Studies Of Media And Head Materials For Ultra-high Density Magnetic Recording

The drastic development of magnetic recording has brought the recording density increasing steadily and close to its physical limit. Nowadays, the most important challenges for magnetic recording are media and head materials.The core materials in writing heads for ultra-high magnetic recording are required to have high saturation magnetization. The bcc Fe65Co35 alloy has been studied extensively because it has the highest saturation magnetization up to 24.5 kG. However, under normal growth conditions, the coercivity of Fe65Co35 thin films is high due to its large magnetocrystalline anisotropy constants (-10 kJ/m3) and large magnetostriction constant (λs= 4.0-6.5×10-5). So, it cannot be applied directly in ultra-high magnetic recording writing heads. Therefore, it is important and essential to improve the soft magnetic properties of Fe65Co35 thin films.L10 phase FePt films show much potential for applications of ultrahigh density perpendicular magnetic recording media because of its large magnetocrystalline anisotropy energy (Ku-7×107 ergs/cm3). It is well known that, as perpendicular magnetic recording media, the easy magnetization axis (c axis) of FePt films should be normal to the films. Nevertheless, the FePt films prepared under normal conditions tend to possess (111) texture because (111) plane is the close-packed plane of FePt alloy, and, consequently, the c axis is not normal to the films. Therefore, one of the key challenges for perpendicular magnetic recording media is how to obtain perfect (001) orientation and excellent perpendicular anisotropy.In this thesis, the microstructure and magnetic properties of FePt thin films, [Fe/Pt]n multilayer films for media and Fe65Co35 thin films for writing heads were investigated, respectively. The thesis includes three parts:I. Investigation of microstructure and soft magnetic properties of Fe65Co35 thin films deposited on different underlayers In this work, the microstructure and soft magnetic properties of Fe6sCo35 (100 nm) thin films deposited on Fe, Ni80Fe20, Co93Fe7 and Cu underlayers were investigated. The magnetocrystalline and magnetoelastic anisotropy energy of Fe65Co35 thin films with (200) and (110) textures were calculated. By comparison, the cause that different underlayer materials have different effect on improving of soft magnetic properties of Fe65Co35 thin films were analysed and explored from the view point of energy. The main conclusions are as follows:(a) The soft magnetic peoperties of Fe65Co35 thin films deposited on underlayers were much improved compared with that of Fe65Co35 thin films deposited directly on glass substrate.(b) The improving of soft magnetic peoperties of Fe65Co35 thin films is correlated with the materials and thickness of unlayers. Different underlayer materials have different effect on improving of soft magnetic properties of Fe65Co35 thin films.(c) The textures of Fe65Co35 thin films deposited on different underlayer materials are different. Furthermore, as the integrated intensity ratio ofⅠ(110)Ⅰ(200) decreases, the coercivities along hard axis of Fe65Co35 films decrease on over trend. In other words, with the texture changing from (110) to (200), the soft magnetic properties of Fe65Co35 films are much improved.(d) The calculations of typical samples with (200) and (110) textures indicate that either the magnetocrystalline or the magnetoelastic anisotropy energy of Fe65Co35 films with (110) structure is larger than that with (200) structure. Therefore, the soft magnetic properties of Fe65Co35 films with (200) structure are better than that with (110) structure.Ⅱ. Effect of magnetic field annealing on microstructure, (001) orientation and magnetic properties of FePt filmsIn this work, a series of FePt films with different thickness were deposited on Si substrate and then post-annealed in a magnetic field at the temperature which is around the Curie temperature Tc of L10 phase FePt. The influence of magnetic field annealing on microstructure, (001) orientaion and magnetic properties of FePt films were investigated. The main conclusions are as follows:(a) The (001) orientaion of FePt films can be effectively improved by magnetic field annealing.(b) The (001) orientaion of FePt films is best when the second step annealling temperature is around the Curie temperature Tc of L10 phase FePt alloy. When FePt (20 nm) films were first annealed at 700℃for 30 min and then at 478℃for 60 min, the integrated intensity ratio ofⅠ(001)/Ⅰ(111) showed a peak value of-7.(c) In industrial applications, this may be a new method to improve (001) orientation and perpendicular anisotropy in FePt films.III. Investigation of the microstructure and magnetic properties of nonepitaxial [Fe/Pt]n multilayer filmsIn this study, the microstructure and magnetic properties of a series of nonepitaxial [Pt/Fe]n (n= 8,16,20,32) multilayer films were invetigated. The influence of interface effect between Fe layer and Pt layer on the perpendicular anisotropy of [Fe/Pt]n multilayer films were discussed. The interactions among grains in FePt films were analysed by measuring theδM (H) curves of typical samples of [Fe/Pt]n multilayer films and FePt single layer films. The main conclusions are as follows:(a) The [Pt/Fe]n multilayer films show a certain perpendicular anisotropy while the dominant texture is (111).(b) The perpendicular anisotropy of [Pt/Fe]n multilayer films has an intimate correlation with the periodicity n and, with the increasing of periodicity n, the perpendicular anisotropy first significantly increases and then decreases slowly. The enhanced perpendicular anisotropy may be caused by the interface anisotropy at the Fe/Pt interfaces.(c) The exchange coupling interaction among grains in FePt films can be effectively reduced by using [Fe/Pt]n multilayer structure. This is beneficial to the improving of signal noise ratio (SNR) for ultra-high density magnetic recording.(d) This may be a new useful and practical method to obtain perpendicular anisotropy and, at the same time, to effectively reduce the exchange coupling interaction among grains in FePt films.