased; however, E
₀
/k
_B
T still had a high of 440 even at 14 at% Pt content. We successfully demonstrated the reduction of H
_r
for CoPt/Ru patterned films on reducing the Pt content, while simultaneously maintaining a high thermal stability. A calculation based on the experimental results suggested the potential recording density of CoPt/Ru dot arrays used for patterned media to be over 1 Tb/in
²