Growing morphology of Co-Cr films on Cr underlayers and its influence on the change in magnetic properties during post-annealing have been investigated. Two kinds of films with different grain morphology are deposited in a magnetron sputtering system by control of sputtering pressure and substrate temperature. The magnetic properties of the films(coercive force; $H_c$, squarness; S, and saturation magnetization; $M_s$) are determined by vibration sample magnetometer. The grain structure is analyzed by XRD, SEM and TEM. The composition analysis of the films is conducted by auger electron spectroscopy(AES) and energy dispersive spectroscopy(EDS). The growing morphology of the films is found to strongly depend on the sputtering pressure and substrate temperature. Development of columnar grains with voided grain boundaries is promoted with higher sputtering pressure and lowering of substrate temperature. The films deposited in reverse conditions(low sputtering pressure and high substrate temperature) exhibit the more or less equiaxed grain morphology and tight grain boundary. Due to this difference in microstructure, the as-sputtered films exhibit the different magnetic properties. The slightly higher coercive force and lower saturated magnetization obserbed in high pressure and low temperature sputtered films are attributed to the isolated grain structure.
A vacuum annealing of the sputtered films gives a result of drastic change in the magnetic properties. Rapid increase in $H_c$ and decrease in $M_s$ and S are observed in the films annealed at 500℃. This annealing effects are more pronounced in the dense array of grain films with tight grain boundary. The composition depth profile determined by AES indicates that considerable diffusion of Cr(about 15 at%) is occurred from Cr underlayer into the Co layer. However, the reverse diffusion of Co into the Cr into the Cr underlayer is found to be negligible. The similar result is obtained in the EDS scanning through cross section of TEM specimen. The increase in $H_c$ after post-annealing is, therefore, belived to be Cr diffusion and formation of non-magnetic Co-Cr alloy in the vicinity of Co grain boundary. A high Cr concentration at the grain boundary could reduce the magnetic coupling between the grains and hence increase $H_c$. The large reduction in $M_s$ also indicates that considerable volume of non-magnetic phase is formed which is most likely to be due to bulk diffusion of Cr from the grain boundaries. Because of this grain boundary diffusion behavior of Cr, the annealing effects are more enhanced in the films of dense grain structure with tight grain boundary and thinner Co layer thickness. In the strongly coupled grain structure, the magnetic separation by Cr-rich phase could be more effective. Since the diffusion length of Cr is proportional to the Co-layer thickness, the annealing effects(increase in $H_c$ and reduction in $M_s$) is more pronounced in the films of smaller thickness.
