Study Of L1₀ FePt-based Magnetic Recording Media

Nowadays, magnetic recording, mainly in hard disk drive (HDD), becomes the most important way to store information. The increasing need in data storage has pushed the HDD into a rapid increase both in areal density and recording speed. Though the ultra-high-density storage with the areal density larger than 100 Gbit/in² can be achieved (134 Gbit/in² has already been achieved in product) by the perpendicular magnetic recording (PMR), the superparamagnetism, like that in longitudinal magnetic recording (LMR), will again limit the density increase seriously when the areal density goes to larger than 500 Gbit/in . One of the new recording methods, so-called heat-assisted magnetic recording (HAMR), has been proposed by Seagate and Hitachi, etc. companies to realize an even higher areal density beyond 1 Tbit/in² in the next 5 or 10 years. This thesis will study a candidate media material based on L1₀ FePt thin films for HAMR, and mainly includes the following contents.Firstly, microstructural and magnetic properties of co-sputtered L1₀ FePt films with different ultra-thin underlayers have been investigated. Two 1 nm thick Fe films are at first deposited on Si substrates with a natural oxidized SiO_x layer, and then one is oxidized by the oxygen plasma. The FePt films of 6 nm nominal thickness are deposited on their top (with Fe and FeO_x underlayers, respectively), using dc magnetron co-sputtering at room temperature. After post-annealed in high vacuum environment at 600 ℃, the fully ordered L1₀ FePt with c-axis perpendicular orientation is obtained for FeO_x underlayer while the c-axis is partially perpendicular oriented for Fe underlayer with a larger coercivity. According to the high resolution Transmission Electron Microscopy (HRTEM), Fe coalesces with FePt film for Fe underlayered sample, while in FeO_x underlayered sample FeO_x is isolated from FePt film after post-annealing. This indicates that the different orientation evolution of FePt films with FeO_x and Fe underlayers under annealing, results from the different thermal history of eutectic reaction between FePt film and underlayers.Secondly, FePt thin films with thickness of 6 nm are deposited onto three kinds of (100)-oriented single crystalline substrates: SrTiO₃, MgO and Si. For Si substrate, there is a 1-nm-thick FeO_x underlayer which is fabricated from sputtered Fe film by plasma oxidation. The annealed FePt films at 600 ℃ are all with (001) preferred orientation, and their magnetic anisotropies are perpendicular to the film plane. The L1₀ ordering process has been studied as a function of annealing temperature. It isfound that the ordering temperature of L1₀ FePt films onto Si with FeO_x underlayer is lower by 100～150 K than those films onto MgO and SrTiO₃ substrates. The stresses resulting from the lattice mismatch, difference of thermal expansion coefficients and residual depositing stress, are responsible to the ordering kinetic difference. High chemical ordered L1₀ FePt with c-axis perpendicular to the film plane has been obtained at lower annealing temperature for the film deposited on Si substrate with FeO_x underlayer.Thirdly, tetragonal L1₀ FePt films are deposited on the pyramid-type Si substrates fabricated by chemical etching process from (100) Si wafers. The films are composed of FePt grains with different easy axis orientations, most of which along substrate normal while the others tilted from the normal direction. The easy-axis tilted grains reverse their magnetization first under an out-of-plane external magnetic field, and then the whole film magnetization reverses for higher field. Small coercivity, suitable α and high squareness are obtained. And we also find that the coercivity increases when the applied field is tilted from the normal direction.Fourthly, the structure and exchange coupling in TbFeCo-FePt bilayer film have been investigated. It is found that FePt has an L1₀ structure and that the easy axis of the FePt film is perpendicular to the film plane. Results of vibrating sample magnetometer (VSM) and magneto-optical Kerr effect (MOKE) measurements show a strong perpendicular exchange coupling between the ferrimagnetic TbFeCo layer and the hard ferromagnetic FePt layer. The magnetization direction of each layer and the process of magnetization reversal are discussed in detail. The swiching field dependence on the exchange coupling has been modeled by micromagnetic simulation.Finally, we have investigated the exchange coupling between soft magnetic Fe (CoFe, NiFe) thin films and hard magnetic L1₀ FePt films, which is the coupling between in-plane and out-of-plane magnetizations. With the increasing of soft magnetic thickness, the coercivity and squareness of the bilayer decrease. And there is a critical thickness between rigid exchange coupling and spring exchange coupling. When a Cu thin film is inserted between CoFe and FePt bilayer, the coercivity is modulated by the Cu thickness, and they are decoupled when the Cu thickness is larger than a critical thickness.