Magnetic Thin Films With High Perpendicular Anisotropy For Magnetic Recording Media Applications

Abstract

In order to meet the basic thermal stability requirement in future extremely high density magnetic recording, magnetic materials with high magnetocrystalline anisotropy constant (Ku) are needed. In this thesis work, the two most-promising candidates of high Ku materials, i.e. CaCu₅-type Sm(Co, Cu)₅ and L1₀-type FePt thin films, have been systematically studied. SmCo₅ has the highest Ku among practical magnetic materials. Prior studies on SmCo₅-based thin films for magnetic recording media applications is reviewed. SmCo₅ thin films with good perpendicular magnetic anisotropy were only grown on Cu underlayer. However, the uncontrollable Cu diffusion from the Cu underlayer and the relatively large Cu grain size make it not suitable for magnetic recording applications. In this study, polycrystalline Sm-Co-Cu films consisting mainly of highly (0001) textured Sm(Co, Cu)₅ grains have been successfully fabricated on non-Cu containing underlayers (Ru or Ru(Cr)) on glass substrates. Strong perpendicular magnetic anisotropy of Sm(Co, Cu)₅ thin films was achieved. It was found that increasing Cu is like increasing deposition temperature - both could improve the SmCo₅ phase formation and the crystallinity of Sm(Co, Cu)5 (0001) films. The median composition of nanocrystalline Sm(Co, Cu)₅ grains estimated from structural unit volume and Curie temperature matches reasonably well with each other. In-plane compressive strain in Sm(Co, Cu)₅ films is inferred from the differences of lattice constants between the thin film and bulk material. The use of Ru(Cr) underlayer with a proper Cr doping could improve the perpendicular anisotropy of Sm(Co, Cu)₅ films. Microstructure of Sm(Co, Cu)₅ thin films was studied using SEM, AFM, and TEM analysis. The grain size of Ru underlayer is about 10-30 nm, much smaller than that of Cu underlayer (~200 nm) reported in the literature. Magnetization reversal in Sm(Co, Cu)₅ thin films is dominated by the domain wall pinning mechanism. Many types of pinning sites are found: voids, grain/matrix boundaries (or crystalline/amorphous boundaries), grain boundaries between crystalline grains, and the composition inhomogeneity in grains. The key finding of TEM elemental mapping analysis is that Cu atoms were found to be rich in the inner part of Sm(Co, Cu)₅ grains or particles, instead of the outer part, such as grain boundaries or edges of voids. Cu served as an alloying element in Sm(Co, Cu)₅ grains, not as a doping element to form Cu-rich grain boundaries. A model of Sm(Co, Cu)₅ films with in-plane graded anisotropy due to composition/crystallization variation can explain the huge difference between the Hc and HK as well as the angular dependences of coercivity and remanence coercivity. A simple analytical expression of the angular dependence of switching field for graded media has been derived and shown to match well with experimental results. Chemical stability of Sm(Co, Cu)₅ thin films has been studied. Ta-capped Sm(Co, Cu)₅ thin films are stable in terms of structural and magnetic properties in a normal laboratory environment (25 °C) over 3 years, but they did not pass the accelerated corrosion test (130°C, 95%RH, 6 hours). A capping layer consisting of a hcp-phased CoPt-alloy layer and carbon overcoat should help Sm(Co, Cu)₅ thin films meet the requirements for future high-density magnetic recording applications. L1₀-type FePt thin films were studied as perpendicular ECC media with potentially high gain factor due to domain-wall assisted magnetic switching. Ultra-thin exchange-coupled-composite (ECC) FePt granular recording media with different soft layer anisotropy were fabricated by controlling the soft layer deposition temperature. The structural and magnetic properties of soft layers (FePt-SiO₂) confirmed the feasibility of controlling the soft layer anisotropy by changing its deposition temperature. The effect of soft layer anisotropy field on the coercivity (<italic>Hc</italic>) and the remanent coercivity (<italic>Hcr</italic>) of ECC FePt thin films showed a "V" shape relationship, with the minimums at T_Soft of 200 °C. It is consistent with the theoretical prediction based on domain wall assisted magnetization reversal mechanism. The ECC FePt thin film with T_Soft of 200 °C may achieve a gain factor larger than 2.