Magnetization Reversal Mechanism And Micromagnetic Simulation Of Ll₀/Al-FePt Exchange Spring Bilayer Films

With the popularity of big data and cloud computing,we will also put forward higher requirements for large-capacity data storage.The areal density of the perpendicular magnetic recording medium is close to Tb/in² scale,the particle size gradually decreases,making the media particles must be faced the superparamagnetic limit and unstable.In order to overcome these problems,magnetic recording materials use high magnetocrystalline anisotropy materials.Ll₀-FePt has a high magnetocrystalline anisotropy value,which is used to store information and can achieve a greater storage area density.Moreover,in the Ll₀-FePt atoms,there are orbital hybridization between 3d?Fe?-5d?Pt?and free electrons,the room temperature H_c of the Ll₀-FePt particle film reached 70 kOe,so it is particularly noticeable.However,a high K_u value material is made of small particles film,although increasing the storage density or reducing the device size,the switching field is too large to rewrite the magnetization state.Studies have shown that interface exchange strength have the effect of reducing the switching field for the exchange-spring?ES?magnets,the focus on magnetoelasticity has shifted hence away from the pursuit of improvement?BH?max to simply seeking to reduce H_c.The exchange spring magnets are composed of hard magnetic?HM?and soft magnetic?SM?materials,and produce exchange coupling at the interface between the two phases.using the high coercivity?H_c?of HM and the high saturation magnetization?M_s?of SM,it can can effectively reduce the condensing the H_c.The ES magnet always nucleates in the soft magnetic phase,and the switching field where the domain wall breaks through the interface into the hard magnetic phase,the switching field of the hard magnetic phase is lower than that of a single hard magnetic phase magnet,so the shape of the magnetization curve is closely related to the thickness of the soft magnetic phase.More research has focused on the FePt/Fe bilayer system.However,using FePt/Fe system,in addition to Fe easily oxidized,the crystal structure types and lattice constants of the two materials are also different,the interface epitaxy is poor,and the distortion stress is large,which affects the interface properties.The best way to solve this problem is to form a heterojunction.In this thesis,Al-FePt replaces Fe that is easily oxidized and has a lattice mismatch with Ll₀-FePt,we obtain the same material heterogeneous Ll₀-FePt/Al-FePt bilayer film,analyzing the thickness of the soft magnetic layer and the degree of order of the hard magnetic layer influence on the nucleation field H_n,the switching field H_s,and the squareness ratio M_r/M_s,etc.,in order to recognize the magnetization reversal mechanism,the magnetization curve was simulated using the micro-magnetic simulation software OOMMF.The results are divided into the following aspects:1.The degree of ordering of Ll₀-FePt prepared experimentally is not thorough enough.By using the OOMMF software to simulate the Ll₀-FePt film,the effect of K_u value on the film is studied.The experimentally prepared FePt thin film has a heat treatment temperature of T_a=700°C.and an order degree of 0.9.This indicates that the degree of ordering of the FePt?30 nm?thin film is not thorough enough,but due to the discontinuity of the thin film at high temperatures,the coercivity is small?0.47kOe?.Using OOMMF to simulate the FePt?30 nm?film,H_c is between 25 kOe³0kOe,and its effective K_u value will not be lower than 1.5×10⁶J/m³.2.The Ll₀-FePt?27 nm?/Al-FePt?3 nm?thin film was simulated by the OOMMF software,and the coercivity of the hard magnetic phase was significantly reduced.Experimentally,isolated Ll₀-FePt particles can be obtained,measuring H_c and K_u,and due to S=1,we can derive the limit H_c and K_u.Furthermore,using the obtained limit K_u,the effective volume is also obtained theoretically that occupied by the soft magnetic phase in the thin film,and the degree of ordering need not be analyzed again by the XRD method.3.Simulating the Ll₀?x nm?/Al-FePt?30-x nm?bilayer films,D<70%.The reverse magnetic domain of the soft magnetic layer advances towards the interface,and the domain wall encounters the interface,and the domain gradually narrows,due to the large storage energy in the domain,the domain wall breaks through the interface,making the hard magnetic layer reverse.D>70%,After nucleation in the soft magnetic phase,the magnetic moment precession of a nucleate point deviates from the direction of the external field,and the deviation angle increases with the increase of the reverse magnetic field,growing into 90°domain wall,it is geting into the hard magnetic layer becomes a 180°domain wall and forms a reverse magnetic domain.Thereby it instantly complete the magnetization reversal process.D>70%,M_r/M_s>0.65,is still large enough to store information,but compared with H_s of pure Ll₀-FePt,its coercivity is greatly reduced.4.Also simulated separately Ll₀?20 nm?/Al?x nm?and Ll₀?30 nm?/Al?x nm?films,the obvious change also occurs when x is between 5 and 7,and the smaller the x,it is more pronounced that the effect of the thickness of the harder magnetic layer changes,but overall it has little effect on coercivity.However,if a 20 nm thick film is actually fabricated,the grain size will be less than 30 nm thick after heat treatment,and the coercive force should instead increase.The Al?x nm?/Ll₀?30 nm?/Al?x nm?films are more reliable than the previous two-layer film simulation method.