PhD Defense: Olatz Idigoras, on 27.09.2013 at 11:00

CIC nanoGUNE Seminars

Speaker

Olatz Idigoras

When

2013/09/27
13:00

Place

Auditorio Edificio Jose María Korta, Avda. Tolosa 72

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_Magnetization reversal behavior of ferromagnetic_ _thin films and nano-structures_ _ _ _ In general terms, this thesis studies the magnetization reversal behaviour of ferromagnetic thin films and nano-structures. In one core part of the thesis, the influence of crystallographic alignment, materials composition and thickness onto the magnetic properties of Co and Co alloy thin films has been studied in detail. Hereby, an epitaxially grown 30 nm thick (1010) Co thin film, which has in-plane uniaxial magnetic anisotropy, has been utilized as a reference structure due to its simple and well-understood behaviour. In order to modify the crystallographic alignment in a continuous and controlled fashion, a method to interrupt the epitaxy has been developed and applied. The magnetic properties of these crystallographically modified samples have been analysed by magneto-optical Kerr effect magnetometry and microscopy. Hereby, it was observed that while in samples with good crystallographic alignment the magnetization reversal is still simple and dominated by uniform magnetization states, non-uniform intermediate stable or meta-stable states emerge in the case of sufficiently disordered samples, even though macroscopic uniaxial anisotropy is still maintained. Furthermore, in samples with partial crystallographic alignment, an anomaly has been found, in which conventional hard axis behaviour disappears in a very narrow range of applied field directions. In such samples, a frustrated magnetic state occurs when the magnetic field is applied along the hard axis, which arises from the competition between ferromagnetic exchange and locally misaligned uniaxial anisotropies of adjacent grains. The existence of such a frustrated state along the nominal hard axis has been theoretically explained in the framework of a two-grain model and has been experimentally corroborated by microscopic imaging. As a function of thickness, epitaxial Co films exhibit a slight variation of their magnetization properties, which is triggered by and consistent with a crystallographic strain release upon increasing the film thickness. For the study of magnetic alloys, CoRu films of different compositions have also been grown epitaxially with (1010) crystallographic orientation. Hereby, it has been necessary to modify the epitaxial growth sequence and incorporate an individualized template for each magnetic alloy concentration, in order to fabricate alloy films of comparable crystal quality. For the so-prepared samples, key magnetic properties have been measured, such as, Curie temperature, saturation magnetization and anisotropy constants as a function of composition. It has been found that the magneto-crystalline anisotropy constant shows a complex and non-monotonous behaviour as a function of Ru content in the alloy, while saturation magnetization and Curie temperature show a more expected monotonic decrease with Ru concentration. In addition to the material oriented studies, Co-films have also been utilized to experimentally investigate the dynamic phase transition and specifically, the influence of a constant bias field onto the phase diagram and phase stability. These experimental studies have been furthermore complemented by theoretical calculations based upon the Kinetic Ising model in mean field approximation. Also here, the role of the constant bias field has been analysed, which was identified as the conjugated field of the dynamic order parameter. Finally, the benefits that the magneto-optical Kerr effect microscope offers for the study of individual structures, even nano-scale structures, has been demonstrated and analysed. Specifically, single cycle hysteresis loop measurements of 30 nm wide Co wires, fabricated by means of focused electron beam induced deposited, have been demonstrated. Also, 100 nm wide wires of only 0.8 nm Co layer thickness (as part of a Pt/Co/Pt-trilayer structure) have been measured with the Kerr microscope with excellent signal to noise ratio, allowing the distinction of uniform vs. non- uniform reversal. These specific Pt/Co/Pt nanostructures have been successfully fabricated, maintaining their original perpendicular anisotropy, by means of focussed ion beam induced layer intermixing. For this purpose, a special TiN hard mask had to be developed, which acts as a sacrificial layer, and opens up a new and promising pathway for directly nano-structuring multilayer materials by means of focused ion beam exposure without damaging their magnetic properties in a wide surface area. _