Micromagnetic modeling of skyrmion dynamics and THz oscillation in antiferromagnets

The main goal of this thesis is the achievement of further understanding on various hot topics related to Spintronics. The name Spintronics is a combination word made of spin and electronics. Electronics concerns the creation and control of electron currents via their charge, which is well known as charge current, while the spin provides a new option to control the charge currents. This connection between electron charge and spin admits to change the electronic transport by spins, and, on the contrary, to alter the magnetic properties by electron charges. Thus, nowadays, the spin is fundamental for some of our technologies due to their interesting properties: nanometer dimension, low energy consumption, non-volatility, high scalability, large speed. Spintronic devices are usually composed of a trilayer where two ferromagnets are separated by a nonmagnetic spacer. One among of two ferromagnets layer can be manipulated by an external applied field as well as by a polarized electric current injected into the layer. This property gives rise to different technological applications of spintronic devices, importantly, as magnetic storages, which have seen a wide range in commercialization. In this thesis, two main topics have been investigated. Firstly, we have investigated the magnetic skyrmion dynamics in ferromagnetic materials. Secondly, we have investigated the antiferromagnetic dynamics. The main contributions of this thesis to the first topic has been about magnetic skyrmions which are chiral spin textures characterized by a non-uniform distribution of the magnetization. They have found widespread range of applications because they can be easily nucleated, moved and shifted by spin polarized current. The center of our attraction is the dynamics of skyrmions driven by the spin-hall effect in a synthetic antiferromagnet. We carried out a theoretical study based on micromagnetic simulations, at room temperature, in presence of thermal fluctuations. We notice that the motion of the skyrmion follows a stochastic flow of motion. Hence, this result paves the way to use skyrmions as building blocks of random bit generators, which can be very useful in modern technology. Another study has been made about the skyrmion topic. We prove that thermal fluctuations excite at least two nonstationary thermal modes which deeply affect the skyrmion dynamics driven by the spin-Hall effect. In particular, to get further fundamental understandings, we perform deterministic simulations where we are able to control the breathing mode of the skyrmion – that in real samples can be due to thermal fluctuations and/or disorder - and therefore we can qualitatively show the effect of such thermal modes on the skyrmion dynamics. We also build up a generalized Thiele equation and introduce an experiment to validate our findings. We have finally shown, in the second topic, the results carryied out on antiferromagnets. We have worked on modeling antiferromagnets by means of a micromagnetic formalism. We have studied the fundamental properties of the model both in term of resonance response and self-oscillation. We have investigated the AFM resonance as a function of the homogeneous intersublattice exchange. We have also executed a ferromagnetic resonance studies, the results indicates that the ferromagnetic resonance frequency coincides with the frequency of the self-oscillation at the critical current. In particular, we also focused on the analytical derivation of the expression of modes of antiferromagnetic order in easy plane and easy axis materials. The antiferromagnet acoustic mode is only shown because the optical mode exists in some specific conditions. In addition, we have worked on the synthesis and the characterization of microparticles of manganese (II) oxide to study its magnetic properties.