Micromagnetic Investigation On Ferromagnetic NiFe Nanodisks

The study of magnetization processes in magnetic materials has been in thelast fifty years the focus of considerable research for its application to magneticrecording technology. In fact, the design of nowadays widespread magnetic stor-age devices, such as the hard-disks which are within computers on our desktopsand laptops, requires the knowledge of the "microscopic"phenomena occurringwithin magnetic media.Meanwhile, magnetic systems with reduced dimensions has attracted a greatdeal of interest for their various potential applications such as high-density pat-terned recording media ,Magnetoresistive Random Access Memory (MRAM) andultrasmall magnetic field sensors. Prior to the technological applications men-tioned above, it is indispensable to understand well fundamental properties of theindividual magnetic elements with reduced dimentions.More and more applications required the spatial scale of magnetic media inthe order of, more or less, hundred nanometers, magnetic phenomena has to beanalyzed by theoretical models with appropriate resolution. This is the case of mi-cromagnetics, which is a continuum theory that stands between quantum theorieslike ab-initio and macroscopic theories. On the other hand, with the read/write fre-quency increased to GHz and more, dynamic effects cannot be neglected, and thetime resolution of our observation should be on the order of picosecond. It is alsothe micromagnetics, which can simulate the ultra-fast magnetic recording deviceson the framework of magnetization dynamics.With the motivation mentioned above, we simulate the demagnetization pro-cesses of submicron-sized NiFe ferromagnetic disks of different shapes. The resultexhibit a magnetic vortex structure, which dominate almost the whole process.This peculiar configuration has been investigated both by Lorentz and magneticforce microscopy on a lot of literatures, which revealed an in-plane spin configu-ration, which closed the magnetic ?ux and a small region of perpendicular magne-tization at the vortex core.From our simulation, it became quite clear that the magnetization reversal ofdisks involves vortex formation, propagation, and annihilation. Although micro-magnetic calculations and experimental carried out previously suggest that vortexformation is preceded by "C" "S" and "W" state in different shape disks, we showhere the detailed mechanism. Different scenarios of vortex nucleation could beuncovered and confirmed by micromagnetic calculations.We study the interaction between magnetic vortices and artificial point defectsin submicron-sized Nife disk by using micromagnetics, which illustrate the inter-action of Pinning and Attracting by plotting the magnetic moment distribution.With the location of the defect changed perpendicular to the direction of the exter-nal field, vortex core will be pinned at the defect on most of the case, though thenucleation field are not the same. Changing defect location parallel to the externalfield will only alternate the degree of the attraction between defect and vortex.A dynamic micromagnetic finite-element simulation on the dynamic responseof a soft-magnetic disc exposed to an oscillatory field applied in the disc planewas presented. The simulation was started in an original-vortex state, and then thevortex core moved around an elliptical orbit and the magnetization vector changedits direction as the same frequency. As for the two resonant oscillations mentionedabove, we concluded that they have a invariable phase difference.