Preparation And Physical Properties Of Perpendicular Magnetic Recording Media

Since perpendicular magnetic recording was proposed in 1977, it has been verified to be one of the leading candidates for an ultra-high density storage system. Many magnetic recording media have been widely investigated including particulate barium ferrite, CoCr-based alloys, Fe-Pt alloys, nanowire arrays and rare-earth alloys. At the same time, GMR (giant magnetoresistance) write-head is an important component in magnetic recording process. Moreover, exchange biasing has played an essential role in GMR device. Based above two facts, we have investigated three kinds of perpendicular magnetic recording media and the exchange biasing in ferromagnetic/ antiferromagnetic (FM/AFM) bilayers. As a basic research, we have investigated the magnetization reversal mechanism in Fe-Cr films.As the following text, we summarize the main contents and results in the thesis.I Co-Pt nanowire arrays Microstructure and magnetic properties of Co-Pt nanowire arrays electrodeposited into self-assembled anodic alumina templates have been investigated. The nanowires with diameter d<80 nm are mainly of (111) preferred orientation along the wire axis. For a specific diameter, the coercivity and the remanent ratio with the external magnetic field parallel to the nanowire axis, i.e., the parallel geometry, are larger than those of the perpendicular geometry. With temperature increasing from 90 K to 600 K, above two physical quantities in the perpendicular geometry decrease monotonically while those of the parallel geometry change little. The dependence of the coercivity and the remanent ratio on temperature and the nanowire diameter can be explained after the shape and the magnetocrystalline anisotropies, and the dipolar magnetic interaction are considered. For the parallel and perpendicular geometries, the coercivity decreases with increasing diameter d as a linear scale of 1/d² and approaches each other in the two geometries for large d. The degradation of coercivity in Co-Pt nanowires with increasing diameter is suggested to come from curling mode of magnetization reversal process.II TbFeCo al loy A TbFeCo film was deposited by DC magnetron sputtering and studied by transmission electron microscopy, polar and longitudinal magneto-opticalKerr effect, and magnetometry measurements. Transmission electron microscopy has shown the existence of lateral compositional inhomogeneity. Magneto-optical measurements have shown that the initial layer at the bottom consists of only magnetic perpendicular component and the top surface layer has a compositional inhomogeneity and consists of in-plane components and perpendicular one. The perpendicular components in the bottom and the surface layers have identical composition. Two in-plane components have been shown by magnetometry measurements. It is shown that phase segregation exists in the TbFeCo film and possible form of compositional inhomogeneity has been discussed. The two in-plane components are exchange-coupled with a magnetization off-alignment of 35 degrees. For the soft in-plane component, the in-plane and out-of-plane angular dependence of the exchange biasing is similar to those of the conventional one. Within temperatures from 100 K to 300 K, the exchange field and the coercivity are both linear functions of temperature.III CoCrPt alloy Magnetically perpendicular CoCrPt films were fabricated by co-sputtering from oblique Cr and Pt targets, and up-right Co target. For comparison, the conventional method has also been employed using single up-right composite target. With the first method, the optimization of magnetic properties can be carried out as a function of the alloy composition by either changing sputtering power of three targets or changing the sampling location due to a compositional gradient. More importantly, the remanent ratio acquires a maximum and a minimum when the applied magnetic field makes angles of 10⁰ and 100⁰ with the film normal direction, indicating that the easy axis makes an angle 10⁰ with the normal of film plane. For the second method, the easy axis is aligned along the film normal direction.IV Fe-Cr al loy A large sample of Fe_xCr_1-x alloy film with a composition gradient (x=0.38-0.52) has been prepared by co-sputtering. Since the magnetization decreases as the Fe content is decreased, the uniaxial anisotropic field H_k is increased although the uniaxial anisotropy energy does not change much. At the same time, the pinning field H_p(0) is expected to change little since the lattice constant of Fe-Cr alloy film does not change much with the composition. Consequently, the H_k is much larger than H_p(0) for low Fe contents while they are close to each other for high Fe contents. As a result, the magnetization reversal mechanism is evolved from the displacementof domain wall to modified Kondorsky model as the Fe content is increased.V Exchange biasing of FM/AFM bilayers We have investigated the exchange biasing in FM/AFM bilayers with various thick FM and AFM layers. The results show that the asymmetric magnetization reversal originates from the different spin structure of AFM, which is expected to elucidate the physics in exchange biasing system.