Properties of amorphous Co–Ta and Co–W films deposited by rf sputtering

Magnetic and thermal properties of Co100−xTax(11≤x≤25) and Co100−xWx(13≤x≤25) amorphous films deposited by rf diode sputtering have been investigated. In both systems, structural changes of these films from crystalline to amorphous lead to a drastic decrease of their coercive force Hc and the absolute value of saturation magnetostriction λs, where λs is negative. Since magnetic properties and structure of the amorphous Co–Ta and Co–W films depend significantly on the argon gas pressure PAr, the PAr must be adjusted to the proper value during sputtering to obtain films with soft magnetic properties. Typical magnetic properties of Co–Ta and Co–W amorphous films are as follows: (i) Co–Ta: 4πMs≤11 kG, −λs≤2×10−6, Hc≤0.4 Oe. (ii) Co–W: 4πMs≤11 kG, −λs≤8×10−6, Hc≤4.0 Oe. The crystallization temperature Tx of the amorphous Co–Ta films is above 700 K and Tx increases with the increase of Ta content in the films. Tx of amorphous Co–W films is above 940 K. In this alloy system, however, structure relaxation occurs at much lower temperature than the Tx, which causes an increase of Hc. This causes the soft magnetic properties of the amorphous Co–Ta films to be more thermally stable than those of the amorphous Co–W films.