Modification and nanostructuring of magnetic materials by ion irradiation


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TIBUS, Stefan, 2010. Modification and nanostructuring of magnetic materials by ion irradiation

@phdthesis{Tibus2010Modif-12383, title={Modification and nanostructuring of magnetic materials by ion irradiation}, year={2010}, author={Tibus, Stefan}, address={Konstanz}, school={Universität Konstanz} }

Tibus, Stefan 2011-04-05T12:19:09Z 2010 eng 2011-04-05T12:19:09Z deposit-license Modifikation und Nanostrukturierung magnetischer Materialien durch Ionenbestrahlung Tibus, Stefan Modification and nanostructuring of magnetic materials by ion irradiation In this work the modification of magnetic materials by ion irradiation and the application of this method to nano-structure magnetic media were investigated. Both aspects are of relevance for magnetic recording applications where the fine tuning of the magnetic material parameters and the creation of well-defined magnetic patterns are crucial for a further increase of the areal storage density.<br /><br /><br />The first part reports on magnetic pattern creation by local ion irradiation. Stripe patterns of 250 nm periodicity were created in a Co/Pd multilayer thin film by focused ion beam irradiation using Ga^+ at negligible surface recession by sputtering. The magnetic reversal behavior of the irradiated medium was investigated using magnetooptical Kerr effect magnetometry and magnetic force microscopy. Most notably, a stripe pattern of alternating magnetization direction was observed at remanence. Its nature can be understood considering a reduced magnetic anisotropy in the irradiated regions such that their magnetization can be reversed by the stray fields of the neighboring nonirradiated areas. To further support this interpretation micromagnetic simulations were performed. Using appropriate (effective) material parameters the characteristic features of the reversal behavior of the stripe pattern could be reproduced. Thus, the simulations confirm the formation of magnetostatically induced alternating magnetization patterns in a medium where the magnetic properties are altered locally. The simulations further support the formation of these patterns at even smaller scales.<br /><br /><br />In the second part the modification of the magnetic properties of magnetic materials by ion irradiation was studied in detail for granular CoCrPt:SiO_2 thin films exposed to Co^+ irradiation. Irradiation simulations were performed beforehand to optimize the ion energy for a maximum impact within the magnetic layer of the samples and evaluate the induced structural damage. Their results indicate a vertical mixing of the magnetic and nonmagnetic layers as well as lateral mixing across the grain boundaries. The irradiated samples were characterized using superconducting quantum interference device magnetometry, magnetooptical Kerr effect magnetometry and magnetic force microscopy. In general, the irradiation was found to affect the perpendicular anisotropy and the intergranular exchange coupling in the medium. This is indicated by a drop in coercivity, a reduction of the switching field distribution and an increase of the typical domain width with increasing irradiation fluence. The transformation from a Stoner-Wohlfarth like to a Kondorski-like reversal behavior observed in the angular dependence of the magnetic reversal further confirms the increase of the intergranular exchange coupling. To assist the interpretation of the experimental results micromagnetic simulations of the reversal behavior of granular media were performed for an extensive range of material parameters. The evolution of the experimental results as a function of fluence could be reproduced qualitatively by reducing the uniaxial anisotropy and increasing the intergranular exchange. Hence, the simulations support the modification of these material parameters as deduced from the experimental observations.<br /><br /><br />The work concludes with a brief investigation on the influence of magnetostatic interactions on the switching field distribution in arrays of magnetic nanostructures by means of micromagnetic simulations. The width of the distribution was found to be independent of the anisotropy and to depend on the geometry of the array and the saturation magnetization only. A hyperbolic dependence of the switching field distribution on the spacing of the array elements was observed. Its exponent was found to depend on the size of the single elements. The study thereby confirms experimental observations of significant contributions of magnetostatic interactions to the broadening of the switching field distribution in high density nanoparticle arrays.

Dateiabrufe seit 01.10.2014 (Informationen über die Zugriffsstatistik)

DissertationStefanTibus.pdf 154

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