Grain Size Reduction by Doping Cr Underlayer with Ru for Longitudinal Magnetic Recording Media

A Cr underlayer was doped with 10 at. % Ru to enlarge the Cr lattice and promote the Co(11.0) formation of the CoCrPtTaB magnetic layer by in-plane tensile stress at a recording layer/underlayer interface. The texture, grain size and magnetic characteristics of CoCrPtTaB/Cr(Ru) were examined as well. The CrRu(002) underlayer exhibit markedly enhanced Co(11.0) orientation, increasing coercivity and preferential alignment of the $c$-axis. The grain size and grain size distribution of the recording layer decreased as the CoCrPtTaB magnetic layer grew epitaxially on the CrRu underlayer. The grain size of the recording layer was reduced from 11.2 nm with a pure Cr underlayer to 5.4 nm with a CrRu underlayer. The latter is the smallest grain size yet reported for CoCr-based alloy recording media.

I. Introduction

Co-Based longitudinal magnetic recording media presently dominate the magnetic recording market. High signal-to-noise ratio (SNR), low media noise at the transition length and narrow signal pulse width (PW₅₀) must be achieved to increase further magnetic recording density. Approaches for reaching these objectives are based on scaling down such physical parameters as grain size, width of the grain size distribution and crystallographic orientation. The appropriate underlayer enables these parameters to be reduced, since the grain size of the underlayer and the lattice mismatch at the recording layer/underlayer interface affect these physical parameters if the epitaxial growth proceeds at the recording layer/underlayer interface [1]–[3]. In a typical Cr-based underlayer, Mo [3]–[5], Ti [3], [6], W [3], [7], Mn [1] and V [3], [8] are soluble, improving the lattice match between the recording layer and the underlayer. However, adding these elements does not significantly reduce the grain size. The Cr underlayer was doped with Zr [1], [2], [4], Nb [2] and B atoms [9] to reduce the magnetic grain size, because these elements are insoluble in Cr lattices, and thus segregate in the grain boundaries, preventing further grain growth. Although doping with these elements reduces the magnetic grain size, the Co(11.0) orientation was poor and the coercivity was low [1]. The expanded Cr lattice has been found to induce in-plane tensile stress in the FePt/CrRu plane and promote the formation of the FePt(001) texture [10]. Thus, in this work, the Cr lattice was doped with 10 at.% Ru to adjust the lattice constant of the Cr(002) underlayer and lattice matching at the recording layer/underlayer interface. Incorporating Ru into Cr expands the body-centered cubic Cr lattice and helps to realize the Cr(002) orientation [10]. Additionally, the CrRu(002)[110] lattice is slightly larger than the Co(11.0)[00.2] lattice and an in-plane tensile stress is present along the crystallographic direction CrRu(002)[110]//Co(11.0)[00.2] [1]–[3], [11]. Therefore, the Co(11.0) plane is expected to be able to develop on a CrRu(002) underlayer, reducing the grain size and the width of its distribution.