Effects of Film Stress and Geometry on Texture Evolution Before and After the Martensitic Transformation in a Nanocrysta
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COBALT (Co) and Co-based alloys have been widely used as thin-film magnetic recording media.[1–7] They undergo an allotropic transformation between two close-packed structures: hcp as a low-temperature phase and fcc as a high-temperature one. The transformation from the fcc structure to the hcp structure exhibits characteristics of a martensitic transformation.[6,8–11] Martensite plates grow by the motion of Shockley partial dislocations (SPDs) along four equivalent types of {111} planes, and the (0001) planes are related to the {111} planes of the fcc structure, revealing a specific crystallographic orientation relationship between fcc and hcp Co.[10–12] For pure Co, it occurs at the equilibrium temperature of ~693 K (~420 C) at ambiSUNG BO LEE, Research Professor, YANGHOO KIM, Research Scientist, HEUNG NAM HAN, Professor, and DONG NYUNG LEE, Emeritus Professor, are with the Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, Republic of Korea. Contact e-mails: [email protected], [email protected] DONG-IK KIM and JI YOUNG BYUN, Principal Research Scientists, are with the Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea. SEUNG JO YOO, Research Scientist, is with the Korea Basic Science Institute, Daejeon 305-806, Republic of Korea. Manuscript submitted September 6, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
ent pressure.[8–12] Bauer et al.[11] examined the kinetics of the martensitic transformation in a polycrystalline Co bulk material using differential scanning calorimetry, and suggested that the transformation is characteristic of athermal nucleation and thermally activated, interface controlled growth because the mobility of SPDs is limited by lattice resistance. The martensitic transformation into hcp is accompanied by a macroscopic distortion due to the change in atomic distances. The lattice expands by 0.021 pct parallel to and shrinks by 0.242 pct normal to the close-packed plane,[13] and as will be discussed later, the macroscopic distortion seems to play a crucial role in the development of a major texture component in the hcp phase. Out of the two phases, the hcp phase is favored in thin films for magnetic recording, because, for Co, its magnetocrystalline anisotropy energy is about one order of magnitude larger than that of the fcc phase.[14,15] Therefore, a mixture of hcp and fcc phases causes poor magnetic properties for magnetic recording. The minimum grain size which can be used in the media is limited by a balance between high signal-to-noise ratios and a thermal effect called superparamagnetism, which require grains to be as small as possible and large enough, respectively.[6,7] It is calculated to be several nanometers in diameter and is experimentally implemented.[1,2,4–7] Therefore, understanding of the martensitic transfor-
mation in a nanocrystalline Co thin film as a model material would be crucial for phase control for the magnetic recording media. Thin films usually develop in-pl
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