The Study On L1₀FePt Anisotropy Graded Media

It has been more than a half century since magnetic recording was applied to computer’s data storage, some new technologies like thin-film inducted heads, magnetoresistive heads, the giant magnetic resistance multilayer membrane had pushed the rapid development of the magnetic recording, so the magnetic recording areal density grew rapidly. After through a "golden time" in1990s, the longitudinal magnetic recording has met its superparamagnetic limitation in the early period of the21st century. The product of the volume of the crystalline grain and the magnetocrystalline anisotropy must be greater than45to insure the thermal stability of the storage media; in order to increase the areal density we have to reduce the size of the crystalline grain while keeping the signal to noise ratio; however, to maintain the thermal stability, we have to choose the materials with high magnetocrystalline anisotropy if we reduce the size of the crystalline grain, but high magnetocrystalline anisotropy will result in the difficulty of the writing of the information (may exceed the writablity). So the thermal stability, writability and signal to noise ratio form the so-called recording trilemma.To overcome the superparamagnetic limitation and further increase the areal density of the magnetic recording, some new technologies have been proposed and put into study, like perpendicular magnetic recording, which has already replaced the longitudinal magnetic recording, and heat-assisted magnetic recording, microwave-assisted magnetic recording, patterned magnetic recording media, shingled write recording and so on. Suess etc. has proposed the concept of anisotropy graded media, in which the domain wall motion replaces the coherent rotation. Anisotropy graded media is promising to overcome recording trilemma in achieving ultrahigh areal density.In order to prove the concept of anisotropy graded media and its advantage experimentally, some experiments have been conducted in this area. However, most experimental works on anisotropy graded media are focused on the continuous films, where the magnetization reversal is dominated by domain nucleation and subsequent lateral propagation. It is not suitable to compare the results of graded films with the theoretical model, which is based on isolated grains with graded anisotropy and the domain wall propagates from soft to hard ends vertically. In order to further prove the concept of "graded" anisotropy media, it is highly desirable to experimentally study patterned media with graded anisotropy based on L10-FePt films.First, we grew10nm FePt uniform anisotropy films and characterized their structure and magnetic properties. We got the real temperature of the sample holder and its relationship with the heating Ta wire by mounting a thermal couple on it; then a series of10nm FePt films with uniform anisotropy were grown on MgO(001) at temperatures ranging from290℃to650℃; The XRD data shows that the growing direction of the film is (001) and the intensity of the superlattice (001) peak increases as the temperature arises, after calculation we got the relationship of the ordering parameter and the growing temperature by the integration of the (001) and (002) peak. The MH curves data indicates that the films are anisotropic and the anisotropy increases as the raise of the growing temperature, the relationship between the anisotropy and the growing temperature was got.Second, based on the former work, we grew45nm linear and uniform anisotropy distribution FePt films and characterized their properties. The films are etched into nanopillar arrays using Electron Lithography Exposure and Ion Milling, which diameter is around80nm, and the inner distance among pillars is100nm. The coercivity of the nanopillar arrays with graded anisotropy is smaller than that of the uniform ones; the magnetic properties of patterned nanopillar arryas with both uniform anisotropy and linearly graded anisotropy are further characterized by the temperature-dependent coercivity measurement, the patterned Fept nanopillar arrays with uniform anisotropy exhibit a larger Ho compared to nanopillar arrays with linearly graded anisotropy; the calculated thermal stability factors KUV/kBT, for both samples are beyond100, indicating that the magnetization could be stable for more than a decade. Furthermore, the calculated relative figure of merit (ζ=2△E/(MSHSWV)for FePt nanopillar arrays with linearly graded anisotropy is61%larger tha that of the FePt nanopillar arrays with uniform anisotropy, which indicates the advantage of the anisotropy graded media; the switching field distribution of FePt nanopillars with linearly graded anisotropy exhibits a relatively narrow peak, which indicates that the soft and hard phases in the sample are strongly coupled.Last, we characterized the microstructure of the linear and uniform anisotropy distribution FePt films. We used FIB, tripod polisher and PIPS make high quality TEM samples. The HRTEM data shows that the interface of the film and the substrate is clear, the film is quasi-epitaxial growth. We calculated the intensity and position of the001and002diffraction spots in the FFT of different areas along the growth direction of the linear sample, which indicates the linear distribution of the anisotropy as expected.