Quantitative Analysis Of The MFM Images On Thin Films And Preparation Of High Performance Probes For MFM

Magnetic domains of the magnetic materials determine the properties of the materials and their applications, so the study of the magnetic domain structure is necessary for the understanding of the magnetic properties and the magnetization reversal mechanism, and it will be clear how to improve the preparing process. Due to the advantages of the manetic force microscopy with high resolution(<50nm), such as simple sample preparation and operation, test environment demand not high, various materials can be tested, the magnetic force microscopy became the most powerful tool for magnetic domain structure observation.This thesis mainly includes three parts:In the first part, the distribution of the magnetic charge and the stray magnetic field was obtained by the Discrete Fourier Transform (DFT) method and the approximation method respectively in order to anlyze the domain structure of the samples. In the the second part, we studied the domain structures of the FcCoAlON thin films by magnetic force microscopy, discussing its origin and appearance condtions of stripe domain. In the third part, the sputtering method was used to prepare high coercivity, high resolution FePt magnetic probe. The optimum preparation conditions were found. The specific research contents and results are as follows:(1) The distribution of the magnetic charge and the stray magnetic field on the surface of the sample was obtained by the Discrete Fourier Transform method. However, the low frequency noise was enlarged in this process, so the filter method was employed to remove the noise of the image. This would make some loss of the signal and the cut-off frequency was hard to choose. Then the approximation method was chosen to calculate the distribution of the magnetic charge and the stray magnetic field on the surface of the sample. (2) For the film material, the film domain structures characteristics were determined through the sample surface magnetic charge and the stray field distribution obtained from the MFM image and the relevant information from the hysteresis loop. For a perpendicular anisotropic thin film, magnetic force microscopy directly gives its domain structure diagram. The component perpendicular to the film plane gives the position of the domain and the domain wall. For the in-plane anisotropy of the film, the distribution of film surface obtained from the magnetic force microscopy can determine the position of the magnetic domain and the domain walls.(3) With the increase of N content, the FeCoAlON films change from in-plane anisotropic to isotropic, and the weak perpendicular anisotropy appeared. when the N content was more than3%, the films possessed the rotational anisotropy and the stripe domain structures appeared at the film surface.(4) We thought that the stripe domain formation in the FeCoAlON thin films was due to the N atoms filling into the (110) plane as the interstitial atoms. The insterstitial atoms may cause the lattice deformation and lead to a compressvie stress which can be the origin of the perpendicular magnetic anisotropy.(5) With increasing thickness, the magnetic domains of the FeCoAlON films changed from the crosstie structures to stripe structures. The relationship curve was fitted from the experimental film thickness and the domain width. We calculated the critical thickess of the stripe domain structure and obtained the thickness of100nm. This matches well with the experimental result of110nm thin film which possesses the obsolete stripe domain.(6) The frequency response characteristics of the FeCoAlON thin films were also measured. The FeCoAlON thin films with the thickness of55nm display a single resonance and the resonance frequency is2GHz and the low permeability is above500. This is similar to the resonance frequency theory of ferromagnetic films without stripe domain. When the thickness are110nm,310nm,320nm,450nm, the resonance are double-peak resonance. The films with the thickness of550nm are multi-peak resonance. (7) The stripe domain structures gradually appears and then disappears during the applied magnetic field which along the direction of the stripe changing from the positive magnetization saturation to the negative magnetization saturation. The domain width increases first and then decreases in the magnetization process.The magnetization process includes a domain wall displacement and moment rotation.The moment rotation was severe near the coercivity field. The direction of the stripe suddenly changes into the direction of the applied magnetic field at the magnetic field above the coercivity field and the domain width decreased when applying the magnetic field perpendicular to the stripe. It means that the domain wall displacement appeared in this process.(8) It must undertake the anti-sputtering process to remove of the surface contaminants of the probe tip during the preparation of FePt probe, so that the combination of FePt magnetic layer and tip framework substrate is firm.(9) It can make the FePt layer more easier to attach on the probe tip by sputtering a thickness of2nm SiO2film layer prior to the FePt magnetic layer, meamwhile, the measured AFM map is more accurate; It is also contributes to make the FePt film epitaxial growth on the SiO2substrate, formate the L10phase high coercivity FePt magnetic layer, and get better MFM image during the test;(10) With different thickness of the FePt magnetic layer, the resolution of FePt MFM probe is different:the resolution of the probe is poor when magnetic layer is thick; On the other hand, the magnetic signal of the probe is inadequate when the sputtered film is too thin, as a result, it will appear too much background noise in the test; When the thickness of the sputtering film is40～60nm, the prepared MFM probes have the best signal to noise ratio and high resolution.