EFFECT OF POST-ANNEALING ON CO-BASED ALLOY LONGITUDINAL MAGNETIC THIN FILM MEDIA AND GRANULAR THIN FILM MEDIA

Abstract

Effect of post-annealing on magnetic properties of Co-based alloy longitudinal thin film media was systematically investigated for the first time. A maximum value of coercivity 5200 Oe of the media Co78Cr10Pt12/Cr deposited on the glass substrate without overcoat has been achieved after rapid thermal annealing in a vacuum at 580?C for 5 minutes from initial 2000 Oe. This is the highest coercivity obtained based on longitudinal CoCrPt/Cr thin films. Improvement in the magnetic properties was found to be dependent on the concentration of Pt and Cr as well as on different sputtering conditions and post annealing process. That is, different sputtering temperature, negative bias, working pressure and annealing temperature, time have different effect on the properties of thin films. An increase in the grain size verified by Atomic Force Microscopy (AFM) and a significant decrease in roughness from magnetic images have been observed in the post-annealed films together with a reduction of the positive peak in the Delta-M curves. Therefore diffusion of Cr from underlayer into magnetic layer may improve the intergranular exchange decoupling in combination with the Pt contribution to the improvement in the coercivity after annealing. Rotational hysteresis loss analysis has been carried out for CoCrPt-X/Cr-Y films in the as deposited and annealed states to study the relationships between Hc, Hkgrain, Hc/Hkgrain, Rh, Ku and Ms. The ratio of Hc/Hkgrain indicates the degree of intergranular magnetic coupling and its maximum ideal value is 0.5. Therefore the improvement in Hc was limited by both value of Hkgrain and the maximum ratio of Hc/Hkgrain. In the present study, it has been found that CoCr- thin films with high Pt content possesses a higher value of Hk and thus experienced a drastic improvement in Hc after annealing. It was also observed that Pt and Cr have different effects on the changes in Hkgrain during the annealing process. The magnitude of Rh was also strongly dependent on the degree of intergranular magnetic coupling and accordingly decreased with the increase in Hc after suitable annealing. CoCrPt-SiO2 granular longitudinal thin films with Cr underlayer have been fabricated by a multilayer deposition method followed by a post-annealing. The microstructure and magnetic properties of the as-deposited and annealed films have been investigated for potential applications in magnetic recording media. The grain size of about 10 nm was obtained for as-deposited films. Post-annealing remarkably enhanced coercivity from about 2000 Oe to 5640 Oe that is the highest value reported in this Co alloy granular system. The peak value of the Delta-M curves changed from positive for as-deposited state to negative for annealed one, indicating an obvious reduction of magnetic exchange coupling upon annealing.