Study On Magnetic Recording Media FePt Films With High Magnetocrystalline Anisotropy

To further improve the density of magnetic recording meadia, the magnetic grain size should be reduced to below 10 nm. But at this level of grain size, the superparamagnetic effect caused by thermal fluctuation will affect the thermal stability of magnetic recording media. With large magnetocrystalline anisotropy ( Ku = 7×106 J/m3), the L10-FePt ordering alloy film has good thermal stability even when the grain size is about 5nm. So the L10-FePt ordering film has recently become a hot issue in magnetic recording field. However, for ultrahigh density perpendicular magnetic recording media, two key issues for the application of L10-FePt nanoparticle films remain to be solved: one is to decrease the ordering temperature of L10-FePt nanoparticle films, the other to obtain the L10-FePt nanoparticle films with perpendicular (001) texture. This dissculation has pyetematically inrestigated the two issues.(1) Study on the reduction of the ordering temperature of FePt thin films Series of FePt and FePtCu thin films with a fixed thickness of 50nm have been prepared by magnetron sputtering. The atomic ratio of Fe to Pt atoms have been accurately controlled by DC co-sputtering. After vacuum annealing at 350℃for 20 min, the coercive force of about 605 kA/m is obtained for the Fe₅₂Pt₄₈ thin films (The atomic ratio of Fe to Pt atom is 1.1:1) with the ordering parameter of about 0.87, which leads to the ordering of the FePt thin films at a lower annealing temperature. The ordering of the FePt thin films is not enhanced by the addition of Cu to Fe₅₂Pt₄₈ films, but the addition of Cu to the Pt-rich or stoichiometric thin films couled promote the L10 FePt ordering. As a rule, we find that L10 phase can be obtained at a lower annealing temperature only for the films with the ratio of the (FeCu) : Pt in the range of 1.1¹.2.The effects of films thickness, annealing temperature and time on the ordering of Fe₅₂Pt₄₈ films have been studied. When the films are annealed at a temperature lower than 400℃, theordering parameter and the coercive force increase with the increase of the thickness of the films. The thickness has little effect on the ordering parameter when the film is thicker than 50 nm.With a fixed composition of Fe49Pt51 films, the ordering temperature of FePt thin films can be lowered by the preparation of Fe/Pt multilayers or FePt/Cu multilayers. Compared with Fe49Pt51 films (40nm), the ordering temperature of Fe/Pt multilayers can be decreased by about 100℃. The coercive force of [Fe/Pt]n multilayers is significantly increased by sputtering Ag or Ti underlayer on the glass substrate. After vacuum annealing for 20 min at 350°C, the coercive force of about 597 kA/m and 645 kA/m is obtained for multilayers with Ag and Ti underlayer, respectively. Meanwhile, the surface roughness and the surface particle size of multilayers with Ti underlayer are close to those of the films without any underlayer.(2) Study on the perpendicular orientation of FePt nanoprticle filmsThe FePt thin films with different thickness have been prepared on (100) MgO single crystal substrates with magnetron sputtering and then vacuum annealed at 600℃for 15 min. The substrate temperature is kept at ambient temperature or 250℃. It has been found that heating of the substrate is useful for the formation of strong L10-FePt (001) texture. The thicker the thin film is, the better its epitaxial growth on MgO (100) single crystal, which leads to oriental growth along the [001] direction normal to the film plane. For the 5nm FePt thin film prepared on the 250℃substrate, the coercive force can reach 240 kA/m and the average grain size is 7nm, showing good perpendicular magnetocrystalline anisotropy.FePt/BN particle films have been prepared on the heated MgO substrates ( 250℃) . L10-FePt/BN particle films with strong perpendicular orientation can be obtained after [FePt(1nm)/BN(0.25nm)]20 multilayer has been vacuum annealed at 700℃for an hour. The results have shown strong perpendicular magnetic anisotropy and good squareness.[FePt (2nm)/Ag (d nm)]10 multilayers have been prepared on MgO (100) single crystal substrates with magnetron sputtering. The low ordering temperature and perpendicular orientation of L10-FePt/Ag particle films with high coercivity have been obtained by controllingaccurately the atomic ratio of Fe to Pt of L10-FePt films and heating the substrate. The average size of L10-FePt grains decreases with the increasing the thickness of Ag layer.A typical example is the [FePt (2nm)/Ag (5nm)]10 multilayers after vacuum annealed at 600℃for 15 min. The interface in multilayers disappears and a particle film is obtained. The average size of L10-FePt grains is about 8nm, and its perpendicular coercive force reaches 979 kA/m.δM results has shown that this particle film has little magnetic exchange coupling.The above results about the ordering of FePt thin films at a lower temperature and control of FePt [001] perpendicular orientation are very useful for the application of future perpendicular magnetic recording media with ultrahigh density.