Magnetic Nanowires Antiferromagnetically Coupled Multilayer Magnetic Properties Of The Micro-magnetic

Micromagnetic distribution and magnetization reversal mechanismin nanowires and antiferromagnetically coupled multilayers is animportant scientific problem; magnetic nanowire array andantiferromagnetically coupled multilayers can be used as idealperpendicular recording media in future. In this thesis, based on themicromagnetic theory, magnetic properties of nanowires andantiferromagnetically coupled multilayers have been investigated bymicromagnetic simulation as follows:The magnetization reversal mechanisms of magnetic nanowires withdifferent diameter were studied. The results show that the magnetizationreversal mechanism significantly depends on the nanowire diameter. Forthe smallest wire, the reversal occurs by coherent rotation; with theincrease of diameter, magnetization reversal takes place via differentnucleation (the transverse domain wall or the vortex domain wall) andsubsequent propagation; for the larger nanowire, multidomain wouldform within the wire. The analytical micromagnetics are used toinvestigate the domain wall structure during the magnetization reversal innanowires. Micromagnetic analysis shows that for the soft magneticnanwire the domain wall structure is mainly determined by thecompetition between the demagnetization energy and exchange energy.The critical diameters of the transverse domain wall and the vortexdomain wall are 40 nm for Ni wire and 20 nm for Fe wire, respectively.The micromagnetic simulations are carried out on the cone-shaped wireand confirm the analytical results.Angular dependence of the switching field Hsw(θ) and thecoercivity Itc(θ) of magnetic nanowires with different diameters hasbeen investigated(whereθis defined as the angle between the fielddirection and wire length). For the nanowire diameter smaller than acritical diameter, at small angle, the switching field is the same with thecoercivity, forθ＞60°, the switching field increase as the angle increases,but the coercivity decrease as the angle increases. For the diameter largerthan the critical diameter, the switching field increase as the angle increases. The angular dependence of coercivity has a complicated form.There is a local maximum at zero angle and a peak at middle angle on theHc(O) curve. Micromagnetic simulations for nanowires show that themagnetization reversal in wires with finite length is significantly differentfrom the reversal in infinite cylinder. The angular dependence behavior isinterpreted in terms of the difference of magnetization reversalmechanism.The magnetization reversal mechanism of antiferromangeticallycoupled ternary layers and multilayers were studied. The phase diagramof two identical coupled ferromagnetic layers is presented by type of easyaxis hysteresis loop and takes into account the effect of uniaxial intrinsicanisotropy, exchange coupling, aspect ratio and thickness. The hysteresisloops of antiferromangetically coupled multilayers are more complex. Inthe reversal process, the even layers reversal firstly, and then the oddlayers occurs reversal. Angular dependence of the hysteresis loop and themagnetization reversal have been investigated (whereθis defined as theangle between the field direction and the easy axis of theantiferromangetically coupled ternary layers). The magnetization reversalof antiferromangetically coupled ternary layers is similar to themagnetization reversal of nanowires. The magnetization reversal mode istransitional behavior from "curling" to homogeneous reversal. Increasingthe intrinsic anisotropy or the thickness of one of the ferromagnetic layers,the hysteresis loops and the switching field show a remarkable change.Above phenomenon are interpreted in terms of reversal mechanism.