Micromagnetic Studies On The Ultrahigh Density Magnetic Recording Thin Films

As a basic theory in applied magnetism, the micromagnetic simulation has been playing an important role in the studies of magnetism and magnetic materials, especially in the research and development of hard disk drives. In this paper, micromagnetic models were built up to investigate the magnetic thin films of ultrahigh density magnetic recording. Four types of recording materials were included:the FeCo soft magnetic thin films of write head, the CoPt-TiO2hard magnetic films, the CoPt-TiO2/Pt/Co-TiO2thin films, the CoPt-TiO2/Co-TiO2thin films. The main results were:1. Micromagnetic Studies on FeCo soft magnetic thin films used in write headsThe magneto-elastic energy of FeCo thin film with texture is carefully analyzed, and it is proved that, under different texture, the magneto-elastic field has a main term Hma and a cross term Hmb. Based on these theoretical understanding of the magneto-elastic field on the interface, the micromagnetic model of FeCo thin film is improved. We founded:The cubic anisotropy of FeCo determines the symmetry of the loops. The main term of magneto-elastic field Hma has significant influence on the easy-axis coercivity. The cross term Hmb has great effects on the hard-axis M-H loop, and the slope of the hard-axis M-H loop is mainly controlled by Hmb. The strong correlation of the coercivity and the interfacial stress is contributed by both the main terHma and the cross term of magneto-elastic field Hmb. Well controlled soft magnetic properties of FeCo thin films can be obtained by using certain underlayers (such as Cu, NiFe, CoFe et al.) corresponding to low values of Hma and Hmb.In the polycrystalline FeCo thin film, lower magnetization4πMs at amorphous grain boundary makes the soft magnetic properties much worse. Meanwhile, the thickness of the thin film has great effects on the magnetic properties of the thin films, due to the interfacial stress.2. Micromagnetic studies on the CoPt-TiO2hard magnetic mediaTwo classes of micromagnetic models were introduced to simulate the M-H loops of the CoPt-TiO2thin films. In the first simple model, the polycrystalline microstructure is not included, and one magnetic grain is modeled as a micromagnetic cell. The perpendicular loop is close to the experiment, except that the tail is longer. However, the in-plane longitudinal hard axis loop is quite different from measurement, this reveals the weakness of the microstructure description in this simple model. In the second model, the polycrystalline structure is simulated, the symmetries in the crystal grains are considered, and the magneto-elastic field related to the underlayer is included. The squareness of easy axis is0.98, the coercivity of easy axis is6.1kOe, and the saturated field of hard axis is20kOe, the simulated results agree well with the experimental results. Besides, we studied the magnetization reversal mechanism and found this has a typical characteristic of the domain-wall-pinning mode as a dominant magnetization reversal mechanism.3. Micromagnetic studies on the CoPt-TiO2/Pt/Co-TiO2thin filmsThe exchange A*3between the hard and soft layer in the exchange coupled composite (ECC) medium is crucial for the ultrahigh density recording performance. An accurate micromagnetic model based on microstructure is built for Co-Ti02/Pt/CoPt-TiO2thin films to investigate the effects of the thickness8of Pt interlayer and the intrinsic magnetic parameters of the soft layer on the magnetic properties of the thin films.In the thin films, the thickness8of interlayer Pt has comprehensive effects on magnetic properties. When we just vary A*3or δ, the coercivity and squareness change little. The simulation agrees well with experiment if we let A*3equals the intra-grain exchange constant A*t, when δis thinner than grain boundary (≤2nm); and A*3should decrease from the inter-grain exchange constant.A*2to0when δ increases from2.2nm to2.8nm. We also found that the coercivity and squareness are decreased when the anisotropy is lower and the saturation magnetization is larger in the soft layer.4. Micromagnetic studies on the CoPt-TiO/Co-TiO2thin filmsA micromagnetic model based on microstructure is built for Co-TiO2/CoPt-TiO2thin films to investigate the effects of the thickness8and the anisotropy field of the soft layer on the magnetic properties of the thin films.The thickness of the soft layer has comprehensive effects on magnetic properties:it affects the magnetostatic interaction and exchange interaction constant A*3between the soft layer and hard layer. The coercivity and squareness change little when we just vary δ, whereas, the coercivity and squareness decrease when the A*3decreases from1.Ox10-6erg/cm to0. The magnetization of the soft layer is easier to be nucleated and reversed, and the squareness of the M-H loop decreases, when the anisotropy field of the soft layer is decreased.