Studies Of Exchange Coupled Media Based On L1₀FePt Thin Films For Ultra-high Density Magnetic Recording

The social progress and the development of technology have brought a drastic information explosion, which urgently require increasing information storage capacity of recording system. Up to now, among the various information storage technologies, hard disk driver (HDD) is the most important technology for high capacity storage. Therefor, it is necessary to increase the information storage density of HDD. To maintain a reasonable signal to noise ratio (SNR) in HDD, the increasing of recording areal density implies a shrunk of the volume of recording bits, and also the volume of magnetic grains. To overcome the so called "superparamagnetic effect" for small grains, it is necessary to use the magnetic materials with higher anisotropy for medium, which results in a difficulty of writing information. This is the "trilemma issue" in the magnetic recording, viz., the compatibility among writing ability, thermal stability and signal to noise ratio. It is an effective approach to resovle the above issue using exchange coupled media, which is composed of a soft and a hard magnetic layer with large anisotropy. The medium coercivity can be tuned by adjusting the exchange coupling interation between soft and hard magnetic layers, while keeping the advantages of larger anisotropy and thermal stability in hard layer. The microstructure, magnetic property and magnetization reversal process are different due to different fabrication condition of hard magnetic layer in exchange coupled media. LI0FePt is considered to be a potential magnetic hard layer in new generation magnetic recording medium, because of its high magnetocrystalline anisotropy and good chemical stability. Thus the aim of this dissertation is to fabricate LI0FePt based magnetic exchange coupled medium and to study its magnetic property. The FePt thin films with different composition and thicknesses were fabricated by magnetron sputtering and annealing. LI0FePt/Fe and LI0FePt-MgO/Fe-MgO exchange coupled thin films medium were also fabricated. The composition, microstructure, magnetic property and magnetic reversal mechanism were studied by using ICP, XRD, XPS,TEM,VSM and MFM.The main conclusions are as follows.1.Fabrication and characterization of FePt thin filmsLI0FePt thin films with highly ordered degree and good perpendicular anisotropy have been prepared. The effects of compositions, thicknesses and annealing temperature on structure and magnetic property of FePt films have been studied.(1)For the FePt films with Fe content varying from31at.%to51at.%, a high ordered degree and good perpendicular anisotropy were abtained in Fe47Pt53thin films. The squareness and the coercivity of out-of-plane loop is0.99and10.4kOe, respectively. The relative round magnetic domains with sizes of100nm-300nm are observed.(2) When the annealing temperature varies from400℃to700℃, higher annealing temperature is beneficial to the formation of LI0FePt thin films. For the film annealed at700℃, the in-plane direction changes into hard magnetization direction and out-of-plane direction changes into easy magnetization direction,the perpendicular anisotropy of FePt thin films are very obvious.(3)For the700℃-annealed films,when the thickness increases from10nm to60nm,the out-of-plane coercivities decreases and the in-plane coercivities increases gradually, revealing that the perpendicular anisotropy of FePt thin films decreases.Ⅱ. Fabrication and studies of LI0FePt/Fe exchange coupled thin filmsZI0FePt/Fe thin films and LI0FePt/Pt/Fe thin films were prepared by using magnetron co-sputtering and annealing. The magnetic property, magnetization reversal mechanism of LI0FePt/Fe thin films and the influence of Pt middle layer on exchange coupled strength between FePt layer and Fe layer were studied. The main conclusions are as follows.(1)When the Fe thickness is less than3nm, the in-plane M-H loops of FePt/Fe bilayer thin films close to straight line, suggesting that the exchange coupled strength is very strong between FePt layer and Fe layer. When Fe thickness is larger than3nm, the in-plane M-H loops of FePt/Fe bilayer thin films similar to soft magnetic thin films due to increasing demagnetization energy. As Fe thickness become thicker, the out-of-plane coercivities decrease, the switch field distribution become wider, the size of magnetic domain become smaller and the domain wall width become wider.(2)When Fe thickness is less than3nm, the strong interface exchange couple exist between FePt layer and Fe layer and the magnetization reversal process dominated by irreversible rotation. When the Fe thickness is larger than3nm, the magnetic reversal process varies from rigid magnetic reversal to exchange spring magnetic reversal with increasing Fe thickness. The magnetization of Fe turns to in-plane direction gradually and reversible Fe magnetization increase due to demagnetization energy. This is to say the magnetization reversal process of LI0FePt/Fe thin films starts from a rigid magnetic reversal, and then change to exchange spring magnetic reversal,finally a two-phase reversal process.(3)LI0FePt thin films are continuous films and exchange couple strength between FePt grains are very strong, demonstrated by Henkel curves and Delta M curves. As Fe become thicker, the magnetic reversal process of sample are influenced by magnetic dipole interaction, thus it is easy to demagnetize sample and hard to magnetize.(4) When the Pt middle layer thickness increases, there are no two-phase step in out-of-plane M-H loops and, the out-of-plane coercivities increase. These results show that the exchange couple is not strong enough between FePt layer and Fe layer when Fe thickness is3nm. So less than3nm Fe thickness is necessary to regulate exchange couple strength between FePt layer and Fe layer.Ⅲ.LI0FePt-MgO hard layer/Fe-MgO soft layer exchange coupled thin filmsThe microstructure and magnetic property of LI0FePt-MgO thin films and LI0FePt-MgO/Fe-MgO exchange coupled thin films were studied.The main conclusions are as follows.(1)LI0FePt-MgO thin films with good perpendicular anisotropy and weak exchange couple between FePt grains have been prepared through doing MgO into FePt thin films, whereas the SiO2doping are not beneficial to the formation of (001)orientation in FePt thin films.(2)The FePt-MgO thin films show good perpendicular anisotropy when the MgO content is20vol.%.The microstructure shows that the FePt grains distribute uniformly in MgO matrix and the average grains size is close to5.4nm.The relatively weak exchange coupling was obtained by the small a value of2.0, which was confirmed by MFM images.This is important for achieving an ultra-high magnetic recording area density.(3)With the soft Fe-MgO layer being added, the in-plane coercivity increases and then decreases due to the weak exchange coupled interaction between hard layer FePt-MgO and upper soft Fe-MgO layer. The magnetization process of the bilayer system transforms from a coherent magnetization to an exchange spring behavior, and finally a two-phase magnetization reversible process with increasing soft layer thickness.(4)When a0.25nm Pt middle layer was inserted into FePt-MgO/Fe-MgO bilayers, the out-of-plane coercivity decreases to9.3kOe from10.1kOe.The out-of-plane coercivity increases with further increasing of the Pt middle layer thickness.It is obvious that the Pt middle layer can effectively regulate exchange couple strength between FePt-MgO hard layer and Fe-MgO soft layer.