Estudio de ferritas duras nanoestructuradas y su acoplamiento con una capa blanda

Resumen

In the last 100 years, permanent magnets have played a key role in advancing many fields of technological innovation. These materials have been used mainly for their application in motors and generators since they allow the transformation of electrical energy into mechanical energy and vice versa. Other uses include recording media, microwave, radiofrequency and magneto-optical device components. However, at present, the best permanent magnets are composed of a considerable proportion of rare earths. Rare earths present two significant problems: their extraction causes great environmental damage, and China controls both extraction and separation. To avoid these handicaps, efforts are devoted to develop new permanent magnets that do not contain rare earths. In this context, these research studies magnetic materials based on hard oxides that can replace magnets that include rare earths. Specifically, the oxide studied in depth in the thesis has been strontium hexaferrite (SrFe12O19, SFO). Since its discovery in the mid-twentieth century, this hexagonal ferrite has become a material of great commercial and technological importance thanks primarily to its high magnetocrystalline anisotropy coupled with its low cost. However, despite its high coercive field, strontium hexaferrite presents a moderate remanent magnetization, which results in energy product values below those achieved in rare earth permanent magnets. A strategy to improve the material magnetic properties would be coupling it with a magnetically soft material. Under the appropriate conditions, this combination allows the soft material to increase the magnetization of the system without significantly reducing the coercive field provided by the magnetically hard material. Thus, a higher energy product is obtained. Hence, an important point in this research will be to understand the magnetic coupling at the interphase of two materials with substantially different coercivities. This is an ongoing and subtle scientific problem underlying the development of future spintronic/nanomagnetic devices and advanced permanent magnets. Importantly, these materials are also very interesting systems for understanding collective and individual magnetization reversal. Their collective behaviour depends on the magnetic properties of the individual layers and the dominant interactions between them: direct exchange coupling and/or magnetostatic interactions. The first stage of the thesis focuses on understanding the structural and magnetic properties of SrFe12O19 and exposes the characterization of this compound in platelets form by different microscopic and spectroscopic techniques. A novel result of this section was to obtain for the first time its X-ray absorption spectrum. The magnetic coupling with a magnetically soft material (cobalt) grown by molecular beam epitaxy (MBE) was investigated. This study was carried out in the photoemission electron microscope (PEEM) by means of X-ray circular magnetic dichroism (XMCD) on the ALBA synchrotron. This technique allowed the determination of the magnetic domains of each layer. In parallel, micromagnetism simulations were performed to understand the magnetic behavior observed in the experimental results. The analysis revealed SFO platelets of hundreds of nanometers in size with magnetization preferential normal to the platelet plane. The platelet-metal system revealed a lack of magnetic coupling due to competition between the magnetocrystalline anisotropy of the platelet with the shape anisotropy of the cobalt layer. To avoid such competition and promote magnetic coupling between the two compounds, the second stage of the thesis consists of growing SFO thin films by sputtering with the magnetic orientation preferentially in-plane for subsequent metal deposition by MBE. Initially, the annealing effect on the formation of the crystalline phase of the thin films was tested, and the parameters involved in modifying the easy axis magnetization of each sample were determined. The composition, structure and magnetism of these films were studied using different characterization techniques such as X-ray diffraction, Raman and Mössbauer spectroscopy. Again the study of the magnetic coupling with the magnetically soft layer was analyzed by PEEM-XMCD. In this experiment, a structural coupling was observed in the bilayer system. Completing this research, cobalt ferrite (CoFe2O4, CFO) has been studied due to its remarkable properties such as a high magnetocrystalline anisotropy and large magnetostriction constant. Like the ferrite discussed above, this compound is magnetically hard and is used in permanent magnet applications. In this third section, MBE-grown CFO thin films have been presented and discussed. These samples have been characterized in an ultra-high vacuum system with microscopy and in-situ spectroscopy techniques with particular emphasis on the discussion of Mössbauer spectroscopy. The variation in thickness and heating with and without oxygen promotes changes in the stoichiometry of the grown phase and consequently to its properties. Finally, to comprehend the origin of the spring-magnet coupling observed in an experimental system consisting of a CoFe2O4 thin film and a iron-cobalt alloy layer, micromagnetic simulations were carried out. These simulations supported the propagation of domain walls in the soft phase as the dominant mechanism for the bilayer magnetic behaviour of the bilayer, in contrast to theoretical models, which do not take into account this effect.