Magnetization reversal behavior of ferromagnetic thin films and nano-structures

Olatz Idigoras has been developing her thesis project during four years at nanoGUNE, under the supervision of Andreas Berger, leader of the Nanomagnetism Group and Research Director of nanoGUNE. Her research work achieved the maximum qualification (cum laude) after the defense and assessment of her work by an international committee that included leading researchers in the field of nanomagnetism.

In general terms, the presented 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. Hereby, epitaxially grown 30 nm thick (1010) Co thin films, which have in-plane uniaxial magnetic anisotropy, has been utilized as a reference structures. In order to modify the crystallographic alignment in a continuous and controlled fashion, a method to interrupt the epitaxy has been developed and applied. Hereby, it was observed that while in samples with good crystallographic alignment the magnetization reversal is dominated by uniform magnetization states, non-uniform intermediate stable or meta-stable states emerge in the case of sufficiently disordered samples. 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 nominal hard axis, which arises from the competition between ferromagnetic exchange and locally misaligned uniaxial anisotropies of adjacent grains. For the study of magnetic alloys, CoRu films of different compositions have also been grown epitaxially with (1010) crystallographic orientation. Here, 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 materials 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.

Finally, the benefits that the magneto-optical Kerr effect microscope offers for the study of individual nano 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 deposition, have been demonstrated. Also, 100 nm wide wires of only 0.8 nm thick Co layers (as part of a Pt/Co/Pt-trilayer structure) have been measured with the Kerr microscope with excellent signal to noise ratio, allowing for 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 focused 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 promising pathway for directly nano-structuring multilayer materials by means of focused ion beam exposure.

An international committee including leading researchers in the field was selected by the UPV/EHU to assess the research project:

• Fernando Plazaola Muguruza (UPV/EHU, Spain)
• Dirk Sander (Max Planck Institute of Microstructure Physics, Halle, Germany)
• Luis A. Morellon Alquézar (Instituto de Nanociencia de Aragon, Spain)
• Josep Nogues Sanmiquel (Institut Catala de Nanociencia y Nanotecnologia, Spain)
• Luis Hueso Arroyo (CIC nanoGUNE, Spain)

The defense consisted of a presentation by the candidate , highlighting the main aspects of the research project followed by a extended discussion based on the questions that each of the committee members raised in reference to the research works that were presented by Olatz Idigoras in her talk or the written thesis. After its final deliberation, the committee decided to award the candidate the Doctor Degree with international doctoral mention and with the highest qualification existing at the University of the Basque Country “apto cum laude”.