Preparation, Structure, Orientation Control And Magnetic Properties Of Co-based Magnetic Recording Thin Films

Co-W (Mo) and Co-Pt alloys are potential candidates for next-generation high density magnetic recording due to their high magnetic anisotropy energy. However, many fundamental issues which greatly hinder their applications remain unresolved, for instance, amorphization of Co-W (Mo) thin films during sputtering deposition and their insufficient phase separation of HCP structured Co-W (Mo) alloys, difficulties in controlling perfect perpendicular orientation and strong exchange coupling among magnetic grains of Co-Pt films. In this thesis, a series of Co-W (Mo) and Co-Pt alloy films with different contents and thicknesses were prepared by DC magnetron sputtering. Structures, crystalline orientation and magnetic properties of Co-W(Mo) and Co-Pt thin films were systematically investigated by Vibrating sample magnetometer (VSM) and X-Ray diffraction (XRD) with the propose to improve their magnetic properties by microstructure control of the films. Correlations between crystalline structures and magnetic properties are clarified. The main conclusions are as follows:Amorphous forming abilities of Co-W, Co-Mo, Co-Cr alloy systems are discussed both theoretically and experimentally. The results show that critical transition compositions from HCP to amorphous are about17.1at.%W and18.7at.%Mo for the Co-W and Co-Mo alloys at RT, respectively, while the Co-Cr thin films are hard to be amorphous at room temperature. It indicates that the Co-Mo thin films exhibit the largest amorphous forming abilities among them and the Co-W thin films are condition-dependent amorphous, in short, Co-Mo> Co-W>>Co-Cr for their amorphorzation forming ability. Increasing substrate temperature and/or decreasing sputtering pressure promote higher atom difusivities of Co-W (Mo) thin films, and thus shift the critical transition composition to a higher W (Mo) content.Effect of Mo content on structures and magnetic properties of Co-Mo films were investigate. Ms values of Co-Mo films decrease linearly with Mo content when the Mo content is less than12at.%, however, they decrease non-linearly in the composition range of12-20at.%Mo, similarly as that of Co-Cr thin films, indicating the existence of phase separation in the Co-Mo system. Ms and Hc values have no obvious change with the addition of Cu and Pb into Co-Mo films. While, Hc value increases with the addition of Pt into Co-Mo films, possibly due to the increasement of magnetic anisotropy energy. It suggests that the insufficient phase separation of Co-Mo is not seriously improved by doping Cu or Pb, which is different from that of Co-Cr thin films.Co-11at.%W films with different thickness (9-80nm) were deposited on MgO (110) single crystal substrate with Cr underlayer (100nm). The epitaxy growth relationships between Cr and Co-W are Cr(112)[111]//Co-W(10.0)[01.0] and Cr(112)[110]//Co-W(10.0)[00.1]. With the thicknesses of Co-W films increase, in-plane strain of Co-W increases from-0.3884%to-0.2711%, while out-of-plane strain decreases from0.7813%to0.5445%, and K1eff decreases from3.82×106erg/cc to2.58×106erg/cc correspondingly. The results show that effective magnetic anisotropy energy can be changed by adjusting the strain between underlayer and magnetic layer.A series of Co-Pt films with different Pt content were fabricated at ambient temperature without any underlayer. Lattice constants (a, c) of the Co-Pt films increase linearly with increasing Pt content, while c/a decreases firstly and then increases. The Hc value has the opposite trend as c/a, highest Hc and modest Ms are obtained in the Co-20at.%Pt thin film.The perfectly perpendicular orientation of Co-Pt thin films can be obtained by the optimized processing of substrate temperature (300℃) and thickness (30nm) of Ti underlayer. Mismatching between Co-Pt layer and Ti underlayer were investigated with different lattice constants of Co-Pt layer by changing Pt content. And the mismatch between Co-Pt and Ti is regarded as the basic reason of affecting the perpendicular orientation. Texture of HCP structured{00.2}<10.0> can be obtained in the Co-26at.%Pt thin films. The grain size of Co-Pt layer can be reduced by introducing Ti-SiO2granular underlayer, together with the improved perpendicular magnetic properties.For Ta/Ru/Co-23%Pt perpendicular thin films, lower working pressure (0.6Pa) and higher power (90W) to deposit Ta layer with several nanometers in thickness are all helpful to control the Ru (00.2) orientation of the Ru underlayer and thus to obtain c-axis of Co-Pt perpendicular thin film. The thickness of Ru underlayer also makes an important role on c-axis orientation and magnetic properties of Co-Pt thin films. Out-of-plane and in-plane hysteresis loops are quite different with different Ru layer thicknesses of Ta/Ru/Co-23%Pt films. Easy axis is normal to the film when Ru layer thickness is40nm, and thus the coercive square ration(-1) and Hc (4000Oe) are obtained.Better perpendicular orientation and magnetic properties of Ta (3nm)/Ru (40nm)/Co-26at.%Pt (15nm) films can be obtained by increasing the Co-Pt layer working pressure to1.6Pa but not higher than2.5Pa. Dependency of Hn and the slope at Hc of out-of-plane hysteresis loop on Co-Pt layer thickness were studied by building a model. Hn transforms from negative value to positive value and the slope decreases with increasing the Co-Pt film thickness.Addition of TiO2or SiO2oxide into the Co-Pt layer for the Ta/Ru/Co-26Pt+oxide multilayer thin films and post-annealing can lead to their good c-axis orientation and perpendicular magnetic properties by a small amount of oxide addition and modest annealing temperature and time.