The magnetic properties of CoCrTa/Cr longitudinal magnetic recording media have been investigated by changing the Cr underlayer thickness, CoCrTa magnetic layer thickness and substrate temperature. The composition of CoCrTa alloy target was $CoCr_{11.92}Ta_{2.08}$. The CoCrTa/Cr layers were deposited on textured NiP/Al-Mg substrates by DC magnetron sputtering and were annealed in a quartz tube with halogen lamp. The thickness of films was determined by DEKTAK IID and X-ray fluorescence thin film measuring system. X-ray diffraction and scanning electron microscopy were used to analyze the texture and surface morphology of the films. Auger electron spectroscopy was used to study the depth profile of the annealed films.
As the substrate temperature increased, the magnetic coercivity of the CoCrTa(500Å)/Cr(1500Å) films was increased. The changes in $H_c$ with $T_{sub}$ may be associated with grain boundary segregation of Cr. The coercivity value of the films was about 1300 Oe at T_{sub}=290℃
As the Cr underlayer thickness increased from 0 to 2000Å, the $H_c$ were sharply increased from 350 to 1400 Oe. Above t_{Cr} = 500Å, the coercivity changed little. The coercivity squareness also changed little with Cr underlayer thickness. Its value was about 0.8. As the Cr underlayer thickness increased, the grain size of CoCrTa film was increased. It result in larger in-plane anisotropy. But, it seems not that the grain size is the major factor of coercivity increasing. XRD pattern of CoCrTa(500Å)/Cr double layer deposited at 290℃ without Cr underlayer on Al-Mg substrate revealed a broad peak with highest intensity near $2\theta$=44.4˚ and the XRD patterns with Cr underlayer showed Co(10\overline{10}$) and Co($10\overline{11}$) peaks by forming texture on the Cr(110) plane.
The maximum $H_c$ of the films at t_{Cr} = 2000Å, deposited at 290℃, was about 1400 Oe, at which the thickness of CoCrTa was about 500Å and then Hc was decreased with further the increase in CoCrTa thickness. The reason for lower $H_c$ above t_{CoCrTa} = 500Å thickness may be the decrease of crystal texture and in-plane anisotropy.
The coercivity of CoCrTa(500Å)/Cr(2000Å) films showed a maximum value of 1600 Oe when they were annealed at 250℃ for 4 hours. The annealed time dependence of magnetic properties may be associated with Cr diffusion from Cr underlayer into magnetic layer or segregation of Cr in CoCrTa magnetic layer during annealing.
