The annual areal recording density of hard disk drive is increasing at a rate of 60 % in recent years. The rapid growth of the recording density became possible by a continuous innovation in head and media. General trend in the media developments is to obtain low noise and high coercivity characteristics at a thinner magnetic layer thickness. To make low noise media, grain size of magnetic layer should be as small as possible and grain size distribution should be as narrow as possible. Also magnetic exchange interaction among grains of the magnetic layer should be low.
The purpose of the resent work is to develope a high coercivity and low noise media with $Co_{74.7}Cr_{15.2}Pt_{10.1}$ magnetic layer by introducing new type of underlayer and to understand magnetic and structural properties by analysing effects of new underlayer on grain size, crystallographic orientation and composition of the magnetic layer. As new underlayer a B2 structured CoTi single layer, CoTi/Cr bilayer, and $Cr_{75}Ti_{25}/CoTi$ bilayer were studied.
Strong CoTi (200) preferred orientation was formed in a CoTi thin film at the substrate temperature of 250 ℃ or higher and at the film thickness of 500 Å or thicker. However, even at 320 ℃ substrate temperature, the CoTi layer showed almost amorphous type nano-crystallization structure was observed at the thickness thinner than 200 Å, and when the layer thickness exceeds 200 Å CoTi (200) oriented grains rapidly grow in conical shapes and cover the whole film surface at 500 Å thickness. The grain size of CoTi underlayer was much smaller than that of Cr underlayer. The lattice constant 'a' of CoTi was 2.98 Å and that of Cr was 2.88 Å. therefore, areal misfit between Co (11$\bar{2}$0) plane and CoTi (110)×(110) plane is 2.7 % while that between Co (11$\bar{2}$0) plane and Cr (110)×(110) plane is 9.3 %.
Longitudinal recording media made on CoTi and Cr underlayers were compared. The 300 Å thick CoCrPt film grown on 700 Å thick CoTi underlayer showed only strong Co (11$\bar{2}$0) diffraction, while CoCrPt(300 Å)/Cr(700 Å) film has a Co (0002) as well as Co (11$\bar{2}$0). The grain size of CoCrPt magnetic layer deposited on CoTi underlayer was about 200 Å, which was a half of CoCrPt grains on Cr underlayer. The cross sectional HRTEM(high resolution transmission electron microscopy) showed that the CoCrPt/CoTi film had an excellent grain-to-grain epitaxial relationship and clear columnar structure. the bicrystal structures of CoCrPt magnetic layer were observed in the CoCrPt/CoTi films. The 300 Å thick CoCrPt film grown on 700 Å thick CoTi underlayer showed the maximum coercivity of 2500 Oe, which was about 600 Oe higher than that of CoCrPt film deposited on Cr underlayer at the same sputtering conditions. The coercivity squareness was 0.9. The improvement of the magnetic properties of CoCrPt/CoTi films can be attributed to the better Co (11$\bar{2}$0) crystallographic orientation induced by a single orientation of CoTi (200), the reduction of grain size and more uniform grain size distribution, and the better areal misfit mentioned earlier.
Coercivity increased from 2200 to 3200 Oe by incorporating the 500 Å thick $Cr_{75}Ti_{25}$ intermediate layer between the CoCrPt magnetic layer and the CoTi underlayer. From x-ray analysis, the rather strong Co (10$\bar{1}$0) diffraction was developed in addition to Co (11$\bar{2}$0) diffraction in the $CoCrPt/Cr_{75}Ti_{25}/CoTi$ film, while only Co (11$\bar{2}$0) diffraction was observed in the CoCrPt/CoTi film. TEM image confirmed that the small grain size of 180 Å in the both type of film. Various observations such as ΔM curve, angular dependency of coercivity, and MFM(magnetic force microscopy) image indicate magnetic exchange among magnetic grains became weaker in the $CoCrPt/Cr_{75}Ti_{25}/CoTi$ films. The enhancement of the coercivity can be mainly attributed to the increase of magnetic decoupling effects through the Cr segregation at the grain boundaries of the magnetic layer. In addition, the presence of Co (10$\bar{1}$10) plane and small grain size due to B2 structure intermetallic compound underlayer should increase the coercivity of the $CoCrPt/Cr_{75}Ti_{25}/CoTi$ film.
Coercivity increased from 3200 to 4000 Oe by applying rf-bias to glass substrate during the deposition of CoCrPt magnetic layer in the $CoCrPt/Cr_{75}Ti_{25}/CoTi$ films. From x-ray analysis, the expansion of lattice parameter was observed in the CoCrPt magnetic layer deposited under the substrate bias. Compositional analysis by RBS(rutherford backscattering spectroscopy) confirmed that the Pt concentration in CoCrPt increased when deposited with higher substrate bias. This enhancement of the coercivity was attributed to the expansion of Co lattice parameter due to the compositional change of the magnetic layer, especially increase of Pt content.
The read/write performance of the CoCrPt/CoTi, CoCrPt/Cr, CoCrPt/Cr/CoTi, $CoCrPt/Cr_{75}Ti_{25}/Co-Ti$ film media on $SiO_2$ substrate was evaluated by GUZIK test. The enhancement of playback signal was obtained in the media grown on CoTi underlayer than that of Cr underlayer. The highest signal was obtained in the films with $Cr_{75}Ti_{25} intermediate layer.
