AFM structure and media noise of SmCo/Cr thin films and hard disks

Abstract

Longitudinal media for ultrahigh density recording require a high coercivity and a low medium noise. While the coercivity is controlled mainly by the chemical composition of the alloy, the medium noise is influenced significantly by the microstructure of the underlayer. We used atomic force microscopy (AFM) to study the microstructure of Cr underlayers and SmCo magnetic films. The AFM study shows that the Cr grains have elongated ‘‘rice’’-like granular features whereas the SmCo grains without an underlayer appear circular with nonuniform grain size. The Cr underlayer grows as well isolated columns with voids. When thin SmCo films (<20 nm) were deposited on the thin Cr (<35 nm) underlayers the SmCo grains were found to replicate the isolated, columnar structure of the underlayer. The intergranular exchange interaction and the recording media noise of SmCo/Cr thin films decreased with a decreasing thickness of the Cr underlayer. The medium noise of high coercivity SmCo/Cr hard disks prepared on thin Cr underlayers remained almost flat with no supralinear increase when tested up to 2760 fc/mm (70 kfci).