Correlation between Noise and Microstructure in CoCrTa Media

A series of identical Co_<82>Cr_<13>Ta_5/Cr thin-film longitudinal media were deposited at substrate temperatues (T_<sub>) of room temperature (RT), 150℃, and 250℃ respectively. Tracks were written at linear densities of up to 250 kfci, and the recording performance was examined. An inverse relationship was observed between T_<sub> and noise levels. A variety of electron microscopy techniques were employed on the same films to investigate the origin of this relationship. Spin-echo nuclear magnetic resonance (NMR), TEM after wet chemical etching, and energy-dispersive X-ray (EDX) analysis reveal the existence of compositional separation in the media deposited at elevated temperatures (250℃). This causes a reduction in the intergranular exchange coupling as detected in ΔM measurements. The reduction can be directly observed in Lorentz images in the form of smaller magnetic clusters. Cluster size has a strong effect on the written tracks. Lorentz images for each medium show a strong correlation between the cluster size and the noise measured.